screw compressor hand book

Bearings in twin screw compressors

Application handbook

Bearings in twin screw compressors

Application handbook

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This application handbook is oneof a series of application handbooksdesigned to provide specific application recommendations forSKF customers to be used with theSKF General Catalog 4000.

It is not possible, in the limitedspace of this handbook, to presentall the information necessary tocover every application in detail.

SKF applications engineers should becontacted for specific bearing recom-mendations. The higher the technicaldemands of an application and themore limited the available experience,the more advisable it is to make useof SKF's engineering service.

We hope you find this handbookinteresting and useful.

Preface

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Contents

Preface

General

Twin screw compressor bearings

Ball bearings in twin screw compressors

Roller bearings in twin screw compressors

Bearings for high speed compressors

Air compressors

Refrigeration compressors

Natural and sour gas compressors

Unit conversion

References

6

1 General - Introduction

General

IntroductionThe twin screw compressor wasinvented in the 1930’s by AlfLysholm, Chief Engineer at SvenskaRotor Maskiner (SRM) in Stockholm,Sweden. SRM acquired several keypatents on the new compressor. Thefirst application of the twin screwcompressor was a supercharger forjet engines for airplanes. After furtherdevelopments, an industrial air com-pressor was introduced in the mid1940’s. At that time, SRM also start-ed to sell technology licenses. Thefirst license was sold in the UK in1946, followed by others in Europe,Japan and the USA. SRM is stillactive in compressor developmenttoday.

The first air compressors werehigh speed machines using externaltiming gears to synchronize therotors. The rotors were designed torotate without contact with the com-pressor housing or with each other.This compressor type was called“dry running” because the rotorsoperate without an injection of a fluidinto the compression cavity.

A new development in the late1950’s was the oil flooded air com-pressor. This design did not use tim-ing gears. Instead, the male rotordrives the female by contactsthrough the rotor flanks. Oil is inject-ed into the compression cavity forpurposes of lubrication, sealing andcooling.

In the late 1960’s, the oil floodeddesign was used in the developmentof industrial refrigeration compressorsusing both ammonia and halocarbonsas refrigerants.

In the early 1980’s, the industrialrefrigeration compressors were fol-lowed by air conditioning compres-sors using primarily CFC-12 refriger-ant and later HFC refrigerants.

The twin screw compressor designcompeted with the reciprocating pis-ton compressor. The twin screw com-pressor design offered potentialadvantages in smaller physical size,lower vibration and noise level, andimproved reliability, efficiency andcost. These benefits were sometimesnot easily realized since the recipro-cating piston compressor was easierto manufacture and could be madewith less sophisticated equipment.Reciprocating piston compressors arestill used today in many applications,at low volumes and high pressures.

From a rolling element bearingapplication standpoint, the twin screwcompressor is very important since alarge number of rolling element bear-ings are used in each compressor.

5

The oil flooded design made it possi-ble to operate at a lower speed whilemaintaining compressor efficiencyand reducing the cost.

1

Photo courtesy ofAtlas Copco

1 General - twin screw compressor function

Twin screw compressorfunctionIn a twin screw compressor, the twomeshing rotors are turning in oppo-site directions inside the compressorhousing. On the suction side of thecompressor, gas is drawn into thesuction opening in the housing andinto the cavity produced between thehousing wall and the two rotors. Asthe rotors turn in opposite directions,the cavity increases in size andmoves forward, drawing in more gasuntil the cavity has passed the suc-tion opening in the housing. At thispoint the cavity begins to decrease insize as it continues to move forwardin the compressor. As the cavityreaches the discharge side of thecompressor, the compressed gas isdischarged through the dischargeopening in the housing (→ fig ■ ).

��cba

��

The position of the suction and dis-charge openings can be varied bymeans of a sliding valve. This makesit possible to control both the sizeand the ratio between the suction anddischarge gas volume. The pressureincrease in the compressor dependson the volume ratio, but for a givenvolume ratio, the pressure ratiodepends on the thermodynamic properties of the gas.

Several compression cavities invarious stages of compression arebeing compressed simultaneously.The number of cavities equals thenumber of lobes of the male rotor.Since each cavity has a differentpressure, a small amount of gas willleak from a cavity with high pressureto one with a lower pressure. Theleakage results in loss of efficiency.By minimizing the clearance betweenthe rotors and the housing, the leak-age is minimized. Three differentclearances must be considered, theclearances between the tips of therotors and the cylindrical surface inthe housing, the clearance betweenthe end faces of the rotors and thehousing ends, and the clearancebetween the rotors. The rotor endclearance is adjusted by axial posi-tioning of the thrust bearing duringcompressor assembly, usually bygrinding a shaft spacer to a widthdetermined from measurements ofcompressor components.

6

Fig 1

1

7

female rotor male rotor

��asymmetric profile

female rotor male rotor

symmetric profile

1 General - twin screw compressor types

Twin screw compressortypesIt is possible to classify screw com-pressor types in many differentterms. The following distinctions ofcompressor types are important:• Flooded compressors• Dry running compressors

Flooded compressorsIn flooded compressors, a fluid isinjected into the rotor cavities duringthe compression process. The pur-pose of the fluid injection is:• To seal the leakage gaps between

the two rotors by filling them with fluid

• To absorb compression heat fromthe gas

• To lubricate the contacts betweenthe two rotors

Oil flooded screw compressorsIn oil flooded compressors, oil per-forms all of the three functions men-tioned above. The sealing of leakagegaps and lubrication of the rotor con-tacts are very efficient.

The injected oil passes out of therotor cavity with the discharge gaswhich passes through an oil separa-tor. This separates most of the oilfrom the gas. The return oil from theseparator is delivered to an oil reser-voir to be used again for injection inthe compression cavities and forbearing lubrication.

Oil flooded compressors operatewith rotor tip speeds in the range 30-50 m/s, which means bearing ndm*values of 250,000-650,000. Typicalshaft speeds are in the range of 3000to 6000 r/min. Typical pressures for oilflooded screw compressors are 7 to13 bar in one single compressionstep.

*ndm is the bearing speed n in r/min multiplied by the

bearing mean diameter dm in mm.

dm =(d+D)/2

1

Fig 2a Fig 2b

Fig 2c

female rotor male rotor female rotor male rotor

Water flooded compressorsWater is also used for injection intothe rotor cavity to absorb compres-sion heat and to seal the leakagegaps. Since water has a high specificheat, water injection is more efficientat absorbing the heat and therebyreducing the discharge temperature.

Water is corrosive and is not anefficient lubricant, therefore the rotorshave to be either made of stainlesssteel or coated with a polymer orsimilar material. An alternate designuses stainless steel rotors which donot touch and external timing gearsare used to facilitate appropriatemeshing.

Water injected compressors musthave efficient seals between therotors and the bearings to preventleakage of water into the bearinglubricating oil.

Liquid refrigerant injected compressorsIn refrigeration compressors, it ispossible to inject liquid refrigerantinstead of oil into the compressioncavities. The heat necessary tovaporize the liquid refrigerant isabsorbed from the compressionprocess. This increases the adiabaticefficiency.

The compressor design can beeither similar to the water injectedcompressor or a few percent of oildissolved in the refrigerant can beused for bearing lubrication.

1 General - twin screw compressor types

Dry running compressorsDry running screw compressors useexternal timing gears mounted on theextended rotor shafts for accuratemeshing of the two rotors. The rotorsare designed never to touch. For thisreason, the clearances between thetwo rotors and between each rotorand the housing have to be larger indry running compressors.

Dry running compressors tend tooperate at higher temperatures thanflooded compressors because no liq-uid is injected between the rotors.Because of the larger clearances andthe lack of fluid for sealing the clear-ances, the leakage rate is higher fordry running compressors, thereforethey are designed to run at highspeed. By running at high speed, thecompression is faster and there isless time for the leakage to occur.

Dry running compressors operateat rotor tip speeds over 60 m/s andbearing ndm* values in the range of650,000-1,200,000. Typical shaftspeeds are in the range of 10,000 to25,000 r/min.

Since there is no contamination ofthe air by oil injection, dry running aircompressors are used in applicationssuch as medical and electronics andothers where air contamination withoil is sensitive or environmentally pro-hibited.

Typically the pressure is 3 to 7 bar.To reach 7 bar, two compressionsteps are needed, with intermediatecooling of the air.

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dm =(d+D)/2

Twin screw compressorbearing function andselection criteriaThe function of bearings in twinscrew compressors is to provideaccurate radial and axial positioningof the rotors and to support the loadon the rotors. These functions are tobe performed reliably, with low fric-tion and low noise generation.

With accurate positioning of therotors, it is possible to design thecompressor with small clearances forhigh efficiency. Radial positioningaccuracy of the rotors is accom-plished by using bearings havingsmall operating clearances and highrunning accuracy (low run-out). Axialpositioning accuracy is accomplishedby small axial bearing clearance orpreload. Axial positioning accuracy isalso affected by the fit between thethrust bearing inner ring and theshaft and bearing deflection and dis-placement due to centrifugal forces.Interference fits will change the axialposition of the outer ring after mount-ing. Axial positioning accuracy is alsoaffected by the accuracy of theadjustment of the rotor end clear-ance during assembly. Thrust bear-ings mounted with interference fitsmake the adjustment more difficult.

In large industrial refrigerationcompressors and in some air condi-tioning compressors, the availablerotor center distance can be a prima-ry criteria in the bearing selection.The rotor profile affects the centerdistance; therefore, the rotor designand bearing selection process issometimes iterative. The center dis-tance limits the size of the bearingouter diameters. Therefore, if onerotor carries a higher load, it is possible to select a larger bearing for this rotor and a smaller bearingfor the other.

This design option is however inconflict with the desire to minimizethe number of different bearings inthe compressor and also requiresadditional tooling for the production ofthe compressor housing.

The rotors can be supported onrolling bearings or on a combinationof hydrodynamic and rolling bearings.The main advantage with rolling bear-ings is their small operating clear-ances. Rolling bearings also havelower friction than hydrodynamicbearings, require less oil for lubrica-tion and cooling, and are less sensi-tive to momentary loss of lubricantand refrigerant flooding than hydrody-namic bearings.

Twin screw compressorbearing loadsBearing loads in twin screw compres-sors are resultants of the followingfactors:• Gas pressure on the rotors• Gear forces from input and timing

gears• Rotor forces from transmission of

torque from one rotor to the other• Induced loads from the inertia of the

rotors at startup• Electric motor rotor weight and

forces for semi hermetic compressors

• Spring preload or balance piston forces

The gas pressure is low at the suc-tion side and then increases towardsthe discharge side. The gas pressurealong the length of the rotor cavityproduces radial forces on the rotors.These forces are heavier towards thedischarge side.

The gas also produces axial forcesfrom the pressures acting on the pro-jected areas at both the suction anddischarge end of the rotors. The dif-ference between these two forces isthe net axial force on the rotor. Thisforce is always directed towards thesuction side and is heavier on themale rotor. In order to reduce the

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1 General - twin screw compressor bearing function,selection criteria and bearing loads

1

axial force from the rotor, stationaryor rotating balance pistons are some-times used. A rotating balance pistonis a disc mounted at the dischargeend of the rotor. Gas at dischargepressure from the compressor isallowed to act on the end face of thedisc, producing an axial force direct-ed towards the discharge side. Thisforce helps to balance out the netaxial gas force on the rotor (→ fig ■).

A stationary balance piston uses abearing for transmission of the bal-ancing force to the rotor (→ fig ■).

If the compressor is gear driven,the forces from the input gear arealso supported by the bearings. Byvarying the gear helix angle, it is pos-sible to control both the magnitudeand direction of the gear axial forces.Sometimes the axial gear force is

used to balance the net axial gasforce. This can cause the net axialgas force on the rotors to reversewhich can cause rubbing betweenthe ends of the rotors and the hous-ing on the discharge side if the axialclearance of the selected bearingarrangement is larger than the rotorend clearance. Too low an axial forceon the bearings can also be detri-mental to the bearings if the loadsbecome less than the minimumrequired load for satisfactory opera-tion. The timing gear forces and iner-tial forces from transmission of torquebetween the rotors are usually small,except at compressor startup.

Analysis of bearing loads in screwcompressors is very complicated and should be performed throughdetailed analysis of compressordesign parameters.

In order to

reduce the

axial force

from the rotor,

stationary or

rotating bal-

ance

pistons are

sometimes

used. A rotat-

ing balance

piston is a

disc mounted

at the dis-

charge end of

the rotor.

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1 General - twin screw compressor bearing loads

Fig 3

��

��

3

4

�� Gas Force

A stationary

balance piston

uses a bearing

for transmis-

sion of the

balancing

force to the

rotor.

11

1 General - twin screw compressor bearing loads

1

Fig 4

Bearing typesFigure illustrates the rolling bear-ings used in twin screw compressors.The most commonly used bearingtypes are the single row angular con-tact ball bearing and the cylindricalroller bearing, however, deep grooveball bearings, four-point contact ballbearings, needle roller bearings, andtaper roller bearings are also used.

Bearing lifeIt is recommended that the basic rat-ing life L10h and the SKF Life Theoryrating life L10aah be used to select theSKF bearings for screw compressorapplications. The SKF Life Theoryconsiders that a rolling bearing canhave infinite fatigue life provided theapplied loads are below the fatiguelimit, the bearing operates in a suffi-ciently clean environment, and wasmanufactured to accurate tolerancesand with high quality steel. The SKFLife Theory enables the optimumbearings to be selected based on theservice life conditions.

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13

2 Twin screw compressor bearings - bearing types

Twin screw compressorbearings

2

Fig 5

Approximate relative load, speed and misalignment capabilities

Misalign-ment

SpeedAxialload

Radialload

X X XXX XX

XX XXXX XXX X

XX XXX XXXX Xone

direction

XXXX — XX X

XXXX — XX XXX

— XXX XXX X

XXXX XXXX XX X

—: No CapacityX: LowXX: Moderate

XXX: HighXXXX: Very High

Single row deep groove ball bearing

BE designsingle row angular contactball bearing pair

Needleroller bearing

CARBTM

Four-point contactball bearing

Taper rollerbearing set

Cylindricalroller bearing XXX — XXXX X

High speedsingle rowangular contactball bearing pair

��

��

��

The equations to use when calculating the bearing rating life are as follows:

1 000 000 (C)p

L10h =60 n

L10aah = aSKF L10h

whereL10h = basic rating life in

operating hoursn = rotational speed, r/minC = basic dynamic load

rating, NP = equivalent dynamic bearing

load, Np = exponent for the life

equationp = 3 for ball bearingsp = 10/3 for roller bearings

L10aah = adjusted rating life according to SKF Life Theory in operating hours

aSKF = life adjustment factor based on SKF Life Theory

Each individual bearing within thecompressor is generally selected toprovide a basic rating life, L10h in therange of 20000-60000 hours. Careshould be taken not to over-dimensionbearings to achieve too long bearinglife due to risks of higher friction andlight load skidding. Because of the highreliability requirements and number ofbearings used in compressors, thesystem life, L10s adjusted with the SKFLife Theory is sometimes considered.System life can be used to compareentire bearing arrangements.System life, L10s can be calculated as follows:

NL10s = [ Σ (1/L10i

es ) ]i =1

whereL10i = life of an individual bear-

ing, hours or revolutionsL10s = system life, hours or

revolutionsN = number of bearings in

the systemei = 10/9 for ball bearings only;

1.35 for roller bearings only, if p = 10/3, es for ball and roller bearing systems

es = exponent for all bearings inthe system; for ball and roller bearing systems, this can be estimated with the formula:

Nes = (Σ ei ) / N

i =1Whenever possible the loading used

to evaluate the selection of the bear-ing should be based on the duty cyclein which the compressor will be oper-ated. The duty cycle considers theperiod or percentage of time that thecompressor will operate at a givenload, speed, temperature, etc. condi-tion.

The adjustment factor aSKF forapplication of the SKF Life Theory isdependent on the viscosity (υ) of thelubricant at the operating conditionscompared to the minimum requiredviscosity (υ1), the fatigue load limit ofthe bearing (Pu), and the contamina-tion level (ηc) in the application. Toenable a systematic and consistentevaluation of the contamination level,an SKF computer program, CADalog®

(available upon request) has beendeveloped for application of the SKFLife Theory. Continued research onthe quantification of the contaminationlevel in applications and the use withthe SKF Life Theory will lead to fur-ther refinement of this program.

Contact SKF ApplicationsEngineering for assistance in selec-tion of the parameters used in thecomputer analysis.

2 Twin screw compressor bearings - bearing lubrication

14

-1/es

P


15

2

If κ > 4, use κ = 4 curveAs the value of ηC (Pu/P) tends to zero, aSKF tends to 0.1 for all values of κ

Factor aSKF for radial ball bearings

50

20

10

2

1

0.5

0.1

0.05 0.01 0.02 0.05 0.1 0.2 0.5

0.1

0.15

0.2

0.3

0.4

0.5

0.6

0.8

Κ=4

1

2

2 1 5 0.005

0.2

5

SKFa

ηcP

Pu

Fig 6


16

Fig 7

If κ > 4, use κ = 4 curveAs the value of ηC (Pu/P) tends to zero, a SKF tends to 0.1 for all values of κ

Factor aSKF for radial roller bearings

50

20

10

2

1

0.5

0.1

0.05 0.01 0.02 0.05 0.1 0.2 0.5 2 1 5 0.005

0.2

5

SKFa

ηcP

Pu

0.1

0.15

0.2

0.3

0.4

0.5

0.6

0.8

κ=4

1 2

cosity with increase in temperature.From this chart, the viscosity of anISO Grade oil can be determined atthe bearing operating temperature.

Synthetic oils are also used in com-pressors. The main reasons arehigher thermal stability which resultsin reduced carbon buildup on hot sur-faces, and in refrigeration compres-sors, miscibility characteristics of theoil and the refrigerant. For example,miscibility with HCFC-134a is the rea-son why polyolester (POE) oils areused with this refrigerant. Syntheticoils have a higher Viscosity Indexthan mineral oils and therefore havea higher viscosity at elevated temper-ature (→ fig ■). The Viscosity Indexof synthetic oils can be in the rangeof 130 to 200. Oils having a highViscosity Index have less viscositydecrease with increase in tempera-ture. Common synthetic oil


Bearing lubricationBearings used in twin screw com-pressors are lubricated by a flow ofcirculating oil. The oil lubricates therolling contact surfaces and the slid-ing surfaces within the bearing. Thelubricant also provides corrosion pro-tection and cooling to the bearings.The oil lubricating the bearings canbe the same oil injected into the com-pressor to lubricate the rotors andremove the heat of compression. Indry running (oil free) compressors theoil is supplied directly to the bearingsand sealed from the compressioncavity. The principal parameter for theselection of a lubricant for the bear-ings is the operating viscosity, υ.

Lubricating oils are identified by anISO Viscosity Grade (VG) Number.The VG Number is the viscosity ofthe oil at 40°C (104°F). The commonmineral oil grades are shown (→ fig ■).This is for oils having a ViscosityIndex of 95. The Viscosity Index ( VI )is indicative of the change in oil vis-

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2

Fig 8

9

8

-20 -10 0 10 20 30 40 50 60 70 80 90 100 120 150

2000010000

5000300020001000

23001250900700470350240190140

10080

60

50040030020015010075504030

2015

10

8

65

4

Temperature, degrees Celsius

NOTE Viscosity classification numbers are according to International Standard ISO 3448-1975 for oils having a viscosity index of 95.

App

roxi

mat

e V

isco

sity

con

vers

ions

sayb

olt u

nive

rsal

sec

onds

(S

US

)

Vis

cosi

ty c

entis

toke

s (m

m

)

Approximate temperature conversions degrees Fahrenheit

50 90 120 140 160 175 190 210

2 / s

ISO VG 680VG 320VG 150

VG 100VG 68VG 48

VG 220

VG 460

VG 32ISO VG 22

air. The actual lubricant selected foran application should ideally providegreater viscosity than the minimumrequired viscosity υ1 (i.e. Kappa >1.0).For bearings operating in air com-pressors, the viscosity ratio, Kappa,should be the guideline for evaluationof satisfactory viscosity. Kappa > 1.5is preferred. The lubricant viscosityshould not be too great since thiscauses excessive bearing friction andheat.

Some synthetic lubricants have dif-ferent effective viscosity in the rollingcontact as compared to mineral oilsdue to a greater or lower viscosityincrease in the pressures of therolling contact. This difference is indicated by the pressure-viscositycoefficient (α) of the lubricant. Theadjusted viscosity, υadj for use in theevaluation of the bearing lubricationcan be determined as follows:

types used in compressors are thepolyalphaglycol (PAG), polyal-phaolefin (PAO), and POE. Syntheticoils are more thermally stable thanmineral oils and therefore have longerservice life. Synthetic oils can reducethe bearing internal rolling friction forimproved compressor efficiency.Furthermore, it should be noted thatfor the same basic viscosity, syntheticoils have different oil film thicknessformation capability compared to thestandard mineral oils. This is becauseof their different pressure viscositycoefficients (see literature on EHLtheory). This coefficient can be higheror lower compared to mineral oils.This point should also be consideredwhen selecting an oil.

The lubricant viscosity requirementsfor a rolling bearing depend on bear-ing size dm and operating speed n,but little on bearing load. The mini-mum required lubricant viscosity υ1needed at the bearing operating tem-perature is obtained from (→ fig ■).The minimum required viscosity fromfigure 10 is for bearings operating in


18

Fig 9

Temperature °C

Vis

cosi

ty m

m

SHC (VI 150) Mineral (VI 95)

6040200-20 80 100 120 140

10000

1000

400

200

100

40

20

10

8

6

4

2 / S

10

The gas in the compressor (refrig-erant, air, natural gas, etc.) can havea significant influence on the operat-ing viscosity of the lubricating oil (seethe section on refrigeration compres-sors). Extreme pressure and anti-wear additives are generally not usedin refrigeration compressors.

Compressor lubrication systemsoften include filtration to remove solidparticle contamination. Filtration isneeded to clean the system of debristhat can clog the small clearances inthe compressor, the orifices of thelubrication and refrigeration system,and damage the bearings. Bearinglife is affected by the cleanliness ofthe compressor and lubricant. Filtersare rated according to a β ratio. The βratio defines the efficiency of the filterto remove particles and the size ofthe particle. The βx ratio is defined asfollows:

No. of Particles entering filterβx =

No. of particles leaving filter

x = particle size, microns (µm)

For example, a rating of β3 = 200means that for every 200 particlesthat enter the filter, only 1 particlegreater than 3 microns leaves the fil-ter. The finer the filter, the morequickly the system is cleaned of cont-amination. The filter specification incompressors is typically in the rangeof β3 = 200 to β12 = 75 . Finer filtersalso increase bearing life, but thedegree of increase depends also onthe viscosity ratio Kappa and thebearing load intensity Pu/P. If theKappa value is high, a change to afiner filter can give significantimprovement in bearing life. If Kappais low, a finer filter cannot compen-sate for the poor lubrication and thebenefit may be questionable. In sucha case it may be more effective toincrease the bearing size.

For evaluation of filter specifica-tions, SKF Applications Engineeringcan consult computer programswhich take into account all of theabove mentioned factors.

υadj = υ (α / αmineral)0.72

where:α = pressure-viscosity

coefficient of the actuallubricant, MPa-1

αmineral = pressure-viscosity

coefficient of mineral oil, MPa-1

The frequency of oil changesdepends on the operating conditionsof temperature, quality of the lubri-cant, and the cleanliness of the bear-ing and lubricating system. Mineraloils oxidize and require shorterreplacement intervals compared tosynthetic oils. The actual replace-ment interval is specified by the com-pressor manufacturer. Longer inter-vals between replacements are pos-sible at lower operating temperatures.Compressors are usually equippedwith oil coolers to remove heat.Synthetic oils are more resistant todeterioration from exposure to hightemperature and allow long servicelife. Lubricants may require more frequent replacement if contaminationis present.


d = (bearing bore + bearing O.D.) ÷ 2υ = required lubricant viscosity for adequate lubrication at the operating temperature

m

1

mPitch diameter (mm) d mm1000

1000

500

200

100

50

20

10

5

3

2300

4600

930

460

230

100

60

40

35500200100502010

Cen

tisto

kes

(mm

/s)

2

(mm /s)2

υ1

υ1

SUS

App

roxi

mat

e sa

ybol

t uni

vers

al s

econ

ds (

SU

S)

100000

50000

20000

10000

2000

1500

1000

500 RPM

200

100

50

20

10

5 5

2

30005000

1

19

2

Fig 10

The permissible operating speed isgenerally dependent on the kinemat-ics of the rolling elements, which isinfluenced by these bearing features.

Consult SKF ApplicationsEngineering for advice and availabilityof bearings suitable for high speedoperation.

Table below shows the recom-mended executions for the differentbearing types for ranges of ndm* val-ues. These recommendations aregenerally valid for twin screw com-pressor applications only.

1

Bearing speed ratingsThe bearing speed rating listed in theSKF General Catalog is an indicationof the speed that the bearing canoperate for an expected operatingtemperature under well defined con-ditions of either static oil bath lubrica-tion or grease lubrication. When thebearing is lubricated with a flow ofcirculating oil, such as in screw com-pressors, the speed ratings in theSKF General Catalog can generallybe exceeded provided considerationis given to specific bearing features:cage construction, internal clearance,and running accuracy and precision.

2 Twin screw compressor bearings - bearing speed ratings

20

Recommended bearing executions for different speed ranges

SpeedRange

ndm

Upto

450,000

450,000to

650,000

650,000to

850,000

850,000to

1,000,000

1,000,000to

1,200,000

single rowangularcontact ballbearingsBE design

Cage Precision

four-pointcontactballbearings

Cage Precision

deepgrooveballbearings

Cage Precision

cylindricalrollerbearings

Cage Precision

taperrollerbearings

Cage Precision

P,Y N 1) MA,FA N 1) J N 1) P,J N 1) J N 1)

M,F P6 MA,FA N 1) J N 1) P,ML N 1) J *

MA,FA P6 MA P6 ML N 2)

MA P6 MA P6 ML N 2)

LA P5 MA P5 ML P63)

Oil jet lubrication required at ndm over 850,000

1) N = normal precision 2) P6 running accuracy required however SKF Cylindrical Roller Bearings have this as standard 3) Shaft roundness tolerance to IT3/2 is recommended

* CL7C design ** An execution for special high speed angular contact ball bearings has been developed by SKF; these bearings have different cage

executions and contact angles than those of the BE design. See section on "Bearings for high speed compressors".



dm =(d+D)/2

Table 1

Speed

is

too

high

Please

contact

SKF

Appl.

Eng.**

Operating temperature is increasedwhen heat is transferred to the bear-ings from compression heat and/or anelectric motor. In refrigeration com-pressors, evaporation of refrigerantflowing through the bearings carries away heat. A lubricationmethod patented by SKF makes itpossible to enrich the oil concentrationin the bearings by the use of bearingfrictional heat to vaporize the refriger-ant and thereby deposit oil in the bear-ings. SKF makes this method availableroyalty free to its customers. ContactSKF Applications Engineering formore information.

It is important to know the expectedtemperature differences between thebearing inner ring and outer ring sothat sufficient initial or unmountedclearances are provided. This is alsodependent on the selected shaft andhousing fits. Contact SKF ApplicationsEngineering for a computer evaluationof the bearing clearance and fitting.

Bearing mountingShaft fits The recommended shaft tolerances for ball and roller bearings in twinscrew compressor applications sup-porting radial load or combined axialand radial loads are given in Table .2

Bearing temperatureIn general, the allowable operatingtemperature of a bearing is limited bythe ability of the selected lubricant to satisfy the bearing's viscosityrequirements (i.e. Kappa). Rollingbearings can achieve their rated lifeat high temperatures provided thelubrication is satisfactory, and otherprecautions such as the correctselection of internal clearance etc.are considered. In some cases, theallowable bearing operating tempera-ture is limited by the bearing compo-nent material, for example, polymercages. Consult the SKF GeneralCatalog for information on the per-missible operating temperature ofbearings. The use of bearings insome refrigerant conditions lowersthe allowable operating temperatureof some bearing components. SeeSection ■ of this handbook for addi-tional detail. In some instances, theoperating temperature is the limitingfactor determining the suitability of abearing for an application.

The operating temperature of abearing is dependent on the bearingtype, size, operating conditions, andrate of heat transfer from the shaft,bearing housing, and foundation.

2 Twin screw compressor bearings - bearing temperature

21

2

shaft diameter. tolerance for tolerance for1 tolerance for tolerance for tolerance for tolerance for mm cylindrical and single row taper roller single row taper roller four-point

needle roller angular contact bearings taking angular contact bearings taking ball bearingsbearings and deep groove axial load only and deep groove combined loads taking axial loads

ball bearings ball bearings onlytaking axial load taking combinedonly loads

< or = 18 m5 h5 j5 j5 j5 j5(18) to 100 m5 h5 j5 k5 k5 j5(100) to 140 m5 h5 j5 m5 m5 j5(140) to 200 m6 h5 j5 m6 m6 j5

1) for ndm greater than or equal to 650,000 use ISO k5 tolerance.

Table 2

7

The recommended shaft tolerances for ball and roller bearings in twin screw compressor applications

Housing fits, bearings takingaxial load onlyIt is common for bearings supportingonly axial load to be mounted radiallyfree in the compressor housing. Thehousing bore should be 1 or 2 mmlarger in diameter than the bearingouter ring. The axial load can varyand be very low or momentarilyreverse direction when the compres-sor is operating at reduced pressuresand volumes or at compressor start-up. For compressors with balancingpistons, the risk for reverse loading ishigher. In these cases, the appliedaxial load can be less than the fric-tion force between the face of thebearing outer ring and the compres-sor housing, in which case the outerring may creep or turn from its initialposition. This can cause wear anddamage to the housing and bearing.This wear can change the axial posi-tioning of the screw rotors changingthe performance of the compressor.

For the bearing outer ring not tomove against the housing, the axialforce on the outer ring must begreater than the internal friction in thebearing. The axial force can comefrom the applied load or additionallyfrom a clamping spring force (→ fig ■).

The recommendations above are thesame as those in the GeneralCatalog with the following exceptions:

• For smaller cylindrical and needle roller bearings, an m5 tolerance is recommended insteadof a k5 to avoid ring creep.

• Single row angular contact ballbearings taking only axial load can use an h5 or a k5 tolerance.The h5 tolerance makes mounting easier and the screw positioning more accurate since an interfer-ence fit will lead to axial displace-ment of the outer ring.

Housing fits, bearings takingradial loadsThe generally recommended housingtolerance for cylindrical and needleroller bearings supporting radial loadis ISO K6. This tolerance results in aninterference between the bearingouter ring and housing. This allowsfor easy assembly and radial clear-ance for bearing expansion withincreases in temperature and is rec-ommended to avoid creeping of theouter ring in the housing bore.

For easier mounting, it is also pos-sible to use J6 or J7 tolerance. SeeTable ■ on page 38.

22

2 Twin screw compressor bearings - bearing mounting

��Fig 11

8

11

MiscellaneousThe diameter of the abutment shoul-ders of the shafts (da) against whichthe bearing inner rings are seatedshould be designed toward the smaller diameter recommended inthe SKF General Catalog. The use ofsmall abutment shoulder diametersminimizes the possibility that formerrors, which might occur in the man-ufacture of the shafts will distort orcock the position and form of thebearing rings.

The shafts and housings in whichthe bearings are seated should bemanufactured within the recommend-ed limits of dimensional form andrunning accuracy according to theSKF General Catalog.

The rings of the bearing should notbe excessively clamped by the mat-ing components. The use of too higha clamp force can distort and deflectthe geometry and reduce the internalclearance in the bearings. The clampforce preferably should not exceedone quarter of the bearing basic static load rating (e.g.C0/4 ).

The necessary axial force can bedetermined from the following equation: Fa > 20 M0/D'

where Fa = necessary axial load to

prevent outer ring rotation, NM0 = load-independent frictional

moment in the bearing, NmmD' = mean diameter of bearing

outer ring side face, mmD' = ( D + D1)/2

The load independent frictionalmoment (M0) for the bearing shouldbe evaluated at the compressor start-up condition of temperature and viscosity.

Another criteria when determiningthe spring force is the magnitude ofreverse axial load. The spring forceselected should be the greater ofthese two forces. The spring forceshould be applied against the outerring of the reverse axial bearing (→ fig ■).

The bearing rings can also be slot-ted ( N1 or N2 suffix ) and fitted withan anti-rotation pin mounted in thehousing. This is common with four-point and single row angular contactball bearings. The bearings can alsobe fitted in a floating housing which ispinned to prevent rotation.

23


��

2

Fig 12

12

Bearing compatibility withgasesExposure of the bearings to gaseswithin the compressor may requirethe selection of certain specific mate-rials for the bearing seals and cages.The gases may adversely affect thematerials, making them age orbecome ineffective. Experience andtests made at the SKF Engineering &Research Centre in the Netherlandshave established the suitability of thecommon bearing materials for use insome gases. The gaskets, paints, andother seals within the compressormay also be affected by the composi-tion of the gases. In some cases, theeventuality that the gases could con-tain contaminates such as moisturemay also dictate the suitability of the material. Below is a list of maximum recommended tempera-tures and suitability for cage and seal materials for use in the variousgases commonly used in compressorapplications.

24


Cage/seal material SKF Suffix Air NH3 HCFC-22 HFC-134a Natural gas

Polyamide 6.6 P, TN, TN9 100 70 100 110 (2)Brass, pressed Y p np p p npBrass, machined M, MA, ML p p p p npSteel, pressed J p (1) p np pSteel, machined F, FA p p p np pNitrile (NBR) RS1 95 60 np np pViton® (FKM) RS2 190 np p (3) p

All temperatures are in C°p = Possiblenp = Not Possible

1.Because of the impaired lubrication, J cages have not been recommended in ammonia compressors. The problems associated with J cages inammonia compressors are cage wear and smearing between the cage and rolling elements. The experience is somewhat mixed, but severalcompressor manufacturers who originally used J cages in angular contact ball bearings and the old generation of cylindrical roller bearings haveredesigned to other types. However, other compressor manufacturers have successfully used ECJ cylindrical roller bearings. Based on thisexperience, the J cage may be used in cylindrical roller bearings, but verification testing by the user is recommended.

2.Natural gas with high concentration of hydrogen sulfide may be too acidic for polyamide materials.3.Viton® is resistant to this gas but is not compatible with PAG oils.® VITON is a registered trademark for DuPont Dow Elastomer’s flourocarbon elastomers.

Table 3

pressors. The mechanical sealrequires a minimum oil submergencefor adequate cooling and satisfactorylife. The closure of the deep grooveball bearing can perform this functionsince the bearing is mounted adja-cent to the mechanical seal. Thematerial of the bearing seal shouldbe checked for compatibility with therefrigerant. Nitrile, Viton®, and othermaterials can be used in the seals.Each material should be examined todetermine its compatibility with therefrigerant and oil used. The standardbearing shields can also perform thisfunction satisfactorily.

The angular misalignment capabili-ty is 2 to 10 minutes, depending onthe clearance and loading.

Single row angularcontact ball bearingsSingle row angular contact ball bear-ings are the most commonly usedball bearings in twin screw compres-sors. They are used to support pureaxial loads or combined radial andaxial loads. The most important fea-tures of this bearing type are its highaxial load capacity combined with ahigh speed rating. Single row angularcontact ball bearings operating with a small clearance or a light preload provide good positioningaccuracy of the shaft.

The most commonly used SKF sin-gle row angular contact ball bearingsare of the 72 or 73 series.

Deep groove ball bearingsSingle row deep groove ball bearingsare sometimes used to support theelectro-motor in hermetically sealedrefrigeration compressors, the gear-ing in high speed air compressors, and as the reverse axial load bear-ings in screw compressors. Deepgroove ball bearings are also used inscroll compressors and reciprocatingcompressors.

Bearings having steel andpolyamide cages can be used inmost cases. The bearing internalclearance depends on the applicationbut is typically greater than Normal(C3 suffix).

The bearing must support at least aminimum radial or axial load for satis-factory operation. The recommendedminimum radial load can be deter-mined according to the SKF GeneralCatalog. An axial load can be provid-ed by an axial wave spring. The rec-ommended minimum axial springforce is determined as follows:F = k d

whereF = minimum spring force, N k = factor between 5 and 10d = bearing bore diameter, mm

Deep groove ball bearings havingseals or shields can be used to main-tain an oil reservoir for the mechani-cal seal of hermetically sealed com-

25

3 Ball bearings in twin screw compressors - deep groove ball bearings,single row angular contact ball bearings

Ball bearings in twin screw compressors

3

® VITON is a registered trademark for DuPont Dow

Elastomer's fluorocarbon elastomers

The GA suffix also denotes that the bearing is universally matchable,but a pair of these bearings will have a light preload when mounted in any of the three arrangementsshown (→ fig ■). The GA preload is generally recommended in screw compressors.

Table ■ lists the values ofunmounted axial clearance and pre-load for the universally matchablebearings. The initial bearing clear-ance or preload is assured when thebearing rings are axially clampedtogether. The initial clearance in abearing pair is reduced or initial pre-load is increased by interference fitsand if the shaft and inner ring operatewith a higher temperature than theouter ring and housing.

SKF universally matchable bear-ings are produced with P6 precisionclass tolerances (ANSI/ABMA ClassABEC 3) as standard.

Caution: Single bearings are not to be used where only radial loadsare present.

Universally matchable single row angular contact ball bearingsSingle row angular contact bearingsare usually mounted in face-to-facearrangements to facilitate easyadjustment of the rotor end clear-ance. They must be manufactured foruniversal matching. SKF universallymatchable bearings are designatedwith one of the following suffixes: CA,CB, CC, GA, GB, GC. The first letterdenotes a clearance (C) or preload(G) and the second letter denotes themagnitude of clearance or preload.

Arrangements of universally match-able bearings usually support axialloads and assure accurate position-ing of the compressor shaft owing tothe small internal clearance or bear-ing preload. If the axial load of thecompressor is heavy, the bearingscan be arranged with a third bearingmounted in tandem, such as shown(→ fig ■).

The standard SKF bearings avail-able for universal matching have theCB or GA suffix, e.g. 7310 BECB or7310 BEGA. The CB suffix denotesthat the bearing is universally match-able, and that a pair of these bear-ings will have a certain axial clear-ance when mounted in any of thethree arrangements shown (→ fig ■).

3 Ball bearings in twin screw compressors - single row angular contact ball bearings

26

��FACE-TO-FACE BACK-TO-BACK FACE-TO-FACE TANDEM��*ndm is the bearing speed n in r/min multiplied by the


dm =(d+D)/2

Fig 13

13

13

13

4

27

3


Table

Clearance / Preload Class

Axial internal clearance of angular contact ball bearings of series 72 BE and 73 BE foruniversal pairing back-to-back of face-to-face (unmounted).

Bore Axial internal clearancediameter Classd CA CB CCover incl. min max min max min max

mm µm

- 10 4 12 14 22 22 3010 18 5 13 15 23 24 3218 30 7 15 18 26 32 40

30 50 9 17 22 30 40 4850 80 11 23 26 38 48 6080 120 14 26 32 44 55 67

120 180 17 29 35 47 62 74180 250 21 37 45 61 74 90250 315 26 42 52 68 90 106

d GA GB GCover incl. min max max min max min max min max min max

mm µm N µm N µm N

10 10 +4 -4 80 -2 -10 30 330 -8 -16 230 66018 30 +4 -4 120 -2 -10 40 480 -8 -16 340 97030 50 +4 -4 160 -2 -10 60 630 -8 -16 450 1280

50 80 +6 -6 380 -3 -15 140 1500 -12 -24 1080 305080 120 +6 -6 410 -3 -15 150 1600 -12 -24 1150 3250120 180 +6 -6 540 -3 -15 200 2150 -12 -24 1500 4300

180 250 +8 -8 940 -4 -20 330 3700 -16 -32 2650 7500250 315 +8 -8 1080 -4 -20 380 4250 -16 -32 3000 8600

Bore Preloaddiameter Class

Preload of angular contact ball bearings of series 72 BE and 73 BE for universal pairingback-to-back of face-to-face (unmounted).

Radial Clearance = 0.85 Axial Clearance(0.0010 in. = 25.4 µm)

4

28

increases friction. This in turnincreases the bearing temperature,reducing the effectiveness of thelubricant and the bearing life.Adequate axial load minimizes therisk of sliding.

Axial displacement of the bearinginner ring relative to the outer ring willoccur depending on the magnitude ofthe axial load and speed (→ fig ■).The axial displacement can causethe compressor rotor end clearanceto decrease. The rotor can contactthe housing if the axial force on thebearing is insufficient. The magnitudeof the the force varies with the rotorspeed. As bearing speed increases(ndm* values 250 000 and greater)the axial load to minimize gyratorymotion of the balls should be applied. Gyratory motion is ballspinning due to a gyroscopicmoment. Gyratory motion willincrease ball sliding and bearing friction. The value of ndm * wheregreater axial load is needed in a particular situation is influenced bythe magnitude of the applied loads,lubrication conditions, and construc-tion of the bearing cage. Insufficientload can also cause variation in theorbital speed of the balls. This willresult in increased loads on the cage and possibly cause damage.

CagesThe SKF BE design bearings areproduced standard with three option-al cages (→ fig ■): the glass fiberreinforced polyamide 6,6 cage (P suf-fix), the pressed brass (Y suffix), andthe machined brass cage (M suffix). The bearings are alsoavailable with a machined steel cage(F suffix).

Bearing minimum axial loadFor satisfactory operation, an angularcontact ball bearing must carry a certain minimum axial load. Atincreased speed, centrifugal forceson the balls will cause a change inthe contact angle between the innerand outer raceways, (→ fig ■). Thesedifferences in contact angle willcause sliding which damages theraceways, balls and cage, and


0 2000 4000 6000 8000 10000

-300

50

0

-50

-100

-150

-250

-200

axia

l dis

pla

cem

ent

(µm

)

axial load (N)��

α

α i

αo

Similar contact angles,inner and outer ring.

Variation in contact angles,inner and outer ring.

��

Fig 15 Fig 16

Fig 14

Axial displacement vs. axial loadSKF 7310 BEP Speed = 3600 rpm

BEP BEM BEY

15

14

16

*ndm is the bearing speed n in r/min multiplied by

the bearing mean diameter dm in mm.

dm =(d+D)/2

29

3

Bearings having small contactangles are better suited for highspeed, lightly loaded applicationsbecause of their lower requirementfor axial load.

During operation, the minimumrequired axial load in a bearing paircan be internally maintained by limit-ing the internal axial clearance. Withsmall axial clearance, the balls areloaded by centrifugal force againstthe raceways with nearly equal innerand outer ring contact angles. As the axial clearance increases, sodoes the difference in the inner and outer ring contact angles. Thisallows increased internal sliding.The minimum axial load can also be maintained by spring preloadingthe bearings.

Tandem mounted single rowangular contact ball bearings Single row angular contact ball bear-ings arranged in tandem are com-monly used in screw compressors tosupport high axial load. The tandemmounted bearings can be positionedadjacent to a cylindrical roller bearingwhich supports the radial load (→ fig ■).

The minimum required axial load forsatisfactory operation of angular con-tact ball bearings can be calculatedfrom the following equation:

Famin = A (n/1000)2

where:Famin = minimum required axial

load, NA = minimum load factorn = rotational speed, r/min

The equation above is more accu-rate than the corresponding equationgiven in the SKF General Catalog.

Values of minimum load factor, A forseries 72 and 73 BE single row angu-lar contact ball bearings are listed in Table .5


��Fig 17

SKF single row angular contact ball bearing minimum axial load factor

A factorsize bore 72 BE 73 BE

diameter (mm)

01 12 0.282 0.53602 15 0.421 0.90603 17 0.686 1.4104 20 1.23 2.6805 25 1.71 4.2906 30 4.07 8.1307 35 7.29 11.108 40 10.9 18.909 45 12.3 29.210 50 14.9 45.611 55 23.5 62.512 60 34.4 84.513 65 47.7 11114 70 56.3 14515 75 63.5 18516 80 85.0 23417 85 114 29218 90 149 36019 95 191 44020 100 239 63021 105 302 72322 110 375 905

17

Table 5

30


The angular contact ball bearingsare mounted radially free (RF) in thehousing, in which case they have aradial clearance of 1 or 2 mm withthe housing bore. In this case, it isappropriate to determine the ratinglife of each angular contact bearingindividually rather than as a set. TheSKF 72 and 73 BE series universallymatchable bearings have satisfactorytolerancing to distribute the axial loadequally in tandem mounted arrange-ments. For tandem bearings support-ing combined axial and radial loadthe bearing rating life is determinedaccording to the SKF GeneralCatalog.

For SKF bearings mounted in tan-dem supporting only axial load, thebearing rating life is determined usingthe applied axial load distributedamongst the bearings and using the basic dynamic load rating, Cfor one single row angular contactball bearing.

An example of the life calculationfor bearings arranged in tandem sup-porting only axial load is as follows:

(2) SKF 7310 BEGAP arranged intandem

Fr = 0, Fa = 10000 N

n = 3600 r/min

C (1 brg.) = 74100 N

P = 0.35 Fr + 0.57 Fafor Fa/Fr > 1.14

Load per bearing = Fa/2 = 5000 N

P = 0.57 (5000 N) = 2850 N

L10h = (C/P)p (1000000/60 n) = [74100 N/2850 N]3x

[1000 000/(60)(3600 r/min)] = 81370 hours

This calculation procedure does notapply to single row angular contactball bearings supporting both axialand radial loads. In these cases, fol-low the procedures of the SKFGeneral Catalog or contact SKFApplications Engineering to perform acomputer analysis.

Bearing preload The purposes of using preload inangular contact ball bearings are:avoidance of light load skidding, con-trol of contact angles, improvement tointernal load distribution, increasedbearing stiffness, and improved shaftpositioning accuracy. Bearing preloadcan increase the fatigue life of abearing by improving internal distribu-tion of the applied external loads (→ fig ■). Too great a preload canreduce bearing fatigue life. In twinscrew compressor applications, bear-ing preload is used in angular contactball bearings for all these reasons.Preloaded bearings are more sensi-tive to misalignment and incorrectmounting than are bearings withclearance.

LIFE

PRELOAD CLEARANCE

Fig 18

18

31

3

achieved by the elastic deflection ofthe bearings against one another.The initial deflection of the bearingsdue to the preloading is δ 0.

When an axial load is applied tothe shaft, only one bearing supportsthis load. This bearing is denoted the"active" bearing. The deflection, δ, of the active bearing reduces the load(i.e. preload) in the adjacent "inactive"bearing also called the back-up bear-ing.

The load-deflection diagram for apair of preloaded bearings rotating at3600 r/min is shown (→ fig ■).

Under rotation, the preload force isincreased, and the force in the inac-tive bearing does not fully reduce tozero due to centrifugal forces. Atincreased speeds (ndm * values 250000 and greater), gyroscopic spinningof the balls will occur if the residualpreload in the inactive bearing is lessthan the minimum required axial load,Fa min.

Figure shows the static load-deflection diagram for two preloadedangular contact ball bearings. Thisdiagram is typical of 40° bearingsarranged either back-to-back or face-to-face. Preload P' in this example is

19


A B

δ

-16 -14 -12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12 14 16

Axial deflection, µm

n = 3600 r/min n = 0 r/min

Axi

al fo

rce

N

7000

6000

5000

4000

3000

2000

1000

Axial force vs axial deflection -(2) SKF 7310 BEGBnominal bearing condition / mean k5 shaft tolerance

Fig 19

Fig 20

20



dm =(d+D)/2

32

Recommended bearing executionsTable below can be used as aguide to select a specific bearingexecution, based on a given shaftand housing fit and an ndm * range.

Consult with SKF ApplicationsEngineering for details of the recom-mendations or if the operating condi-tions are different from those listed inthe table below.

To avoid significant bearing lifereduction, the angular misalignmentof the bearings should be limited to 4minutes. This applies to preloadedsingle row angular contact ball bear-ings in the face-to-face arrangement,mounted radially free in the housing(→ fig ■).

6


��Fig 21

Recommended bearing executions for given shaft and housing tolerances

Shaft tolerance k5 k5 j5 j5 h5

Housing tolerance J6 H6,RF J6 H6,RF RF

ndm value BECBM BECBM BECBM BECBM BECBM BEGAPup to BECBY BECBY BECBY BEGAM BEGAM BEGBY

250,000 BECBP BECBP BECBP BEGAY BEGAY BEGBPBEGAP

250,000 BECBM BECBM BECBM BECBM BEGAM BEGBYto BECBY BEGAM BEGAM BEGAM BEGAY BEGBP

450,000 BECBP BEGAY BEGAY BEGAY BEGAPBEGAP BEGAP BEGAP

450,000 BECBM BEGAM BEGAM BEGAM notto recommended

650,000 area

RF = Radially free. This means that there is a 1-2 mm radial gap between the bearing outer ring and the housing.For ndm values lower than 450,000, housing tolerances J6, H6 may be replaced with J7, H7 respectively.Applies to solid steel shafts/steel or cast iron housings. Applies to bearing bore size 20 to 100 mm (including 100 mm).Applies to applications with inner ring temperature no more than 10 degrees C warmer than outer ring temperature.Circulating oil lubrication or other means for improved cooling may be necessary for control of the bearing operating

temperature, in particular. at high speeds.Contact SKF Applications Engineering for details of the recommendations.

*ndm is the bearing speed n in r/min multiplied by the bearing mean diameter dm in mm.dm = (d + D)/2

Table 6

21

33

3

Four-point contact ball bearingscan theoretically carry combinedloads up to a minimum ratio of axialto radial force of 1.27. Since in screwcompressors it is difficult to predictthe ratio of axial to radial force, it isrecommended to use them for thrustloads only, in combination with anoth-er bearing which takes the radialforce(→ fig ■).

The recommended shaft toleranceshould be j5 to allow the shaft tolocate the two inner rings concentri-cally and to minimize screw runout athigh speed. If a tighter fit is used,bearing clearance has to be analyzedaccurately, especially if the bearingclearance is small (eg. C2L).

It is recommended to clamp theouter ring axially by means of aspring. In this way, it is possible tocontrol the clamping force accurately.Excessive clamping can lead to radialloading and outer ring distortion. Toolittle clamping can lead to outer ring rotation. For calculation of the clamp-ing force, please refer to page 22, section ‘Housing Fits, Bearings taking axial loads only’. The angularmisalignment should be limited to 2 minutes .

Four-point contact ballbearingsFour-point contact ball bearings (QJprefix) are used in oil flooded andhigh speed dry air compressors tosupport axial load. The QJ bearinghas a two piece inner ring allowing ahigh number of balls and 35° contactangle for support of high axial load.It has an outer-ring guided machinedbrass cage. The larger sizes of QJbearings have an anti-rotation slot(N2 suffix) as standard. These fea-tures make the bearing well suited forhigh speed applications.

Four-point contact ball bearingsshould carry a minimum axial load forsatisfactory operation. The formula forcalculating the minimum requiredaxial load to minimize gyratory motionof the balls is the same as for BEdesign single row angular contact ballbearings:

Famin = A (n/1000)2

whereFamin = minimum required axial

load, NA = minimum load factorn = rotational speed, r/min

The equation above is more accu-rate than the corresponding equationgiven in the SKF General Catalog.

Values of minimum load factor A for series QJ 2 and QJ 3 four-pointcontact ball bearings are listed inTable ■.

Four-point contact ball bearingscannot be preloaded like a pair of sin-gle row angular contact ball bearings.It is necessary to maintain an operat-ing clearance at all times. Excessiveclearance should be avoided since itnegatively affects screw positioningaccuracy. In oil flooded twin screwcompressors, C2L clearance is acommon choice for this reason. Fordry air twin screw compressors,which operate at very high speeds, itis necessary to use a larger clear-ance, for example C3.

3 Ball bearings in twin screw compressors - four-point contact ball bearings

��Fig 22

22

7

34

small, the inner ring, balls, and shaftwill displace axially towards the cen-ter of the outer ring ball groove. Thiscan allow the ball to contact bothsides of the outer ring raceway at thesame time, while still contacting theinner ring raceway. This three pointcontact can result in severe slidingfriction in the contacts, damaging theraceways, balls, and cage.

The axial load necessary to preventthe axial displacement of the ring andballs due to centrifugal forces hasbeen calculated with an advancedcomputer program. With the use ofregression analysis, it was found that the force increases with thespeed raised to an exponent ofapproximately 1.45.

The minimum axial force to preventthree-point contact due to centrifugalforce is calculated according to thefollowing equation:

Famin = B (n/1000)1.45

whereFamin = minimum required axial

load, NB = minimum load factorn = rotational speed, r/min

Three-point ball contact is prevent-ed when the load applied to the bear-ing by the compressor or additionalforces from springs or balance pis-tons exceeds the force calculated bythe above equation.

Values of the minimum load factorB for series QJ 2 and QJ 3 four-pointcontact ball bearings are listed inTable ■.

For four-point contact ball bearings,a second criteria for minimum axialload must also be considered. Thecentrifugal force acting on the ballswill tend to move the balls radiallyoutward, similarly to that shown infigure . This movement is prevent-ed by the contact with the outer ringat its primary rolling position.However, if the axial force is too

15

3 Ball bearings in twin screw compressors - four-point contact ball bearings

SKF four-point contact ball bearing minimum axial load factors

A factor B factorsize bore QJ2 QJ3 QJ2 QJ3

diameter (mm)

03 17 0.427 0.8690 1.989 3.720

04 20 0.877 1.428 3.793 5.728

05 25 1.261 2.770 4.841 10.10

06 30 3.082 5.077 11.06 16.64

07 35 4 724 7.374 15.24 22.93

08 40 7.098 11.76 21.50 34.08

09 45 10.04 21.93 29.10 59.86

10 50 10.27 28.93 28.58 74.81

11 55 17.28 40.35 45.47 99.32

12 60 24.17 54.84 60.45 129.0

13 65 32 94 72.98 78.63 164.7

14 70 39.93 95.12 92.59 206.4

15 75 45.24 122.2 102.1 255.8

16 80 62.80 154.6 136.3 312.7

17 85 76.61 192.8 160.0 377.7

18 90 105.8 259.6 214.0 493.4

19 95 136.8 316.9 268.0 585.3

20 100 176.0 404.3 334.5 720.2

22 110 276.5 645.6 496.9 1085

24 120 353.4 784.0 608.6 1260

26 130 410.5 1057 679.5 1629

28 140 554.4 1395 878.2 2068

30 150 641.4 1645 975.4 2352

32 160 909.9 N/A 1332 N/A

34 170 1036 2918 1464 3904

36 180 1317 3379 1820 4386

N/A: Please consult SKF for availability

7

Table 7

The NU type cylindrical roller bear-ing is commonly used since it allowsseparate assembly of the inner ringsand outer ring/roller assemblies ontothe shaft and into the housing,respectively. The NU type bearingaccommodates axial displacementdue to thermal expansion of theshaft. The use of the NU type bearingallows both the inner and outer ringsto be mounted with a transition orinterference fit for more precise posi-tioning of the bearings and rotors.

Cylindrical roller bearings aresomewhat sensitive to misalignment.The maximum allowable misalign-ment is three to four minutes,depending on the bearing series.

Important: For satisfactory opera-tion, cylindrical roller bearings shouldbe subjected to a given minimumradial load. The required minimumradial load to be applied to cylindricalroller bearings is estimated from theequation provided in the GeneralCatalog, which is:

Frm = kr (6 + 4n/nr) (dm/100)2

Frm = minimum radial load, NKr = minimum load factor

= 100 for bearings of series 10= 150 for bearings of

series 2,3, or 4= 200 for bearings of series 22= 250 for bearings of series 23

n = operating speed, r/minnr = speed rating for oil

lubrication, r/min, see bearingtables in the SKF General Catalog

dm = mean diameter of bearing= 0.5 (d + D), mm

Cylindrical rollerbearingsSKF cylindrical roller bearings of ECdesign are used in twin screw com-pressors for their high speed andhigh radial load capability. The SKFEC cylindrical roller bearing has alarge number and size of rollers, log-arithmic roller profile, and optimizedflange geometry.

The SKF EC design cylindricalroller bearings are produced withthree basic cages(→ fig ■): the glassfiber reinforced polyamide 6,6 cage(P suffix), the pressed steel cage (J suffix), and the machined brasscage (M and ML suffix). The bearingsare optionally available with a highspeed light alloy cage (LP suffix) forhigh speed air compressors and amachined steel cage (F suffix) for gascompressors. The bearings are avail-able with ranges of internal radialclearance for optimization of thebearing position accuracy. The SKFEC cylindrical roller bearing is pro-duced standard with ISO P6 runningaccuracy.

35

4 Roller bearings in screw compressors - cylindrical roller bearings

Roller bearings inscrew compressors

4

23

Fig 23

designed to give optimum operatingradial internal bearing clearance. Ina screw compressor, it is important tokeep the operating radial internalclearance of a bearing small for thefollowing reasons:

• The operating radial internal clearance affects the position-ing accuracy of the screw and therefore, also the compressor efficiency. The ability to oper-ate with a small radial internal clearance is a major advantageof a cylindrical roller bearing over a hydrodynamic bearing.

• A smaller operating radial inter-nal clearance results in low noise and vibration level in the compressor.

• A smaller operating radial inter-nal clearance results in longer bearing life, increased stiffness,and reduced deflection.

It is important that the radial internalclearance is not too small for the following reasons:

• With too small a radial internal clearance, there is risk of radialpreload and premature bearing failure.

• Too small a radial internal clearance can cause difficulties in assembling the compressor and damage to the bearing.

• If an interference fit is used in the fitting of the bearing into the housing, the housing seat may be ovalized due to the effect of the press fit on the unsymmetrical housing stiff-ness. This may increase the possibility of preloading the bearings and cause difficulties with the assembly.

The required minimum radial loadfor a cylindrical roller bearing in agiven application is further depen-dent on the bearing cage type andlubrication conditions. Consult SKFApplications Engineering for details.

• for bearings having roller guid-ed cages (P, J, M), use the value calculated for Frm above as the minimum radial load.

• for bearings having outer ring guided cages (ML), double the value calculated for Frm above as the minimum radial load.

If the minimum load is not main-tained, the rollers will in part slide, inpart roll on the bearing raceway.This is called skidding and can, butdoes not have to, lead to smearing.With good lubrication, smearing canbe avoided even if the bearing isskidding. To minimize the risk forskidding, good oil drainage of thebearing cavity is important.

Selection of fits and radialinternal clearance for cylindri-cal roller bearings in screwcompressorsIn order to achieve maximum posi-tioning accuracy of the screws in acompressor and maximum bearinglife, it is necessary for the bearings tohave a minimum operating radialinternal clearance. However, the ini-tial bearing clearance and fitting ofthe bearing into the compressor must be designed to avoid the riskthat the bearings become preloadedin operation.

The operating radial internal bear-ing clearance is a function of the ini-tial radial internal bearing clearance,shaft and housing tolerances, andtemperature of the bearing rings. Therecommendations in Table ■ are

36


8

half of the greater than Normal range.Bearings having reduced clearanceranges should be used for optimumscrew positioning accuracy. The radialinternal clearance ranges in the SKFGeneral Catalog and the reduced clearances ranges (CNM, C3L) arevalid even if rings from different bear-ings are interchanged. This is aunique feature of SKF EC designcylindrical roller bearings.

Radial internal clearancerangesThe internal clearances in cylindricalroller bearings are according to DIN620, part 4. The standard ranges ofclearance are the Normal range andgreater than Normal range (C3 suf-fix). Additional clearances having areduced range are also available asnon-standards. The reduced clear-ance ranges use the lower, middle,and upper half of the standardranges. For compressor applications,the common reduced clearanceranges are the CNM and C3L ranges.The CNM suffix denotes the clear-ance is centered on the mean of theNormal range. The C3L suffixdenotes the clearance is the lower

37


-20

5000

350000

300000

250000

200000

10000

150000

bear

ing

life

(hou

rs)

400000

450000

500000

-15 -10 -5 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70

operating clearance

mounted clearance

unmounted clearance

4

Fig 24

Bearing life as a function of radial internal clearance for an SKF NU310 ECP/CNM (m5/K6 shaft and housing tolerance) C/P = 20; speed= 3600 rpm; Kappa = 1Operating clearance based on Inner Ring temperature 10°C warmerthan Outer Ring temperature

38

Application recommendationsRecommended shaft and housing tolerances and cylindrical roller bear-ing executions are defined in Table ■The table includes the recommendedcage execution, precision, bearingseries, etc., since all these featureshave to be defined in the final bearingdesignation.

AvailabilityNot all cylindrical roller bearing vari-ants (CNM, C3L, etc.) are currently inproduction. Availability may depend onusage by other customers, requiredvolumes and delivery schedules.Please contact your local SKF salesoffice for details on availability.


Application Recommendations for SKF Cylindrical Roller Bearings in Screw Compressors

Speed and Design Bearing Bore Diameter

ndm Bearing Compressor (20) to 40 mm (40) to 100 mm (100) to 130 mmSeries Design

Up A ECP/CNM ECP/CNM ECP/C3Lto m5/K6 m5/K6 m5/K6

650,000 B ECP ECP/C3 ECP/C3k6/J7 m6/J7 m6/J7

650,000 A ECML/P6CNM ECML/P63L ECML/P63Lto NU 2, NU 3 m5/K6 m5/K6 m5/K6

1,000,000 B ECML/P6CNM ECML/P63L ECML/P63Lk6/J7 k6/J7 k6/J7

NU 2, NU 22NU 3, NU 23

ndm is the bearing speed n multiplied by the bearing mean diameter dm, mm.Compressor Design A: For maximum screw positioning accuracy/high compressor efficiency.Compressor Design B: For lowest manufacturing cost/best bearing availability.P63L = P6 + C3L.

The table is valid for the following conditions:- Solid steel shafts/steel or cast iron housings- The inner ring is no more than 10°C warmer than outer ring. If the temperature difference between the

inner and outer ring is more than 10°C, the operating clearance should be calculated for verification- Bearing temperature is controlled by circulating oil lubrication. Maximum bearing temperature 100°C.- The compressed gas is air, R22. R123 or R134a,- Special limitations apply for ammonia compressors,- For more details on these recommendations, contact SKF Applications Engineering.

Table 8

8

39

Taper roller bearingsTaper roller bearings (TRB) are usedin oil flooded air screw compressorsat the male and female dischargepositions, (→ fig ■). They can also beused to support the gearing in both oilflooded compressors and high speeddry air compressors. The taper rollerbearing has high axial and radial loadcapability. Taper roller bearingsarranged face-to-face (DF suffix) areused to limit the axial play of therotors. A limiting factor is the speedcapability of taper roller bearings.The speed rating is limited by the sliding friction between the rollers andthe inner ring flange. Taper roller bear-ings are not recommended for refrig-eration compressors because of thedifficulty in lubricating the roller/flange contact and at the steel cageand roller contacts. This is especiallydifficult in ammonia compressors butalso in other refrigerants, eg HCFC-22and HFC-134a. The SKF Q line taperroller bearing is well suited for com-pressor applications. The Q line taperroller bearing features logarithmicroller profile, low friction steel cagedesign, and optimized roller end andflange profiles.

The SKF taper roller bearing is alsoavailable with the CL7C execution.This bearing has improved roller end and flange geometry to reducerunning-in wear. The SKF taper roller bearing is available in the common ISO series. The 313 series,having high contact angle is well suited for the thrust position in compressor applications.

A taper roller bearing must operatewith a certain minimum axial load forsatisfactory operation. The axial loadmay be applied by the compressor orinduced by the taper roller bearingmounted adjacent to it. The axial loadmust be greater than

Fa > 0.5 Fr/Y

where Fa = total axial load on bearing, NFr = applied radial load, NY = bearing axial load factor,

according to the SKF General Catalog

See also the SKF General Catalogfor additional details.

It may be necessary to apply addi-tional force to the bearing outer ring to prevent its rotation with the housingface if the force on the bearing ring istoo low. See the section "Housing fits,axial loads only" on page 22. The face of the bearing can also be slotted to mate with an anti-rotationpin mounted in the housing. This is anon-standard feature.

Screw compressors fitted with taper roller bearings are relativelycomplicated to assemble, since theinner ring has to be mounted with aninterference fit. The interference fitcauses an axial displacement of theouter ring relative to the inner ring.This displacement may need to beconsidered when the rotor end clear-ance is adjusted.

4 Roller bearings in screw compressors - taper roller bearings

��

4

Fig 25

25

40

When a reverse thrust taper rollerbearing is used, then the clearancebetween the two taper roller bearingsmust be adjusted by means of shim-ming. A small axial clearance must bemaintained since taper roller bearingscannot operate with preload in screwcompressors.

For the reasons mentioned above, itis difficult to set a small rotor endclearance with TRBs, therefore screwcompressors fitted with TRBs typicallyhave larger rotor end clearance.

Needle roller bearingsNeedle roller bearings (→ fig ■) areused in oil flooded air and refrigera-tion compressors because of theirhigh radial load capability and com-pact size. The needle roller bearinghas separable rings similar to thecylindrical roller bearing. The needleroller bearing has limited availabilitywith special clearances, cages or withhigher precision.

To avoid significant bearing lifereduction, the angular misalignmentshould be limited to 1 minute.

CARBTM BearingSKF has developed a new ToroidalRoller bearing type called CARBTM

(→ fig ■) (Compact Aligning RollerBearing). This is an entirely new bear-ing which is a combination of the bestfeatures of the spherical roller bear-ing, the cylindrical roller bearing, andthe needle roller bearing.

The CARB bearing is capable ofaccommodating axial expansion andmisalignment while providing highradial load carrying capabilities, lowfriction and compact cross section.

CARB bearings are produced in thesame dimension series of many popu-lar spherical roller, cylindrical roller,and needle roller bearings.

CARB bearings with cages arerecommended for screw compressorapplications.

4 Roller bearings in screw compressors - taper roller bearings, needle roller bearings,bearings for high speed compressors

��

Fig 26 Fig 27

26

27

41

High speed compressors, which aretypically dry air twin screw compres-sors, operate at ndm * values greaterthan 750,000. Normal design and pre-cision bearings are not suitable foroperation at such high speeds sincethe centrifugal forces are too greatand can lead to very high inducedloads and thus low bearing lives.

SKF has developed special highspeed bearings suitable for operationto ndm * values of 1,200,000. Thebearings are SKF EC cylindrical rollerand special sets of angular contactball bearings which have high preci-sion and running accuracy (→ fig ■).The roller bearing supports the radialload. The angular contact ball bearingset consists of a pair of bearings,mounted either face-to-face or back-to-back, with the thrust bearing havinga contact angle between 30° and 40°and the back-up bearing having acontact angle between 15° and 20°.The difference between the contactangle of the thrust bearing and back-up bearing is between 10° and 20°.With this design, the internal forcedue to centrifugal force is small, sothe induced axial force in the bearing system is minimized.

This arrangement of cylindrical rollerand angular contact ball bearings forcompressors is patented by SKF. Thearrangement is available royalty freeto customers. The bearings are madeto order and inquiries should bedirected through SKF ApplicationsEngineering.

In high speed applications, it is alsopossible to use a four-point contactball bearing as a thrust bearing. Insuch cases, the four-point contact ballbearing should have P5 running accu-racy. The cage should be either amachined brass or machined lightalloy outer ring guided cage.

In comparison with the above men-tioned bearing sets, a single four-pointcontact ball bearing has larger axialclearance and thus requires a largerrotor end clearance since the bearingaxial position is affected by the cen-trifugal loads of the balls.

Bearings having ceramic rolling elements (hybrid bearings) can alsobe used. The reduced mass of theceramic rolling element compared tosteel elements allows the hybrid bear-ing to be used at higher speeds. Thiswill allow lower preloads since the riskof skidding due to gyroscopic ballmovement is reduced.

5 Bearings for high speed compressors

Bearings for highspeed compressors

��

5

Fig 28

28



dm =(d+D)/2

6 Air compressors - Oil flooded compressors, dry air compressors

Air compressors

Oil flooded aircompressors

Bearing arrangements

6

��

Fig 29

43

Photo Courtesy ofGardner Denver

A typical oil flooded air compressorbearing arrangement is shown in(→ fig ■).29

44

BearingsOn the inlet side the most widely usedbearing type is the cylindrical rollerbearing in an NU execution. This typeallows free axial movement of therotor due to elongation under temper-ature influences and eliminates therisk for parsitic axial forces. For keep-ing tight control of the rotor end-play,the axial location of the rotor takesplace on the outlet side.

Various bearing arrangements are used depending on speed, loads, mounting options, and rotorend-play setting procedure. Typicalcombinations are shown in (→ fig ■).

Dry air compressorsBearing arrangements

�� SUCTION SIDECOMPRESSION SIDE

FGAX


Fig 30

Fig 31

��

��

��

��30

A typical dry air compressor bearingarrangement is shown in (→ fig ■).31

BearingsBecause of the high rotational speedof dry running air screw compressors,the bearings most commonly used arefour-point contact ball bearings andcylindrical roller bearings. The patent-ed high speed compressor bearingset is also used. These bearings canbe fitted with the necessary featuresof internal clearance, cage construc-tion, and tolerancing for suitable usein the compressor. Consult SKFApplications Engineering for selectionof the bearing execution.

The lubricant must be jetted axiallyinto the bearing between the bearinginner ring and cage(→ fig ■).

Orienting the jet this way will over-come the air resistance of the highspeed bearing. The jet should have aminimum 1 mm diameter to preventthe risk of being clogged by debris inthe oil. The oil viscosity and flow ratemust be suitable to lubricate and cool

the bearing. The jet speed should beat least 15 to 20 m/s to avoid deflec-tion from the air resistance of thebearing. With a 1 mm jet diameter anda velocity of 15 m/s, the flow is usual-ly more than enough to lubricate andcool the bearing.

The shaft and housing of the dry aircompressor have high temperaturessince the heat of compression is notremoved by oil injection. Typically thebearings are lubricated by a syntheticlubricant having ISO VG 32 to 68depending upon speed and tempera-ture. The lubrication system shouldinclude fine filtration. The oil flow tothe bearing should not be so greatthat it causes excessive bearing fric-tion and temperature rise.Experimentation should be used tooptimize the oil flow requirements.Drainage holes on both sides of thebearing may be required to drain offexcessive oil.

��

��

gap 0.030-0.050mm

�


6

Fig 32

45

32

Fluorocarbon basedrefrigerant compressorsBearing arrangements

BearingsThe selection of bearings for fluoro-carbon based refrigeration (HCFC,HFC) applications requires considera-tion of the effects of the refrigerant onthe oil selection and oil properties. Allfluorocarbon based refrigerants do notmix freely with mineral oils. The refrig-erants, when used with compatibleoils, are dissolved in and dilute theviscosity of the oil and in some casesreduce their property to increase inviscosity with pressure in the rollingcontact. This diminishes the oils capability to develop the elastohydro-dynamic (EHD) film in the rolling contact. The capability of the oil tosupport sliding friction (cage androller/flange contacts) are also diminished.

��

��

7 Refrigeration compressors - fluorocarbon based refrigerant compressors

Refrigeration compressors

7

Fig 33

47

A typical fluorocarbon based refriger-ant compressor bearing arrangementis shown in (→ fig ■).33

48

rolling bearings must be compatiblewith the refrigerant and compressor.Incompatibility can cause chemicalreactions between the refrigerant andthe compressor oil. These reactionscan destroy the compressor oil andthe result is corrosion and deposits inthe compressor. SKF has preserva-tives specially defined for refrigera-tion conditions within HFC-134a/POEand HCFC-22/mineral oil applica-tions. Preservatives can be testedwith other refrigerant and oil combi-nations upon request.

Bearings having ceramic rollingelements (hybrid bearings) can alsobe used in refrigeration applications.The smooth surface finish and highhardness of the ceramic rolling ele-ments make them suitable for use in very high concentrations of refrig-erant (e.g. low oil volume)

Ammonia compressorsTypical bearing arrangementFigure 34 shows the bearing arrange-ment of the locating bearing side foran industrial ammonia screw com-pressor. Plain bearings to support theradial loads are used in combinationwith rolling bearings for the axialguidance of the rotors. The smallaxial clearance of the rotors neededto obtain the high efficiency of thesecompressors can be achieved eco-nomically by using paired angularcontact ball bearings. These bearingsare mounted in a back-to-backarrangement in such a way that thebearings are able to move free in theradial direction. To prevent outer ringrotation and creeping, the bearingsare fitted in special bushings with aslot, so that a pin stop can be used.The paired angular contact ball bear-ings have light

In normal applications of bearingsoperating in an air environment, anoxide layer is developed on the sur-faces in the rolling contacts. Thislayer acts as an anti-wear protectivecoating to the steel surfaces. The nor-mal selection of lubricants for rollingbearings, e.g. the SKF GeneralCatalog, is based on the presence ofthe protective oxide layer. This lack ofoxide layer in HCFC-22 applicationsis to some extent compensated for bythe chlorine content in HCFC-22.HFC-134a has no chlorine contentand therefore no anti-wear properties.

For the application of bearings inHCFC-22 and HFC-134a, it is neces-sary to adjust the viscosity of thelubricant to account for the dilution ofthe oil by the refrigerant and also forthe reduced pressure-viscosity rela-tionship. It is also necessary to adjust( increase ) the minimum requiredviscosity ( υ1) from figure 10 toaccount for the lack of anti-wear protection in the rolling contact.Consult SKF ApplicationsEngineering for more details.

The most commonly used bearingsin refrigeration screw compressorsare the single row angular contactball bearing and the cylindrical rollerbearings. These bearing types havesmooth surface finishes and low slid-ing contact for best formation of theEHD film. Customers have had wearproblems with bearings having highsliding speed such as taper rollerbearings (roller end /flange contact).

The bearings can be fitted withpolyamide ( P, TN9 suffix ) cages orbrass cages ( Y, M, MA, ML suffix ).Tests at the SKF Engineering &Research Centre have shown thepolyamide cages to be suitable foruse in the fluorocarbon based refrig-erants up to bearing operating tem-peratures of 100°C (212°F). Thepolyamide cage is the optimum cagein low viscosity applications providedthe speed and temperature is withinthe allowable limits.

The preservation oil used on the

7 Refrigeration compressors - ammonia compressors

��

��

� ��

��

��

��

��

�� BearingsPreferred bearing typesBearing types with low friction, such

as angular contact ball bearings andcylindrical roller bearings, are pre-ferred. Customers have experiencedsevere wear in taper roller bearings,between the roller end face and theinner ring flange. This wear phenom-ena has been reproduced in laborato-ry testing. For this reason, taper

preload and a machined cage. Inorder to prevent unloading of theinboard bearing at unexpected work-ing conditions the outer rings are preloaded by springs so that therequired minimum axial load is given.The bearings are lubricated by oilinjection into the outboard bearingand oil leaves the bearing arrange-ment at the inboard bearing wherethe oil flow is supported by a flinger.


7

Fig 34

49

50

Polyamide 6,6 cage (P and TN9 suffix)The presence of ammonia acceler-ates the aging of polyamide cagematerial at elevated temperatures.Polyamide cages are successfullyused in ammonia compressors, butthe bearing operating temperatureshould be limited to 70°C (158°F).This limit has been established byexperience and laboratory testing.

Pressed brass (Y suffix) and machinedbrass cages (M, MA, and ML suffixes)In the ammonia refrigeration industry,specifications such as the AmericanSociety of Heating, Refrigeration andAir-Conditioning Engineers(ASHRAE) Handbook do not recom-mend the use of copper or brassalloys in ammonia systems, sincethese materials are subject to stresscorrosion cracking. For example,since copper tubes have residualstresses from bending, steel tubing isused instead of copper tubing.

roller bearings, and other bearingtypes with high sliding friction suchas cylindrical roller thrust bearings,spherical roller thrust bearings, andthrust ball bearings, are not recom-mended in ammonia compressors.

Pressed steel cage (J suffix)Because of the impaired lubricationdue to the undissolved ammonia gasin the lubricating oil, pressed steelcages have not been recommendedin ammonia compressors. The prob-lems associated with J cages inammonia compressors are cage wearand smearing between the cage androlling elements. The experience issomewhat mixed, but several cus-tomers who originally used J cages inangular contact ball bearings and theold generation of cylindrical rollerbearings have redesigned to othercage types.


Fig 35

Tests using ammonia have beendeveloped to determine the presenceof residual stresses in copper andbrass. One such test is DIN 50916.Because of the industry's generalrecommendations regarding copperand brass, ammonia compressormanufacturers have traditionallyavoided pressed brass and machinedbrass cages in compressor bearings.

This position is technically correctfor pressed brass cages, sincepressed brass cages contain residualstresses from the pressing operation.SKF solid, one-piece machined brasscages, however, are stress free andare not subject to stress corrosioncracking. Testing at the SKFEngineering & Research Centre andlonger term experience has shown

that solid, one-piece machined brasscages do not crack or corrode in aconcentrated ammonia environment.

Machined steel cage (F and FA suffix)Because of the residual stressesfound in pressed and some machinedbrass cages, ammonia compressormanufacturers hesitate to use them,and they prefer machined steelcages. The machined steel cageoffers good contact geometrybetween the rolling elements and thecage, and stress corrosion is not anissue with steel. However, limited pro-duction limits the availability of themachined steel cage.

A typical failure of a bearing due toexcessive quantities of ammonia isshown (→ fig ■).


7

51

Fig 36

��

35

52

Electro-motors in semi hermeticcompressors can have very compactdesign since the refrigerant providesgood cooling conditions.

An important consideration in thecompressor design is the weight ofthe motor rotor mounted on theextended screw shaft. This designmust be carefully analyzed from arotor dynamics standpoint to avoidproblems with vibration. To avoidsuch problems, it is desirable to use a large shaft diameter and tomount the rotor as closely to thescrew as possible.

To allow a large shaft diameter, thesuction side bearing should have athin section height, since the avail-able space is also limited. Cylindricalroller bearings of dimension series 10or 2 are used commonly. ContactSKF for more information.

Semi hermetic compressorsSince refrigerant compressors oper-ate in closed systems, it is importantto minimize leakage of refrigerantfrom the compressor. The mainsource of leakage is the shaft sealwhich is necessary if an externaldrive motor is used. Semi-hermeticcompressors enclose the compressorand the electro-motor in a commonshell. This eliminates the need for aseal and improves the compressorreliability.

The motor rotor is mounted on the extended rotor shaft on the suction side. The motor stator ismounted in the extended compres-sor housing. The construction materi-als of the motor must be carefullyselected to withstand the refrigerantenvironment.


Caution must be used since sourgas is not a well defined compound.Sour gas containing excessive acidscan destroy the polyamide cage,however, the problems are not limitedto the bearings. All compressor com-ponents must be designed to with-stand the acid. The most reliable, but also the most expensive com-pressor designs, incorporate efficientmechanical seals for separating thecompression cavity and the bearings.

Sour gases are natural gases havinghigh concentrations of hydrogen sul-fide gas (H2S). Hydrogen sulfide gasin the presence of the moisture (H2O )often found in gas compressors canform sulfuric acid (H2SO4 ). This acidcan corrode the bearing steel anddamage the cages.

Brass components have historicallynot been used in natural or sour gascompressors. Bearings havingpressed (J suffix) or machined steel(F or FA suffix) have been used. Thebearing rings are protected by thelubricating oil. Tests at the SKFEngineering & Research Centre haveestablished that bearings havingpolyamide 6.6 cages (P and TN9 suffix)can be used successfully in sour gascompressors to an operating temperature limit of 70°C (158°F).

Natural and sour gascompressors

8

8 Natural and sour gas compressors

53

Photo Courtesy ofFrick/York International

54

Comparative viscosity classifications

9 Comparative viscosity classifications

Fig 37

Power 1 W = 1.36x10-3 HP1 HP = 736 W

Pressure 1 MPa = 1 N/mm2 = 145 lbf/in2 (psi)1 MPa = 10 bar1 atm = 1.01 bar1 psi = 6.89x10-3 N/mm2 = 6.89x10-3 MPa

Kinematic 1 mm2/s = 1 cStviscosity

Velocity 1 m/s = 3.28 ft/s1 ft/s = 0.305 m/s

Flow rate 1 ft3/min = 4.72x10-4 m3/s1 US Gallon/min(GPM)= 6.31x10-5 m3/s1 m3/s = 15850 GPM1 ft3/s = 449 GPM

Temperature °F = (°C x 1.8) + 32°C = (°F - 32) / 1.8

Length 1 mm = 0.039 inch1 inch = 25.4 mm0.001 inch = 25.4 µm1 m = 3.28 ft1 ft = 0.305 m

Area 1 m2 = 10.8 ft2

1 ft2 = 0.093 m2

Volume 1m3 = 35.3 ft3

1 ft3 = 0.028 m3

1 liter = 0.264 US Gallon1 US Gallon = 3.79 liter1 Imperial Gallon = 4.55 liter

Mass 1 kg = 2.20 lb

Force 1 N = 0.225 lbf1 lbf = 4.45 N

Moment 1 Nmm = 8.85x10-3 in.lbf1 in.lbf = 113 Nmm1 Nm = 0.738 ft.lbf1 ft.lbf = 1.36 Nm

Unit conversion

10 Unit conversion

9

55

56

1] Jacobson, Bo. "Ball Bearing Lubrication In Refrigeration Compressors." SKF Engineering &Research Centre B.V., The Netherlands.

2] Svenningson, Kurt and Dr. Ulf Sjolin. "Svenska Rotor Maskiner AB; The History of SRM Screw Compressor Development."

3] SKF General Catalog 4000.

References

11 References

10

57

Although care has been taken to assure the accuracy of the data compiled in this publication, SKF USA inc. does not Publication 100-956assume any liability for errors or omissions.

©1998 SKF USA Inc. 7.5M 10/98 AN Printed in USA

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