Low friction cylindrical roller bearings

Dipl.-Ing. (FH) Roland Lippert and Dipl.-Ing. (FH) Bruno Scherb

INA reprint from „antriebstechnik“April 1993, No. 4Vereinigte Fachverlage GmbHRevised April 1995

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Low friction cylindrical roller bearingsDipl.-Ing. (FH) Roland Lippert and Dipl.-Ing. (FH) Bruno Scherb

The high performance density ofmany modern machines placesenormous demands on the rollingbearings used; these often exceedthe capabilities of conventionaldesigns of bearings, necessitatingearly and frequent replacement.In such machines bearings withhigh limiting speeds even underhigh radial loads are required. In general, convential cylindricalroller bearings cannot meet theserequirements.INA has developed two new typesof cylindrical roller bearing – series LSL with a disc cage andseries ZSL with plastic spacersbetween the rollers – which canfulfil the requirements. This allowsreliable and economical operationof modern high-performancemachines.The following paper presents thesetwo new developments from INA,which combine the advantages offull complement roller bearings(high load carrying capacity) andcage-guided bearings (high limitingspeed).

1 IntroductionCylindrical roller bearings have proved tobe very successful in both mechanicalengineering and the automotive industry.They ensure reliable power transmissionin both the radial and axial directions.Modern aggregates are becoming increa-singly compact due to optimized manu-facturing processes, high performancematerials and accurate calculations. There is also a corresponding increase in the power density. This bearing is nowvirtually indispensable for designs withhigh power transmission in limitedspaces.There are two basic designs for this typeof bearing:• Full complement cylindrical roller

bearing (figure 1)• Cylindrical roller bearing with cage

(figure 2)

Each of these bearing systems has parti-cular features and is therefore assignedto certain applications accordingly. When selecting the appropriate bearingtype, compromises must often be madeand the advantages and disadvantages weighed out. The design engineer has tochoose between the caged cylindricalroller bearing and the full complementcylin-drical roller bearing for the design of the bearing arrangement. In makingthis decision, the main criteria for correctselection should be precisely determined.In the case of caged bearings forexample, the cage requires about 15 % to 35 % of the space between inner andouter ring. The theoretical 100 % utilizationof the load carrying capacity of the rollingelements is thus reduced to approx. 85 % to 65% and the basic rating lifedecreases accordingly from 58 % to 24 %compared with the full complement rollerbearing.

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In the case of full complement bearingsmore heat is generated due to the highpower density. The limiting speed is therefore reduced to 50 % compared with the caged bearing. The permissibledynamic and static axial loads are directlyproportional to the lateral contact facesbe-tween the rolling elements and thecorresponding ribs on the inner and outerrings. In this case, the full complementbearing is preferable as it has more rollingelements.Frictional torque, heating, cooling,prevention of smearing, wear, radialdimensions and bearing power densityshould also be taken into considerationwhen selecting the appropriate cylindricalroller bearing.

As far as the development of cylindricalroller bearings is concerned, INA’s idea isto combine the positive features of bothfull complement cylindrical roller bearingsand cylindrical roller bearings with cage tocreate a new generation of cylindricalroller bearings.This new idea has created two highlyadvanced bearing systems which can be described as follows:• Cylindrical roller bearing with disc cage

(figure 3)• Cylindrical roller bearing with spacers

(figure 4)

Figure 4 Cylindrical roller bearing with spacers, series ZSL 19 23

Figure 1 Full complement cylindrical rollerbearing SL 1923

Figure 2 Cylindrical roller bearing with cageNJ 23

Figure 3 Cylindrical roller bearing with disc cage, series LSL 19 23

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2 New bearing series LSLand ZSL

2.1 StructureCylindrical roller bearings with a solid disccage guided by ribs on the outer ring andcylindrical roller bearings with spacers aremainly bearings of series NJ. The stan-dard design of these bearings is in thedimensional range 23. ZSL bearings aredesigned for the bore reference numbers05 to 24 and LSL for 20 to 60. They arefully interchangeable with knowncylindrical roller bearings of seriesNJ 23 and NJ 23 VH. The inner ring,rolling elements and outer ring of seriesZSL are identical to those of ourestablished series SL 19 23.By removing a cylindrical roller from therolling element unit, space has beencreated for a spacer to separate theremaining rolling elements from oneanother (figure 5).For series LSL, the inner ring and rollingelements are identical to series SL 19 23and ZSL 19 23. The outer ring is radiallysplit and has an additional groove in theraceway between the ribs; a speciallydeveloped brass disc cage is located inthe groove (Figure 6).In conjunction with heat treated retainingrings, the outer ring with the integral disccage forms a bearing unit.The rolling element unit in bearings ofseries LSL 19 23 with this new disc cage also has only one rolling element lessthan the full complement bearing ofseries SL 19 23. As a result, extremelyhigh load carrying capacities are obtainedcompared with cylindrical roller bearingswith cage.

LSL and ZSL bearings are also availableon request as serial bearings providedeconomically viable quantities are requi-red. Figure 7 shows the main bearing of a D.C. engine as used in high speedrailway applications. The bearing is fittedwith this cage.

2.2 Series LSLThis new one-piece cage has beendeveloped as a plane disc with the rollingelements held in pockets. The insidediameter of the cage has been pulleddown below the pitch circle line in order toprovide retention for the rolling elements,i.e. the inner ring can be mountedseparately. The disc cage is located atthe exact centre point between the ribs ofthe outer ring by means of a groove cut inthe outer raceway. The pockets of thecage have a special shape (like teeth).They contact the rolling elements exactlyin the plane of symmetry. All the tangentialforces are therefore directed very safelyinto the cage. Unlike conventional cages(figure 8) which come into contact withthe rolling elements at any point on thesurface, the disc cage cannot exert anygyroscopic effect on the rolling elements.In other words, a conventional cage mayin some respect force the rolling elementsto skew whereas the new disc cage does

not interfere with the guidance of therolling elements through ribs or shoulders.The cage minimizes the distance betweentwo rolling elements which means thatthere is a greater number of rollingelements in the available space comparedwith conventional caged bearings.

Notch stress which can arise withconventional solid cages does not occurany more with the disc cage. As usual therolling elements are guided between theribs on the outer ring. As the disc cage,unlike conventional cages, does notinterfere with this guidance, the velocitydrop of rolling elements outside the loadarea is very small.The required accelerationmoment for the rolling elements istherefore much lower and less frictionaltorque and heat are generated in thebearing. Due to the lower coefficient offriction and fric-tional torque of these disccage bearings, much higher limiting speedsare achieved, even with normal accuracy.This theory has been proven by varioustests on INA’s heavy-duty bearing test rig.

The cage which is guided in the outerring groove represents an optimal slidingbearing: a sufficient oil film is formed atlower speeds to separate the contactsurfaces and provide perfect hydro-dynamic running conditions.

Figure 5 Spacers made of composite material

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Lubricant exchange occurs by means ofradial lubrication holes arranged centrallyaround the outer edge of the cage whichjoin with axial through holes in the cage.In rolling bearings where heat dissipationis provided by the lubricant, the free axialspace should be as large as possible to ensure a regular and high volume oilflow in the bearing. With full complementbearings, the gap remaining between tworolling elements is relatively small. Withconventional caged bearings, this freespace may be filled by the cage itself,thus reducing and restricting the oil flow.The open disc cage bearing provides the solution: the lubricant is allowed toflow freely with minumum obstruction.Bearings of series LSL are black-furnished,a further positive feature which provides alevel of protection against smearing to adegree not achieved before.

2.3 Series ZSLThese spacers specially developed forthis series are designed in such a waythat the bearing and the inner ring can bemounted separately.Due to the special design of the spacer(PES) the point of contact on the rollingelement outside diameter is always at thepitch diameter circle of two adjacentrolling elements irrespective of theoperating condition. This preventsuncontrolled radial force componentswhich usually occur in other spacerapplications. This has a positive effect on the frictional behaviour in the bearingor between the spacers and rollingelements. Furthermore, no additionalconstraining forces have to be overcome;this can be seen in the low frictionaltorque of the bearing.The spacers are axially guided betweenboth ribs on the outer ring. The radiallimits in the mounted condition areformed by the raceways. The spacerswith radial clearance are pushed outwards,even at low speeds, and lie on the outerring race-way. The special design of theradial abutment surface facilitates hydro-dynamic separation between the spacerand the raceway as the speed increases. The central recesses at the inner and

outer diameter provide optimum bearinglubrication. Stripping the oil film at theraceways can be avoided and thefrictional torque independent of the load is reduced due to the free flow of oil.Every effort has been made in the designof the spacers to obtain the best axiallubricant flow.

Figure 6 Solid brass disc cage Figure 7 Special bearing with brass disccage

Figure 8 Conventional solid brass cage11

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Figure 10 Test matrix

3 Research and tests

3.1 Test rigVarious tests have been carried out onthe operating behaviour of the LSL andZSL bearings as shown in figure 9.In this case INA has used a test rig with aso-called „flying“ test piece arrangement.This test rig has already been describedin detail by Giese and Scherb [1], [2]. The test rig has been designed to provideeasy access to the test piece for themeasuring sensor application and forgood visual examination by means ofstroboscopic video recordings orrecordings using high speed cameras.Easy mounting and a clear measurementof the frictional torque in the test bearingshould be possible without the inter-ference of any external influences.

The influence of the load direction in theload area with the force of gravity oragainst it, as well as intermediate positions,together with the influence of a super-imposed thrust loading should be able tobe investigated. These requirements ledto the test rig as shown in figure 9.

The test bearing is loaded via the centralbearing unit which exerts a defined testload through servohydraulically controlledpressure cylinders; this test load is con-stant throughout an extremely wide rangeof temperatures. In order to determinethe limiting speeds of the new bearings itwas necessary to replace the two angularcontact ball bearings of the centralbearing unit with an LSL bearing. It wasthen possible to set the load and speedconditions without damaging the bearingexerting the load.

The central bearing unit can be rotatedthrough 360° in a supporting ring in orderto achieve various load directions. Theaxial load on the test bearing is appliedby means of four servohydraulic control-led pressure cylinders which act on thetest bearing through the stationary outerring of the central bearing unit on theshaft and then through the rib on theinner ring. Variable test load may besimulated by a special actuator. Constantlubricant viscosities are guaranted for alltest conditions due to extremely efficientoil coolers or heaters.

Figure 9 Heavy-duty bearing test rig

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The variable speed drive motor permitsmeasurements and tests up to a speedof 4000 min–1. This corresponds to avalue over 200 % higher than the limitingspeed given in the catalogue for a cagedbearing of series NJ 23 32 E.

3.2 Running testsA comprehensive series of tests on thefriction and temperature behaviour as wellas the kinematics of the test bearing havebeen carried out on the test rig shown infigure 9. The required kinematic revolu-tions are calculated according to [3].A matrix of values from previous tests hasbeen compiled starting with the highestload and the lowest speed as shown infigure 10.Two versions of series 19 2332 bearingswere provided for comparison purposes:one as a disc cage design, the other withspacers. A lubrication oil with a viscosityν40 = 215 mm2 · s–1, ν100 = 17,5 mm2 · s–1

was used for lubrication. Figure 11 con-tains an overview of the acquisition andprocessing of the measured values andof the sensors and monitoring equipmentused. All measured values have been

monitored and stored by means of acomputer so that a subsequent evaluationof the recorded measuring signals can becarried out.The following measured values werecontinuously recorded to assess theoperating behaviour of these bearings:– Temperature at the inner and outer ring

of the test bearing– Temperature of oil at inlet and outlet– Oil flow through the test bearing– Bearing load– Bearing frictional torque– Size and direction of the load area– Displacement of the shaft– Structurally–borne noise from the rolling

element when entering the load area– Shaft speed– Speed of the rolling element mid point– Cage slip or rolling element slip– Rotational velocity of an individual

rolling elementThe most important characteristics forassessing the operating behaviour of largebearings are provided by the frictionalbehaviour and the sensory analysis ofthese bearings.

As described in 3.1, the test bearing ismounted in the hydraulic friction measu-ring balance. A load sensor is attachedvia a lever arm which is located on themovable part of the friction balance: the measured reaction force and thecalibrated length of the lever arm lengthprovide a scale for the frictional torque of the bearing.Observations at the test rig have shownthat particularly in case of large bearingsthe rolling elements do not always rotatewith their kinematic nominal speed butare subjected alternately to accelerationsand decelerations.Figure 12 shows the various kinematicareas in a bearing.The rolling element is provided withenough radial load to produce a kinematicgyratory movement around the axis of the rolling element. Only in the area ofnominal speed in the deceleration area,the rolling element is slowed down due tothe friction and is accelerated again to itskinematic nominal speed in the accelera-tion area by the frictional forces whichoccur. This acceleration and decelerationprocess is repeated periodically with eachrotation. Furthermore, cage or rollingelement slip can occur with lower exter-nal loads, i.e. the cage does not achieveits kinematic nominal speed either.

Figure 12 Kinematic areas in a bearing

Figure 11 Measuring chain with sensory analysis and monitoring equipment

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Figure 13 LSL 19 2332 frictional torque curves Figure 14 ZSL 19 2332 frictional torque curves

Determination of the self-rotation of themeasuring rolling element is by means ofan incremental shaft encoder which has avery low mass moment of inertia as wellas a very low internal friction. It isconnected coaxially to a rolling elementvia a universal joint. The incremental shaftencoder is mounted on a disc which isdriven by another rolling element remotefrom the measuring rolling element andwhich rotates at the speed of the cage.This rotating disc is provided with holesand supplies a measure for the cagespeed via inductive proximity switchscanning. The signal from the incrementalshaft encoder was supplied by an F-Utransducer; the transformed stress signalprovides a measure for the current speedof the rolling element. Another hole on the rotating disc situated at the angularposition of the measuring rolling elementallows to locate the coning angle of themeasuring rolling element to be createdvia a stationary inductive proximityswitch.

3.3 Test resultsAs described in section 3.2 the operatingbehaviour of large bearings is essentiallyassessed from the frictional behaviourand bearing kinematics. The followingdeals in greater detail with the frictionalbehaviour of those new bearings.

3.3.1 Frictional behaviourFigures 13, 14 and 16 show the charac-teristic frictional torque curves dependenton speed and load.The graphs in figures 13, 14 and 16 showa significant frictional torque advantagefor the disc caged bearing LSL 19 2332.Figure 15 clearly shows the frictionaltorque difference at operating point nr. 25(Fr = 100 kN; n = 960 min–1).The progression over a period of time ofthe dynamic frictional torque allows con-clusions to be drawn regarding the ope-rating behaviour of the bearing. Figure 17shows the comparison between thedynamic frictional torques at operatingpoint nr. 25 (Fr = 100 kN; n = 960 min–1).

The slight frictional torque fluctuations of the disc cage at a very low absolutefrictional torque are to be taken intoconsideration.These low fluctuations in the frictionaltorque lead to the assumption that thereare also differences in the kinematics.

3.3.2 KinematicsAs already described in 3.2 the rollers areperiodically accelerated and decelerated.Figure 18 shows the speed characteris-tics of the measuring rolling elementdependent on its relative position for thevar-ious bearing types LSL (disc cage),ZSL (spacers), NJ (solid brass cage) andSL (full complement).

Figure 15 Frictional torque comparison ofvarious bearings

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FRICTIONAL TORQUE in Nm

SLFULLCOMPLEMENT

LSLDISCCAGE

ZSLSPACERS

NJBRASSWINDOWCAGE TYPE

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20

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Figure 16 NJ 23 32 E frictional torque curves Figure 17 Dynamic frictional torque curves

Under extreme conditions the rolling ele-ment slip can reach 100% which meansthat the rolling element is decelerated to astandstill. The maximum rolling elementslip is influenced, on the one hand, by theinternal frictional conditions of the bearingat the roller-rib, roller-roller or roller-cagecontact points and on the other hand, by lubricant friction. This influence waseliminated in the tests by using a lubricantwith exactly the same viscosity and thesame oil feed as was used for the testbearing. As a consequence, the internalfrictional conditions and the cage designremain the only variables which have aninfluence on the maximum slip of therolling element.

Figure 19 shows the comparisonbetween the maximum accelerations atthe test point matrix.The low frictional torques and the verylow values for the rolling element slip leadto very low rolling element accelerationsand to a very favourable thermalbehaviour in these bearings. An additionalcriterion for low frictional torques andoptimum kinematics is the oil flow andtherefore the heat dissipation. This hasalso been taken into consideration in thedesign of the disc cage and the spacerswhich provide an optimum solution dueto the shape and the design.As a result, the limiting speeds of thesenew bearings can be increased.

Figure 18 shows that very low rollingelement slip values are achieved due tothe design of the disc cage. The bearingwith spacers also has low values for therolling element slip compared to theconvention-al cage (figure 18).When the rolling elements enter the loadarea they are accelerated within a fewmilliseconds from a starting speed to theirkinematic nominal speed. The energywhich occurs during this accelerationmust be transferred to the lubricant film.The tests have established that thisabrupt acceleration process is a criticaloperational condition for smearing and isknown as the so called smearingacceleration.

Figure 19 Comparison of the maximumrolling element accelerations

Figure 18 Speed curves of the measuring rolling elements

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ROLLING ELEMENT ACCELERATION POWER in W

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ZSLSPACERS

NJBRASSWINDOWCAGE TYPE

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Bearing type LSL 19 2332 ZSL 19 2332 NJ 2332 E SL 19 2332

Cage type Disc cage Spacers Window type cage Full complementbrass composite brass

material

Rel. dyn. load rating (%) 121 121 100 129

Rel. stat. load rating (%) 110 110 100 120

Rel. life (%) 190 190 100 232

Rel. friction value (%) 75 95 100 150

Rel. speed factor (%) 210 147 100 42

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Figure 20 Frictional torque curves – Determining the limiting speed

Figure 21 Limiting speed – Bearing series NJ 23 32

Figure 22 Speed parameter – Bearing series NJ 23 32

3.3.3 Determining the limitingspeeds

The limiting speeds of the new bearingsystems have been established accord-ing to the test conditions for determiningthe limiting speed specified in step [4].Figure 20 shows the measured frictionaltorques at C0/P = 20 (the speeds wereincreased incrementally).Figure 21 shows the comparison betweenthe limiting speeds for bearings of seriesNJ 23 32.Figure 22 shows the speed parametersfor the established limiting speeds withrespect to the average diameter of thesebearings.The speed parameter n · dM for the fullcomplement cylindrical roller bearing of 200 000 mm/min could be increasedfive-fold, which means an increase in thespeed parameter of over 200 % com-pared with the brass window type cage.This speed increase can be achieved withbearings with normal accuracy (PN).

Table 1 Comparison of the main sizes for bearing selection

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LIMITING SPEED in min

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LSLDISC CAGE

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4000

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NJBRASSWINDOWCAGE TYPE

–1

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ZSLSPACERS

NJBRASSWINDOWCAGE TYPE

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4 SummaryThis article shows that the new cylindricalroller bearings developed by INA exceedby far the application potential of conven-tional radial cylindrical roller bearings.In designing the bearing with spacers andthe disc cage bearing INA has succeededin combining the advantages of the extre-mely high load ratings of full complementbearings with the advantages of a cagedbearing. Furthermore, the frictional torqueand the rolling element slip can beminimized due to the shape of the disccage and the spacers. The result is a veryfavourable dynamic frictional behaviourwith minimal frictional torque fluctuations.

Table 1 shows a summary of the compa-rison of the different dimensions used forthe evaluation of main sizes and bearingselections as shown for the bearingseries 23 32.The advantages of the cylindrical rollerbearing with disc cage and of thecylindrical roller bearing with spacers canbe summarized as follows:– high static and dynamic load carrying

capacity– high limiting speed– low frictional torque over the whole

speed range– quiet running due to optimum rolling

element kinematics– high protection against smearing– high permissible axial load carrying

capacity– thermal stability due to optimum heat

dissipation

Bibliography[1] Giese, P. and Scherb, B.: “Bestim-

mung der Knatterlast von vollrolligenZylinderrollenlagern”, antriebstechnik 31 Nr. 3, S. 64 – 73

[2] Giese, P. and Scherb B.: “Wälzkörper-satzschlupf bei Zylinderrollenlagern”,antriebstechnik 31 (1992) Nr. 5,S. 54 – 60

[3] Eschmann, P., Hasbargen, L. andWeigang K.: “Die Wälzlagerpraxis”, R. Oldenbourg, Verlag München-Wien1978

[4] Needle roller and cylindrical rollerbearing catalogue 307: INA WälzlagerSchaeffler oHG, Herzogenaurach,edition 1997

Authors:Dipl.-Ing. (FH) Roland Lippert is head ofApplication Engineering in the IndustrialSector Management for mechanicalengineering, construction machinery,plastics processing and agriculturalmachinery.Dipl.-Ing. (FH) Bruno Scherb is head ofthe Radial Bearing Sector in the researchdepartment. He is responsible for plan-ning, calculating and the conduction ofthe tests related to his sector. He dealsmainly with tests on the frictional beha-viour and bearing kinematics of radialbearings.Both employed at INA WälzlagerSchaeffler oHG, Herzogenaurach

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