Cylindrical Roller Bearings with Cagefor Vibratory Machines

Roland Lippert

INA reprint from „antriebstechnik“Vol. No. 3, March 1999Vereinigte Fachverlage, Mainz

2

Cylindrical Roller Bearings with Cagefor Vibratory MachinesRoland Lippert

Rolling bearings in modern vibratoryequipment must be capable ofsupporting heavy shock loads,centrifugal forces and accelerations,and also allow high speeds even invery small design spaces. Circular,elliptical or linear vibrations ofbearing supports also occur duringrotation about the bearing’s ownaxis with intense radial accelera-tions, which mainly exert loadson cages. Poorly aligned bearingpositions and shaft deflectionscause additional loads to be appliedto bearings. This article attemptsto explore some important charac-teristics of bearings in vibratorymachine applications.

1 IntroductionVibratory machines and equipment areessentially used in earth moving, civilengineering, road construction, themanufacture of precast concrete parts,and in separating and conveyingapplications. Typical examples in theseareas include: Top vibrators, vibratorycompactors, concrete pipe and blockmachines, vibrating screens and vibratingmotors. The construction of these machinesis basically the same, consisting of aspring-mass system. Depending on theapplication, this spring-mass system canbe designed as either a circular or linearvibrator (Fig. 3 and Fig. 4).

2 Choosing the right bearingsystem

Choosing the proper bearings is importantfor the reliable operation of vibratoryequipment. In general, heavy series 23bearings are used because of theextreme loads that vibratory machinesmust withstand. Due to intense accelera-tions and harsh operating conditions,these applications require caged bearings.The main type used here is the cylindricalroller bearing. For low-accelerationapplications, spherical roller bearings arealso used. For a long time, choosing theright bearing system meant compromisingbetween load rating and cage strength. In order to provide a strong, reliablesolution for these extreme bearing arran-gements and to achieve maximum servicelife, INA Wälzlager Schaeffler oHG hasdeveloped two bearing series, theLSL (cylindrical roller bearing with heavy-section disc cage, Fig. 1) and the ZSL(cylindrical roller bearing with spacers,Fig. 2).

The following discussion will briefly dealwith the most important criteria for theefficient design of such bearing supportsand will compare conventional bearingsystems with INA’s new LSL and ZSLbearing series for vibratory machines.

2.1 AccelerationIn terms of acceleration and application,vibratory equipment can be divided into4 machine types

AccelerationApplication a (m/s2)

Vibrating motorsVibrating screens 30 to 70

Vibratorycompactors 70 to 120

Concrete blockmachinesConcrete pipemachines

150 to 250

Downhole vibratorsTop vibrators 300 to 550

These accelerations (based on theequation a = r · ω2) have a significanteffect on the service life of a bearing.This means that the choice of the rightcage design is very important. Cages areusually made from brass, aluminum orplastic. Metal cages are available as rivetedplate disc cages or as heavy-sectionwindow cages.If the wrong cage is selected, a servicelife will result that is far below thecalculated rating life.

3

2.2 CagesBesides the normal function of a cage,i.e. the separation of rolling elements, acage designed for vibrations must primarilybe capable of supporting the mass forcesthat impinge on the cage itself due to itsown natural weight, as well as the massforces exerted by the rolling elements onthe cage pockets.

Here, the following distinctions are made:

2.2.1 Cage guidanceCage guided by rolling elementsThese cages are supported directly onthe rolling elements. The most commontype of support is on the pocket webs.Cage webs are subjected to extremeloads. Depending on the contact anglebetween the web and the rolling element,the latter is subject to increasing decele-

ration in the no-load zone. When the rollingelement enters the load zone, it must bere-accelerated within milliseconds. Thisincreases bearing frictioned torque andthe risk of smearing. These types ofcages are often unsuitable for vibratoryequipment.

Cage guided by outer ribsBoth sidewalls of the cage are radiallysupported directly on the outer ring ribs.This means that the natural rotation of therolling elements is less subject to decele-ration by the cage webs outside the loadzone, resulting in a decrease in frictionaltorque. A further reduction in bearing frictionaltorque is achieved by a snug fit betweenthe cage and the rips of the outer ring,combined with centrifugally driven lubri-cation on the outer ring.

2.2.2 Cage designSealing shield cageDue to the intense vibration present, theheat-deformed cage rivets lose their axialpreload to the sealing shield. The connec-tion becomes loose and the sealing shieldstrikes radially against the rivet stems.This induces intense shear stresses in therivet pad and ultimately leads to fatiguefracture.

Heavy-section window cageThis cage is machined from a single piece,making it somewhat costly. Due to theradial acceleration present, the cage’ssize and weight are sources of additionalstrain, which induce high residual stresses.It also takes up a great deal of the availableenvelope, which has a detrimental effecton the potential number of the rollingelements, resulting in a load ratingdecrease and a shorter service life of thebearings.

Fig. 3 Circular vibrator

mu

rur

Fig. 4 Linear vibrator

mur u

r umu

rr

Fig. 1 Cylindrical roller bearing with disccage, series LSL 1923

Fig. 2 Cylindrical roller bearing with spacers,series ZSL 1923

4

INA brass disc cage [1]

The new disc cage design calls for a flatdisc (Fig. 5). Pockets to hold the rollingelements are located along the insidediameter. The cage's inside diameter isdesigned below the pitch circle line,which allows a retention for the rollingelements to be achieved, so that the innerring can be mounted separately. On theoutside diameter, the disc cage ispositioned midway between the rips in agroove in the outer ring. Due to the specialshape of the cage pockets, there areoptimally shaped webs between therolling elements. These webs efficientlyconduct any tangential rolling elementforces outward to the closed ring. In con-trast to conventional, costly heavy-sectioncages, this design provides maximumreduction in web width. Notch stressesthat are present in the web connectionsof window cages do not occur in the disccage. Because of its low mass, the cageis only minimally stressed by accelerations.Thus, the disc cage is an optimum solutionin terms of separating the rolling elementswhile sustaining mass stresses. Thisallows a larger number of rolling elementsto be used in the rolling bearing than ispossible in conventional heavy-sectioncage bearings. The snug fit of the cage in

the groove in the outer ring makes it anoptimal plain bearing, which completelyseparates the rolling elements radiallyeven at low speeds. It can thus run as ahydrodynamic plain bearing with lowfriction characteristics. Lubricant exchangeoccurs through central lubrication holeslocated radially in the cage, which matchup vertically with the axial through holes.Rolling bearings must have very large freeaxial surfaces to dissipate heat from thebearing during lubrication. This is the onlyway a sizeable volume of oil can flowthrough the bearing evenly. In a conven-tional heavy-section cage bearing thisfree cross-section is quite small, due to thelimitation of O.D. piloted cage, the innerring rip and two adjacent rolling elements.This optimally designed, axially open disccage bearing is also unique in terms of oilflow, which is additionaly supported byaxial holes in the disc cage.

INA spacers [1]

The plastic spacers (Fig. 6) speciallydeveloped for the ZSL 1923 seriesbearings are designed in such a way thatthe complete set of rolling elements isself-retaining, so that the bearing and theinner ring can be monted separately. Dueto the special design of the plastic spacers,

the surface of the rolling element (in anyoperating situation) is clearly in tangentialcontact on the connecting line consistingof the center point of two adjacent rollerelements. This means that there are noundesirable radial force components,which is beneficial in terms of the frictioncharacteristics in the bearing, and thespacers and rolling elements. As a result,there are no additional constraining forcesto overcome, resulting in minimal bearingfrictional torque. The spacers are guidedaxially between the two outer ring ribs. Inthe mounted condition, the two racewaysconstitute the radial limits. Even at lowspeeds, the rotational centrifugal forcepresent causes the spacers (which haveradial internal clearance) to be lifted fromthe inner ring raceway and contact theouter ring raceway.

As speed increases, the specific shape ofthe radial contact surface promotes thehydrodynamic separation of spacer andraceway. The virtually “segmented” cagecannot induce internal bending stressesduring radial accelerations. Only forcesfrom rolling element acceleration have tobe supported, as Hertzian stresses fromthe spacers.

Fig. 5 Heavy-section brass disc cage Fig. 6 Spacers for cylindrical roller bearings series ZSL 1923

Fig. 8 Comparison of calculated rating life [%]

5

2.3 Load ratingsThe dynamic load rating is very importantin the design of vibratory machines. In thecase of the high mass forces and speedsrequired by these machines, conventionalbearings generally yield low rating lifevalues. Because of the internal structureof series LSL 1923 and ZSL 1923 bearings,these cylindrical roller bearings canaccommodate more rolling elements thanconventional bearings. This means anincrease in the dynamic load rating andcalculated nominal rating life that is sub-stantially higher than in conventionalcylindrical rolling bearings. For the valueNJ 2324, the graphs above illustrate theload rating and bearing life advantages ofthe LSL series with heavy-section brassdisc cage and the ZSL series, with plasticspacers compared to a conventionalbearing NJ 2324 E.M with brass cage.For a 22% dynamic load rating advanta-ge (Fig. 7) a 193% increase in thenominal rating life compared to theconventional cage bearing results (Fig. 8).

2.4 Bearing tiltingLong, thin shafts and heavy loads invibratory machines cause tilted bearingpositions that must be supported orcompensated. For the compensation ofimpermissibly high edge loading betweenthe rolling elements and the ring raceways,LSL and ZSL bearings for vibratorymachines are provided with speciallycrowned inner ring races [2]. Tilting up to0.1° between the inner and outer ringscan be supported. If greater tilting occurs,the INA Applications Engineering Depart-ment should be consulted. Appropriatedesign programs are available for thesecases.

2.5 Radial internal clearance and fitIn addition to the common method oflocating the bearing ring with a circum-ferential load, the ring must also be locatedwith point load using an interference fit. Ifthis is not done, a deflection of the shaftor housing seat may occur. This procedureand temperature differences between theinner and outer rings generally require theradial internal clearance group C4 [2].Series LSL and ZSL cylindrical rollerbearings for vibratory machines thus haveC4 as the standard radial internal clearancegroup.

3 SummaryIn the machine building industry, vibratorymachines place high requirements onrolling bearings.Bearing cages are subjected to enor-mous dynamic mass forces and are thusof central significance in the implemen-tation of these applications. Besides thestringent requirements for dynamic loadratings and long rating life, cylindrical rollerbearings for vibratory machines must beable to compensate or support shafttilting. Series LSL 1923..BIR C4 andZSL 1923..BIR C4 cylindrical roller bearings,specially developed by INA WälzlagerSchaeffler oHG for vibratory machines,are an optimal solution to these require-ments [2]. These bearings have proventhemselves in numerous applications formany years.

Literature:[1] Lippert, R. and Scherb, B.:Reibungsarme Zylinderrollenlager,„antriebstechnik“ Vol. No. 4, April 1993[2] INA publication LVM, Bearings for Vibratory Machines, Herzogenaurach,August 1998

Fig. 7 Comparison of dynamic load ratings

About the author:Roland Lippert is DepartmentManager for Application Engineering,Power Transmissions (Non Automotive),Construction and Plastic Machinery atINA Wälzlager Schaeffler oHG inHerzogenaurach, Germany.

1000 200

NJ 2324 E.M

ZSL 192324

ZSL 192324

150

100

100 193 193

50

0

800

600

400

200

0

780 950 950

Dynamic load ratings [kN] Calculated service life [%]

Sac

h-N

r. 0

05-1

61-3

12/IL

V U

S-D

049

92 ●●

·Prin

ted

in G

erm

any

INA Wälzlager Schaeffler oHG

D-91072 HerzogenaurachTelephone (+49 91 32) 82-0Fax (+49 91 32) 82-49 50http://www.ina.com