Cage Bearing Conceptfor Large-scale Gear Systems

Roland Lippert and Bruno Scherb

INA reprint from „Der Konstrukteur“Vol. No. S 4, April 1999Verlag für Technik und Wirtschaft, Mainz

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Cage Bearing Concept for Large-scale Gear SystemsRoland Lippert and Bruno Scherb

Manufacturers of modern gearsystems are increasing productpower density without changinghousing space. In the face ofsteadily increasing service lifedemands, the torques and speedsthat must be transmitted requirerolling bearings that are capable ofsupporting heavy loads and veryhigh speeds as well as high axialloads. Expertise in the features andcharacteristics of rolling bearings isbecoming more and more importantfor successful applications. Like thecurrent trend in gear manufacturing,the INA cylindrical rolling bearingspresented in this article are designedfor increased power density.

1 IntroductionThe design of compact, efficient gearsystems requires minimum toothing pitchcircle diameters with maximum gearcutting technology. Shaft diameters andspacing are specified by the gear manu-facturer. To keep shaft deflections at thepinion as low as possible, bearing supportdistances must be minimized. Performanceoptimized gear systems are designedmoreover weight optimized and thusrelatively thin sectioned. Accordingly,available bearing seat width is designedon the basis of single-row bearingswherever possible. Together with therequired bearing rating life the product ofmaximum bearing height times maximumwidth is given.

2 Radial bearing designs forlarge-scale gear systems

When designing bearing arrangementsfor a gear system, the design engineergenerally has only three options:• cylindrical roller bearings with cage or

full roller complement• self-aligning roller bearings• tapered roller bearings.All three designs and their respectivevariants are of proven efficiency. In makinga choice, the designer must be guidedby his own specifications. The internalstructure chosen for the gear dictatespossible bearing space, bearing speedsand axial and radial reaction forces on the

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bearing. Given the required bearing ratinglife, a preliminary load rating design choicecan be made. Due to the high efficiencyof modern gear systems mentioned above,series 23 bearing sizes are being usedmore and more often, especially for inter-mediate shafts. To meet this demand,INA Wälzlager Schaeffler oHG now offerstwo new bearing concepts: the LSL 19 23and the ZSL 19 23 series, in addition tothe long proven full-complement seriesSL 19 23.

2.1 Powerful components from INAThe two bearing series introduced in1993 [1], the LSL (cylindrical roller bearingwith heavy-section brass disc cage, Fig. 1)and the ZSL (cylindrical roller bearing withspacers, Fig. 2), are now being usedworldwide. They have become well estab-lished wherever quality, reliability, andmaximum power density are required.The following discussion is restricted tothe selection of the dynamic load carryingcapacity and nominal rating life, featuresthe design engineer must consider inselecting the right bearing system.

2.2 Load ratings and bearingrating life values

When bearing designs are compared,dynamic load ratings alone do not saymuch about the resulting nominal ratinglife values. They can merely serve asreference values for a preliminary choice(Fig. 3). When calculating the nominalrating life, DIN ISO 281 and the bearingmanufacturer's data [2], [3] must be used.Due to their design, tapered roller bearingsare subject to axial load components evenunder purely radial loads, and these havea direct effect on bearing life. Additionalaxial loads, such as those generated byhelical gearing, are also applied directly tothe raceway contact and significantlyreduce the expected bearing rating life.In the case of self-aligning roller bearings,externally applied axial loads also cause areduction in the nominal rating life. Herethe external axial loads are applied directlyto the rolling contact through a single rowof bearings.

Fig. 1 Cylindrical roller bearing with heavy-section disc cage, series LSL 19 23

Fig. 2 Cylindrical roller bearing with spacers,series ZSL 19 23

Fig. 3 Comparison of dynamic load ratings [2], [3]

Compared load ratings C [kN]

950 950900

780670

1000

800

600

400

200

0

LSL 19 2324 ZSL 19 2324 2 2324 NJ 2324 3 2324

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Radial cylindrical roller bearings are theexception. Radial and axial load supportmust be considered separately.Only radial loads are conducted throughthe rolling contact. They are the onlydeterminants of the rated fatigue life. Onthe other hand, additional axial loads areimposed via the rib-to-rolling elementsliding contact. This rib contact is calcu-lated as f (axial load, speed, lubrication,rib geometry) with respect to wear. Thereis no DIN ISO standard calculation method.The bearing manufacturer's data must beused to calculate wear limit loads [2], [3].Fig. 4 shows a comparison of nominalrating life Lh in relation to radial load Fr.Fig. 5 shows the relationship when addi-tional axial loads are applied at givenoperating points. The wear limit load equations given in theINA rolling bearing catalogs are basedon years of experience, comprising bothtesting and practical applications. Thefollowing is one example of a series of testsconducted at INA. The effects of speed,radial load and axial load on frictionalbehavior are briefly summarized.

3 Experimental testing

3.1 Test rigsTo investigate the operational behavior ofcylindrical roller bearings, INA uses thelarge bearing test rig (see detaileddescription in [4]) and the newly developedfrictional torque test rig shown in Fig. 6.A prime consideration in the developmentof the test rig were friction tests, withparticular attention devoted to the develop-ment of friction in the axial contact of thecylindrical roller bearing. The result is thethree-bearing test rig shown in Fig. 6.The test bearing is positioned in the centralbearing unit, which is designed as ahydrostatic friction device.

This allows the bearing’s frictional torqueto be measured separately. Friction-inducedtangential force is measured using alever bar and a dynamometric device(MR = Ft · r). Radial and axial loads areapplied by hydraulic cylinders and aremeasured using load-sensing devicesacting as strain gauges. The axial load isintroduced to the rotating shaft by theaxial hydrostatics. Heavy-duty oil coolersand oil heaters are used to maintain thebearing lubricant at constant viscositiesacross the wide temperature range.Drive is provided by a dynamic converter-fed asynchronous motor ata speed of up to 9000 rpm.

Fig. 5 Rating life vs. ratio Fa/Fr

Fig. 4 Rating life Lh vs. radial load Fr

100 000

80 000

60 000

40 000

20 000

060 65 70 75 80

Radial load Fr [kN]

Nominal rating life Lh [h]

n = 2000 rpm LSL/ZSL 19 2324

2 2324

NJ 2324

3 2324

85 90 95 100 105 110

100 000

80 000

60 000

40 000

20 000

00 0,1 0,2 0,3 0,4

Nominal rating life Lh [h]

Fr = 60 kN n = 2000 rpm LSL/ZSL 19 2324

2 2324

NJ 2324

3 2324

Load ratio Fa/Fr

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3.2 The testsComprehensive testing of the friction andtemperature behavior of radial cylindricalroller bearings under combined loadswere conducted using the above testrigs. For this purpose a test matrix wasset up and carried out after test bearingrun-in. The samples tested were series19 23 LSL, ZSL and SL bearings.An ISO VG class 220 gear oil was usedas lubricant. The measured values werestored on a computer, so that a detailedanalysis could be made later.The following test parameters are significant in evaluating the frictionalbehavior of cylindrical roller bearingssubjected to combined loads:• temperature of the test bearing’s

outer ring• temperature of the loaded ribs• temperature at oil inlet and oil outlet• amount of oil flow through the test

bearing• radial load on test bearing• axial load on test bearing• shaft speed• frictional torque of test bearing.

3.3 Test results3.3.1 Effect of speed on frictional

behavior exemplified by theLSL 19 2316 C3

Fig. 7 shows the characteristic frictionaltorque and speed curves for different axialloads under a constant radial load. Thecurves exhibit typical Stribeck characteris-tics with a solid body friction componentat n = 0, a subsequent mixed-frictioncomponent up to the minimum frictionaltorque, the so-called trip point, and amore or less salient hydrodynamic frictioncomponent as a function of the axial loadlevel.

3.3.2 Effect of radial and axial load onfrictional behavior examplified bythe LSL 19 2316 C3

Fig. 7 also shows that higher frictionaltorques occur as the axial load increases.As axial loads increase, the speed curveshows a sharp rise in frictional torque atlower speeds, caused by an increase inthe mixed-friction component in the axialcontact. At these extremely high axialloads ( Fa/Fr > 0.5 ) a continuous drop infrictional torque can be observed in thespeed curve as speed increases. Theminimum frictional torque is not achievedfor the curve (Fa/Fr = 1, Q = 5 l/min).Under these conditions the test bearingoperates under mixed-friction conditions.

AMK-Motor DH13-80-4 (luftgekühlt)

Fig. 6 Frictional torque test rig

Fig. 7 Frictional torque vs. speed – LSL 19 2316 C3

Frictional torque MR [Nm]

Speed n [rpm]

25

20

15

10

5

00 1000 2000 3000 4000 5000 6000

Q = 5 l/minFa/Fr = 0

Q = 15 l/minFa/Fr = 0

Fa/Fr = 0,25

Fr = 20 kN

Fa/Fr = 0,25

Fa/Fr = 0,5

Fa/Fr = 0,5

Fa/Fr = 1,0

Fa/Fr = 1,0

➀ hydrostatic plain bearingfor applying axial load

➁ measurement of axialload with load cell

➂ lubrication of test bearing

➃ hydrostatic frictionalbalance

➄ cantilever beam to measure frictionel torque

➅ test bearing

➆ measurement of drivingtorque with torque meter

➇ measurement of radialload with load cell

➀

➁

➂➃ ➄

➅

➆

➇

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This means that the axial contact may besubject to wear. The graphs of frictionaltorque vs. load ratio Fa/Fr, (Fig. 8), andaxial load, (Fig. 9), provide a clear analysisof the effect. Under radial loads and forequal load ratios it can be seen that thereis a disproportionate rise in frictional torqueeven under high axial loads (Fig. 8).The effect of the loads is evident in Fig. 9.

A nearly linear increase in frictional torquewith the axial load can be seen. The effectsof radial load are less significant.

3.3.3 Comparison of frictional torqueresults

The frictional torques measured for com-bined loads Fa/Fr at different speeds arecompared in Fig. 10.

4 SummaryThe results demonstrate that the INAcylindrical roller bearings support veryhigh axial loads due to their favorableinternal friction characteristics. With goodlubricating conditions it is possible toreach load ratios of up to Fa/Fr = 1.0. Inaddition, as a result of the comparativelylow bearing temperatures, the low frictionaltorque promotes higher lubricant viscosi-ties, which leads to earlier developmentof hydrodynamic conditions, especiallyin the axial contact. As a result, the INAcylindrical roller bearing guarantees wear-free operation earlier than conventionalbearing designs.Compared to conventional cylindricalroller bearings, these results point to ahigher axial load carrying capacity forINA series LSL and ZSL cylindrical rollerbearings.

Fig. 8 Frictional torque vs. load ratio Fa/Fr – LSL 19 2316 C3

Fig. 9 Frictional torque vs. axial load – LSL 19 2316 C3

50

40

30

20

10

00 0,2 0,4 0,6 0,8 1

50

40

30

20

10

00 0,2 0,4 0,6 0,8 1

Frictional torque MR [Nm]

Load ratio Fa/Fr Load ratio Fa/Fr

Frictional torque MR [Nm]

Fr = 5 kN

Fr = 10 kN

Fr = 20 kN

Fr = 50 kN

Fr = 5 kN

Fr = 10 kN

Fr = 20 kN

Fr = 50 kN

n = 500 1/min; Q = 5 l/min n = 4000 1/min; Q = 5 l/min

50

40

30

20

10

00 10 20 30

50

40

30

20

10

00

Frictional torque MR [Nm]

Axial load Fa [kN]10 20 30

Axial load Fa [kN]

Frictional torque MR [Nm]

Fr = 5 kN

Fr = 10 kN

Fr = 20 kN

Fr = 50 kN

Fr = 5 kN

Fr = 10 kN

Fr = 20 kN

Fr = 50 kN

n = 500 1/min; Q = 5 l/min n = 4000 1/min; Q = 5 l/min

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5 Future prospectsThe analytical study of the rating life orservice life of radial cylindrical rollerbearings under combined loads revealsextremely complex relationships in termsof the effects of additional axial loads. Forthis reason further analysis of the frictioncontacts in the rolling bearing is neededin order to examine how friction occurs.On the basis of lubrication conditions, thepossibility of calculating wear life in theaxial contact must be developed.

Literature:[1] Lippert, R. and Scherb, B.:

Reibungsarme Zylinderrollenlager,„antriebstechnik“ (1993) Vol. No. 4

[2] INA catalog 307

[3] FAG Wälzlager Katalog,WL 41 520 DB, May 1995

[4] Scherb, B.: Zusammenhang zwischenKäfig- und Wälzkörperdrehzahl beiZylinderrollenlagern,„antriebstechnik“ (1997) Vol. No. 2

About the authors:Roland Lippert is Department Managerfor Application Engineering, PowerTranmissions (Non Automotive),Construction and Plastic Machinery.Bruno Scherb is in Charge of theRadial Bearing Section in the TestingDepartment with special focus on frictionalbehavior and bearing kinematics in radialbearings. Both are employed atINA Wälzlager Schaeffler oHGin Herzogenaurach, Germany.

Fig. 10 Comparison of frictional torque MR, LSL / MR, NJ and MR, ZSL / MR, NJ

1

0.8

0.6

0.4

0.2

0

LSL ZSL NJ

1

0.8

0.6

0.4

0.2

0

LSL ZSL NJ

Relative radial load dependent frictional torque

Series 2316

Relative axial load dependent frictional torque

Sac

h-N

r. 0

05-1

98-0

38/K

LG U

S-D

049

92 ●●

·Prin

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in G

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any

INA Wälzlager Schaeffler oHG

91072 Herzogenaurach (Germany)Telephone (+49 91 32) 82-0Fax (+49 91 32) 82-49 50http://www.ina.com