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MATERIALSTitanium

This paper focuses on the perfor-mance enhancement of taperedroller bearings in automotiveaxle and transmission applica-tions. It also provides anobjective comparison, basedon test results, with alter-native ball bearing arrange-ments. The influence of differ-ent design parameters is quanti-fied in the analytical and experimental investi-gation. The bearing power loss is calculated andmeasured for operating conditions that are relevant in avehicle fuel economy cycle.

1 Introduction

Additional specific aspects related to theuse of tapered roller bearings, such as set-ting and mounting, are discussed undersimilar considerations. The paper containsa system analysis of the pinion bearings ina rear axle, and of a Continuously VariableTransmission (CVT) transfer shaft interact-ing with the variator output shaft and withthe differential. Shaft and gear mesh de-flections are evaluated in the general con-text of system stiffness for improved dura-bility and noise, vibration and harshness(NVH) behavior.

The rear axle pinion bearing arrange-ment and the CVT transfer shaft exampleswere selected based on their high emphasison fuel economy, but the conclusions ofthis paper can be extended to the taperedroller bearings employed in various typesof automotive transmission and drivelineapplications. Better fuel economy is one ofthe main objectives in the automotive de-velopment process. For the driveline andtransmission component suppliers thistranslates into more challenging require-ments with respect to the power capacity,weight, volume and power loss characteris-tics of their product. Rear axles are a major

By Mircea Gradu,

Mark Joki and

Harold Hill

Kegelrollenlager für

Anwendungen in

Antriebsstrang und

Getriebe

You will find the figures mentioned in this article in the German issue of ATZ 3/2004 beginning on page 230.

Tapered RollerBearings for Automotive Driveline and Transmission Applications

16 ATZ worldwide 3/2004 Volume 106

contributor in the overall fuel efficiencypicture of the vehicle. Continuously vari-able transmissions currently represent oneof the leading technologies, offering an im-portant overall vehicle fuel economy im-provement, due in principal to the steplessratio change and to the operation of the en-gine closer to the optimum efficiency. Anoptimized design of the tapered roller bear-ings employed in these applications, couldsignificantly contribute to reduce theirpower loss level, as well as their mass anddimensions.

The case studies presented in this paperare the pinion bearing arrangement in arear axle and the transfer shaft bearings ofa commercially available CVT, Figure 1.

2 Power Density a Prerequisitefor Fuel Efficiency

Over decades of continuous innovation andimprovements in the bearing design, in-creased power density was one of the maindesign and development goals of TheTimken Company. The power density canbe defined as the ratio between the powertransferred by a machine component divid-ed by its weight, and the following featuresbeneficially influence it:■ high load capacity per unit of radial sec-tion■ clean material■ optimized stress distribution over thebearing race profile■ low roughness finishing.

Applied consistently, the above designprinciples lead to a significant reduction inthe bearing envelope dimensions and tothe concept of “bearing-in-a-bearing”, Fig-ure 2. The small bearing can carry the sameamount of load as the large one, but it fitsinto its bore.

The evolution of the Timken steel clean-liness is shown in Figure 3. By utilizing ad-vanced manufacturing processes, the totallength of inclusion stringers was reducedby several orders of magnitude. There wasalso a constant progress related to profilingthe bearing races in order to improve thestress distribution, Figure 4.

3 Fuel Efficient Design Strategy Applied to PinionBearings

Taking a proactive approach to addressthe global need for improved efficiency inautomotive applications, The TimkenCompany has defined a coherent bearingdesign strategy that significantly reducesthe power loss and simultaneously main-tains or improves some of the other sys-tem performance characteristics like stiff-

ness, operating temperature and debris re-sistance.

The basic concept is to convert the exces-sive bearing life margins into advanced de-sign features resulting in better efficiency.The other main principles promote a reduc-tion of the unnecessary bearing spread andthe optimization of the raceway contactthrough adequate profiling.

An important aspect is defining correct-ly the operating conditions used for the de-sign optimization exercise. This has to bedone under the consideration of typicaldriving behaviors and of a fuel economycycle, Figure 5.

For an axle application, the vehiclespeed – time chart describing the fuel econ-omy cycle can be easily converted into a di-agram specifying the cycle percentage driv-en at a certain pinion gear speed range andinto a diagram depicting the relative ener-gy lost by a unit of torque. It is clear fromthis analysis that higher speeds bring themost significant contribution to the energyloss over the fuel economy cycle. Accord-ingly, the optimization will target the de-sign parameters directly involved with thehigh-speed torque reduction.

The typical torque behavior of a taperedroller bearing is shown in Figure 6. At rela-tively low speeds, the rib-roller torque dom-inates, while at higher speeds, the rollerraceway contact generates a higher torquecontribution.

Based on the specific durability, stiffnessand efficiency targets the Fuel EfficientBearing arrangement offers also the oppor-tunity for axle housing weight and size re-duction, Figure 7.

The Fuel Efficient Bearing design wastested under various speed, pinion torqueand temperature operating conditions, andcompared to the standard design, Figure 8.The test fixture is presented in Figure 9.

In general the Fuel Efficient Bearingsyielded 30 % lower torque values over theentire speed range, in a clean oil environ-ment. At 2500 RPM and 60 °C, this repre-sents a 0.33 kW powerloss reduction.

Similar tests were conducted in a debris-laden environment, demonstrating addi-tional benefits related to the wear charac-teristics and operating temperature of theFuel Efficient Bearings. The amount ofworn material and the temperature levelswere significantly lower compared to thestandard product.

The up to 36 % parasitic torque reduc-tion obtained with the Timken Fuel Effi-cient tapered roller bearings represents thehighest achievable efficiency level in theindustry even when compared with thetandem angular ball bearing solutions, at asignificantly lower cost.

4 Tapered Roller Bearing Selection and Design Optimization for a CVT TransferShaft Application

The second case study presented in this pa-per is the transfer shaft of a commerciallyavailable CVT, Figure 2. The transmissionmanufacturer has chosen a fixed-floatingball bearing solution. The flow chart in [4]indicates the different analysis and designphases leading to an arrangement based ontapered roller bearings, with a higher pow-er density and improved efficiency. Basedon the ball bearing ratings and on some ofthe imposed envelope requirements, a con-ventional tapered roller bearing catalog se-lection has been initially made. The resultof this selection was refined, accordingly tofatigue life and rolling torque criteria, in or-der to retain the optimum tapered rollerbearing design.

A comparison between the dimensionalcharacteristics and the weight of the ballbearings and of the proposed tapered rollerbearings is presented in the Figure 10.

The most objective validation of the ta-pered roller bearing selection can be ob-tained only through the comparison withthe ball bearing performance over a vehicleor transmission duty cycle. Using sophisti-cated simulation techniques, the influenceof the environment and design characteris-tics on the bearing performance was accu-rately quantified. The SysX software pack-age developed by the Timken Company al-lows the analysis of a complete system con-sisting of shafts, gears and bearings under awide range of load / speed / temperatureoperating conditions. The output of the pro-gram contains fatigue life, contact stressesand running torque results for the bearings,but also deflection results for the shafts andgears, representing valuable informationfor the transmission designer.

The target fatigue life (L10) on the dutycycle was 250 hours. Several tapered rollerbearing design variations were consideredin this study, but for simplification, the pa-per includes only the comparison betweenthe fixed and floating ball bearings used inthe original transmission layout and two ta-pered roller bearing (TRB) designs: a stan-dard design currently commercially avail-able and a new design (ND) featuringamong others a special race profile. The re-sults of the simulation over the duty cycledemonstrate that both tapered roller bear-ing designs meet the application require-ments, and validate their selection. Com-bined with the dimensional and weight ad-vantages shown in Figure 10, this translatesinto a remarkable power density of the ta-pered roller bearings.

DEVELOPMENT Bearing

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MATERIALSTitanium

The special profile tapered roller bearingdesign, being optimized for the present ap-plication, reduces the stress level along thecone race (especially the edge stress at theroller-cone contact, Figure 4), increasing thefatigue life significantly compared to thestandard design.

The differential thermal expansion be-tween the transmission housing (Alu-minum) and the transfer shaft (Steel) is mi-nor, in this case, due to the compactness ofthe design, and does not affect the bearingsetting and implicitly the life performance.Over the operating temperature range, as-sumed from -40 to 120 °C, the differentialthermal expansion results in a maximumsetting variation of 0.18 mm. The preloadforce corresponding to the extremes of thedimensional setting does not exceed 800 Nper bearing. At nominal dimensional set-ting, the preload force per bearing reaches300 N, being relatively small compared tothe gear loads. For the duty cycle, the resul-tant gear axial load varies from 12,500 N to2500 N (helix angle 33° and 38° for the largeand respectively small gear). However, forcompensating the thermal effects in alu-minum or magnesium transmissions withlarger bearing spacing, adequate taperedroller bearing design solutions are commer-cially available, Figure 11.

An important aspect is the reduction ofthe radial deflections at the gear meshpoints, achieved by the both presented ta-pered roller bearing designs, with a verybeneficial impact on the noise and durabil-ity parameters of the gear train.

5 Ball versus Tapered RollerBearings Efficiency Evaluation

The emphasis of the CVT case study is notonly on the superiority of the tapered rollerbearings from the power density point ofview, but also on their better efficiencycharacteristics. The overall vehicle efficien-cy is reflected in the test results over a fueleconomy cycle. The currently used fueleconomy cycles are FTP75 in USA and ECEEC96 in the European Community, Figure 5.A detailed analysis of these cycles revealsthe percentage from the total cycle time foreach transfer shaft speed operating condi-tion. Taking into account the dependencyof the bearing running torque on the oper-ating speed, this analysis is essential for thebearing efficiency evaluation.

In order to isolate the bearing power lossfrom other transfer shaft system losses,caused by the gears, drag, etc., the bearingrunning torque measurements were con-ducted on a test rig configuration similar tothe one represented in Figure 9. The exter-nal loads applied on the rig were intended

to simulate the loading conditions to whichthe bearings are subjected on a transfershaft transmitting two reference torquevalues of 100 Nm and respectively 200 Nm.The test bearings are located in the centralpositions, carrying theoretically the entireapplied load. Two ball bearings similar tothe floating position on the transfer shaftwere used as slave bearings.

When using tapered roller bearings un-der nominal setting (300 N per bearing), the100 Nm and 200 Nm applied torque on thetransfer shaft translates to four loadingconditions. These conditions, plus the purespin condition, were used for a “one-to-one” comparison between the taperedroller bearings and the ball bearings, aswell as the basis for the summation of thetotal measured torque of the bearing setssupporting the transfer shaft. The measure-ments were conducted at three tempera-ture levels: 30, 50 and 80 °C being represen-tative for the different phases of the fueleconomy cycles. The torque increase for thehighest preload of the tapered roller bear-ings was within 5 % of the total runningtorque due to external loads. The measure-ment results prove the superior efficiencyof the standard tapered roller bearing de-sign compared to the floating ball bearing(which, at its turn, is more efficient thanthe fixed ball bearing). The tapered rollerbearing efficiency can be enhanced furtherthrough the special new design (ND),which contains some elements derivedfrom the fuel-efficient design strategy. Themeasurement results also show a verygood overlap with the torque values calcu-lated using the SysX package.

The results of the running torque evalu-ation for the bearing sets supporting thetransfer shaft are presented in detail in [4].The impact of these torque characteristicson the total power loss over the two repre-sentative fuel economy cycles is reflectedin Figure 12.

The tapered roller bearing resists debrisby its continuous, self-cleaning, lubricant-pumping action. A ball bearing, with itsdeep groove race traps and retains debris.The clean-sealed ball bearings are betterprotected from contaminants, but the run-ning torque increases significantly due tothe seals.

6 Conclusions

Recent tapered roller bearing design en-hancements reduce the running torque,meeting simultaneously the other techni-cal requirements with respect to system lifeand stiffness. The up to 36 % parasitictorque reduction compared to a standardtapered roller bearing design obtained with

the Timken Fuel Efficient Bearings (FEB)represents the highest achievable efficien-cy level in the industry even when com-pared with the tandem angular ball bear-ing solutions, at a significantly lower cost.

Compared to ball bearings, in automo-tive driveline and transmissions applica-tions, at the same load carrying capacity,tapered roller bearings offer the advan-tages of a significant weight and packagesize reduction.

Tapered roller bearings provide a bettergear system stiffness, leading to an im-proved NVH behavior and durability of thetransmission.

The running torque of the tapered rollerbearings, under the same operating condi-tions, is considerably lower than the torqueof the equivalent ball bearings supportingthe CVT transfer shaft analyzed in thisstudy.

In compact designed automotive trans-missions and axles, minimal setting varia-tions, due to thermal effects, maintain thesystem life and the efficiency performanceof the tapered roller bearings to a superiorlevel compared to the equivalent ball bear-ings.

Under the typical operating conditionsof the most popular fuel economy cycles(FTP75 and ECE EC96), the tapered rollerbearings provide major improvements (ap-prox. 27 % power loss reduction on the CVTtransfer shaft analyzed in this study).

The debris resistance of the taperedroller bearings contributes to a better run-ning torque performance, by eliminatingthe need for the clean-sealed technology,which involves the additional seal torque.

References

[1] Krabill, T. J.: “Fatigue Life Comparison of Balland Tapered Roller Bearing under WheelBearing Load Conditions”, The Timken Com-pany, SAE paper 890869

[2] Houpert, L.; Merckling, J.: “A successful tran-sition from physically measured to numerical-ly simulated bearings, shafts, gears and hous-ing deflections in a transmission”, TheTimken Company, Global Powertrain Con-gress, Detroit, October 1998

[3] Houpert, L.: “A uniform analytical approachfor ball and roller bearing", The Timken Com-pany, ASME Jour. Trib., Vol. 119, p. 851-857,Oct. 1997

[4] Gradu, M.: “Tapered Roller Bearings withImproved Efficiency and High Power Densityfor Automotive Transmissions”, The TimkenCompany, SAE paper 2000-01-1154

[5] Leibensperger, R. L.: “So you think you knowall about bearing preload”, The Timken Com-pany, Machine Design, Cleveland, August1972

[6] Zhou, R. S.; Hoeprich, M. R.: “Torque ofTapered Roller Bearings”, The Timken Com-pany, Transactions of ASME, Vol. 113, July1991

[7] Witte, D. C.; Hill, H. E.: “Tapered Roller Bear-ing Torque Characteristics with Emphasis onRib-Roller End Contact”, The Timken Compa-ny, SAE paper 871984