Validation of Wheel Bearing Systems in Biaxial Wheel.pdf

400 Commonwealth Drive, Warrendale, PA 15096-0001 U.S.A. Tel: (724) 776-4841 Fax: (724) 776-5760 Web: 2005-01-1827 Validation of Wheel Bearing Systems in Biaxial Wheel/Hub Test Facilities Gerhard Fischer and Rdiger Zinke Fraunhofer-Institute for Structural Durability and System Reliability LBF Reprinted From: Steering and Suspension, Tires and Wheels (SP-1915) 2005 SAE World Congress Detroit, Michigan April 11-14, 2005 SAE TECHNICAL PAPER SERIES THIS DOCUMENT IS PROTECTED BY U.S. AND INTERNATIONAL COPYRIGHT It may not be reproduced, stored in a retrieval system, distributed or transmitted, in whole or in part, in any form or by any means. Downloaded from SAE International by Brought to by the J. Robert Van Pelt Library / Michigan Technological Univ. , Friday, December 21, 2012 11:03:57 AM The Engineering Meetings Board has approved this paper for publication. It has successfully completed SAEs peer review process under the supervision of the session organizer. This process requires a minimum of three (3) reviews by industry experts. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of SAE. For permission and licensing requests contact: SAE Permissions 400 Commonwealth Drive Warrendale, PA 15096-0001-USA Email: Tel: 724-772-4028 Fax: 724-772-4891 For multiple print copies contact: SAE Customer Service Tel: 877-606-7323 (inside USA and Canada) Tel: 724-776-4970 (outside USA) Fax: 724-776-1615 Email: CustomerS ISSN 0148-7191 Copyright 2005 SAE International Positions and opinions advanced in this paper are those of the author(s) and not necessarily those of SAE. The author is solely responsible for the content of the paper. A process is available by which discussions will be printed with the paper if it is published in SAE Transactions. Persons wishing to submit papers to be considered for presentation or publication by SAE should send the manuscript or a 300 word abstract to Secretary, Engineering Meetings Board, SAE. Printed in USA THIS DOCUMENT IS PROTECTED BY U.S. AND INTERNATIONAL COPYRIGHT It may not be reproduced, stored in a retrieval system, distributed or transmitted, in whole or in part, in any form or by any means. Downloaded from SAE International by Brought to by the J. Robert Van Pelt Library / Michigan Technological Univ. , Friday, December 21, 2012 11:03:57 AM ABSTRACT Validation of the complex wheel/hub assembly has been carried out in the Biaxial Wheel/Hub Test Facility since the early 1980s, developed at Fraunhofer LBF. This test procedure was applied as standard at most of the European vehicle producers and wheel suppliers and was also introduced as SAE wheel standard J 2562, for wheels of passenger cars, issued in 2003. By extensive test series and investigations a suitable load file has been developed which is able to create fa- tigue failures and damages on wheel bearings, compa- rable to real service failures within an acceptable testing time and so far to prove their operational durability. The load program is based on the existing test program for durability approval of wheels and hubs, simulating differ- ent driving sections such as straight ahead driving, cor- nering and, if required, off-road driving and braking operations. The influence of different load programs on the bearing damage is described in this paper. Finally, a new bearing test rig is presented which is based on the Biaxial Wheel Test Rig, but without using the tires for testing. Load files for bearing tests, devel- oped by the German car producers on the “Eurocycle” basis can be simulated as well as special programs for high-performance sport cars. INTRODUCTION Wheel bearings are important functional components of vehicle suspensions. Although a great amount of re- search has been undertaken by the bearing manufactur- ers in calculation and testing of bearings, the individual application in specific car systems makes it difficult to apply these results properly. The wheel bearings are a part of the complex wheel/hub system (Fig.1) comprising various components of differ- ent materials, treatments and press fits. The variable loading conditions, caused by operational wheel forces and superimposed brake and torque moments, may re- sult in additional time-varying tolerances and press fits during operation and, consequently, in different damage mechanism. Fig. 1: Validation Tests on Driven Wheel/Hub Compo- nents For many years, validation tests of the wheel/hub as- sembly for passenger cars and commercial vehicles have been carried out in the Biaxial Wheel Test Facility (Fig. 2) applying load program tests 1-4. Fig. 2: LBF Biaxial Wheel Test Rig 2005-01-1827 Validation of Wheel Bearing Systems in Biaxial Wheel/Hub Test Facilities Gerhard Fischer and Rdiger Zinke Fraunhofer-Institute for Structural Durability and System Reliability LBF Copyright 2005 SAE International THIS DOCUMENT IS PROTECTED BY U.S. AND INTERNATIONAL COPYRIGHT It may not be reproduced, stored in a retrieval system, distributed or transmitted, in whole or in part, in any form or by any means. Downloaded from SAE International by Brought to by the J. Robert Van Pelt Library / Michigan Technological Univ. , Friday, December 21, 2012 11:03:57 AM The test procedure was accepted as test specification by German car and truck producers at an early stage and also became a technical standard as SAE J 2562 in 2003 5. In order to cover the decisive damage mechanism of wheel bearings during validation tests in the Biaxial Wheel Test Rig, additional test series have been carried out in order to - develop a suitable load program to reproduce service failures within an acceptable testing time and to - monitor first initial failures. From many validation tests on car and truck wheels as well as hubs in the LBF Biaxial Wheel Test Rigs, some important results related to the behavior of car wheel bearings are presented below. TEST PROCEDURE IN THE BIAXIAL WHEEL/HUB TEST FACILITY The validation tests are carried out in the well known Biaxial Wheel Test Rig (Fig. 2). The standard LBF-test procedure in the Biaxial Wheel Test Facility is based on the load program Eurocycle 1. Based on these tests it was possible to detect weak points for both newly developed truck hub units and new passenger car bearings. It must be emphasized that service-like load-time histories are necessary to repro- duce all damage influences. High peak stresses due to off-road operations, severe driving on uneven or poor surfaces, parking operations (trailer axle etc.) may cre- ate local plastic deformations, which can consequently lead to fatigue failure. On the other hand, also a high number of cycles has to be simulated in order to create failures caused by the rolling contact fatigue. Superimposed high temperatures caused by braking operations or severe mechanical loading cases influence the bearing clearance, especially if hub carriers are made of light alloy materials (Fig. 1). Last but not least, any muddy water is able to reduce dramatically the life if seals are not working properly. Therefore, at Fraunhofer-Institute LBF the tests are car- ried out including the braking operations, if necessary 2. The test requirement for the load program Eurocycle is based on a comparative damage calculation as well as data derived from on experience. The test require- ment for bearings should include the specific damage criteria for rolling contact fatigue, which consequently limits the possibility of accelerated testing by decreased testing time in relation to the testing time for wheels and hubs (Fig. 3). The preliminary derived test requirements (Fig. 4) result from comparison to proving ground tests and were confirmed through customer usage. Comparative damage calculations on system points very close to the bearing also confirm these experiences. Fig. 3: Basic Design Spectrum for 300,000 km of Ser- vice and Test Spectrum Fig. 4: Test Requirements for Load Program “Eurocycle” A further reduction of testing time by simulating corner- ing driving only (like super-racing tracks) or even con- stant amplitude loads may of course lead to early fail- ures due to the high loading and the reduced testing time, but it is impossible to investigate other damaging influences like contact fatigue under low stresses and high number of cycles. Experience of validation testing in the Biaxial Wheel Test Rig and results from customer usage and proving ground tests with existing and new developed bearing units indicated a good correlation, but additional efforts must be undertaken to increase the reproducibility of the method. Therefore, additional test series have been carried out in order to detect the limits of test acceleration and the fail- ure reproduction. RESULTS OF BEARING TESTING Results from testing and optimization of two bearing types, a non-driven front axle hub unit and a driven rear- axle hub (Fig. 5) are presented. Both test results have been related to the test life achieved on proving grounds as a reference base. THIS DOCUMENT IS PROTECTED BY U.S. AND INTERNATIONAL COPYRIGHT It may not be reproduced, stored in a retrieval system, distributed or transmitted, in whole or in part, in any form or by any means. Downloaded from SAE International by Brought to by the J. Robert Van Pelt Library / Michigan Technological Univ. , Friday, December 21, 2012 11:03:57 AM Fig 5: Validation Testing on Bearings Front Axle Hub Unit For the validation testing and optimization procedure of a car non-driven front axle hub unit, where early failures occurred during proving ground tests, three different load programs were applied for validation testing in the Biaxial Wheel Test Rig, in order to find the optimal ver- sion (Fig. 6). Fig. 6: Load Program for Bearing Testing 1. The “Hockenheimring” short racing track (2.6 km) consisting mainly of left and right bends. 2. Load program “Eurocycle” (original version 30 km). 3. Shortened load program “Eurocycle” (20.4 km) by omitting straight driving sections with result- ing low stresses. The stress-time histories, recorded on the hub, are shown in Fig. 6. During the tests, also different data are recorded, such as temperature close to the bearing seat, circumferential walking actions of the bearing races, loss of bolt pre- tension or grease and increase in the clearance during the test etc. For a failure detection the frequency analy- sis of accelerations is used. The test series under the original load program “Eurocy- cle” led to an early failure by edge marks (Fig. 7). The temperature variation depending on the cornering forces can clearly be observed. The long test duration of the original “Eurocycle” made it necessary to accelerate the testing time by program modification. Fig. 7: Contact Fatigue Failure on Front Wheel Bearing, Load Program “Eurocycle” EC On the two modified versions A and B+) of the bearing system validation tests have been carried out using the “Hockenheimring” test track-program and the “shortened Eurocycle” load program. Under the “Hockenheimring” program excessive damaging occurred to the bearing already after only a short test life of some hundred kilo- meters (Fig. 8). Due to high local loads and stresses, no significant dif- ference occurred between design A and the optimized version B (only 20% life increase with design B). During the switch-off of the test a local temperature of 150o C was recorded. (Fig. 8). Fig. 8: Contact Fatigue Failure on Front Wheel Bearing, Load Program Racing Track “Hockenheimring” HR Contrary to this result the “shortened Eurocycle” pro- gram led to reasonable test lives. Design A did not pass the test requirement, while design B achieved a seven- times higher sufficient test life of more than 40,000 km (Fig. 9). A slight difference between the first defect indi- cated by the frequency analysis of acceleration and the final damage, accompanied by rough running, was ob- served. The maximum temperature at the bearing amounted to approx. 100o C. +) - reinforced flange - modified number of balls - increased contact diameter of stub axle THIS DOCUMENT IS PROTECTED BY U.S. AND INTERNATIONAL COPYRIGHT It may not be reproduced, stored in a retrieval system, distributed or transmitted, in whole or in part, in any form or by any means. Downloaded from SAE International by Brought to by the J. Robert Van Pelt Library / Michigan Technological Univ. , Friday, December 21, 2012 11:03:57 AM Fig. 9: Contact Fatigue Failure on Front Wheel Bearing, Load Program “Eurocycle-Shortened”, Design B, EC-S All test results are plotted in Fig. 10. Fig. 10: Results of Bearing Testing on non-driven Front Axle Hub Units Compared to the reference proving ground tests the fol- lowing main conclusions can be drawn from these tests: 1. The shortened “Eurocycle” program can be regarded as a reasonable load program for bearing testing in terms of damage simulation and testing time. Differ- ences of design variations can be clearly pointed out. The optimized version B indicates a significant life in- crease compared to the first batch and the reference proving ground tests. 2. The severe “Hockenheimring” racing track program turned out not to be not suited because the damage mechanisms differ from the service conditions. In this specific case, even the decisive design differences could not be detected in comparison to the “short- ened Eurocycle” program. However, further statistically based investigations are necessary to confirm these results. Rear Axle Hub The results of investigations on bearings of a driven rear axle are shown in Fig. 10 to 13. Based on results of proving ground tests as reference tests, where bearing failures occurred below the life re- quirement, comparative tests were carried out with the original “Eurocycle” load program (30 km per lap) and the shortened “Eurocycle” program (20.4 km per lap), see Fig. 6. Fig. 11: Contact Fatigue Failure on Rear Wheel Bearing, Load Program “Eurocycle” EC Fig. 12: Contact Fatigue on Rear Wheel Bearing, Load Program “Eurocycle Shortened” EC-S Fig. 13: Results of Bearing Testing on driven Rear Axle Hubs At the simulation with the original “Eurocycle” load pro- gram damages occurred between approx. 15,000 km and 29,000 km (Figs. 11, 13) on the inner race and outer race. While on the higher loaded inner race a severe damage could be observed, the outer race only showed pittings. The failures correspond very well to those from proving ground tests. The first initial failures were de- tected by an improved frequency analysis of accelera- tions (Fig. 14). THIS DOCUMENT IS PROTECTED BY U.S. AND INTERNATIONAL COPYRIGHT It may not be reproduced, stored in a retrieval system, distributed or transmitted, in whole or in part, in any form or by any means. Downloaded from SAE International by Brought to by the J. Robert Van Pelt Library / Michigan Technological Univ. , Friday, December 21, 2012 11:03:57 AM Fig. 14: Frequency Analysis of Acceleration at Bearing Tests After simulation of the “shortened Eurocycle” load pro- gram the same damage was found after approx. 6,000 km and 9,000 km (see Fig. 12, 13). The failure mode in the contact surface of the inner and outer race is similar to the original “Eurocycle” and to the proving ground tests. Only the number of cycles and testing time are lower than at the original “Eurocycle”. These results confirm that the shortened “Eurocycle” program with 20.4 km per lap is able to reproduce ser- vice-like failures within a reasonable testing time. For a statistical evaluation, the results have to be confirmed by a higher number of tests. Although a good correlation between proving ground tests and accelerated lab tests has been achieved, the correlation of the damage mode and service-life has still to be verified. NEW BIAXIAL BEARING TEST RIG The Biaxial Wheel/Hub Test Rig is a reliable road simu- lator for validation testing of the entire wheel/hub as- semblies. But due to tire wear and wheel failures, spe- cific time and cost saving tests on bearings and hubs can better be carried out in a newly developed biaxial bearing test rig. The test principle is shown in Fig. 15 and 16. Fig. 15: LBF Biaxial Bearing Test Rig Fig. 16: LBF Biaxial Bearing Test Rig The hub is mounted to the original wheel or an adapter having the same stiffness as the wheel, which itself is attached to a drum. Both vertical and lateral forces are applied to the hub via a loading frame similar to the Bi- axial Wheel/Hub Test Rig. Additionally, the line of action of the vertical force actuator can be shifted by a sepa- rate actuator, corresponding to the operational condi- tions on the car wheel. Within this test rig many tests on various load programs were carried out and compared to the previously de- scribed tests, which indicated a good correlation. Also the newly developed bearing load program of the five German car producers (Audi, BMW, DaimlerChrys- ler, Porsche, VW), based on the “Eurocycle”, can be simulated with high accuracy. Additionally, a special load program to be applied for high-performance sports cars, like Porsche, was developed at LBF, which includes both the corresponding high wheel forces and the shift- ing line of the vertical force action. Both parameters are decisive for the stress duplication on hub and bearings. Especially the line of vertical force introduction is not included in most existing bearing test rigs. CONCLUSIONS The approval of modern bearing systems can be carried out in biaxial wheel/hub test rigs, developed at LBF. All influences of adjacent components and their interaction and c