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400 Commonwealth Drive, Warrendale, PA 15096-0001 U.S.A. Tel: (724) 776-4841 Fax: (724) 776-5760 Web: SAE TECHNICALPAPER SERIES2002-01-1691Developing Next Generation Axle Fluids:Part I Test Methodology to MeasureDurability and Temperature ReductionProperties of Axle Gear OilsEdward S. Akucewich, James N. Vinci,Farrukh S. Qureshi and Robert W. CainThe Lubrizol CorporationInternational Spring Fuels & LubricantsMeeting & ExhibitionReno, NevadaMay 6-9, 2002The appearance of this ISSN code at the bottom of this page indicates SAEs consent that copies of thepaper may be made for personal or internal use of specific clients. This consent is given on the condition,however, that the copier pay a per article copy fee through the Copyright Clearance Center, Inc. OperationsCenter, 222 Rosewood Drive, Danvers, MA 01923 for copying beyond that permitted by Sections 107 or108 of the U.S. Copyright Law. This consent does not extend to other kinds of copying such as copying forgeneral distribution, for advertising or promotional purposes, for creating new collective works, or forresale.Quantity reprint rates can be obtained from the Customer Sales and Satisfaction Department.To request permission to reprint a technical paper or permission to use copyrighted SAE publications inother works, contact the SAE Publications Group.No part of this publication may be reproduced in any form, in an electronic retrieval system or otherwise, without the prior writtenpermission of the publisher.ISSN 0148-7191Copyright 2002 Society of Automotive Engineers, Inc.Positions and opinions advanced in this paper are those of the author(s) and not necessarily those of SAE. The author is solelyresponsible for the content of the paper. A process is available by which discussions will be printed with the paper if it is published inSAE Transactions. For permission to publish this paper in full or in part, contact the SAE Publications Group.Persons wishing to submit papers to be considered for presentation or publication through SAE should send the manuscript or a 300word abstract of a proposed manuscript to: Secretary, Engineering Meetings Board, SAE.Printed in USAAll SAE papers, standards, and selectedbooks are abstracted and indexed in theGlobal Mobility DatabaseABSTRACTLight trucks and sport utility vehicles (SUVs) havebecome extremely popular in the United States in recentyears, but this shift to larger passenger vehicles hasplaced new demands upon the gear lubricant. The keychallenge facing vehicle manufacturers in North Americais meeting government-mandated fuel economyrequirements while maintaining durability. Gear oils mustprovide long-term durability and operating temperaturecontrol in order to increase equipment life under severeconditions while maintaining fuel efficiency.This paper describes the development of a full-scale lightduty axle test that simulates a variety of different drivingconditions that can be used to measure temperaturereduction properties of gear oil formulations. The workpresented here outlines a test methodology that allowsgear oil formulations to be compared with each other whileaccounting for axle changes due to wear and conditioningduring testing. Results are shown from a variety ofdifferent axle configurations and loading conditions. Thistest method shows the importance of accounting forchanges in the axle when comparing test resultswhenever severe conditions are experienced.INTRODUCTIONWithin the last few years, there has been a reneweddesire to make fuel economy improvements in NorthAmericas light trucks and sport utility vehicles (SUVs).Vehicle manufacturers have set aggressive fuel efficiencyimprovement objectives for these vehicles. Because ofthis, gear lubricants have been targeted to contribute fueleconomy improvements over the current products used inthese applications.This is not as easy as it may seem. In addition toacceptable fuel economy, gear lubricants are required toprotect axle components under a variety of stressedconditions. These include high speed scuffing, low speed,high torque wear, corrosion and oxidation. In light truckand racing applications, gear oils must provide long-termdurability and operating temperature control underextreme conditions, such as trailer towing or extendedhigh speed applications. Higher operating temperaturesfor prolonged periods can adversely affect metallurgicalproperties and reduce fluid film thickness, both of whichcan lead to premature equipment failures. In our view,operating temperature is an important indicator ofdurability.While fuel economy is now the driving force in nextgeneration lubricant development, it is clearly recognizedthat any improvements in fuel economy must not be at theexpense of axle durability or performance.Fuel economy improvements can be measured via theU.S. EPA 55/45 driving cycles(1). Automotivemanufacturers use this test to certify a vehicles fueleconomy. This test can also be used to show fueleconomy improvements in gear oil lubricants.Many manufacturers feel that stabilized operatingtemperature under the proper controlled conditions is animportant indicator of the durability performance of alubricant under severe conditions. In the case of operatingtemperature assessment, there exists no standard testmethod or methodology. Typically, when applied in alaboratory test stand a single axle is broken-in and thenused repeatedly to evaluate many lubricants. Undersevere conditions, the stabilized operating temperaturesfor a given reference oil decreases each time it is run in anaxle. As the number of test runs on an axle increases thestabilized operating temperature of the reference oil islower. This poses a problem when evaluating candidatelubricants. With a changing target, how can a lubricantbe accurately evaluated?This paper describes a laboratory test method thataccounts for test-to-test changes in the axle and gives thelubricant formulator an accurate way of comparing testresults. In addition, common pitfalls of this method andoperating guidelines will be described.2002-01-1691 Developing Next Generation Axle Fluids: Part I Test Methodology to Measure Durability and Temperature Reduction Properties of Axle Gear Oils Edward S. Akucewich, James N. Vinci, Farrukh S. Qureshi and Robert W. Cain The Lubrizol Corporation Copyright 2002 Society of Automotive Engineers, Inc.2The remainder of this paper is divided into four parts.First, the test stand used to develop and utilize the testprocedure is described. Second, the test methodology isdiscussed in detail. The third section focuses onpresenting test results that demonstrate the usefulness ofthe test methodology. Finally, the last sectionsummarizes the papers findings and offers someconclusions.PART 1 - AXLE TEST STAND CONFIGURATIONThis full-scale axle dynamometer test stand was designedand set up to simulate a variety of operating conditions. Aschematic of the test stand is shown in Figure 1. Thisfigure illustrates the axle rig and its major components.Figure 2 shows a picture of the test stand.Figure 1: Schematic of Axle Test StandInputtorqueOutput TorqueMeter (2)Output TorqueMeter (1)Box shroud + fan(optional)ENGINE: V8 GASOLINEDynamometerDynamometerSpeedIncreaserSpeedIncreaser3Figure 2: Photograph of Test StandSTAND CONFIGURATION - Power is supplied to the axleby a gasoline fueled 7.4 liter V8 engine through a heavyduty 4-speed automatic transmission that can beautomatically shifted by the data acquisition and control(DAC) system. The axle used for lubricant evaluation isrigidly mounted to the stand. The power driven throughthe axle is absorbed by two air gap eddy currentdynamometers. A speed increaser is placed between theaxle wheel end and the dynamometer to boost outputspeed to the dynamometer for low speed applications.The stand used is flexible and with a quick change oftorque meters and/or axle fixtures is able toaccommodate a wide range of axle sizes, from smallpassenger vehicle axles to large on highway truck axles.TORQUE METERS - A single in-line torque meter integralto the drive shaft measures the input pinion torque to theaxle. Two in-line torque meters measure the outputtorque from the axle to the dynamometers. One outputtorque meter has been placed between each axle wheelend and speed increaser.In addition, the torque meters used are the enhancedaccuracy, DC operated models. This was done toincrease and maintain a high degree of accuracy andrepeatability. These torque meters are periodically deadweight calibrated to insure accurate torquemeasurements.AXLE COOLING AND TEMPERATURE MONITORING -Behind the axle a fan is positioned to provide airflowacross the axle. This was done to simulate the actualairflow cooling experienced in field tests. The fan speed,size and position were selected to produce temperaturesin the axle which match field test data for the axle beingtested.In addition, two water spray nozzles are positioned aroundthe axle. These spray nozzles are used for two purposes.First, they are used to control the lubricant temperatureduring axle break-in. Second, they provide protectionagainst high axle lubricant temperatures. Depending uponthe lubricant under evaluation, this test procedure has thepotential of experiencing very high axle lubricant4temperatures. To protect the axle, high temperature limitshave been put in place for each test stage.Another major concern is the measurement of the ambientair and axle lubricant temperatures. Thus, care was takento properly position the thermocouples. The axle lubricanttemperature is measured by a thermocouple positioneddirectly next to the axle ring gear. The thermocouple isheld in place by a specially modified axle cover. Theambient air temperature is measured by placing athermocouple in the air stream produced by the fan. Boththermocouples are periodically calibrated to insureaccurate temperature measurements.DATA ACQUISITION AND CONTROL SYSTEM - A DSPRedline ADAPT / MRTP system is used to control theoperation of the stand and to acquire data throughout thetest. In addition to the ambient and lubricanttemperatures, this system monitors and recordsadditional temperatures (engine oil, transmission oil,dyno, gear box, fuel, and coolant), torques (input and twooutputs), speeds (engine, pinion, axle shafts, and dynos)and axle efficiency (ratio of output torque to input torque)throughout the test. Data is logged periodically.This system controls the operation of the stand with fivecontrol loops. Two control loops are used to maintain the desiredpinion speed. This is done by modulating eachdynamometer current to achieve a desired pinion rpm. The load on the pinion is maintained by adjusting theengine throttle. A fourth control loop is used to control the axlelubricant temperature during axle break in and toprevent high temperatures from damaging the axleduring lubricant evaluations. Finally, a fifth control loop is used to insure that theautomatic transmission is running each test stage inthe appropriate gear.It is important that the automatic transmission isoperating in the proper gear. Some of the test stagesduring this test run at relatively high loads. Prematurefailure will occur if the transmission does not operate inthe appropriate gear for a given test stage.PART 2 - TEST METHODIn general, the evaluation of the lubricants durability wasassessed by determining its stabilized operatingtemperature and axle efficiency at a number of discretespeed / torque conditions. The test procedure used isdescribed below.REFERENCE OILS - Reference oils are critical to thistest methodology. For the development of this testprocedure and evaluation of lubricants, two reference oilswere used. The fluids used as reference oils are asfollows:Good Reference: Synthetic SAE 75W-140Poor Reference: Synthetic SAE 75W-90The good reference has been shown to provideoutstanding performance in a wide variety of severeservice applications. This fluid provided excellenttemperature reduction in a controlled severe duty fieldtest. This reference oil is used to break-in the axle and isperiodically tested on a given axle to track any changesthat might occur in stabilized operating temperatures.The poor reference was also field tested and did notprovide the same level of durability or temperaturereduction in severe conditions as the good reference.Testing has shown however that this lubricant providesmeasurable fuel economy benefits. This reference oil isused to verify that the test procedure can distinguishbetween oils that provide different levels of performance inthe field.AXLE BREAK-IN - Before an axle can be used forlubricant evaluation, a break-in procedure is run. Thisprocedure consists of a series of controlled load andspeed conditions. The axle lubricant temperature iscontrolled throughout the break-in procedure where it isnot allowed to exceed 250F (121C). The good referenceoil is used for the break-in procedure.Running an adequate break-in is critical in preparing theaxle for accurate lubricant evaluations. Once broken in anaxle can run multiple candidate lubricant evaluations.TEST STAGES - Following the break-in procedure,candidate lubricants are evaluated by determining thestabilized operating temperature and efficiency at fivecombinations of speed and loads (stages) to approximatedifferent severe operating conditions. Table 1 outlines thetest conditions used.5Table 1Durability and Operating Temperature Test ConditionsSTAGE GENERAL CONDITION CORRELATIONI High torque / low speed Heavy Load - Start-UpII Moderate torque / high speed High Speed - Flat SurfaceIII Moderate torque / moderate-high speed Heavy Load - Flat SurfaceIV Moderate-high torque / moderate speed Heavy Load - Moderate GradeV High Torque / low-moderate speed Heavy Load - Steep GradeEach of the load stages is run until a stabilized lubricanttemperature is achieved. This typically takes 1.5 to 2.5hours. Once a stabilized temperature is reached, thenext test stage is started. This cycle is repeated until alltest stages have been evaluated. At the completion ofeach test stage, the ambient air temperature, stabilizedlubricant temperature and stabilized axle efficiency isrecorded(2).AMBIENT AIR TEMPERATURE ADJUSTMENTS - It hasbeen observed that changes in ambient air temperatureaffect the stabilized operating temperature of the axlelubricant. Since this test method was run in a laboratorywhere the ambient air temperature may vary, changes inambient air temperatures must be accounted for.Adjusting the axle lubricant temperature to account forambient air temperature changes is done by normalizingthe axle lubricant temperature relative to an ambient airtemperature of 80F with the following equation:Tcorrected = Taxle + (80F Tambient)Where,Tcorrected = lubricant temperature (F) correctedfor the ambient air temperature.Taxle = measured lubricant temperature(F).Tambient = measured ambient air temperature(F).Before applying any of the methodology described below,the axle lubricant temperature is adjusted to account forambient air temperature differences.REFERENCE TEMPERATURE CHANGES - As thenumber of tests run on an axle increases, the stabilizedoperating temperature for a given load condition of anysingle oil is lower. This fact poses a problem whenevaluating a candidate lubricant.To solve this problem in the past, reference oil is testedperiodically and the candidate result is compared to thelast reference test result. However, if the reference testtemperature gets lower after each test run, comparing thecandidate to the last reference result will make thecandidate seem better than it actually is relative to thereference.Figure 3 shows the change in stabilized operatingtemperatures for stage V conditions on a test axle whenthe good reference oil is tested. The stabilized operatingtemperature goes down as the number of test runs on theaxle increases. For this test procedure, this trend occurson all 5 test stages.6Figure 3: Stabilized Operating Temperature For the Good Reference Oil Over the Life of a Test AxleUnder Stage V ConditionsREFERENCE TARGET TEMPERATURE - To make a faircomparison between a reference and a candidate, thereference oils stabilized operating temperature used forcomparison should be adjusted for the number of runsmade on the axle. This adjustment must be done foreach test stage and lubricant evaluated on an axle. Theadjusted reference oil temperature or “reference targettemperature” can then be compared to the candidate oilsstabilized operating temperature for the load stage inquestion.Based on the reference test data, an equation for eachtest stage can be generated taking into account thereduction in the reference stabilized operating temperatureas the number of test runs increases on an axle. Thismust be done for each axle tested. Once generated,candidate results can be accurately compared toreference oil performance. Figure 4 shows a curve fittedto the stabilized operating temperatures of the goodreference oil for Stage V test conditions.Stabilized Axle TemperatureGood Reference OilStage V Conditions180200220240260280Increased Axle RunsCorrected Temperature (Deg F)7Figure 4: Curve Fitted To Stabilized Operating Temperature Results for Good Reference Oil on a TestAxle Under Stage V ConditionsFrom the equation developed in Figure 4, a referencetarget temperature can be calculated for each test run onthe axle for each test stage. Candidate test results cannow be accurately compared to reference test results.In addition, this method allows the formulator to moreaccurately compare results that were tested on differentaxles since your comparison is relative to the referenceoil.AXLE EFFICIENCY CHANGES - Just as with thestabilized temperature, a similar effect occurs with theaxle efficiency measurements on test axles. The axleefficiency gradually increases as the number o