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Research Papers: Mixed and Boundary Lubrication

Predicting Boundary Friction of Aging Limited Slip Differentials

[+] Author and Article Information
K. Berglund

Division of Machine Elements,
Luleå University of Technology,
Luleå SE-97187, Sweden
e-mail: kim.berglund@ltu.se

P. Marklund, R. Larsson

Division of Machine Elements,
Luleå University of Technology,
Luleå SE-97187, Sweden

R. Olsson

BorgWarner TorqTransfer Systems,
Landskrona SE-26151, Sweden

1Corresponding author.

Contributed by the Tribology Division of ASME for publication in the JOURNAL OF TRIBOLOGY. Manuscript received November 8, 2013; final manuscript received August 12, 2014; published online September 19, 2014. Assoc. Editor: Jordan Liu.

J. Tribol 137(1), 012101 (Sep 19, 2014) (7 pages) Paper No: TRIB-13-1228; doi: 10.1115/1.4028403 History: Received November 08, 2013; Revised August 12, 2014

The prediction of friction is a challenge for scientists and engineers in a wide variety of applications in industry today. One such an application is the limited slip differential. The friction characteristics of the wet clutch are central to the performance of the limited slip differential system. Frictional changes with aging of the limited slip differential affect both the torque transfer accuracy and the tendencies to vibrations and noise generation due to stick-slip or shudder. Therefore, the objective of this work is to establish a method to predict the frictional changes of aging limited slip differential systems. In this study, a number of experiments were performed to establish a method to predict the changes in boundary friction with time due to aging. Accelerated aging was performed for different sets of operating conditions. Results from the tests were used to establish and verify a model to predict friction increase in limited slip differentials. The method assumes that frictional changes with aging are caused by decreased concentrations of friction modifying additives. The decrease in concentration was assumed to depend on the lubricant bulk temperature according to the Arrhenius equation. The model agreed well with tests performed at operating conditions close to the real operating conditions of the limited slip differential. The developed method can be implemented in a vehicle where it can be used to compensate for frictional changes and to indicate when service should be made.

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References

Figures

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Fig. 1

Schematic sketch of the wet clutch test rig

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Fig. 2

Example of variation in sliding speed and torque during one engagement

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Fig. 3

Overview of the wet clutch test rig used in the low power case. 1, Electric motor drive; 2, torque sensor connected to stationary output shaft; and 3, wet clutch housing.

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Fig. 4

Friction increase with time, sliding speed = 0.03 m/s, the first 48 h of testing removed

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Fig. 5

Possible explanations on the effect of temperature on friction

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Fig. 6

Additive reserves measured according to ASTM D4927 after completion of tests 1–3. (a) Wear reserve, (b) friction reserve, (c) extreme pressure reserve, and (d) detergency reserve.

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Fig. 7

Protection against oxidation results measured according to ASTM D6971

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Fig. 11

Curve fit of I/Im versus time at v = 0.16 m/s, test 2

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Fig. 10

Curve fit of friction coefficient increase at the end of life versus sliding speed, test 3

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Fig. 9

Curve fit of friction increase versus time at v = 0.16 m/s, test 3

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Fig. 8

Schematic of frictional change over time for a given sliding speed, μ1 and μm are shown

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Fig. 12

The predicted effect of aging time and lubricant sump temperature on the α-value for the investigated limited slip differential system

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Fig. 13

Comparison of model prediction to tests 1 and 4. (a) v = 0.17 m/s and (b) v = 0.02 m/s.

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Fig. 14

Comparison of model prediction at v = 0.05 m/s, test 5

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Fig. 15

Comparison of model prediction at v = 0.05 m/s, test 6

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