Research Papers: Biotribology

Friction Measurement in the Biaxial Rocking Motion Hip Joint Simulator

[+] Author and Article Information
Vesa Saikko1

Department of Engineering Design and Production, Helsinki University of Technology, P.O. Box 4300, FIN-02015 TKK, Finlandvesa.saikko@tkk.fi

J. Tribol 131(1), 011201 (Dec 02, 2008) (8 pages) doi:10.1115/1.2991121 History: Received February 12, 2008; Revised August 21, 2008; Published December 02, 2008

The established biaxial rocking motion (BRM) hip joint simulator was complemented by a novel friction measurement accessory. This simple and practical system, which was easily added to an existing BRM design, is described in detail and shown to perform well in long-term wear tests and in comparative tests of 24 h duration involving several different bearing couples. The system was based on the measurement of frictional torque about the leaning axis of the lower component, the femoral head. In the 28 mm CoCr-on-ultrahigh molecular weight polyethylene articulation with diluted calf serum lubricant at body temperature and 1 kN static load, the maximum value of frictional torque during a cycle was 1.2 N m on the average. The alternative system based on the measurement of torque about the vertical loading axis was shown to be less sensitive and highly insensitive when contact area was small, as was the case with alumina-on-alumina.

Copyright © 2009 by American Society of Mechanical Engineers
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Figure 1

Novel friction measurement accessory for the BRM hip joint simulator: (a) lever of the leaning shaft, (b) linear bearing, (c) rail, (d) button load cell, (e) roller, (f) reaction plane, and (g) bearing housing of leaning shaft

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Figure 3

Schematic of the new friction measurement principle. The four positions correspond to those of Fig. 2. Viewing direction is always along the rail of the linear bearing. For simplicity, point contact is assumed in this illustration. The resultant friction force vector (Fμ) has a constant moment arm (r sin α) relative to the IER axis but no moment arm relative to the vertical loading axis. The force measured at the end of the lever of the leaning shaft (FIER) has a constant moment arm (A=100 mm) relative to the IER axis. In (d), the contact point is behind the head.

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Figure 4

Variation of Tleaning,max and Tleaning,min during the first 17,000 cycles after a restart with fresh lubricant in wear test 2

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Figure 5

Variation of frictional torque Tleaning,max, gravimetric wear, linear wear, radius of bearing surface of cup, and increase of lubricant temperature relative to room temperature ΔT with number of cycles: (a) test 1 and (b) test 2

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Figure 6

Close-up of test chamber: (a) at start, filled with fresh lubricant; (b) 6 days and 520,000 cycles later, showing degraded serum with abundant, coagulated protein lumps

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Figure 7

Typical simultaneous measurement of Tleaning and Tvertical together with their sum that is always nearly constant due to equal amplitude but opposite phase. In this particular measurement, Tleaning,min≈Tvertical,max and Tleaning,mean≈3×Tvertical,mean.

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Figure 2

BRM simulator with two friction measurement systems shown in four different positions of a cycle. Tleaning,min coincides with positions (a) and (c) and Tleaning,max with positions (b) and (d). Load is applied vertically from above. Note lever of loading bar, thrust bearing and universal joint of loading bar.



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