TLfriction2009 - Tribol Lett (2009) 35:1723 DOI...

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METHODS PAPER Addressing Practical Challenges of Low Friction Coef±cient Measurements D. L. Burris Æ W. G. Sawyer Received: 26 December 2008 / Accepted: 25 March 2009 / Published online: 10 April 2009 Ó Springer Science+Business Media, LLC 2009 Abstract A friction coefFcient is deFned as the ratio of the friction force to the applied normal force. Despite the disarming simplicity of its calculation, there are practical challenges that make low values of friction coefFcient difFcult to accurately quantify. The connections of imperfect parts in friction measurement devices (called tribometers) produce small misalignments between the transducer and counterface axes. According to Schmitz et al. (J Tribol Trans ASME 127:673–678, 2005), ‘‘ the measurement of friction coefFcient is extremely sensitive to misalignments’’ and ‘‘for materials with friction coefF- cients below 0.05 the alignment becomes hopelessly dif- Fcult if the goal is to have uncertainties below 1%.’’ This method article reviews the challenges of low friction measurements and presents a robust reversal technique that eliminates misalignment bias. Experiments with controlled misalignment angles demonstrate the bias sensitivity and validate its elimination using a low uncertainty tribometer in conjunction with the described reversal technique. Keywords ±riction coefFcient ± Tribometer ± Uncertainty ± Measurement ± Statistics 1 Friction Coef±cient Measurements ±riction force measurements provide insights into the fundamental interactions of surfaces and an empirical basis for machine design. A friction coefFcient is deFned, as shown in Eq. 1 , as the ratio of the force resisting motion (friction force, F f ) to the applied normal force ( F n ). l ¼ F f F n ð 1 Þ In general, friction coefFcients range from about 0.2 to 1 for typical material pairs under standard conditions. Values much greater than 1 are not uncommon in vacuum and a number of modern materials have exhibited ‘super lubricity’ with friction coefFcients well under l = 0.01. Values of friction coefFcient are often reported for common material pairs in handbooks to guide preliminary designs. Engineers mistakenly use these values as they would use tabulated values of yield stress. In reality, material pairs do not have singular characteristic values of friction coefFcient. The friction coefFcient is extremely sensitive to the lubrication, environment, and contact conditions, and under nominally constant conditions, it can exhibit large time dependent variations over a range of time-scales. Scientists often calculate statistics from measurements to provide an indication of the population mean and vari- ance, both of which are valuable to design engineers. In addition to inherent variations in the interfacial friction coefFcient, the measured value always deviates from the true value of the measured quantity. The measurement uncertainty describes the ‘‘dispersion of values that could reasonably be attributed to the measurand’’ [ 2 ]. Measure-
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This note was uploaded on 08/22/2011 for the course EGM 4313 taught by Professor Mei during the Spring '08 term at University of Florida.

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TLfriction2009 - Tribol Lett (2009) 35:1723 DOI...

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