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Unformatted text preview: The Difficulty of Measuring Low Friction: Uncertainty Analysis for Friction Coefficient Measurements Tony L. Schmitz, Jason E. Action, John C. Ziegert, and W. Gregory Sawyer University of Florida, Department of Mechanical and Aerospace Engineering, Gainesville, FL 32611 The experimental evaluation of friction coefficient is a common laboratory procedure; however, the corresponding measurement uncertainty is not widely discussed. This manuscript examines the experimental uncertainty associated with friction measurements by following the guidelines prescribed in international standards. The uncertainty contributors identified in this analysis include load cell calibration, load cell voltage measurement, and instru- ment geometry. A series of 20 tests, carried out under nominally identical conditions, was performed using a reciprocating pin-on- disk tribometer. A comparison between the experimental standard deviation and uncertainty analysis results is provided. @ DOI: 10.1115/1.1843853 # 1 Introduction Laboratory experimentation remains the only practical method available for the accurate identification of friction coefficients for arbitrary material pairs. However, accurate and repeatable friction coefficient measurement remains challenging due to the depen- dence of friction coefficients on the material, surface, environ- ment, and measuring equipment. The purpose of this paper is to critically examine the experimental uncertainty associated with the instrumentation used in dynamic friction coefficient measure- ments. In order to enable the confident use of experimental data, it is necessary to provide a quantitative, defensible statement regard- ing its reliability. The topic of data uncertainty has been addressed by researchers for several individual tribological measurement conditions @ 16 # . In this paper, we focus on establishing a sys- tematic framework for the evaluation of instrument-related uncer- tainty in tribological testing. Our example application is the deter- mination of low dynamic coefficients of friction. We consider friction coefficient measurements carried out using a traditional pin-on-disk tribometer, where a pin is pressed against a reciprocating counterface. This tribometer uses a pneumatic cyl- inder and a multichannel load cell located directly above the pin to continuously monitor the contact force vector, which is then decomposed into friction force and normal force vectors. The un- certainty analysis follows the guidelines provided in Refs. @ 7,8 # . Uncertainty contributors include load cell calibration, where both the applied load and voltage measurement uncertainties have been considered, and instrument geometry. 2 Description of Tribology Experiments 2.1 Reciprocating Pin-on-Disk Tribometer. The tribom- eter shown schematically in Fig. 1 creates a reciprocating sliding contact between two surfaces of interest. A four-shaft pneumatic thruster, model 64a-4 produced by Ultramation ~ Specific commer- cial equipment is identified to fully describe the experimental pro-...
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