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Unformatted text preview: MEASUREMENT UNCERTAINTY IN TRIBOLOGICAL WEAR RATE TESTING Tony L. Schmitz, Jason E. Action, David L. Burris, W. Gregory Sawyer, John C. Ziegert Department of Mechanical and Aerospace Engineering University of Florida Gainesville, FL KEYWORDS Tribology, wear rate, uncertainty ABSTRACT Wear due to relative motion between surfaces is a primary failure mode for many manufactured systems. Unfortunately, it is difficult to accurately predict component life due to wear because reported wear rates generally exhibit large scatter. This paper outlines an uncertainty analysis for single point wear rate measurements carried out using a reciprocating tribometer. Analytic and Monte Carlo-based uncertainty evaluations are compared to the standard deviation obtained from repeated tests to verify that significant contributors are included in the uncertainty evaluations. It is shown that measurement confidence can be improved by modifying the measurement hardware to reduce the dominant uncertainty contributors. WEAR RATE MEASUREMENTS Manufactured systems are subject to several modes of failure, including plastic deformation, fracture, fatigue, excess deflections, and wear. Of these, wear is generally the least predictable using current design methodologies. This is partially due to imperfect knowledge of the appropriate wear rate for the selected material pair to be used in calculating component life. Wear rate is normally experimentally determined using a tribometer, which attempts to mimic the contact conditions of the material pair and system under study. Wear rate values reported in the literature for many material pairs and contact conditions often show wide variation, even for nominally identical tests. The source of this variation in measured wear rates is currently unknown. It may be due to actual variations in the wear rate of the material pair or it may be due to intrinsic factors in the experimental apparatus and procedure which lead to variations in the reported wear rate value. The purpose of this paper is to 1) outline a method for determining the uncertainty of the measured wear rate for a given apparatus as a function of uncertainties in the input quantities and 2) compare the result with experimental deviations. Description of Wear Rate Experiments The tribometer shown schematically in Fig. 1 creates a reciprocating sliding contact between the two surfaces of interest. A four-shaft pneumatic thruster, model 64a-4 produced by Ultramation (specific commercial equipment is identified to fully describe the experimental procedures), creates the contact loading conditions using a 61.2 mm bore Bimba pneumatic cylinder. The cylinder is nominally protected from transverse loads by four 12 mm diameter steel rods. An electro-pneumatic pressure regulator controls the force produced by the thruster. The pneumatic pressure output is controlled using a variable voltage input in combination with an active control loop within the electro-pneumatic system. A linear...
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