UNCORRECTED PROOF Please cite this article in press as: R.S. Colbert, et al., Edges, clearances, and wear: Little things that make big differences in bushing friction, Wear (2009), doi:10.1016/j.wear.2009.06.030 ARTICLE IN PRESS G Model WEA 99236 1–8 Wear xxx (2009) xxx–xxx Contents lists available at ScienceDirect Wear journal homepage: www.elsevier.com/locate/wear Edges, clearances, and wear: Little things that make big differences in bushing friction 1 2 Rachel S. Colbert a , Luis A. Alvarez a , Matthew A. Hamilton a , Jason G. Steffens a , John C. Ziegert b , David L. Burris c , W. Gregory Sawyer a,∗ 3 4 a Department of Mechanical and Aerospace Engineering, University of Florida, United States 5 b Department of Mechanical Engineering, Clemson University, United States Q1 6 c Department of Mechanical Engineering, University of Delaware, United States 7 8 article info 9 10 Article history: 11 Received 6 March 2008 12 Received in revised form 12 June 2009 13 Accepted 18 June 2009 14 Available online xxx 15 16 Keywords: 17 Wear 18 Bushing 19 Solid lubrication 20 Friction 21 abstract Traditional pin-on-flat tribometers are necessary instruments for making direct measurements of tri- bological properties, but mechanical design of even simple systems often requires application-specific information that only component level testing can provide. This paper uses the design and operation of a bushing tribometer to elucidate geometric effects that plague bushing systems. For example, the nature of the bushing edge has a dramatic influence on the performance of the system in practice where the system is necessarily over-constrained. It is shown mathematically that the torque requirements increase as the wrap angle of the contact spreads for a constant friction coefficient; at steady state the equation for calculated torque requirement is found. This is demonstrated using ultra-high molecular weight polyethylene bushings that showed increasing torques at decreasing clearances. A polytetraflu- oroethylene (PTFE) bushing with 150 m interference initially required 50% more torque than a PTFE bushing with 350 m clearance. As the two different bushings ran into steady state wear, the torque asymptotically approached the mathematically derived value. The results of these experiments provide designers insight into the design of successful bushing pairs and the ability to tune frictional torques without changing material through the selection of clearance. © 2009 Published by Elsevier B.V. 1. Introduction 22 There are ever increasing applications where traditional fluid 23 lubrication techniques are precluded; solid lubricants are often 24 the only available solution [1,2]. Space applications, for exam- 25 ple, present a myriad of design challenges including temperature 26 extremes, near perfect vacuum, and atomic oxygen. As a result, the 27 design engineers are faced with the unenviable task of providing 28 effective and reliable lubrication using sparse empirical data and 29 incomplete knowledge of past successes [2,3]. 30 In bushings, appropriate constraints and degrees of freedom 31 are provided via the tribological interface [4,5]. This results in 32 the contact geometries being substantially more complicated than 33 those of general tribology experiments. In the simplest revolute 34 joint, a shaft rotates within the slightly larger journal or bushing. 35 This ubiquitous design is subject to evolutions of geometry, con- 36 ∗ Corresponding author at: Department of Mechanical and Aerospace Engineer- ing, University of Florida, 237 Mechanical Engineering Building, Gainesville, FL 32611, United States. Tel.: +1 352 392 8488. E-mail address: wgsawyer@ufl.edu (W.G. Sawyer). tact area and pressure distribution, which result in a deceptively 37 high degree of complexity and evolution in friction coefficient and 38 torque. Further, during application, variations in temperature and 39 stress results in distortion and misalignment. Engineers frequently 40 desire component level testing for accurate prediction of the joint 41 and mechanism behavior, life, performance, and ultimately failure 42 [3]. 43 This paper reports on the design, construction, and operation 44 of a component-level bushing tribometer that can operate in vac- 45 uum and support studies on the effects of bushing geometry and 46 misalignments on torque, friction, and wear. The results provide 47 design engineers with guidelines for the design of bushings. 48 2. Tribometer design 49 The basic design concept and primary bushing tribometer com- 50 ponents are illustrated in Fig. 1. The most critical requirement of the 51 design is the ability to accurately measure low friction coefficients. 52 The load measurement strategy used here follows the methodology 53 described by Schmitz et al. [6,7].A six-channel load cell is rigidly 54 mounted to the bushing sample. This design promotes low uncer- 55 tainty measurement because, as shown in Fig. 2, the loads on the 56 0043-1648/$ – see front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.wear.2009.06.030