Evaluation of the ultra-low-speed friction characteristics of precision molybdenum disulfide coated ball bearings

Almost all precision positioning mechanisms and tracking systems contain instrument ball bearings that operate at ultra-lowspeeds. Successful operation of these sensitive systems largely depends upon predicting their bearing friction torque characteristics. It is known that ball bearings undergoing ultra-low-speed motion exhibit Dahl hysteresis friction behavior which is characterized by two parameters: the rest slope, {dollar}sigma{dollar}, and the steady friction torque, T{dollar}sb{lcub}rm s{rcub}.{dollar} There is a crucial lack of information in the open literature on the rest slope and steady torque in thin solid film lubricated surfaces. This information is needed for control algorithms that must accurately compensate for friction.; In this dissertation fundamental tribological bearing friction torque relationships for {dollar}sigma{dollar} and T{dollar}sb{lcub}rm s{rcub}{dollar} are established for coated bearings which are directly applicable to enhancing the control of precision systems. Specifically, the ultra-low-speed frictional forces between silicon nitride and steel balls and sputtered MoS{dollar}sb2{dollar} coated surfaces are experimentally and theoretically investigated. Using a specially designed apparatus that is similar to a radial thrust bearing, 84 distinct sets of low-speed operating conditions were examined at rotational speeds ranging between.01 and 50 deg/sec, normal loads varying from 81.6 to 185.4 N per ball, and MoS{dollar}sb2{dollar} coating thicknesses of 4000 Aand 10000 A. Experimental rest slope and steady friction torque results are tabulated at each operating condition tested and general trends for the hysteresis friction behavior of coated surfaces are established and discussed. Furthermore, results of the MoS{dollar}sb2{dollar} coated friction measurements are directly compared to that of oil-lubricated bearings operating under identical conditions.; A three-dimensional finite element model which realistically characterizes the hysteresis friction experienced by a thin solid film lubricated ball bearing element is also presented. The finite element model, designed to mimic the operating conditions experienced by the experimental apparatus, was generated using ANSYS 5.1{dollar}spcircler.{dollar} Both the problems of a cylindrical rolling element and a ball confined between two disks subject to normal and tangential loading were investigated. The findings of the model were verified using Hertzian contact theory. MoS{dollar}sb2{dollar} coated and uncoated finite element rest slope and steady torque results are analyzed and discussed for 72 operating conditions that include normal loads of 81.6, 133.5, and 185.4 N/ball, ball materials of Si{dollar}sb3{lcub}rm N{rcub}sb4{dollar} and steel, substrate materials of Si{dollar}sb3{lcub}rm N{rcub}sb4{dollar} and steel, and ball diameters of.0127m and.0191m. Utilizing the finite element results as a basis, useful analytical expressions for T{dollar}sb{lcub}rm s{rcub}{dollar} and {dollar}sigma{dollar} are derived for coated bearing surfaces. Excellent correlation was found between finite element expression predicted and experimental results.