219 Experimental Investigation of Forces Pr oduced by Misaligned Steel Rollers 

Timothy Krantz*, Christopher DellaCorte* and Michael Dube** 

Abstract 

The International Space Station Solar Alpha Rotary Joint (SARJ) uses a roller-based mechanism for  positioning of the solar arrays. The forces and moments that develop at the roller interfaces are  influenced by the design including the kinematic co nstraints and the lubrication condition. To help  understand the SARJ operation, a set of dedicated experiments were completed using roller pairs. Of primary interest was to measure the axial force dire cted along the axis of rotation of the roller as a  function of shaft misalignment. The conditions st udied included dry and clean surfaces; one surface  plated by a gold film, and greased surfaces. For the case of a bare 440C roller against a nitrided 15-5 roller without lubrication, the axial force can be as great as 0.4 times the normal load for a shaft angle of 0.5 degree. Such a magnitude of force on a roller in  the SARJ mechanism would cause roller tipping and  contact pressures much greater than anticipated by the designers. For the case of a bare 440C roller against a nitrided 15-5 roller with grease lubrication, the axial force does not ex ceed about 0.15 times the  normal load even for the largest misalignment angles tested. Gold films provided good lubrication for the short duration testing reported herein. Grease lubricat ion limited the magnitude of the axial force to even  smaller magnitudes than was achieved with the gold f ilms. The experiments demonstrate the critical role  of good lubrication for the SARJ mechanism. 

Introduction 

The International Space Station makes use of a roller-based mechanism for positioning of the solar arrays. The fundamental concept of the roller-bas ed mechanism of the Solar Alpha Rotary Joint is  described by Loewenthal and Schuller [1]. A careful study of the SARJ system rev eals an interesting and  significant interplay of forces at the interface where the rollers of the rotary joint mechanism touches the  large rotating ring. It is well established in the lit erature that forces and moments can develop at the  interacting surfaces in rolling and sliding contac t [2,3]. The forces and moments that develop are  influenced by the details of the design including the kinematic constraints. Proper understanding of the  influence of roller misalignment has proven to be important in the development of a roller mechanism used for positioning of a radio astronomy antenna [4,5,6]. The influence of roller misalignment is likewise important for the operation of the SARJ mechanism.  The roller and ring of the SARJ mechanism and the meaning of the term “shaft misalignment” is depicted in schematic form in Figure 1. The SARJ rollers are c onstrained to rotate about shaft axes that nominally  intersect the rotational axis of the ring (Figure 1 [a-b]) . Any deviation from perfect alignment of a roller axis  and ring axis (Figure 1[c]) will give rise to a force at t he contact interface in the di rection of the roller shaft  axis (termed herein the axial force). The magnitude of the axial force will depend on the magnitude of  misalignment, the normal load on the roller, the stiff ness of the system, the to rque transmitted by the  roller, and frictional condition of the contacting surfaces. The SARJ mechanism was built to high precision, and the installed roller shaft angle (misal ignment) was within a fraction of a degree to perfect  alignment. As will be evident from this study, misa lignments of even such small magnitude can produce  significant forces that influence the performance of the mechanism.                                                       * NASA Glenn Research Center, Cleveland, OH  ** NASA Goddard Space Flight Center, Greenbelt, MD 

Proceedings of the 40th Aerospace Mechanisms Symposium, NASA Kennedy Space Center, May 12-14, 2010  NASA/CP-2010-216272