425 Wear of Steel and Ti6Al4V Rollers in Vacuum 

Timothy Krantz* and Iqbal Shareef** 

Abstract 

This investigation was prompted by results of a qualification test of a mechanism to be used for the James Webb Space Telescope. Post-test inspections of the qualification test article revealed some loose wear debris and wear of the steel rollers and the mating Ti6Al4V surfaces. An engineering assessment of the design and observations from the tested qualification unit suggested that roller misalignment was a controlling factor. The wear phenomena were investigated using dedicated laboratory experiments. Tests were done using a vacuum roller rig for a range of roller misalignment angles. The wear in these tests was mainly adhesive wear. The measured wear rates were highly correlated to the misalignment angle. For all tests with some roller misalignment, the steel rollers lost mass while the titanium rollers gained mass indicating strong adhesion of the steel with the titanium alloy. Inspection of the rollers revealed that the adhesive wear was a two-way process as titanium alloy was found on the steel rollers and vice versa. The qualification test unit made use of 440F steel rollers in the annealed condition. Both annealed 440F steel rollers and hardened 440C rollers were tested in the vacuum roller rig to investigate possibility to reduce wear rates and the risk of loose debris formation. The 440F and 440C rollers had differing wear behaviors with significantly lesser wear rates for the 440C. For the test condition of zero roller misalignment, the adhesive wear rates were very low, but still some loose debris was formed. 

Introduction 

This investigation was prompted by results of a qualification test of a mechanism to be used for the  James Webb Space Telescope. The mechanism is used to move a magnet for certain operations of the telescope’s near infrared spectrometer (NIRSpec) instrument. The motion of the magnet is guided by a set of preloaded steel rollers in contact with anodized Ti6Al4V. The qualification testing was accomplished in a cold vacuum chamber to match as closely as possible the extreme deep space environment. Post-test inspections of the qualification test article revealed some wear of the steel rollers and mating Ti6Al4V surfaces, and some loose debris was found. The NASA Engineering Safety Center (NESC) is investigating the potential risk of the wear and debris toward hindering the full capability of the NIRSpec instrument. This article will discuss the NESC investigation of the wear phenomena including dedicated laboratory experiments.   Qualification Testing Article and Testing Results.  The mechanism of interest is a translator assembly that features a set of 11 rollers to guide the motion of  a magnet (Fig. 1). The translation of the magnet is used to affect the positions of micro-shutters. A motor and linkages provide the motive force. The translator assembly undergoes oscillatory motion in a straight line. The translating distance is approximately 200 mm on each stroke (total of 400 mm of travel during back and forth motion) and translation time in one direction during qualification testing was 10.5 seconds. The qualification test included 96,000 of such forward and return excursions.  The translator assembly motion is guided by a set of eleven rollers. The in-plane guide is provided by a set of four rollers (two of these rollers are visible to the far right side of Fig. 1). These four rollers contact both sides of a guide rail defining the direction of motion. The out-of-plane position and motion is guided by a set of seven rollers. Three rollers are in contact with a base plate defining one plane and four rollers are in contact with a cover defining a second parallel plane. The roller preloads are set by a shimming                                                    * Glenn Research Center, Cleveland, OH  ** Bradley University, Peoria, IL 

Proceedings of the 41st Aerospace Mechanisms Symposium, Jet Propulsion Laboratory, May 16-18, 2012