426 procedure to a nominal normal load of about 18 N. The roller is a cylinder with diameter of 21.2 mm. The  roller profile is a crown radius with a flat feature in the center of the crown. Assuming line contact across the flat feature, the Hertz contact maximum pressure is about 240 MPa. The rollers are made from 440F steel (a free machining variant of 440C). The roller material was in the annealed condition and passivated. A pair of deep groove ball bearings supports each roller. The fastener securing the roller bearings to the axle is a locking style bolt assembled in a manner allowing for a small amount of axial play. The rollers are in contact with Ti6Al4V that had been stress relieved and anodized.  After completion of the qualification test, wear was visible on the rollers of the translator assembly and on the mating surfaces. Fig. 2 shows the condition of a qualification test roller. Some loose particulate debris was also found, which was collected and analyzed [1-2]. The worn surfaces and wear debris provide evidence that the wear process was primarily adhesive wear. In total it appears that the steel has transferred to the mating Ti6Al4V surfaces. The loose wear debris included both the steel and titanium alloy materials. The severity of wear was not the same on all rollers.   From observations of the wear patterns and wear debris of the qualification unit, study of the literature, and engineering assessment of the kinematic configuration, manufacturing tolerances, and roller mounting details, the roller alignment seemed likely to be a key variable influencing the wear rate. To better understand the wear phenomena and to explore possible mitigation strategies, a set of laboratory roller tests were conducted in vacuum. The next sections will discuss the testing apparatus, specimens, procedures, and the corresponding results.           Figure 1 – Translator assembly [1].  Figure 2.  Condition of roller after   completion of qualification test [2]. 

Laboratory Test Apparatus, Specimens, and Procedure 

Test Apparatus for Roller Pairs  Testing was done using the NASA Glenn Research Center Vacuum Roller Rig (Figure 3). The rig allows  for application and measurement of a load pressing the rollers together while having a purposely misaligned and adjustable shaft angle. The rig is depicted in schematic form in Figure 4. A drive motor provides motion to the driving roller. A magnetic-particle brake attached to the output shaft imposes torque on the driven roller. The rig can be operated with the brake not energized. For such a condition the torque transmitted through the roller pair is the drag torque of the output shaft (drag of the seals and support bearings). The normal load pressing the rollers together is provided by an air cylinder. The cylinder acts through a gimbal point to rotate the plate that mounts the driving shaft and drive motor. The rotation of the drive motor plate displaces the driving roller toward the driven roller shaft in an arc motion. The pressure to the cylinder, and thereby the load between the contacting rollers, is adjusted by a hand-operated valve  (open-loop control). A turbomolecular pump assisted by a scroll pump provides vacuum in  the test chamber. The typical condition in the test chamber is a pressure of about 3x10 -7 Torr. The most