226 Some tests were done to investigate the influence of grease lubrication of the performance of the  contacting rollers. For these tests the roller surfaces  were lubricated using a space-qualified grease. The  base oil of the grease is a stable perfluorinated polyether. The gelling agent is a tetrafluoroethylene  telomer. The grease contains molybdenum disulfi de. The grease was applied using a syringe. The  difference in the mass of the syringe before and after applying the grease was 0.34 gram. The grease was distributed on the rollers by positioning the rolle rs with only a small gap between them and the shafts  were rotated. As the grease became distributed by  the shaft rotation the gap between the rollers was  slowly reduced in increments. This action proved effective to distribute the grease about the roller circumference, and by visual inspection the distribution of the grease about the roller circumference appeared uniform.  Once rollers were installed and the chamber pressure test condition was established, the next step of the  test procedure was to “run-in” the roller surfaces. In general contacting surfaces will quickly “run-in” via wear and deformation of asperity features. The test  rig shaft misalignment angle was set to 1.5 degrees  and the rollers were brought into contact with ap proximately 667 N (150 lb ) normal force between the  rollers. The test rig was operated for at least 1000 shaft revolutions to run-in the test surfaces.  The third step of the test procedure was to sm oothly and continuously change the misalignment angle  while recording data to investigate the traction capability of the contact. This step was done to establish  the relationship of the misalignment angle to the developed axial force. The misalignment angle was changed by hand-turning of a threaded rod to rotate  the turntable relative to the rig bedplate. The  misalignment angle was swept from a position of approx imately 1.5 degrees to a position of -1.5 degrees,  and after a short pause the direction reversed and the angle adjusted again in a smooth fashion back to the starting angle of 1.5 degrees. The angle adjustment occurred over a time of approximately three  minutes. During these sweeps of the misalignment angl e the magnetic-particle brake on the output shaft  was not energized, and the recorded torque on the output  shaft was in the range 0.8~1.2 N-m (7~11 inlb). The procedure to adjust the misalignment angle was repeated for 3 values of the normal load,  approximately 445, 667, and 890 N (100, 150, and 200 lb).  The preceding paragraphs described the procedure to investigate the behavior of the contact with small  torque transmitted buy the roller pair. Next, a test wa s completed to assess the relationship of axial force  as a function of the torque transmitted by the rollers for a condition of  shaft misalignment of 1.5 degrees.  The rollers were the same roller pair as described in  the preceding paragraph. The rig was operated at a  speed of approximately 15 rpm. The test was done in vacuum of 5x10 -6 Torr. The normal load between  the rollers was 436 N (98 lb). With the test operating , the braking torque on the output shaft was adjusted  until the output shaft was at a near stall condition because of the high braking torque. The data was  processed to determine the measured axial force as a function of the torque transmitted by the roller pair.   Test Results 

Axial Force as a Function of Shaft Misalignment Angle

Data were recorded and processed to determine the axial force created in the direction of the rotational axis of the output shaft as a function of the operating condit ion. The array of testing that was performed is  documented in Table I. Each of the test conditions of Table I were repeated for 3 levels of applied normal  load, the targeted loads being 445, 667, and 890 N (100,  150, and 200 lb). A Hertz contact analysis was  completed for each of the three targeted load using the calculation method of Hamrock and Brewe [ 7].  The maximum calculated contact pressures for thes e loads are 1.55, 1.79, and 1.97 GPa (225,000,  260,000, and 285,000 psi) respectively. The measured l oads (as opposed to the targeted test loads) were  used for the processing and reporting of data and results.  The axial force that developed in the contact was measured and plotted as a function of the misalignment angle. Figure 9 is a plot of data for the case of un-nitrided 15-5 roller with a 440C roller in a vacuum environment for three levels of normal load. The data  has been plotted as a ratio of the measured axial  force to the measured normal load as a function of shaft misalignment angle. The data shows that, for  NASA/CP-2010-216272