Roamer E. Predmore

A New Apparatus to Evaluate Lubricants for Space Applications: The Spiral Orbit Tribometer (SOT)

Lubricants used in space mechanisms must be thoroughly tested prior to their selection for critical applications. Traditionally, two types of tests have been used: accelerated and full-scale. Accelerated tests are rapid, economical, and provide useful information for gross screening of candidate lubricants. Although full-scale tests are more believable, because they mimic actual spacecraft conditions, they are expensive and time consuming. The spiral orbit tribometer compromises between the two extremes. It rapidly determines the rate of tribochemically induced lubricant consumption, which leads to finite test times, under realistic rolling/pivoting conditions that occur in angular contact bearings.

In-situ, On-demand Lubrication System for Space Mechanisms

Many of todays spaceraft have long mission lifetimes. Whatever the lubrication method selected, the initial lubricant charge is required to last the entire mission. Fluid lubricant losses are mainly due to evaporation, tribo-degradation, and oil creep out of the tribological regions. In the past, several techniques were developed to maintain the appropriate amount of oil in the system. They were based on oil reservoirs (cartridges, impregnated porous parts), barrier films, and labyrinth seals. Nevertheless, all these systems have had limited success or have not established a proven record for space missions. The system reported here provides to the ball-race contact fresh lubricant in-situ and on demand when the ball bearing is close to failure. The lubricant is stored in a porous cartridge attached to the inner or the outer ring of a ball bearing. The oil is released by heating the cartridge to eject oil, taking advantage of the greater thermal expansion of the oil compared to the porous network. The heating may be activated by torque increases that signal the depletion of oil in the contact. The low surface tension of the oil compared to the ball bearing material is utilized and the close proximity of the cartridge to the moving balls allows the lubricant to reach the ball-race contacts. This oil re-supply system avoided a mechanism failure, reduced torque to an acceptable level, and extended the life of the component.

Probabilistic Analysis of Space Shuttle Body Flap Actuator Ball Bearings

A probabilistic analysis, using the two-parameter Weibull-Johnson method, was performed on experimental life test data from space shuttle actuator bearings. Experiments were performed on a test rig under simulated conditions to determine the life and failure mechanism of the grease lubricated bearings that support the input shaft of the space shuttle body flap actuators. The failure mechanism was wear that can cause loss of bearing preload. These tests established life and reliability data for both shuttle flight and ground operation. Test data were used to estimate the failure rate and reliability as a function of the number of shuttle missions flown. The Weibull analysis of the test data for the four actuators on one shuttle, each with a two-bearing shaft assembly, established a reliability level of 96.9% for a life of 12 missions. A probabilistic system analysis for four shuttles, each of which has four actuators, predicts a single bearing failure in one actuator of one shuttle after 22 missions (a total of 88 missions for a four-shuttle fleet). This prediction is comparable with actual shuttle flight history in which a single actuator bearing was found to have failed by wear at 20 missions.

Sliding friction of copper alloys in vacuum.

Sliding friction experiments involving copper and Cu-Be alloy plates in contact with copper, Cu-Be alloy, steel and titanium alloy sliders were performed in air and vacuum. Micro-hardness and metallographic examination of plate and slider interface suggests friction and wear by the adhesive friction mechanism with prow formation. Transfer of strain-hardened metal wear debris across the sliding interface leas observed. The friction coefficient and the prow size generally increased in vacuum. Presented as an American Society of Lubrication Engineers paper at the ASME/ASLE Lubrication Conference held in Atlantic City, New Jersey. October 8–10. 1968.

Friction and wear of steels in air and vacuum

The friction and wear behavior of steel (soft, medium, and hard), titanium, aluminum, copper and copper beryllium sliding on a wide hardness range of steel plates was measured in both vacuum and air. The wear mechanism was identified for each test by visual and metallographic inspection and by microhardness traverses below the wear interface. Four mechanisms were observed and their occurrence correlated with alloy couple, hardness variations, and vacuum conditions. In addition to prow formation, which has been discussed in the literature, three distinct types of rider wear mechanism are herein identified and defined as severe, intermediate and mild rider wear. The mechanisms are: 1) Prow formation: Characterized by a build up on the slider tip of work hardened wear debris (usually from the plate), which separates the plate from the slider. The sliding interface is in the plate below the prow and friction is governed by plate metal properties. 2) Severe rider wear: Metal is worn off the rider and is transf...