157 Prototypical Gearbox Rated Capacity  The prototypical gearbox design was analyzed per ANSI/AGMA 2001-C95. Allowable bending stress  versus cycles and allowable contact stress versus cycles was extrapolated from the AGMA standard based on the material hardness used. Stress factors were selected to match typical space planetary gearbox applications as much as possible. Based upon the analysis, the following capacities were predicted (assuming a 1.5 Reliability Factor for 99.99% reliability):     38.6 N-m (342 in-lb) operating torque against the endurance limit (1E7 Cycles)   56.2 N-m (497 in-lb) operating torque at 3.33 RPM for 205.1 hr (Pitting Life = Bending Life)    89.5 N-m (792 in-lb) for short term operation of 4 hr or less   137.5 N-m (1217 in-lb) operating torque for momentary operation (bushing dynamic limit,   ≤5 minutes rated gear operation at this load)   205 N-m (1812 in-lb) static torque limit for gears (2.0 Safety Factor)   Test Evaluation Parameters  Bushing Materials  For applications in which rolling element bearings are not suitable due to either capacity or packaging constraints, heritage planetary gearbox designs have traditionally utilized oil-impregnated SAE 841 bronze bushings. SAE 841 bushings have a Pressure-Velocity (PV) rating of 1.75 MPA-m/s  (50 kpsi-ft/min) with a peak intermittent pressure of 28 MPa (4 kpsi). Often the bushing will be the limiting factor in the dynamic and/or static rating of the gear stage.   An alternate material under consideration is Toughmet 3AT by Materion Brush, a  spinodal/copper/nickel/tin alloy. Toughmet 3AT has a published PV rating of 4.6 to 9.0 MPa-m/s (132,000 to 260,000 psi-ft/min) depending upon the surface finish of mating parts. Maximum low-speed pressure was not provided and the testing that generated the PV ratings for the Toughmet 3AT material was performed at speeds of 1.5-2.0 m/s (300-400 ft/min) with additional lubricant added during the test. Space planetary gearboxes do not allow any lubrication to be added during life and often operate at speeds at or below 0.5 m/s (100 ft/min).  It was decided to test both SAE 841 bronze and Toughmet 3AT bushings without providing additional  lubrication and over a speed range closer to the typical space planetary applications. The purpose of the test was to determine if published PV ratings are valid and also to show a direct comparison between the two materials.  Gear and Journal Surface Treatments  The literature for Toughmet AT recommended Metalife Industries MLP as a surface treatment that could  provide lower friction, higher PV ratings, and longer life in lubricant starved conditions. Metalife MLP is a thin dense chrome coating with a proprietary polymer compound added.  Oerlikon recommended adding their Balinit C coating to gear teeth and mating surfaces of bushings to  extend life and capability. Balinit C is an amorphous carbon-tungsten carbide coating (WC/C) with a high surface hardness and a low coefficient of friction that claims higher bearing load capacity, lower sliding wear, improved scuffing resistance, and reduced pitting particularly in applications with boundary lubrication conditions such as slow moving gears in contact.   REM Chemicals recommended their Isotropic Superfinish (ISF) process which claims to eliminate breakin, reduce friction, and reduce contact fatigue. The ISF process involves a chemical conversion creating a micro thin soft layer that is removed by ceramic media. Multiple passes through the process resulted in removal of the peaks from the surface, while leaving the valleys as location for lubricant to remain during contact operation.