160 Conclusion  SAE 841 Bronze proved to perform at rated levels even in a lubrication-starved environment with space  grade lubricants. SAE 841 Bronze also provides extensive additional margin between initial friction increase and catastrophic failure due to the ability to replenish lubricant and absorb debris.  Toughmet 3AT did not perform to rated capacity most likely due to the inherent lubrication starved  environment and the use of space grade lubricants instead of higher performing commercial alternatives.  Unlike the SAE 841 bronze, initial onsets of friction quickly turned to catastrophic friction increases . 

Metalife MLP provided lower initial friction than bare steel, but the coating broke down at relatively low operating levels, generating abrasive debris which resulted in a high wear rate. This coating may be suitable for bearing journals in high speed/low load applications but extreme caution would need to be taken. Metalife MLP did not appear suitable for use on gear teeth with any significant load.  Balinit C displayed higher friction in journal bearings than bare steel and was detrimental to the  performance of the SAE 841 bronze material. It is therefore not recommended for use against bushings. While Balinit C was more durable on gears than Metalife MLP, once the coating was broken down the debris was very abrasive and resulted in extreme wear. Balinit C may help fatigue life on higher speed, lower torque gear stages, but should be avoided in higher torque stages of fine pitch gears to prevent coating breakthrough.  Shot peening resulted in significant surface roughness that could not be overcome by Superfinishing and  resulted in swifter degradation of subsequent coatings of Metalife MLP or Balinit C. Shot peening is not recommended on 64DP or 96DP gears unless the process can be refined with extra fine media and reduced pressure, in which case additional testing should be performed.   REM Isotropic Superfinishing provided mixed results. Initial friction levels were improved and there was less debris observed in the lubricant after gearbox run-in. However, overall gearbox life was not conclusively improved by the process. Superfinishing should be considered for 64DP and coarser gearing that cannot be cleaned and re-lubed after run-in. There remained concern over the impact of Superfinishing on the gear profile for 96DP gears (and finer).  Gearbox rated capacities for short term, momentary torque, and static capacity were all validated through  the testing of the standard No Finish gearboxes with additional margin shown. This data gives confidence moving forward that calculation techniques and assumed parameters are valid and gear designs can be pushed closer to their calculated limits to minimize mass and volume.   Flight Application  Sierra Nevada has incorporated design elements from this SBIR study into several programs that have been qualified, acceptance tested, and delivered. Significant programs included JPL Mars Science Laboratory Descent Braking Mechanism, JPL Mars Science Laboratory Low Torque Actuator Gearboxes (5 different gearbox designs), as well as other commercial Aerospace applications.  References  1. “D21507 Final Report: Lightweight Gearbox Technology Program Phase II SBIR.” SNC  2. “ToughMet Plain Bearings – Performance with High PV.”  Materion TechBrief   3. “Enhancing Toughmet 3 Bearing Performance with Metalife Hard Coated Steel.”  Materion TechBrief    4. “Coated Components: Greater performance and reliability.” Oerlikon Balzers, 2010. 5. Winkelmann, Lane et al “The Effect of Superfinishing on Gear Micropitting.” Gear Technology  March/April 2009 .