211 Lubricant Degradation in High- Load, High- Cycle Actuator Test Using Heritage  Harmonic Drives for the Multi -Angle Imager for Aerosols Instrument  

M. Michelle Easter* 

Abstract 

The Multi -Angle Imager for Aerosols (MAIA) instrument features a pushbroom spectropolarmetric camera  on a two- axis gimbal, which is actuated throughout the mission duration for multi -angle imaging, target  revisiting, and inflight calibration. Each gimbal axis is driven by an existing design Mini Dual Drive Actuator  (MDD A), which  featur es a duplex brushless DC motor and redundant size 10 pancake Harmonic Drive (HD)  gears et within its drive outputs. As the size 10 pancake assembly is no longer available from Harmonic  Drive , LLC , units were acquired from long- term flight st orage. Due to the load and speed profiles, as well  as the continuous operation required of the actuators, an endurance test was conducted to verify  acceptability of the mechanism for the application, based on  fatigue life. Approaching the two- life mileston e,  the test failed due to breakdown of the Braycote 601EF lubricant, with a final apparent overall gearbox   efficiency of 7%.  

Disassembly and SEM inspection revealed two distinct flavors of degraded and polymerized lubricant, tooth  and race wear, material adhesion onto rolling wave generator bearing surfaces, and non- trivial amounts of  corrosion. Due to the dither behavior and relatively high loads, it seemed breakdown due to high contact  stress was the  likely culprit. This led to the decision to change the drive output subassembly lubricant from  Braycote 601EF grease and Bray Oil 815Z, to Rheolube 2000 grease and Nye Synthetic Oil 2001. Since  the motors were already built using Braycote, and the lot of Harmonic Drives (HD) and duplex bearings had  already been processed with Braycote, testing to verify compatibility and re- wettability of the two lubricants  began. Meanwhile, a complete disassembly and inspection was performed, down to the individual bearingball level.  

The inspection revealed corrosion withi n the Wave Generator bearings, even on inner diameters of inner  races that were bonded onto the Wave Generator hubs, despite having been stored in hydrocarbon oil.  Specialized inspection also brought the black oxide coating application of the HD circular s plines into the  spotlight, just as  chemical analysis revealed an unexpected presence of copper in the degraded lubricant.  Black oxide, old adhesive, and corrosion were removed from the HD assemblies, they were processed with  Pennzane- based lubricant, and t he endurance test began again. This paper provides recommendations  regarding use of mechanisms from long term storage, even if stored in seemingly ideal conditions, and  document s a discovery process unique to work with heritage hardware. Also addressed is  a performance  comparison between Braycote 601EF and Rheolube 2000, as well as the Pin on Disc Test results regarding  their compatibility  along with evidence that, contrary to prevailing belief, it is possible to functionally rewet  with Pennzane hardware th at was once processed with Braycote . 

Introduction  

MAIA Instrument   The Multi -Angle Imager for Aerosols (MAIA) mission objectives are to assess the impact of mixtures of  airborne particulate matter of different sizes and compositions on human health, and to collect multi -angle  spectropolarmetric imagery over targets of interest with respect to air quality and climate research. The  MAIA instrument is class 3, and features a pushbroom spectropolarmetric camera on a two- axis gimbal,  which is actuated througho ut the mission for multi -angle imaging, target revisiting, and inflight calibration.  

* Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA  

Proceedings of the 45th Aerospace Mechanisms Symposium, NASA J ohnson Space Center, 2020