256 conditions and accelerate damages . The main focus of the experiment is on the lubricant s ability to limit  surface and subsurface damage in the gears operated in lubricant starved conditions.  

Concept lubricant   A concept lubricant was prepared by dissolving 0.4 wt% of a hydrocarbon- mimicking ionic liquid   (tetraalkylphosphonium trimethylsilaalkylsulfonate)  [4] in m ultiply alkylated cyclopentane. Adequate  performance with regar ds to thermal vacuum outgassing and solubility was recently demonstrated by the  this lubricant  [8], which we hereafter designate as P-SiSO . Two reference lubricants were employed during  this work ; Syntheti c Oil 2001a , a multiply alkylated cyclopentane supplied by Nye Lubricants, Inc.   (Fairhaven, MA), and Fomblin Z25, a perfluoropolyether  supplied by Solway S.A. (Brussels, Belgium). The  reference lubricants are designated as MAC and PFPE respectively. Neat t etraalkylphosphonium  trimethylsilaalkylsulfonate was synthesized by Nisshinbo Holdings Inc. (Tokyo, Japan).   

Actuator Gear box Lubrication   The geared actuators consist of a planetary gearbox (GP32) and a DC -motor (RE30) with encoder and  servo  controller (ESCON 36/2), all acquired from Maxon Motor AG, ( Sachseln, Switzerland). The gearboxes  are 3-stage planetary gearbox es with 51:1 gear ratio, with max continous torque rating of 4.5 Nm . The  servo controller can provide a maximum continous current of 2 A at 25  V, which corresponds  to a max  continous torque of 2.6 Nm, ensuring that the sintered steel gears are not mechanically overloaded. The  gearboxes are dissasembled and cleaned, before relubricated with the test lubricants, as shown in  Figure 1(a-b). The amount of lubricant applied is significantly reduced in order to provoke starved  conditions; the original grease fill of 1.6 g is replaced with 0.060 g of test lubricant (5 µl to each planet gear  and 15 µl to output bearing).  After applying the test lubricant the gearbox is rotated under zero load at low  speed (5 min at 800 rpm followed by 5 min at 4000 rpm) to achieve a consistent lubricant distribution within  the gearbox.  

Geared Actuator Test Rig (GATR)   A custom made geared actuator test rig (GATR) , shown in Figure 1(c -d), was designed and manufactured  for the purpose of evaluating the lubricants in a component scale experiment.  In this setup, the actuator is  mounted in a refrigerated chamber filled with N 2 gas. The main purpose is to subject the actuator gearboxes  to operat ion in lubricant -starved and oxygen- reduced conditions in order to perform post test damage  evaluation and boundary film analysis . The GATR is equipped with a dynamometer and temperature  sensors in order to monitor effects on gearbox efficicency and temperature  evolution while running  the  actuator against a braking torque. The efficiency is defined as the ratio  of electrical power input, 𝑃𝑃𝑖𝑖𝑖𝑖, to  mechanical power output, 𝑃𝑃𝑜𝑜𝑜𝑜𝑜𝑜. The power input can be determined by motor speed, current, and torque  constant, while power ouput is determined by output speed and applied brake torque.  

Figure 1. (a) Dissasembled 3 -stage planetary gearbox. (b) Procedure of appl ying lubricant to planet gear.  (c) Atmospheric chamber enclosing actuator  and sensors . (d) Overview of Geared Actuator Test Rig.