255 Lubrication Concept Evaluated for Geared Actuators under  Starved  Conditions  

Erik Nyberg*, Jonny Hansen** and Ichiro Minami* 

Abstract  

Lubricant starvation leads  to the risk of a shift in the lubrication regime from (elasto)hydrodynamic towards  boundary conditons. Effective tribofilm formation is essential to limit surface damages in these conditions,  but additive technology for space- grade lubricants  is lacking. T his work evaluates the feasibility of a novel  type of multifunctional ionic liquid lubricant , for use with multiply alkylated cyclopentane (MAC). Actuator  gearboxes are operated under starved conditions in nitrogen atmosphere to evaluate the effectiveness  of  the tribofilm forming lubricant  (designated P -SiSO). The effectiveness of P -SiSO was evaluated from macro  to micro scale in both surface and sub- surface analysis by use of microscopy (optical , interferometric , SEM)  and X -ray microtomography (XMT) , and mechanisms of effective lubrication are discussed.  

Introduction  

Conditions faced in robotic space exploration missions pose significant challenges to lubrication of complex  mechanisms. Geared actuators operated in low temperatures require extensive preheating before startup  [1], but once in operation they may suffer from lubricant starvation due to limited resupply of lubricant to the  contact [2]. In vacuum conditions, native oxide layers quickly wear out and if the lubricant does not form a  protective tribofilm, there is high risk of seizure. Perf luoropolyethers (PFPE) and multiply alkylated  cyclopentanes (MAC) are heritage lubricants used in space applications. They both have benefits and  drawbacks; the main benefit being outstanding resistance to outgassing, but their tribofilm forming  properties  are problematic. PFPE forms iron fluourides in tribocontacts, which prevents seizure but  eventually degrades the system autocatalytically [3] . MAC on the other hand is a neat hydrocarbon, and i s  not generally tribochemically active. Additives are possible, but finding effective additive s that  are miscible  and non- volatile  is challenging, and few options are currently available. As space exploration missions are  demanding increasing performance of mechanisms, new solutions are urgently required. This paper aims  to establish the feasibility of using hydrocarbon- mimicking silicate forming ionic liquid (P -SiSO) as triboimproving additive in MAC.  

Recent work on P -SiSO   In our previous work  [4], we described the molecular design of a hydrocarbon- mimicking synthetic lubricant  composed of a tetraalkylphosphonium cation and trimethylsilalkylsulfonate anion, and found that it provides  excellent lubricating performance under boundary lubrication conditions  [5] as well as  elastohydrodynamic  conditions [6]. The hydrocarbon- mimicking structure enables miscibility with a range of hydrocarbon base  fluids, while the ionic structure of P -SiSO  enables reduced volatility. Surface analysis has shown that the  excellent performance correlates with formation of a  novel type of tribofilm, mainly based on silicate .  Preliminary studies in vacuum tribometers  and outgassing tests [7]  have produced positive results and  therefore the next step is to evaluate the lubricant under increasingly realistic configurations .  

Materials and Methods  

In this work, P -SiSO  was evaluated in commercial off -the-shelf (COTS) geared actuators. Sintered metal  gears, reduced lubricant fill, and reduced temperatures  was employed to provoke lubricant starved 

* Luleå University of Technology, Luleå, Sweden; erik.nyberg@ltu.se   ** Scania CV AB, Södertälje, Sweden  

Proceedings of the 45th Aerospace Mechanisms Symposium, NASA Johnson Space Center, 20 20