287 Laboratory Studies of Spacecraft Fluid Lubricant Mobility and Film Thickness  

Peter Frantz*, James Helt * and Steve Didziulis * 

Abstract  

Spacecraft mechanisms often rely upon very small amounts of lubricant to survive in the harsh conditions  of space.  In critical applications, thin oil films must provide both low drag and longevity, with ball bearings  in some devices lasting hundreds of bil lions of loaded mechanical cycles. Our lab has developed several  unique test facilities to measure the physical properties of thin liquid lubricant films, and to monitor their  performance in realistic bearing level tests, such as ball bearings operating up to 6000 rpm. To understand  and optimize lubricant performance, our groups strategy is to correlate bearing level test data with the  chemical -physical properties of the lubricant and counter -body interfaces as they evolve during operation .  This talk will p rovide an overview of our capabilities and highlight some unique aspects of thin film rheology  and additive function.  

Introduction  

During the acquisition phase of spacecraft mechanisms, we are often asked to estimate the  assets  usable  lifetime. Similarly, during operation we are asked to assist with extending mechanism life by critically  assessing telemetry data  that informs to the state of the lubricant . In both cases, mechanism life is usually  determined by the life of the lubricant in the critical rolling and sliding interfaces. Once the lubricant is  depleted, friction and wear lead to deteriorating performance and failure. This separates space mechanisms  from many terrestrial applications, where lubricant may often be reappli ed until failure occurs by other  processes, such as rolling contact metal fatigue. For spacecraft mechanisms, retention of lubricant is of  paramount importance.  

When managing these mechanisms during operations, we periodically estimate  the relative quanti ty of oil  to monitor component health. Bearing drag torque, which is typically proportional to spin motor current, can  be separated into three terms: 1) a “Coulombic” term, due to sliding and interfacial slip, 2) a hysteresis term,  due to losses in compres sion of the Hertzian contact, and 3) a viscous term, due to displacement of the  lubricant in the ball path. This final term, the viscous drag torque,  has been shown to scale with viscosity to  the 2/3 power [1]. Viscosity decreases with increasing temperature according to a model with functional form  similar to the Arrhenius Equation[2 , 3]. Taken together, over small changes in temperature, we find that the  dependence of viscous drag on temperature is approximately linear , with the constant of proportionality  referred to as the viscous coefficient (Q).  This value is used as a relativ e measure of the lubricant in the  critical interfaces of a spinning bearing. It is usually large at beginning of life, and tends towards zero late  in life. Thus , it can be used as an indicator of bearing health.  

When monitoring this value in a large population of similar high- speed mechanisms, we have found that  (after lubricant run in) the value remains nearly constant for most of life. As the lubricant is depleted, the  value gradually falls to zero. If there are  two bearings in the component, we find that  Q initially falls to half  of its saturation value, and then later falls to near zero. A graphic representation of this process is shown  in Figure 1 for a typical mechanism. For the first 14 years of operation of  this hypothetical mechanism, the  viscous coefficient (Q) hovered around 0.2 mA/ °F. During the 7h year, the Q value fell to approximately  0.1 mA/°F, and three  years later it fell to near zero. At that point, retainer chatter and instability may begin  to oc cur. The late stage behavior is interpreted as sequential starvation of the two bearings. The first one  becoming depleted of oil at 7 years, indicated by the viscous drag falling to half of its original value. T he 

* The Aerospace Corporation, El Segundo, CA  

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