Whilst such changes are temporary (as the fluid passes though the ball/raceway contact for example)  permanent viscosity loss has been reported [8] at mean contact stress > ~2 GPa (~3 GPa peak) for  815Z/Z25. Though a high contact stress for most space mechanism applications, this phenomenon should  nevertheless be borne in mind.   Rheological changes as discussed are cited by some authors, notably Vergne, Bair et al (9,10) as the  dominant reason for the common failure to correlate experimental and calculated specific film thickness.  Indeed, the implication of their work is that the kind of “Quantitative EHL assessment” needed in order to  define and demonstrate valid accelerated testing can only be achieved by a fully detailed assessment of  the lubricant rheology. To our knowledge such an assessment is not yet available for the most common  space lubricants, and the situation for greases seems still more complex – requiring both the rheological  properties of the base oil, its flows and thickener behaviors to be well understood.  Batch Variability and Degradation Effects  For fluid lubricants, intra-batch property variations, usually assumed small, are not well understood or  quantified in the open literature. Perhaps more significant, but also not well understood at component level,  are the effects of degradation/deterioration due to the nature of the fluid lubricant (for example grease bleed  (oil separation); evaporation of volatile constituents (e.g. additives which typically have a vapor pressure  an order of magnitude higher than the base oil); oxidation or tribo-degradation (viscosity loss or even autocatalysis)).  Lubricant Availability  The actual quantity of lubricant available at a given contact (e.g. ball/raceway) is dependent on various loss  routes which deplete the local supply, primarily migration due to 1g orientation, surface energy driven creep  and evaporation. Of these, only the latter is considered calculable, losses due to evaporation being typically  estimated using a Langmuir equation approach. However, in general a lubricant may have components of  different molecular weights and a range of volatilities. This could render an analysis using the Langmuir  equation and published vapor pressure data too simplistic, the true situation being that the rate of mass  loss, even for nominally single component fluids without additives is dependent on the thermal history of  the lubricant. Indeed, there is evidence [11] that the Langmuir equation may predict higher loss rates than  found in practice by approximately an order of magnitude at typical operating temperatures as shown in  Figure 2 suggesting a high uncertainty on the quantity of oil to be initially added or remaining within a  mechanism at the end of life.  Figure 2. Comparison of experimentally measured evaporative loss rates for Nye 2001A (left) and  Fomblin Z25 (right) with the predictions of Langmuir theory [11]  For cotton phenolic cages, which are usually vacuum impregnated over a period of hours to saturate the  cotton fabric with the lubricant oil prior to use, there is evidence [12] that even “fully impregnated” cages  may absorb a significant mass and a disproportionately high percentage of bearing free oil during  subsequent storage as the phenolic matrix itself absorbs oil. The infiltration of oil into the phenolic matrix is  a very low rate process, taking months, but as shown in Figure 3 the total mass absorbed can be relatively  high and assuming this were to be absorbed entirely from the free oil quantity typically added to a bearing,  NASA/CP—2018-219887 515