Incorrect Assessment of Lubricant Regime  Even in the absence of lubricant rheological effects such as solidification or shear thinning there are other  challenges in the correct assessment of lubricant regime. The λ−ratio is usually assumed fixed and based  on the specified or measured un-deformed ball/raceway surfaces prior to run-in/asperity modification or  wear. In reality however, this will likely be incorrectly assessed. A higher λ may be applicable under bearing  preload due to contact asperity flattening and λ−ratio may even be variable throughout test, increasing as  surfaces run-in or decreasing due to thermally-driven contact angle changes or wear. It may only be  possible to assess the impact of the test on the bearing surfaces by sophisticated pre- and post-test  profilometry and analysis.   Changes During Operation  For ball bearings, the cage is rarely fully benign and its stability fundamentally impacts both torque  performance and wear. For a valid accelerated test, the cage behavior needs to match that of the application  at nominal operating conditions. However, changes to ball/cage and cage/land friction and cage geometry  (e.g. due to wear) may have a considerable impact on subsequent cage stability or wear. Meaning that at  the beginning of bearing life a cage could be stable, but develop instability due to oil absorption or changes  in the surface friction or geometry of pockets or lands.  Non-Calculable Performances  Finally, there is a range of “non-calculable”, even non-predictable performances, meaning behaviors that  are not predictable by analysis, but apparently real and commonly observed phenomena. In this context  we might list:  •Evidence for the regimes of starved/parched lubrication – where viscous losses are low, films theoretically of implausibly low thickness, but wear also low [15]. •Differences between fully-flooded and starved films in grease. When fully flooded greases tend to have a thickness higher than predicted based on oil properties alone (by a factor ~ 2), whereas when starved (and indeed there is evidence that greases may “self-starve”) the film thickness resulting is ~ 50-70% of that of the base oil under the same speed, load and thermal conditions. (see [16] for example). Indeed, as greases age, both types of behavior may be observable. Given the above, it was highlighted [5] that in some cases it may not be possible at the start of an  accelerated test involving a fluid lubricant to be certain that the test would ultimately be valid, even if the  target life was achieved. In the intervening years the proposed “tribologically valid” approach has been little  used and given the new knowledge exemplified above now seems to require update since new knowledge  renders this kind of accelerated test likely less, rather than more, representative.  Perhaps due to the above uncertainties, accelerated tests of fluid lubricated mechanisms are not common,  and it seems that, where used, either the speed and the acceleration factor adopted remain relatively low,  or there is an implicit acceptance and engineering judgement that the above effects may be second order  or neglectable in the overall performance. Such judgements are not normally supported by experimental  data and so the tests themselves remain, in the absence of appropriate experimental data on the fluid  lubricant behaviors, to some greater or lesser extent, flawed.  Uncertainties Surrounding Accelerated Test – Solid Lubricants and Self-Lubricating Bearings It is often assumed that, in contrast to the quite complex situation with oils and greases, the accelerated  test of solid lubricated or self-lubricating bearings is much more likely to be valid.   Solid lubricated bearings usually rely on shear of anisotropic lamellar solids (e.g. MoS 2) or low shear  strength Face Centered Cubic (FCC) metals (e.g. lead, silver, gold). This “primary lubricant” is  supplemented often by a cage material which undergoes the double-transfer mechanism to transfer cage  material via the surface of the balls to the raceway, a “secondary lubricant” phenomenon which may  NASA/CP—2018-219887 517