effect), it can be stated that the life joint was over-tested. The testing validated the mechanism and flex  cable’s ability to function properly far beyond the full life requirement of this mission. Moreover, it demonstrated a capability that far surpasses previous NASA flight experiences with similar space flight mechanisms. During the entire life test, the drive stalled only once at 2.3M cycles at -2°C. The cause was increased viscosity of the lube because of a slight degradation of the lube due to aging. Cold testing was resumed at +3°C and finished without any further incidence. Figure 7 shows a close up picture of the harmonic drive, taken after the life test: No mechanical wear  could be detected at any mechanical parts, nor was there any graceful degradation evident, which would  have expressed itself, for instance, by substantial increases in backlash. But the lube’s color had changed from brown to black in the engagement zones between gear teeth and in the bearings. Chemical  analyses, however, revealed that lube samples taken from inside the engagement zone had retained their lubricating capabilities. The joint thus maintains its full capabilities.  The overall conclusion based on life test results (for life test conditions), is that the joint demonstrated its capability to survive  the required mission error free, pointing away from a mechanical  cause for the anomaly: The life test joint is still being used on  occasion today, proving its mechanical integrity and the lube’s  lubrication capabilities time and time again. It runs error free, provided it receives the proper electrical input signal and current.  Figure 7: Close up View of Mechanical Components after  the Life Test  Keystoning Effects  During the assembly of Diviner’s elevation joint, it was noticed when the partially assembled joint was  rotated freely (actuator not attached yet), that the output bearings occasionally experienced some hard spots for no obvious reasons. The bearings ran smoothly again when part of the inner ring preload, applied by the bearing retainer (at arrow tip of item 4 in Fig. 5) was removed. Initially, a non-flat retainer surface was suspected that could have deformed the thin cross section bearings. But even after re-machining the retainer shoulders and checking flatness and perpendicularity of every shoulders of the bearing assembly (all within ±0.0003 in, 0.008 mm), hard spots could still be detected occasionally when the full retainer preload of approximately 120-lb (54-kg) axial load was applied. This resistance was somewhat position dependent and lasted over a short distance of up to 30 degrees, but could disappear and reappear at different locations.  Not finding another cause, the bearing supplier [8] was contacted. Their engineering department related  that they had experienced similar behavior on other applications with full complement bearings as well.  The phenomenon is called keystoning. Here is a somewhat personal explanation of this writer for what  happens: If the balls are not rolling exactly on the same race diameter, which can happen with angular  contact bearings, the rotational velocity of the balls inside the bearing are not exactly the same. Hence,  with no retainer to separate the balls, they progress from contacting each other to pressing against each other in the high load zone. That causes sudden frictional forces between balls. This friction force is not  sustained over a long distance because the external load on balls fluctuates (i.e. if the bearing’s external load is applied radially, then the balls roll away from being under the high radial load zone). After the external load on a ball is removed, it is free to re-align itself in the race grooves and w.r.t. other balls.  Thus, a friction load may or might not build up when balls enter the high load zone. With a retainer present, a ball might temporarily push against the retainer’s finger that engages between  two balls. But since the retainer is much more compliant, it will not produce excessive friction forces in the  high load zones. Thus, the retainer prevents keystoning from occurring. Lesson learned: avoid using full  278