215 Test Results  

Ring Durability   The first task was to determine if the damaged flight race could suffer a structural failure if continued on-orbit operation was resumed. It was decided to run the previously damage test plate dry to bound the problem. This was in line with the test philosophy of doing ring durability testing prior to the main test for lubrication life.   The ring durability test was started with the rollers set at flight-like mistrack angles based on  measurements made on flight trundles at NASA MSFC. LVDT’s were set up on the rollers to monitor tipping and the load cells in line with the roller axis were set up to monitor sideload. Unfortunately, due to  the sensitivity of the load cell and t he higher drag in the bushings through which the bearing shaft rides,  the exact friction values couldn’t be extracted from the data. The load cell data however, was still very useful for monitoring trending of the sideload of each roller. With the preload set at a flight-like value, roughly 4 kN (900 lb) per roller, the test was started.   Initial testing was performed with no lubrication in order to characterize the track dry. It was also performed at ambient pressure to create a baseline to which data could be compared to during the transition to vacuum. The sideload values were high relative to the later runs with grease. Also, edge  loading was observed upon examination of the tracks but no tipping was evident. It was decided to limit the amount of dry running to preserve the Datum A tr ack so as to ensure a flight-like surface for the  lubricated portion of the test. The surface was run on just long enough to obtain some wear but not long enough to cause damage since the on-orbit datum showed minimal signs of damage.     After approximately 16 equivalent flight months of te sting, the previously dam aged outer track showed no  significant structural degradati on other than generation of additional surface wear debris and a deeper  groove under the edge of the tipped roller. The test results showed that struct ural damage even at the  high stresses associated with tipped rollers and dry running would not jeopard ize the structural integrity of  the SARJ.  Lubrication

Grease was then added to the track using a grease gun similar to the on-orbit gun and in similar quantity. A single bead of the Braycote 602 flight grease wa s added to the center of the track around the entire  circumference. The test was restarted at ambient pr essure for more data and then the lid was put on the  fixture and the chamber taken to vacuum. The grease effect on the track was an immediate reduction of  close to 50% of the sideload as ex pected. After a short period of time , the grease was pushed out of the  way and the sideload increased and then stabilized (see sideload drop at 6000 revs in Figure 11). 

Another important variable was the te st speed. The test speed needed to be fast enough to perform the  test in a reasonable time frame but slow enough to ensu re a flight-like test environment. Running too fast  could cause the track to starve itself of grease and that is exactly what was observed during initial grease  runs.  

As shown in Figure 11, the sideload friction levels climbed back to the pre-grease levels after only a few  thousands revs at 1 rpm. This test speed is approximately 20X faster than flight. Based on previous SARJ  coupon testing, it was known that speed affected gr ease performance so it wa s clear that the speed  would have to be reduced in order to allow the grease to produce a flight-like affect. The test rate was then lowered to 0.4 rpm at roughly 17000 plate revs. The sideloads then started to drop. The track was then re-greased and the sideload dropped even more as expected. They remained reasonably stable until at about 21000 revs when an operator error inadvertently switched the test speed back to 1 rpm. Figure 11 shows that the sideloads continued climbed again until about 29000 revs when the speed was reduced back to 0.4 rpm which lowered the sideload once more. Clearly there was a direct connection between surface speed and grease effectiveness. The explanation for this is that the grease needs  NASA/CP-2010-216272