The focus mechanism life test started 12-Mar-97 and lasted until 29-May-97. During nearly all this time the focus mechanism was operating in a vacuum chamber making 5000 focus position changes per day. Each focus position change was 60 steps (position 0 to 60). After about the end of April (240,000 moves) the focus mechanism started to show signs of increased friction. The mechanism continued to operate for another month or so, until a total of about 400,000 focus position changes had been made. At that time the mechanism would no longer operate. The chamber was opened and the mechanism would then operate. Nothing was done for a couple of months. Then yesterday Tom & I disassembled the focus mechanism. The apparent cause of the failure was that the nut & the leadscrew just wore out. The leadscrew was stainless & the nut was brass. There was a fair amount of brass particles visible. We did not see any cracks in the flexures. Plot of analysis
Dave Akin 13 Aug 97