S.J.Metcalfe, 5/4/00
The test is to simulate an estimated jaw movement of 2 cycles per shift which is 9000 cycles over the 5 year design life, running in a vacuum of 10-7
The
test equipment cycles at 29rpm with a stroke of 0.175”. It will take 5 hrs 10
mins to complete the first stage of the test (9000 cycles). At this point the
area under the fingers should be checked for dust and the sample plate should be
inspected for wear, if necessary it may be removed and inspected.
The
travel of the collimator jaw is 0.65” whereas the test fingers travel only
0.175” therefore the fingers will not have traveled the full distance required
by the test.
After
reassembly of the plate the test should be repeated for a further 24390 cycles
giving a total of 33390 cycles (a factor of 3.71 derived from 0.65/.175)
Plating
details:
Fingers: Rhodium max .0002”
Coupon: Silver .0006/.0007"
The Test
It soon became apparent that there was a problem with the fingers because the Coupon was showing signs of extreme wear after just a few strokes. There were deep scratches at point of contact with the top and bottom of each finger. The test was aborted after 9000 cycles and the fingers and coupon were removed for examination.
It was determined that the fingers had not been elecropolished prior to rhodium plating, so the sharp edges caused by the EDM machining were acting like saw teeth, causing the removal of the silver plating on the coupon.
Preparation for the second test
A second set of fingers were ordered, this time they were elecropolished. Examination of the fingers prior to plating showed that all the sharp edges had been removed, there were no cracks in the material and that the line of contact was smooth .
The coupon had been plated on both sides and would be reversed although it was noted that the surface finish of the coupon was not as smooth as the inside of the collimator and this would have to be taken into account during the final examination.
Test 2
After running for a few minutes it became apparent that for some reason, the fingers were not parallel in 'Y' to the coupon because the top six fingers were not making contact with the coupon whereas the remainder of the fingers were progressively making more contact, as shown by the burnishing of the silver plating. It was decided that valid results could still be obtained from the test so it continued up to 9000 cycles.
Close examination through the window showed that no significant wear was taking place on the coupon, even on the lowest fingers which were being pressed harder against the coupon. The coupon was considered to have passed its five year lifetime test, subject to measurements being taken at the end of the test to determine the actual amount of compression on the lower fingers.
The apparatus was switched on again to continue the test of the fingers, however the drive bearing failed after 24374 cycles and the test was aborted. Following removal of the vacuum chamber, measurements showed that the top of the fingers were 0.005" further away from the coupon than the bottom, the actual figures being: top=0.075", bottom= 0.070". The correct (calculated) dimension should have been 0.068".
Examination showed no more noticeable wear on the coupon even though it had run for twice its design life (10 yrs equivalent). If the fingers had been mounted at the correct height of 0.068" (0.002" closer) this would probably have increased the wear rate slightly but I judged that they would still have passed the five year test.
The fingers showed no noticeable wear at their contact points and were judged to have passed the test.
