53 Table 3. Mean tribological data for re- lifed vs. fresh Maplub SH051- a grease [5]  Grease  Lifetime  (orbits/µg)  Initial friction  coefficient  Mid-life friction  coefficient   Re-lifed grease  11,050  0.07 0.093   Fresh grease  9,569  0.07 0.100  

Life Tests 

Life testing of  Scanner and Derotator  was performed in order to demonstrate that the mechanisms met  the  specified lifetime requirements of 201 million revolutions for the Scanner and 101 million revolutions for the  Derotator, respectively.  

Life test approach   The bearings were  considered the only life- limited  (in terms of revolutions)  items  of the mechanisms . The  following approach was chosen:   • Build up full mechanism mo del (with optics dummy  and industrial encoder)   • Perform cumulated environmental testing   • Perform  accelerated  life test in thermal -vacuum conditions   Full models of Scanner and Derotator were built up. This was  necessary as different bearing layouts and  types had been selected for the two mechanisms (refer to sections Design Overview  and Lifetime). Life - testing of standalone bearings was ruled out; such an approach would not have been flight -representative:  pre-conditioning of the bearings by representative environmental testing would not have been possible and  mounting conditions of the bearing in the mechanisms could not have been considered.  

Environmenta l testing of the life test models  was performed in order to stress the bearings in a similar  manner as the flight bearings. The worst -case environmental loads of a flight mechanism would be three  times proto- flight testing (mechanism level, instrument level, satellite level) plus launch loads and in- orbit  loads.  Therefore, t he following environmental pre- conditioning was performed :  • Vibration testing along three axes with qualification loads and 4x proto- flight duration (= two times   qualification duration)   • Shock testing along three axes (3 full -level shocks per axis)   • Thermal -vacuum testing (1 cycle non- operating temperatures, 7 cycles operating temperatures)  

The predicted contact stresses during vibration were much higher than the ones during non-operational   temperature cycling (2,000 MPa vs. 1, 500 MPa for the Scanner bearings ). Therefore, no additional benefit  would have been obtained from doubling the thermal vac ( TV) test duration (and cost) .  The life test was performed under consideration of the following constraints:   • The life test has to be accelerated in order to finalize it in reasonable time. The duration of the life  tests at nominal speed would have been 11 years.   • The baseline temperature of the life test should not be overly conservati ve; the average  temperature over one orbit , including hot case margins , was considered a reasonable choice.   • Analytical predictions yield ed that the bearings operate in the mixed lubrication regime; a ny  acceleration of the life test (= speed increase) requi red an increased temperature during the test  in order to keep the oil film thickness  constant   • It was decided to limit the temperature to 60°C at the bearing; there is little experience with fluidlubricated bearings permanently operated beyond this temperature for such a long duration.   • The Hertzian pressures in the life test environment had to be similar to the ones on orbit   • The number of load changes (rotational acceleration/deceleration, stops) in the accelerated test  had to be the same as in a non- accelerated test   • The rotational acceleration should be the same for the non- accelerated test and the accelerated  test in order to avoid overstressing the bearing cages