Table 5: Four load cases run for each test block   Load Case #1 Load Case #2 Load Case #3 Load Case #4  SRT Op. Mode Calibrate / SeekScan Brushing Rock Grinding Easy &  Medium Rocks Grinding Difficult  Rocks  Max. Mean Stress 1.28 GPa 1.28 GPa 1.39 GPa 1.59 GPa  F axial 0 N 10 N 37.5 N 100 N  F radial 6.8 N 10.9 N 27.7 N 52.3 N  Moment 0.19 Nm 0.25 Nm 0.73 Nm 1.67 Nm  Speed 220 rpm 1500 rpm 750 rpm 750 rpm  Op. Mode Duration 464 minutes 1500 minu tes 6489 minutes 3600 minutes  Bearing Revs 1.02 x 1052.25 x 1064.87 x 1062.7 x 106 Stress Cycles 2.97 x 1056.52 x 1061.41 x 1077.82 x 106 Block N Duration 13292 minutes = 221.5 hours (or 0.5x required life) 

Results and Discussion  Data and Naming Convention   Data collected during the test included average, minimum and maximum bearing torque  (torque sensor),  bearing speed and motor current. Data was stored once every two minutes. The average torque was  calculated from a 4 Hz buffer filled over the two-minute storage period. The minimum and maximum  torque values represent the minimum and maximum to rque sensor signal ov er that same two-minute  storage period. Finally, the grease-plated bearings are designated Bearing Pair ‘A’ (BP-A) while the  grease-plated plus grease-filled bearings are designated Bearing Pair ‘B’ (BP-B).   The average torque sensor data from all six blocks of the bearing lubrication life test are shown in Figure  6. Note that a single block represents 0.5x required lifetime and is approximately 9.92 x 10 6 revolutions  (2.87 x 107 stress cycles). Other than  catastrophic events, it was difficult to see gradual trends in the  average torque signal for a couple reasons. First, adjusting the load case four times per test block  complicates the torque signal (i.e., running torque should change with bearing load) and introduces set-up  error. While the set-up procedure was consistently followed, small variations in load could produce  noticeable differences in the torque se nsor signal. Second, it is suspected that the tapered bearing  between the load application housing and the non-rotating bearing shaft introduced a certain amount of  error that varied with the load case. Friction in the tapered bearing would subtract from the amount of  torque registered on the torque sensor. That being said, trend information can be seen pretty clearly by  looking at the torque noise in the bearing. Torque noise is the difference between the maximum and  minimum torque levels.   Bearing Pair ‘A’ (BP-A) – grease-plate only

From Figure 6, it is evident that over the length of the test that BP-A (blue line) ran with a higher parasitic  torque than BP-B (red line). Through 2x life (3.97 x 107 revs), the average torque for BP-A was between 5  and 10 mNm.  Figure 7 shows the torque noise in BP-A.  After 1.1x life (2.2 x 107 revs) the noise in the  bearing pair started to steadily increase. At approximately 2.3x life (~4.5 x 107 revs) there was a  significant spike in the torque exceeding the 28-mNm limit that stopped the test. After increasing the limit  and restarting the test, the noise and average torq ue continued to increase rapidly until a catastrophic  torque increase stopped the test for good just before 3x life. 

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