The reason for sensitivity to load is not yet fully understood, and is the subject of ongoing investigation. One possible explanation being explored is based on the mechanism of heat transfer. Once the bearing moves at a significant speed, the heat transfer process is very different than a static bearing. Significant motion would prevent conduction process through the contact points and ball. Instead, we believe that mass transport takes place, where the lubricant picks up heat at the hotter race, gets carried by the ball as it rotates, then deposit heat at the cooler race. As the axial load increases, this band of lubricant exposed to the mass transport process increases, resulting in greater heat transfer rates, thus higher bearing conductance. 4.0 Test Results:  PART 2 –Bearing Lubricant Behavior Observations Based  on Thermal Conductance Measurements /g3 This section explores lubricant behavior observed through conductance changes during the run-in  process. The impact of changing the maximum run-in speed was studied. The impact of bearing rest was also described below. From these measurements, we infer changes in lubricant quantity. 4.1 Test Method The bearing inner ring raceway was rotated in vacuum at constant speed and environmental temperature. After thermal equilibrium was established, the conductance was measured while operating at the  indicated speed. The speed was then changed, and the system was allowed to establish a new  equilibrium before the next data point was measured. The resulting thermal conductance was collected  for speeds ranging from a minimum of 0 rpm to a maximum of 10,000 rpm. Because we were establishing the influence of run-in, a test matrix was /g81/g82/g87performed on each data point, making it impossible to separate out the influence of temperature and speed. Hence the temperature was  allowed to vary with speed in the data set shown below.  4.2 Influence of Run-In on Bearing Lubricant Distribution Two medium-sized bearings were tested as described above. The first bearing, labeled medium A in  Figure 4.2.1, was freshly lubricated and had not been previously run-in. The second bearing, labeled medium B in Figure 4.2.2, had been run-in and tested previously but rested for months before the tests  shown here. The curves on each of the plots show a set of runs that has the bearing tested at  successively increasing or decreasing speeds.   The initial running cycles of the virgin bearing (blue diamonds, Figure 4.2.1) show erratic behavior caused  by rapid changes in lubricant distribution. The high initial conductance at rest indicates a significant amount of oil in the menisci. As the first speed cycle returned to rest from 10,000 rpm, the conductance was less than initial, but still above the fully run-in value. With additional speed cycles, the conductance settled down to a continuous decrease with speed, showing the hypothetical effects of heat transport  described above. After the multiple speed cycles, the static conductance leveled off at approximately 0.10 W/°C. At this point, we believe the bearing was fully run-in, as excess oil between the ball and race  interfaces were displaced. The used bearing, Figure 4.2.2, had been through a similar suite of testing, and then rested for months. We found that although the high-speed conductance changed little with additional run-in cycles, the conductance at rest (and, by inference, the quantity of oil in the static menisci) fell slowly with progressively greater accumulated operating time and as the bearing became run-in. The static conductance began at 0.13 W/°C, and fell with additional speed cycles to a stable value nearly equal to  the static conductance of the virgin bearing after several speed cycles. 124