Figure 6.  Hydrodynamic Load Capacity for the Rear Bearing  on the Failed Pump. Green and Red Ranges Indicate Bearing  Margin with Respect to the Lubricant Jet Force. Post-Failure Inspection Findings The pump module was returned from orbit in 2011 on the final Space Shuttle mission, STS-135. After  post-flight processing and ammonia decontamination were completed, a comprehensive inspection procedure was conducted on the returned hardware. This inspection covered electric power and controls,material microscopy and crack progression analysis, debris analysis, x-ray, ultrasound and visual examinations of the physical  hardware. The electrical and  control systems were deemed to be in acceptable condition given the long-term exposure to  ammonia. No evidence was  found to further pursue these components as the root cause.The most significant finding was  severe wear of the carbon bearings at all three bearing locations, two journals and one  thrust bearing. The most severe wear occurred at the rear journal bearing. An additional significant finding was that the wear was organized in a manner that indicates an angular misalignment. In other  words, a specific load acting in a specific direction on the rear bearing was the most likely root cause. Typically, in high speed rotating machinery the expectation is that wear would occur throughout the circumference of the bearing either in a cylindrical or conical shape. However, in this case the wear was concentrated in one quadrant and mainly on one bearing. Again, this is a strong indication a specific static force was acting directly on the rear bearing. Uncharacterized Bearing Loads The severe damage found on the bearings in the post-failure investigation initiated a quest to identify and  quantify all potential loads that may act on the bearings. For most pumps that are operated on the ground  in standard gravity conditions and for these pumps during ground-based qualification testing, the most  significant force that must be reacted by the bearings is the deadweight load of the rotating mass. This  load typically dominates the bearing performance calculation and all other forces are considered insignificant. In fact, the gravity load often tends to stabilize a rotating system because it provides a preferred location for minimum film thickness and hence health monitoring instrumentation can be located accordingly. However, once this load is removed by operating the machine in orbit, the remaining small forces can contribute to instabilities, vibrations, and even a static overloading of the bearing. During the course of the failure investigation, five loads were discovered that were uncharacterized during the design phase and qualification testing of these pumps. It is recommended that vertical qualification testing beimplemented for future pumps designed to operate in microgravity environments. Vertical testing relieves the radial system of deadweight forces and uncovers potential deleterious effects of an unloaded system. Residual Unbalance of Rotating Assembly Residual unbalance exists in all rotating systems and exists when the line of mass centers does not  coincide with the rotational axis. This offset when combined with the rotational speed of the machine manifests itself as a synchronous, rotating force on the bearings. Typically, high-speed rotating  equipment is dynamically balanced after final machining and prior to assembly. The dynamic balance  process aims to return the mass center to the rotation axis by adding mass or removing material from the  456