NASA - National Aeronautics and Space Administration

When NESC was asked to help investigate an anomaly with one of the auto-tracking solar arrays on the International Space Station (ISS), it responded with the NESC Mechanical Systems Technical Discipline Team (TDT) and gear and tribology experts from NASA’s Glenn Research Center, Goddard Space Flight Center and Marshall Space Flight Center.

This expanded NESC team supported the ISS Program Anomaly Root Cause Investigation Team with a combination of records review, inspections of hardware returned from ISS, hardware testing and consultations.

Solar Alpha Rotary Joints (SARJ) are a key element of the solar array auto-tracking system that powers ISS. One half of the system is on the port side, the other half on the starboard side.

Due to an anomaly, the starboard side was shut down in 2007.

Figure 1a. SARJ location on ISS.	Figure 1b. Locations of TBAs and DLAs present on SARJ.

The hardware and the problem

The SARJ consists of 12 Trundle Bearing Assemblies (TBA) and two Drive Lock Assemblies (DLA). The TBA consists of three gold plated 440C rollers each of which contacts a race present on a 10 foot diameter race ring composed of three nitrided 15-5 PH steel surfaces. Figure 2 shows the SARJ Structural Test Article (STA) including the SARJ race ring, TBA, and DLA. As a result of high current, auto-tracking of the starboard SARJ was halted in 2007. EVA inspection of the starboard SARJ revealed severe damage to the outer 45 degree nitrided 15-5 PH steel race with large quantities of magnetized debris present on both the race and TBA as shown in Figure 3.

Figure 2. Members of NESC SARJ root cause team inspecting the SARJ Structural Test Article (STA) at NASA MSFC.

Figure 3. Section of damaged outer 45 degree race adjacent to Trundle Bearing Assembly (TBA) S/N 1037 on starboard Solar Alpha Rotary Joint (SARJ). Note magnetized debris.

NESC Role

The NESC team of mechanical systems and drive mechanisms experts provided testing to address root cause and consulted on identification of lubrication solutions. NESC members provided a detailed review of the SARJ build records from both government, and subcontractor archives to help determine likely root causes. The team’s preliminary conclusions are that the damage is a result of high friction and SARJ kinematics. NESC specific support for this investigation included inspections performed at NASA MSFC of the STA and starboard SARJ TBAs returned from ISS, vacuum roller traction rig testing performed at NASA GRC (Figure 4), and block on ring testing performed at NASA MSFC (Figure 5). The NESC SARJ Support Team provided key data to the SARJ Analysis, SARJ Root Cause, SARJ M&P, and SARJ Recovery Teams.

Figure 4a. Schematic of NASA GRC vacuum roller traction rig.

Figure 4b. Traction curve generated.

Figure 5. Schematic of NASA MSFC block on ring test.

NESC Results

Results of the NESC’s portion of the investigation provided the necessary data to exonerate potential root causes such as incorrectly installed TBA rollers, and also lead to the discovery that gold plating adhesion was an issue on several TBAs during the initial build. After further investigation, it was revealed that the gold adhesion issue might be a distinguishing characteristic between the current nominal operations of the ISS port SARJ versus the anomalous starboard SARJ. This team backed up their hypotheses with laboratory testing which resulted in physical proof of the low friction benefit of the gold.

In Summary

The joint NESC-ISS Project Team conclusion was that a major contributor to the root cause of the starboard SARJ failure could be attributed to the lack of reliable long-term lubrication preventing high friction (i.e. 0.4) and the kinematic susceptibility of the SARJ design to tipping due to roller mistracking. The combination of high friction and roller mistracking satisfied the tipping condition leading to high roller edge stresses and subsequent damage and failure of the nitrided 15-5 PH surface. Lubrication maintains low friction and prevents this type of damage. The NESC team continues to support the ISS Project on the corrective action for short and long term SARJ operations.