188 orbit, larger than any previous space station. It was designed as an orbital scientific platform and is  intended to operate continuously while supporting a crew of six in pressurized modules . The ISS offers an  advantage over spacecraft such as NASA's Space Shuttle because it is a long-term platform in the space  environment, allowing scientific experimentation as well as long-duration studies on the human crews that  operate them . Long -term expedition crews conduct science daily (approximately 160 man-hours per  week), across a wide variety of fields, including human research, life sciences, physical sciences, and Earth observation, as well as education and technology demonstrations . The power required to support  the scientific and life sustaining functions of the ISS is provided by arrays of solar panels.  The ISS has a backbone or set of trusses that house several ISS systems. These trusses are joined to a set of pressurized modules that house the crewmembers living and working aboard the ISS . Figure 1  shows the ISS after assembly mission 17A by the Space Shuttle. The pressurized modules are located along the center of the truss structure, extending forward and aft . The power generating solar arrays are  located on the port and starboard sides of the truss structure outboard of the SARJs. The location of each Solar Alpha Rotary Joint (SARJ) is indicated in Figure  1.     Solar Alpha Rotary Joint Overview

The SARJ is a single-axis pointing  mechanism that allows orbital-rate sun-tracking rotation of the  outboard trusses and solar arrays of the  ISS. The SARJ completes one full rotation per orbit of the ISS,  approximately every 90 minutes . Figure 2 shows a drawing of  the SARJ with the major components labeled . The SARJ is  capable of transferring 60 kW of electrical power, spare low power (300 W), and data channels across the rotary joint . The  total weight of the SARJ is  1161 kg (2561 lb) . Two SARJ  mechanisms are installed onboard the ISS - Port (activated  December 2006) and Starboard (activated June 2007) . The  SARJ serves as the structural joint between the ISS inboard and outboard truss elements via twelve Trundle Bearing Assemblies (TBA) . The trundle bearings straddle between an inboard and  outboard triangular cross-section race rings . The race rings are  approximately 3.2 meters (10.5 ft) in diameter. TBAs are nominally mounted to the stationary inboard ring while their rollers track against the three surfaces of the outboard race ring .  These rollers are highly pre-loaded against the race ring to allow them to react ISS structural loads. The  bearing race is made of a 15-5PH stainless steel forging with a nitride hardened case. TBAs are designed  for individual on-orbit replacement to protect the mechanism against a roller bearing failure. The SARJ is  driven by one of two redundant Drive Lock Assemblies (DLAs) that interface with an integral bull gear on  the race ring via a motor-driven pinion . Each DLA is controlled by a Rotary Joint Motor Controller (RJMC)  which, in conjunction with processors in the ISS computing infrastructure, performs closed loop control of  the joint’s motion . SARJ system health and status data is relayed by the processors to the ground in the  ISS telemetry stream .    Trundle Bearing Assembly Overview

The TBA contains three roller bearing assemblies: two identical upper rollers and a lower, slightly wider, roller . A picture of a TBA is shown in Figure 3 . The roller assemblies consist of an internal double row  tapered roller bearing whose cup is shrunk fit inside of an outer roller . The outer roller is the physical  interface with the SARJ bearing race . The outer roller is made of 440C and is lubricated with 1 250-2250  angstroms of gold applied via an ion deposition process. The gold plating on the rollers serves as the sole lubrication scheme for the roller/race interface (the internal tapered roller bearings have a grease lubricant) . At the time of the SARJ preliminary design it was believed that the mechanism would be  exposed to the atomic oxygen present in the low Earth orbit environment. The final design included thermal shrouds around the entire circumference of the mechanism, but these were not part of the baselined design at the time the lubrication system was being selected . Due to the long life requirement  Figure 2. SARJ Overview  

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