How do the roll-rings at the ISS work?

Question

I get that on the ISS they need to keep the solar panels constantly rotating to keep them looking towards the sun.

To transmit the electricity and data to and from the Solar panels truss structure they use a piece of equipment called roll-rings, that transmit data and energy through an external strip to a rotating conductive middle circunference, and then to an internal conductive strip.

My question is:

How do you keep that middle circunference rotating, making electric contact and not fusing (soldering/welding) with any of the two conductive strips?

If there is vacuum in space, then when you send something to space, there is no small air cushion between the parts, and that's why I think it makes sense that 2 metallic parts touching each other ,in the (almost) vacuum of space, would solder.

Answer 1

The ISS Utility Transfer Assemblies (UTAs) are located in the center of the Solar Alpha Rotary Joints (SARJs) - the continuously rotating interface between the outer truss segments that support the solar arrays, and the inner truss segments that support the pressurized modules. The UTAs contain the roll rings. Here is a picture of one of the roll rings.

The UTA has a roll ring structure, which consists of multiple stationary metal plates surrounded by rotating metal rings. Flexible metal rollers, called “flexures,” between each plate and ring maintain a continuous conducting path to pass electrical power or computer signals between the stationary plate and rotating ring. One plate-roller-ring set is required for each power or data connection that must pass through the rotating joint. These roll rings allow for 360^o continuous rotation with seamless power and data conduction.

From The International Space Station - Operating an Outpost in the New Frontier

A side view schematic of the UTA is shown here (annotation mine)

From Roll Ring Assemblies for the Space Station which also gives details of the materials used:

The ring tracks and flexures are plated with a gold/cobalt alloy, which acts as a dry lubricant during vacuum operation and ensures the integrity of the electrical contact surfaces. The gold plating is backed by a nickel plating to enhance the wear life, reduce porosity in the gold plating, and act as a migration barrier to the copper in the base metal. Wear and flexure fatigue testing has been conducted to over 3.2 x 10⁷ revolutions of the inner ring in a vacuum environment and 1.6 x 10⁸ revolutions in air. The resultant wear debris of the latter unit was of extremely low volume and consisted of gold dust adjacent to the running tracks. In summary, the roll ring design exhibits low and consistent torque, has near zero wear debris, and has no time-related effects; thus, it is an excellent choice where long-life requirements are to be met.

The paper ELECTRICAL CHARACTERIZATION OF A SPACE STATION FREEDOM ALPHA UTILITY TRANSFER ASSEMBLY describes early ground testing of the roll rings in a vacuum chamber.

The report concludes

In summary, the UTA developmental unit testing performed at LeRC accomplished all of the goals established for this effort with no major problems having been identified. The testing yielded valuable information to support calibration of system models and to aid in finalizing the UTA design for subsequent qualification and flight units.

The DC resistance and steady state temperatures recorded are evidence of the UTA's high electrical transfer efficiency. The thermal interface between the Space Station primary power distribution cables operating at predicted temperature extremes and the UTA was not verified, as further analysis and facility modifications would be required to support such a test. The impedance test results suggest the total inductance of a UTA crossing pair is substantially higher than originally specified in the Space Station Electrical Power System Architectural Control Document and consequently should be considered in the design of the primary distribution network. Electrical rolling noise was shown to be extremely low in magnitude, possibly to the point it could be neglected in simulation and modeling efforts.

Corona test results indicate no inherent problem with this particular UTA design. Since the onset of corona is very much dependent on geometry, successful results for this testing do not necessarily guarantee that subsequent designs will comply with corona specifications. The corona results are believed to be conservative because the corona onset probably occurred in the external circuitry instead of within the UTA, especially for the testing under vacuum. Onset values above 1 kV would seem sufficient for the Space Station application.

The primary mode of crosstalk coupling was determined to be capacitive and appears to be sufficiently attenuated so that power transients do not cause disruption to the MIL-STD-1553 data bus. The post-test performance measurements demonstrated the unit did net suffer adverse effects from exposure to normal system transients. Off-normal transients were not investigated however, and would be a logical follow on activity to the testing reported on here.

Additional ground testing was performed to determine the effects of a large overcurrent event on the roll rings. The tests described in Large Transient Fault Current Test of an Electrical Roll Ring applied almost 4000 amps to the system with only minor degradation resulting.

The successful testing and a long operational history would appear to rule out inadvertent "soldering" of the rings.