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 360o 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.
