Metal shavings seen in solar array rotary joint
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: October 28, 2007
Spacewalker Dan Tani, inspecting a massive solar array rotary joint that has been experiencing high vibration and power usage, reported a large number of metal shavings inside the mechanism after removing a thermal cover and to get a glimpse at the joint's inner workings.
"Great discovery, Dan," spacewalker Scott Parazynski radioed. "I didn't think you'd be able to see anything, but..."
"It's quite clear," Tani said. "There's metal-to-metal scraping, or something, and it's widespread."
Engineers had speculated a thermal blanket or perhaps a misaligned bolt was causing unusual friction in the rotating mechanism of the right-side solar alpha rotary joint, or SARJ. The space station is equipped with two SARJ units, one on each side of the main power truss, to turn outboard solar arrays and keep them face-on to the sun. The port, or left-side, SARJ is operating normally and while the starboard unit still functions, engineers have been monitoring high vibration and current use in recent weeks.
It was not immediately clear what might be producing the metal shavings Tani reported. He did not immediately see what might be causing the problem.
Tani initially simply floated around the perimeter of the joint and inspected its outward appearance, checking each of 22 thermal blankets and making sure no bolts were out of position.
"I see you've done everything from 1 to 22," Italian astronaut Paolo Nespoli said. "You don't see any missing bolts, the gap looks more or less constant everywhere.
"All the swing arms are locked, the gap looks very constant, yes," Tani reported. "And nothing obvious, no scratching. The only thing I could see maybe is these big SLR, the outboard, the SLR parts that rotate, the corner braces at the corners? They might rub up against the MLI (multi-layer insulation), but I don't see any marks on the MLI. So I don't know if that's happened. And it looks like it's designed for the MLI to stay out of there."
"OK, Paolo, is there anything you or the ground want me to do while I'm out here? I guess, unfortunately, I have nothing to report in terms of anomalous conditions."
Tani then was asked to peel back one of the insulation blankets to look inside the joint. That's when he saw the shavings.
"There are lots of very fine metal shavings, and I can see that because the motor that is the, what is that, that's a trunnion bearing, has a lot of those iron shavings. It's like the (result) that you get with the metal, iron filings and you put a magnet under it and they stand straight up."
Footage from Tani's helmet camera showed the large gear inside the starboard solar alpha rotary joint, but the filings he reported were not obvious.
The shavings, he said, could be seen "on this big motor, it looks like a big magnet that is this upper roller, I don't know why there'd be a magnet here but there's a magnet. I see microswitches, there's two microswitches on it and a metal squarish 2- by 2-inch by 2-inch cube that I believe holds the roller and it looks like it's magnetic because it looks like there are these iron filings, or shavings, on it.
"There's quite a bit and then the outer race, the flat part, is discolored, if you can see that, from the teeth, the angled portion that are between the teeth and the thin part of the race is all sort of discolored. I would almost say corroded in some way. And there are even some filings... I'm going to look up, there's a connector here that says J-15 outboard, looks like there are some filings there. There must be something magnetic to hold those on."
In a worst-case scenario, the starboard SARJ could be locked in a position to maximize solar energy production while engineers assess various options for future troubleshooting. Lead station flight director Derek Hassmann said Friday the station could operate with one stationary set of solar arrays without any major problems.
"One of the concerns we have about the way the SARJ is behaving is the potential it might stall in a position that's not optimal for power," he said. "So one of the things we have been talking about ... is to find a current (power) value at which we're going to stop rotating the SARJ and put it in a position that's good for power consumption. We've basically got constraints to find that will allow us to park the SARJ in a good power producing attitude before it would stall.
"As long as we can get it into an attitude that's reasonably good for power generation, combined with what the other SARJ can produce, we wouldn't have any significant power impacts that we couldn't deal with."
Tani was asked to collect samples of the shavings to help engineers troubleshoot the problem. His camera, meanwhile, malfunction and prevented him from immediately completing the desired photo documentation.