Spacewalker inspects station's other rotary joint
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: June 5, 2008; Updated after status briefing
Toward the end of today's Kibo outfitting spacewalk, astronaut Michael Fossum removed a thermal cover over a section of the space station's left side solar alpha rotary joint for a quick inspection of its drive gear and bearing surfaces. He reported what looked like streaks of built-up grease but no signs of the sort of metallic filings and surface damage that have forced NASA to stop normal use of the station's right-side SARJ.
The port SARJ has been operating normally and Flight Director Annette Hasbrook said engineers believe the drive gear is in good condition. The grease, she said, may have originated with one or more of the 12 trundle bearing assemblies that hold the gear and allow it to smoothly rotate. A similar grease build up was spotted the last time the joint was inspected last year.
"What they saw, the trundle bearing as it's riding may not ride completely flat, there may be a little bias so you could see some, it's not grooving, but in a sense streaking, or wearing of material. And then the rest of it, it looked like a grease that was potentially on the bearing surface.
"They're pretty sure these trundle bearings have leaked a little bit of grease and that could be one of the reasons that we're not seeing any of the wearing on this assembly. It could be a lubrication factor. It could be something that's helping the rolling mechanism so you don't get any friction build up and you're not seeing any degradation."
But all in all, she said, the port SARJ appears to be in good shape.
The two SARJ mechanisms are critical to space station operations. They feature 10-foot-wide motor-driven gears gripped by powerful roller bearings to turn outboard solar arrays like giant paddle wheels to track the sun and maximize power generation.
Last summer, engineers noticed the right-side SARJ drive motor was working harder than expected. That, coupled with high vibration levels, led engineers to suspect some sort of interference or friction in the mechanism. An inspection during a subsequent spacewalk revealed extensive metallic shavings on the drive gear and bearing surface degradation.
Extensive troubleshooting has not yet been able to pin down what might be causing the damage. But engineers suspect a small crack or defect in one bearing surface might have resulted in cracks or debris that, as it ground through the bearings, caused further damage. Engineers want to make sure no similar process gets started on the left-side SARJ or the right-side SARJ's backup drive gear.
During a spacewalk Tuesday, Fossum experimented with scrapers, grease and towels to help engineers determine the best way for future crews to clean up the metal shavings and permit normal, or near-normal operation for as long as possible. Each SARJ is equipped with a backup drive gear and while many engineers think they will be forced to switch to the other, undamaged right-side gear sooner or later, they would like to get as much mileage as possible out of the damaged gear before taking any last-resort steps.
During today's spacewalk, Fossum was asked to take a quick look at the port SARJ gear just to make sure nothing unusual was going on there. His initial report caught everyone's attention but it later became apparent what he was seeing was similar to what was observed the last time the SARJ was inspected in 2007.
"OK. I'm looking at the outer ring, datum-A, and I see two features, which unfortunately look a whole lot like what we had on the other side," Fossum radioed, peering into the SARJ mechanism. "The overall condition, I do not see filings. There is one line in the datum A (bearing surface) that appears to be a drag line. It's about a quarter inch, three eighths of an inch in from the inside edge. There's a definite wear line at that location. It looks to me just like other damage I've seen to bearings the brakes and such. It's not totally uniform, there are some striations in it. There's some marks, and this around the, you can see the wear... can you see my WVS (helmet camera video) at all?"
"I can see something that looks like a groove," pilot Kenneth Ham replied from inside the shuttle Discovery.
"Yeah, there' a groove on the inside edge."
"Yes, on the inside edge, I can see that," Ham agreed.
"OK. There's also some features right under my finger," Fossum said. "Do you see my finger?"
"Right under that. And I can't tell if it's (raised) up or (depressed) down. It looks to me like it's a deposit. And as I get along the edge... yes, it's a deposit. It's not a divot. It's on the surface."
"OK, now is that in the path of where the bearing goes?" Ham asked. "Or is the bearing inside of that?"
"It's right on the edge," Fossum replied. "You can see the wear line where the bearing goes and it's right on the outside edge of that wear line."
"OK, so the bearing does not go over that."
"That's affirmative. That's affirmative," Fossum said. "And the other one is on the inside edge. They almost look alike, I mean it almost even looks like some grease where more of it got squished to the inside and just a few bits kind of gummed up and rolled out on the outside."
"OK, a little bit earlier you said it looked like a drag line," Ham said. "Now you're thinking it's possible it might be a, like grease piled up there?"
"There are features that run along the ring that you can see," Fossum said. "It's a dragging mechanism of some kind, it appears to me. You can see lines in it, it's not a uniform gray line."
A few minutes later, he reassured flight controllers, saying "again, I do not see any signs of metal shavings or any of the other kind of stuff that we had all over under the covers on the other side. This is a lot cleaner."
"It appears there's none of the kind of surface damage we had on the starboard SARJ?" Ham asked.
"No, there's not," Fossum replied. "There's not. There are a few lines that almost, again, almost look like grease or someting that's kind of squished out to the outside edge."
Hasbrook said engineers will study photographs of the area inspected by Fossum, but "it looked very similar to what was seen (last year)."
"We believe we got lubrication on that bearing from the TBA. They didn't sit there and wipe those rings with grease pre launch," she said. "So are the TBAs leaking? I don't know that they're actively leaking, but we believe the grease that we're seeing is coming from the TBAs. On the starboard SARJ, we have not seen that at all."