No obvious problems found with beta gimbal assembly
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: December 18, 2007
Five hours into a planned 6.5-hour spacewalk, engineers have not yet spotted a smoking gun that might explain what is causing problems with a critical solar array rotary joint on the international space station. But it appears that metallic shavings contaminating the mechanism's main gear are slightly more concentrated near two bearings on either side of a drive motor. Whether that observation by spacewalkers Peggy Whitson and Dan Tani is significant or not remains to be seen.
Based on vibration data, engineers suspected possible problems with trundle bearing assembly No. 5, one of 12 bearings mounted around the 10-foot-wide solar alpha rotary joint race ring. The SARJ features two drive motors, called drive lock assemblies, and motor No. 2 is sandwiched between bearing assemblies 4 and 5.
As it turned out, the highest concentration of metallic debris was found near bearings 4 and 5, which might mean some sort of problem with the load one or both assemblies bring to bear on the race ring. That remains to be seen, but Whitson and Tani plan to bring one of the bearings back into the station for eventual return to Earth.
Working around the perimeter of the 10-foot-wide race ring, Whitson and Tani reported widespread damage to one surface of the ring similar to what was noticed during earlier inspections. The damage presumably was caused by the same process that produced the metal shavings seen throughout the joint. Using adhesive tape to collect samples, Tani reported the material was fine grained and powdery.
"When I went out with the tape, the tape of coures adhered to only what it came directly in contact with and the rest is sort of like a pile of flour," he said. "It just sort of disintegrated. I don't know if that was your experience..."
"It seems like it breaks up," Whitson agreed. "The debris breaks up really easy and becomes this powdery gray stuff."
"A little clump of it floated away and I tried to capture it with my tape, sort of let it float into the tape thinking I might get the whole clump," Tani reported. "And again, when it hit the tape it just disintegrated like it had almost no attractive force to itself, you know. It's very powdery debris, very, very fine except for the little flakes that we get. I got the surface fairly clean, but I probably only captured half of it, not even half, on the tape. But the tape is saturated. It won't take any more."
"Dan, that was an excellent description," astronaut Tom Marshburn radioed from mission control. "Sounds like some of the particles have some substance to them and the others are very small particle size, basically just fine dust."
"Fine dust that's ferrous, which is held by the only force out here, which is magnetic force," Tani said.
The bearing race/gear has three sides that are gripped by 12 trundle bearing assemblies positioned around the circumference. One surface is mottled, clearly damaged by some sort of metal-on-metal interference. But Tani reported the inner side of the race was clean and undamaged.
"I've got a good view here," he said at one point. "Nice, shiny surface, no mottling, there's some debris on the trundle bearing assembly but the surface itself, from the reflection I can see here, is clean."
"OK, let me make sure I've got that right," Marshburn said. "The outer race inner surface is clean."
"It's clean, it's not damaged," Tani confirmed.
"OK, that's highly significant."
Whitson, meanwhile, set a new record at 8:27 a.m. today, accumulating more spacewalk time over five spacewalks - 29 hours and 17 minutes - than any other female astronaut. The old record was set by astronaut Sunita WIlliams earlier this year.
"Congratulations, there is no pressure now because you are the queen of EVA," Marshburn radioed. "You are the world record holder in EVA time."
"Thanks," Whitson replied. "Just being in the right place at the right time."
"You know, that's the same thing Suni said."
"Glad I'm here," Whitson said.