Just 1000 hours of use in 35 years of service? How can anyone justify that kind of mobile equipment utilisation rate? NASA can.
A thousand hours in over 35 years is what they have logged for their two space-shuttle crawler transporters. And unlike construction contractors, NASA is less concerned with utilisation rates than with reliability. One concept NASA shares with contractors is that preventive maintenance is a wise investment. So despite the low hours on their crawlers, NASA has had them rebuilt. Denison and its distributor, Hydradyne, helped.
Working on a crawler is no easy feat. Each crawler is 40 metres wide, 35 metres long, close on 8 metres high, and is driven by 16 electric traction motors.
The motors are powered by four 1 MW generators which are driven by two 2,05 MW diesel engines. Two 750 kW generators powered by two 800 kW diesel engines are used for jacking, steering, lighting and ventilating. Maximum travel speed is 3,2 km/h when the crawler is unladen. With a fully fuelled shuttle atop the mobile launch platform, a load of over 6000 tons, maximum travel speed is cut in half. The crawler rarely achieves its maximum rated speed, however, so the two-mile trip from the Vehicle Assembly Building to the launch pad takes about five hours. During the trip, the orbiter must remain within plus or minus 10° of arc (the tip can sway no more than the diameter of a basketball). This tolerance must be maintained even while climbing the 5% grade to the launch pad surface.
When the crawlers were built in 1966, Denison provided components for the steering and jacking systems: four PV08 and eight PXC-1223 pumps per crawler. Those pumps were recently rebuilt by the Tampa branch of Hydradyne Hydraulics, Denison's largest distributor.
"They are large displacement, pressure-compensated piston pumps. We replaced most of the moving parts," says Bob Nasca, general manager of the branch office. "These pumps were among the units routinely supplied by Denison to the navy for shipboard use. They had a set of electronic drivers, which was pretty high tech 35 years ago." Rebuilding these pumps is a daunting task. "The PXC-1223 was made obsolete 30 years ago," says Sam Fujii, applications manager at Denison. "We dug down until we found old drawings, then we worked with some of our vendors to come up with replacement parts. The barrel assemblies were forged in Europe and had initial machining done there, with final machine work done here in the States."
Why not just replace the obsolete pumps with a current model?
"We looked at that," says Nasca. "But the cost of replacement was about double the cost of repair." Some of the difference was the cost of the parts, but some was due to increased labour. "The pumps are hard welded to the system," said Nasca, "so you would have to replace not only the pump, but also a good portion of the welded pipe. Flushing the system after all that welding and cutting would be a problem, too."
One advantage of using updated product would be an easier rebuild next time. "There will not be a next time," says Nasca. "By the time these would be due for rebuilding again, the Shuttle program will be pretty much over. NASA will have new vehicles and new launch methods."
For more information contact Walter Heijboer, Parker Hannifin, +27 (0) 11 961 0700, [email protected]
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