One of two crawler-transporters at Kennedy Space Center. Photo: NASA

Each of the crawlers uses 16 massive 20-inch-wide JEL cylinders to lift a shuttle and its mobile launch platform and to keep the 12-million-pound load level while climbing the 5-degree incline to the surface of the shuttle's launch pad. Each JEL cylinder includes two large bearings, one at the top and one at the bottom.

During recent work to refurbish two JEL cylinders on one of the crawlers, engineers were surprised to discover severely cracked bearings. Additional inspections were ordered and so far, nearly half of the bearings checked show signs of cracks. The concern is that an outright bearing failure on a fully loaded crawler would be extremely difficult to repair.

It appears likely at least some of the bearing damage occurred years ago and it's possible the crawlers can safely operate "as is." An engineering analysis is underway. But sources say NASA does not have enough spare parts to immediately replace all of the cracked bearings if repairs ultimately are ordered. In that case, the shuttle program almost certainly would face a launch delay. But the full impact of such a worst-case scenario is not yet known and NASA managers are hopeful it won't come to that.

The Apollo-era crawlers are used to move the space shuttles to the Kennedy Space Center launch pads. Photo: NASA

The crawler-transporters are impressive machines, built on site in the mid 1960s to move Saturn 5 moon rockets from the VAB to the launch pad. Each crawler measures 131 feet long and 113 feet wide. They weigh 5.5 million pounds each and can travel at a top speed of 2 mph unloaded (1 mph loaded) using 16 375-horsepower electric traction motors powered by four disel-driven 1,000-kilowatt generators.

Each crawler features four "trucks," each one with two cleated belts made up of 57 "shoes" per belt. The shoes weigh one ton each. Each truck, in turn, is equipped with four traction motors. The JEL cylinders use hydraulic power, provided by eight pumps, to lift a shuttle and its 8.8-million-pound mobile launch platform for the trip form the VAB to the launch pad. The cylinders can be extended about six feet.