Maintenance of treadbelt shoes for the shuttle transporter

摘要

The vehicle that transports the space shuttle to the launch pad has 8 tracks, each consisting of 57 shoes. Each shoe is made of AISI 86B30 cast steel and weighs about 0.9 metric ton. So far, twelve shoes from a batch of 228 had a premature failure by cracking at a service mileage of less than 700 miles. In contrast, the shoes of the other batches, with service mileage in excess of 1000 miles, fail primarily by the wear of the pin connection areas. Since these areas can be repaired by weld overlay, cracking is considered especially damaging. A statistical analysis based upon the Weibull distribution indicated an early wear-out of the subject batch. A subsequent investigation revealed that the subject batch had higher hardness and lower fracture toughness than the other batches and that cracks were initiated from the same area. It was also found that the cracking process involved significant plastic deformation and the crack mouth opening was substantial. Since the cracking process was not sudden and brittle, it was concluded that reliability of the vehicle would not be compromised by continued use of the subject batch; cracked shoes could be found and removed before a complete failure occurred. A continuous monitoring routine was established and refurbishment of the worn-out shoes was expedited. Presently, less than 60 shoes from the subject batch remain in service. They are mounted in one track for ease of inspection and maintenance. Failure to provide transportation services for the shuttle launch operation has been successfully prevented by a continuous maintenance plan based upon simple visual and boroscope inspection.