On March 10, 1997, at 1321 central standard time, a Boeing 727-2K5, N909PG, registered to Pegasus at San Francisco, California, operated by Aeromexpress as a Title 14 CFR Part 129 on demand cargo flight, experienced the separation of the number 3 main landing gear wheel during the takeoff at Dallas/Fort Worth International Airport, DFW Airport, Texas. Visual meteorological conditions prevailed and an instrument flight plan was filed. The 4 crew members were not injured and the airplane sustained minor damage. Air Traffic Control personnel reported that the flight departed runway 35R with a planned destination of Mexico City, Mexico. During the departure climb, ATC personnel informed the crew that a wheel assembly had departed the aircraft. The crew reported to ATC that all cockpit indications were normal and they would return to the DFW Airport for landing. The flight landed on runway 35L without further incident. Aeromexpress has operated the aircraft since July 1994, under an approved foreign air carrier aircraft maintenance program for 6,783:30 hours with 2,709 cycles. The last installation of the wheel and brake was accomplished on February 10, 1997. Total aircraft time was 40,423.07 hours with 17,036 cycles at the time of the wheel separation. Inspection of the wheel by the FAA inspector and the investigator-in-charge revealed that the outer bearing race, retainer ring, and axle were damaged and the inner ring of the outer wheel bearing exhibited deformation, rubbing, cracking, and galling. None of the bearing rollers were recovered. Metallurgical examination at Boeing disclosed that the inner wheel bearing of the right hand inboard main landing gear wheel assembly migrated approximately 1.5 inches in the inboard direction. The outer bearing ring showed rub damage on the circumference of the inner and outer diameter and the bearing cage. The bolt that locked the wheel retaining nut was fractured; however, it could not be determined if the fracture of the bolt contributed to or was the result of the bearing damage. The inner ring of the outer wheel bearing displayed heat damage and localized melting. The damage of the inner ring "appeared to be more consistent with bearing deterioration caused by inadequate or loss of preloading rather than a bearing seizure event." See the enclosed report for details of the examination.