On November 18, 1994, at 0706 central standard time, a Boeing 737-200, N988UA, sustained minor damage during takeoff from Springfield, Missouri. The crew of five and 52 passengers were not injured. Instrument meteorological conditions prevailed for United Airlines Flight 1487, scheduled Title 14 CFR Part 121 service to Denver, Colorado. During interviews and on the enclosed Pilot/Operator report, the information in this paragraph was reported. The crew heard a "slight scraping sound" during the rotation from runway 02. A witness reported to the Springfield Air Traffic Control Tower (ATCT) that airplane debris was found on the runway. The parts were identified as brake assembly components from a Boeing 737 and Springfield ATCT notified the flight crew. The crew reported that all cockpit indications were normal and that they would continue the flight to the destination. Upon arrival at Denver, the tower confirmed the left outboard main wheel was missing. The crew was cleared for a landing on runway 35R and taxied to the ramp without further incident. The airplane components were visually examined on November 23, 1994, at Denver, Colorado, by company maintenance personnel (enclosed statement), a Federal Aviation Administration (FAA) inspector, and a NTSB investigator (enclosed statement). The axle nut retaining ring and nut were not available. Threads on the outer spindle were damaged and "appeared to be rolled outboard." All components were forwarded to the United Airlines facility at San Francisco, California, for further examination. On January 7, 1995, at San Francisco, California, the components (enclosed report) were reexamined at the company engineering facility by company engineers, metallurgists, a FAA inspector, and a NTSB investigator. Six recovered rollers had "severely damaged rolling surfaces; however, all the roller ends were relatively free of scuffing damage, which indicated adequate lubrication and bearing axle preload." There was no indication in or around the keyway of "reportable defect or damage pre-existing the failure event." The left hand axle outboard "end had severe resultant damage consistent with a primary outboard wheel bearing failure." On February 17, 1995, company personnel (statement enclosed) reported that the axle nut lock washer and lock ring were never recovered; however, there is physical evidence "that they were installed at the time of the incident." They further stated that the "primary cause of this incident was a failure of either the inboard or outboard wheel bearing assembly."