| Abstract | A high unit-load, relatively poor lubrication, and a harsh environment characterize operating conditions for traction motor bearings. Despite proper maintenance and condition monitoring, their failures are too frequent and usually result from failure of the lubricating system. Improving safe operation of traction motor journal bearings, and thus train safety, was the reason for this project. The hope was that a composite bearing material could not only perform well when lubricated, but also allow for a certain period of safe operation in a case of lack of normal lubricant flow in the bearing gap.
Contemporary composite materials usually consist of thermoplastic matrix and solid lubricant with a fiber providing creep resistance and strength. Evaluation of a selection of composite bearing materials was carried out on a Falex block-on-ring wear tester, in accordance with ASTM Standard G77 (Standard Practice for Ranking Resistance of Materials to Sliding Wear Using Block-on-Ring Wear Test). Out of the six materials tested, Vespel® SP214X, a polyimide resin filled with graphite, and/or PTFE, and/or molybdenum disulfide, was selected for the full-scale experimental investigation. This material is made by DuPont de Nemours.
The experimental investigation of the bearing was carried out on a test rig developed by National Research Council Canada. The rig accommodates a full-size traction motor bearing, has a maximum load capacity of 41,000 lb., and can run at speeds of up to 600 rpm. The instrumentation included 14 thermocouples monitoring the temperature of the tested bearing, eddy current proximity probes, and a force transducer recording the changes to the friction torque. As a reference for the Vespel®-lined bearing, data were collected from a babbitt-lined bearing, which was tested for a full range of operating conditions. These tests also validated the design of the test facility.
Initially, the new traction motor bearings were fabricated using Vespel® in the form of 1.25 in. wide rings. The last tested bearing was made from one cylindrical piece of Vespel®. The process of bearing fabrication included an elaborate procedure for bonding the composite material to the brass substrate.
The tests on the Vespel®-lined bearings were unsuccessful, even in the presence of lubricant. The maximum load achieved was 15,000 lb. at a speed of 106 rpm. All of the tested bearings experienced severe friction-induced vibration and damage to the bearing surface, and eventually seized on the shaft. Changes to the bonding procedure and bearing clearance did not improve the situation. In total, four bearings were tested.
It was concluded that the most likely cause for the bearing failures was that the material’s thermal properties were insufficient to transfer the large amount of heat generated in the bearing. The resulting decrease in oil viscosity and thermal growth of the bearing led to the bearing seizure. |
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