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Case Study #7 — Fastener Failure Workshop

Train / Traction Motor
 

Industry:

Locomotive Traction Motor

Problem:

The fasteners bolting on the motor's lubrication system were coming loose. This caused failures to the main shafts and bearings, and subsequent train derailments.

Injuries:

Some minor injuries to railroad crew members reported.

Damages:

Seven trains within six weeks derailed before calling for assistance. Track was twisted, locomotive engines and numerous cars were damaged. No estimates were provided.

Observations:

None of the traction motors were new. Only a few of the fasteners that failed to hold were recovered, and they were found unbroken inside the traction motors.

Investigation:

Lubrication of the main bearing in the traction motor is via a simple device called a wick plate. A fibrous pad transfers lubricant to the bearing by the process of wicking. The pad is attached to a plate bolted to the top of the traction motor. This wick plate assembly is attached by wick plate bolts. The plate covers an access hole in top of the motor about 6' x 12' (150mm x 300mm). There are four 1/4" (6mm) hex bolts (one at each corner) attaching the plate. A gasket seals between wick plate and traction motor case. With the wick plate bolted tight, the lubrication system works fine. If the plate loosens and comes off, as happened here, the bearing runs dry and seizes, causing the drive wheels to lock up and the train to derail. The trains that wrecked had four things in common: all carried the heaviest loads; all had traveled on the roughest terrain in their track system; all had been in service for about the same time (several years); and when we checked the tapped holes in the traction motor cases used for the wick plate, the no-go thread gage fit was extremely loose.

Conclusion:

The traction motors don't have a suspension and are jarred by every rough section of the track. The vibrations wore out the softer cast iron threads, allowing bolts to come loose and fall out, causing big problems.

Recommendations:

The railroad preferred not to invest in the thread inserts to strengthen the tapped holes; therefore, special fasteners were made up with a heavy nylon patch and cross-drilled holes through the heads to allow safety wiring. In addition, the soft gaskets were replaced with a thinner, harder type to allow more preload to be accomplished upon bolt installation.

 

As appeared in Fastener Technology International magazine.

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