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. |
