Industry:
Elevated Train Ride
Luxury Resort Hotel
Problem:
The fasteners securing the wheels to the axles on the tram were
coming
loose. Three of the wheels fell off. Two of them fell into the lobby of
the hotel, barely missing people in the area. This was from the fourth
floor, where the tram ran through the building. The cars screeched loudly
to a stop, with sparks flying wildly.
Injuries:
No one was physically hurt; however, the trauma of the near misses,
the
spectacle created by the sparks and the noise from the tram, coupled with
the screaming hotel guests, ruined a number of vacations.
Damages:
There was negligible damage to the tram and the tracks, as the operator
brought the vehicle to a stop quickly. The wheels bouncing
in the lobby
took out chunks of the floor, and went on to damage tables, lamps, and
a large glass display case.
Observations:
The fastener application holding on the wheels couldn't
be simpler.
As a part of the suspension for the cars, the axle shafts had an all-metal
locknut holding on each wheel. They appeared to be zinc plated with three
locking deflections equally spaced at the top of the nuts. On the wheels
still in place, there was approximately 1/2" (12mm) of threaded
axle protruding through the nuts. The nuts that came off the axles were
found on the grounds outside the hotel, as well as one that fell into
the lobby. The nuts looked shiny and new.
Investigation:
The maintenance department said these nuts had just recently been
installed
as part of a refurbishing program. They said that these nuts looked just
like nuts they'd used before, but that they were purchased through
a new source that offered them a cost savings. Next, an independent laboratory
evaluation was conducted. The thread size of the nuts
was 1"–14"
and from the back entrance of the nut they gauged properly. The dimensions
and hardness conformed to Industrial Fasteners Institute (IFI) 100/107
for a Grade C 1" hex locknut. The next test was proof load. While
the nuts passed the Grade C specifications for this test, an interesting
discovery was made. The nuts screwed onto the test mandrels with almost
no resistance. When the nuts were installed onto the test bolts for the
purpose of checking the prevailing torque, the First Removal showed 15
in-lb (1.7N•m) maximum. This was less than a fifth of the minimum
required value. In other words, the locknuts did not lock.
Conclusion:
The nuts did not meet industry specifications. Consequently, they
loosened
and the wheels fell off. They apparently were made by a source lacking
process controls and lab testing.
Recommendations:
Buyer beware! Cost saving opportunities need to be supported with
proven
product performance. Detailed purchase specifications are essential, and
so is proof of conformance. In this case, maintenance could have ascertained
the locking ability by screwing a nut onto a bolt clamped
in a vice. With
a good quality source, this isn't necessary. Additionally, for safety,
a secondary security method should have been used, such as a cotter pin
outside of the nut.
As appeared in Fastener Technology
International magazine. |
