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Case Study #15 — Fastener Failure Workshop

Elevated Tram Ride
 

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.

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