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Case Study #4 — Fastener Failure Workshop

Coaster Ride
 

Industry:

Amusement – Large Park
Coaster Ride Attraction (in a Rugged Setting)

Problem:

Hex socket shoulder screws, chosen as link pins between cars, were losing their heads. The remaining portions would then fall through the holes, causing the cars to separate. Ultimately, some of the cars would jump the track and then crash while carrying occupants.

Injuries:

Undisclosed.

Damages:

Undisclosed.

Observations:

Aided by a hand-held magnifier, the fracture surfaces of the failed products appeared to be clearly fatigue related. We were told that this ride had run for years and never had a failure of this type. We needed to find out what was different.

Investigation:

Independent lab tests confirmed the visual observation as to the failure mode. An inspection of the park's stock fasteners showed a mixture of two different manufacturers' products. A microscopic examination of the items showed that while both technically met the specifications, they were made with two distinctly different manufacturing methods. The undercut area under the head was machined in one case, and centerless ground in the other. Ironically, under separate orders, both types were supplied by the manufacturer who used the ground method (they had no stock and bought some from another company). All failures were from the other manufacturer that used the machining method.

Conclusion:

The machining process left small score lines in the area under the head. The score lines were stress risers that led to cracking under the violent forces of the ride. The cracks propagated until breakage, and the body of the screw dropped through.

Recommendations:

1. With magnifier, the bin stock was sorted to use just the ground items.

2. The longer-term solution was the elimination of this type of fastener with built-in weaknesses. A stronger pin was suggested with safety features.

 

Note: In such applications, redundant backup systems must be employed for security.

 

As appeared in Fastener Technology International magazine.

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