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Case Study #1 — Fastener Failure Workshop

Water Ride
 

Industry:

Amusement – Large Park
Peaceful Water Ride Attraction – On Tracks

Problem:

Bolts holding the wheels onto the axles were breaking at the head-to-shank juncture. The wheels would fall off, allowing the ride to float free. Some riders needed to be rescued by a boat and divers.

Injuries:

Undisclosed.

Damages:

Undisclosed.

Observations:

The bolts that broke had head markings indicating that they were made of 300 series stainless steel. A collection of broken bolt heads was found under the water at a drain location. This showed the breakage had gone on for some time. A visual check of fracture surfaces, using a hand-held magnifier, seemed to indicate a mix of fatigue and fatigue to shear failures. Removed from water, a check of the ride's wheel attachment showed six bolts going through the wheel hub into the axle. There was no apparent interference under the bolt head, and of particular interest, the axle was devoid of a suspension.

Investigation:

Independent lab tests confirmed visual observations as to the failure mode. The fasteners were normal as to chemical, physical, and dimensional properties. Review of the ride's drawings and subsequent discussions with staff engineers confirmed the absence of a shock-absorbing suspension for the axle.

Conclusion:

The forces acting on the head-to-shank juncture of the bolts were unreasonable and excessive, as the bolt heads had to absorb the force and shock of all movement cause by passengers entering and exiting the ride. These actions were compounded by all movements to the ride caused by the force of water in contact with it.

Recommendations:

1. An immediate temporary fix was to install high-tensile, heat-treated, 400 series stainless steel bolts in place of the softer 300 series items.

2. The follow-up long term solution was to design a suspension into the ride. The suspension would absorb the forces and the bolt heads would be left unaffected.

 

As appeared in Fastener Technology International magazine.

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