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. |
