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. |
