Industry:
Amusement Park
360° Looping Ship Ride
Problem:
During the first two weeks of assembling a large new ride purchased
overseas,
large diameter bolts were breaking one to three days after they had been
installed. Workers were nearly hit by a number of bolts that were shooting
from the ride.
Injuries:
None.
Damages:
While there were no damages to the ride, it was time consuming to
replace
the broken bolts. Some of the replacement bolts also broke.
Observations:
This ride is essentially a large boat with seating for forty people.
It
is connected via steel structural members to a central pivot point, much
like a very large swing set. Directly opposite the "ship,"
above the pivot point, there is a connecting metal structure with a massive
counterweight attached. When the ride starts, the ship is rocked back
and forth in increasing amounts until the ship makes complete revolutions.
The bolts that were failing held the counterweight to the structural frame.
The
bolts are 1 1/4" x 10", SAE Grade 5, and
plain finished.
Flat washers were used under the bolt heads and under
the matching strength
structural hex nuts. The fracture surfaces had a craggy,
rock candy appearance.
Investigation:
All of the fasteners were shipped with the ride from the ride's
manufacturer, and it came complete with spares. All
of the bolts were
made by the same company. The method of installation was evaluated and
found to induce normal stress levels.
An independent laboratory viewed the fracture
surface with a scanning
electron microscope and assessed intergranular cracking,
as is seen with
hydrogen embrittlement. Four bolts were put into stress
durability testing
per MIL-1312-5. Three of the four lost their heads before
the 72 hours
were up. Wedge tensile tests were passed in a normal
manner. However, the
most revealing test was core hardness. The readings were
very low; in
fact, when Rockwell hardness readings were made directly
in the center
of the bolt, there were so soft it was as if the bolts
were never heat
treated at all.
Conclusion:
Although the products weren't electroplated, these were
hydrogen
embrittlement failures. In the processing of steel, a pickling operation
in sulfuric acid is normal, but it creates atomic hydrogen. However, these
fasteners weren't heat treated correctly. With partial hardening,
residual hydrogen was still inside. Once the products were stressed in the
application through tightening, delayed fracture embrittlement failures
occurred.
Recommendations:
Replace the fasteners with products of known good quality, and protect
them from the elements with a type of safe plating. If
these were left
plain, they could have resulted in a future Stress Corrosion Cracking
(SCC) failure, and another disaster.
As appeared in Fastener Technology
International magazine. |
