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Case Study #19 — Fastener Failure Workshop

Looping Ship Ride
 

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.

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