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Case Study #13 — Fastener Failure Workshop

Car Carrier
 

Industry:

Vehicle Transportation
Car Carrier Truck

Problem:

The fasteners bolting on the chains that pull and secure cars into their loaded positions on the truck were breaking. When failure occurred, subsequent damage took place.

Injuries:

None.

Damages:

Eight trucks reported damage to the vehicles transported because of the fastener failures. In addition, while it was possible to have a bolt/chain failure that could have allowed a car to slide out onto the pavement in front of traffic, it fortunately didn't happen. Ten cars did sustain damage to their bodies and windows, resulting in a lot of money paid out in repairs; as a result, there were ten people who didn't receive delivery of their cars as they had expected.

Observations:

Hex flange head bolts were used to clamp the chains to the shafts. These bolts passed through the first link of the chain, and through the hole in the shaft. Hex flange nuts were tightened onto the bolts on the opposite side of the shaft. There were no flat washers used in this application, and as such, the contact area with the chain was minimal. The chain wound around the shaft with a hook at the other end. The shafts rotate via motors, and the hooks are attached to a car's undercarriage, holding the vehicles in place; that is, until the bolts broke.

Investigation:

Two hex flange head bolts were marked with the six equally spaced radial lines that were consistent with the SAE Grade 8 or ASTM A 354 Grade BD. There were no manufacturer's identification marks. Locking serrations were on the bolt's flange, and extended from near the fillet radius to nearly the edge of the head. Although the bolts were long enough that they would normally have a full body with partial threads, they were fully threaded. In addition, the failure point of the bolts was at the head, in the fillet radius area. The flange nuts also had locking serrations; however, they did not have grade identification markings. Both fastener products appeared to be zinc plated. Further, it was learned that these fasteners were newly purchased products that represented cost savings to the company, and that they had just recently been installed. A laboratory evaluation of the fasteners was conducted. It was found that the nuts were made of 1018 steel, and that the bolts were made of 1022 carbon steel; not an alloy material as Grade 8s would require. Further, and surprisingly, both the bolts and the nuts were case hardened!

Conclusion:

The small contact areas and the high surface hardness allowed these bolts to be easily cracked to failure, and without manufacturer's identification marks, there was no traceability.

Recommendations:

Replace the fasteners with the factory-developed method that used real alloy fasteners, through hardened, and with thick through hardened flat washers supporting the loads.

 

As appeared in Fastener Technology International magazine.

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