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Case Study #5 — Fastener Failure Workshop

Heavy Mining
 

Industry:

Heavy Mining – Limestone Quarry
Vehicle: Huge front-end loader,

bucket capacity of 50 tons (45,360kg)

Problem:

Special hex cap screws, minimum 180,000 psi (1200MPa) tensile strength, were used as wheel-to-axle mounting bolts. Their heads were randomly breaking off during mining production. The operator, due to the high noise levels and the rugged terrain, didn't notice anything until the vehicle fell over when one of its wheels, more than 8' (2.4m) diameter, fell off.

Injuries:

Minor cuts and bruises to the operator.

Damages:

Axle destroyed. Cost: US$110,000, plus labor and parts.

Observations:

Most rear axle fasteners on both sides of the vehicle were broken. All failed bolts broke at head-to-body juncture — the fillet radius. With a magnifier, the fracture surfaces appeared fatigue related.

Investigation:

The quarry had a number of front-end loaders of this type and size with no history of fasteners breaking like this. The maintenance log showed that the rear axle bolts had been replaced on the subject vehicle days earlier. It was also found that an aftermarket bolt was substituted for the OEM. These new bolts were used with the OEM flat washer under the head. Visual observation and field measurements showed samples of the OEM bolts had a very small fillet radius — like an aircraft fastener. The OEM flat washer had a correspondingly small ID to match, while the aftermarket bolt had a standard fillet radius. There was clearly a mismatch of a small hole size in the washer, which would interfere with the aftermarket bolt's fillet radius.

Conclusion:

While a flat washer with normal clearance would have likely worked, the inside edge of the OEM flat washer scored the fillet radius under the head. The score lines then became stress risers that began cracking. This caused the heads to break off, resulting in the damage.

Recommendations:

A flat washer should provide a uniform flat bearing surface, prevent embedment, and should spread bolt loads over a greater surface area. It should also protect a bolt's critical fillet radius from damage. It is very important to select flat washers that are truly compatible with bolts put into applications.

 

As appeared in Fastener Technology International magazine.

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