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. |
