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Case Study #24 — Fastener Failure Workshop

Baggage Conveyors
 

Industry:

Commercial Aviation
Baggage Handling Conveyors

Problem:

A luggage conveyor at a busy international airport stopped working while it was fully loaded with bags. Everything had been working fine up to that moment.

Injuries:

None.

Damages:

None, but the customers had a tough time getting their bags and this incident would temporarily put the carousel out of service.

Observations:

After routine electrical problems were ruled out, the large drive motor was sent to the manufacturer for repairs. At the plant the motor was disassembled. It was soon discovered that broken fasteners prevented the motor from working correctly. The fasteners breaking were M6 x 45mm (1.8") socket cap screws, Property Class 12.9. These were being used in a circular pattern of eight equally spaced screws in what is called the drive drum. The remains of all screws were still within the tapped holes.

Investigation:

We began with a careful visual observation under a stereomicroscope. This showed that the fractured surfaces were clearly fatigued in nature, with the accompanying striations (beach marks). The screws broke about 6 to 8 mm (0.236" to 0.315") from the end, and all screws had a nearly identical fracture appearance. The point end of the screws were immobilized with what looked like a chemical locking product. The hardened residue was apparent. At the end of the threads on the long section of the screws, the major diameter appeared slightly necked down. We placed some of the pieces on a video measuring machine at a medium high magnification. This showed that distention clearly was present. A study of the thread engagement indicated there were barely enough threads to prevent stripping. And, a discussion with the installers on the assembly line showed that they didn't have a good understanding of torque or bolt tension. They put the chemical on so vibration wouldn't loosen the screw, but had no idea what frictional properties the chemical had on the assembly. However, they did point out the 45mm (1.8") length screw was "just temporary until the new screws came in," as a 50mm (2") length screw was normally used.

Conclusion:

In a bolted connection there is a balance between the force created by tightening the fastener (clampforce) and the force that the machine will put out. Generally, when the clampforce is greater than the force made by the machine, the bolt lives. If we assume that a good design practice was used on this motor, then the torque was unreasonable for the screw's strength with the chemical locking product. Yes, they would have been better off with the longer screws, but that didn't cause the failures. The fact that the screw had "necked out" slightly reduced the stress area, and thus made an imbalance. The machine force now was greater than that provided by the screw, therefore an opportunity for a fatigue failure.

Recommendations:

While a tensile-to-fatigue failure is relatively rare, you can expect any number of things to go wrong if each aspect of the joint and assembly aren't carefully understood, and successfully addressed. It pays to take the time to do so.

 

As appeared in Fastener Technology International magazine.

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