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Synthetic Textiles
 

Case Study #26 — Fastener Failure Workshop

Synthetic Textiles
 

Industry:

Synthetic Textiles
Polyester Fiber Processing

Problem:

Fasteners holding drums that stretch the fibers to gain strength were breaking. When all fasteners holding a drum broke, the drum would be thrown to the floor and production would be stopped.

Injuries:

None, but this is because they've had very good luck.

Damages:

The exterior surfaces of the drums are highly polished. When a drum is thrown onto the concrete floor they sustain significant surface damage whereby they then must be repaired or replaced. When notified of the problem, more than one drum per week was coming off at a replacement cost of more than US$10,000 each.

Observations:

The breaking fasteners were 1 1/4" - 12 x 6 1/4" socket cap screws. Failures were occurring at the head to body juncture and in the threads. Wear marks were directly adjacent to the fillet radius.

Investigation:

There are 23 drums on each processing machine. They're positioned in a zigzag fashion — one high and the next one low. Each drum is approximately 2' (0.61m) in diameter and 4' (1.22m) long. There were 12 fiber "ropes" evenly spaced across the drums, and were each about the width of a small finger. These polyester fibers streamed through the drums in a serpentine manner. Each proceeding drum turns a little faster than the one before it, thus stretching the fibers to maximize their strength. The first drum rotates at about 250 rpm and then progresses until the last one is turning at 400 rpm. As the fiber is getting stronger, bending forces increase on the drums. Each drum is fastened to its individual motor roll shaft by 16 of the socket screws. They're torqued through the open end of the drum by an elaborate, custom extension using a torque multiplier. While the method was sound, the assembly of these large screws in an "as received" condition may not have resulted in consistently developed clamp force.

 

There's a hardened backing ring with 16 holes under the screws' heads. The top of the ring is countersunk to protect the fillet radii. Examination of the ring showed some of the tapped holes weren't perpendicular. Lab tests on the screws showed proper core hardness and a well-tempered martensitic structure. A scanning microscope (SEM) view of the fracture surfaces showed ductile fatigue failures on all.

Conclusion:

The cyclical bending stresses placed on the screws were too much for standard fasteners to absorb. The poor machining practices added to the screws' demise.

Recommendations:

1. Keep the countersinks concentric with the holes.

2. Drill and tap the roll shaft holes perpendicular.

3. Change to an UNJ thread style and roll the threads after heat treatment.

4. Undersize the body and build in a much larger filler radius, and then roll the fillet after heat treatment.

5. Use a non-metallic anti-seize compound on the threads to reduce variation in clamp force.

6. Use a thread-locking patch to minimize vibrational loosening.

7. In the future, if needed, change to alloy steel that has nickel content for added toughness.

 

As appeared in Fastener Technology International magazine.

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