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Case Study #20 — Fastener Failure Workshop

Medical Equipment
 

Industry:

Medical Equipment
Linear Motion Pushers

Problem:

A large medical equipment producer conceived a method of patient transport through a scanner, designed to be smoother than anything before. Using this method, they anticipated better image resolution, but, on the initial assembly line run the hex recesses of the stainless steel flat countersunk head socket screws rounded out.

Injuries:

None.

Damages:

There was no damage to the equipment, but the long anticipated new machine couldn't be assembled, and there was lost production time and potential sales.

Observations:

The stainless screws were #10-32 x 1/2". In an unused condition, they appeared entirely normal. There was also ample evidence of screws that had their hew recesses grossly rounded out. There were inch pound torque wrenches available by the machine. They had short sections of hex keys held into the sockets on the wrenches.

Investigation:

Samples of the fasteners were taken to a lab for review. A test with a magnet showed the parts non-magnetic. As the fasteners were made of a stainless steel that couldn't be heat treated, the sole source of any added strength was from cold working. While forming a hex recess imparts added toughness to the socket, this is still a relatively weak material, and the screws can't be installed with a great amount of force. We decided to conduct a test to evaluate these screws for assembly. The test consisted of screwing the fasteners into a hardened test block by hand until the heads bottomed out and stopped turning. Next, a new hex key in a calibrated torque wrench was used to assess the ability of the hex recess to hold a prescribed torque amount. It's not a test required by ASTM F 879, the specification covering this product. Our intent was to determine if the hex would cam out, that is strip or round out, prior to achieving the manufacturer's recommended installation torque of 34 in-lb (3.8N-m), which was 11 in-lb (1.2N-m) less than what was used at the plant. Three specimens from the lot were tested. Each of the screws easily held the assembly torque without damage. We then tried increasing the torque to see what value the hex would round out. Surprisingly, we took the torque up to 68 in-lb (7.7N-m) on new specimens. That's twice the installation torque! Still there was no damage. We next checked the cut-off hex keys being used. The dimensions were right, but the hardness at the ends where force was exerted was low. After asking the workers, we found that the keys were shortened by grinding without coolant; therefore, the excess applied heat annealed and softened the keys.

Conclusion:

The screw's recesses were very strong, but hex keys were poorly modified making the screws appear to be the problem. Additionally, the torque used was too high.

Recommendations:

Hex keys are high hardness items that must be cut with an abrasive cut-off disk that is water cooled. The cooling will preserve the hardness. Further, manufacturer's torque recommendations normally take into consideration a number of variables; therefore, unless you have truly proven your assembly method, it's a good idea to follow their advice.

 

As appeared in Fastener Technology International magazine.

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