Wednesday, August 11, 2010

Broken Bolts

These bolts were tightened by hand and failed at an unreasonably low torque. I suspect that this was due to cost cutting on the part of the bolt manufacturer.

It appears that they can save about 8% on the cost of fastener production by not heat treating the bolts. This would enable the bolt vendor to under bid the competition by selling bolts below the cost of making properly heat treated bolts while still making a profit.

This is the second time I have seen bad bolts in recent years, and I hope that manufacturers are testing the strength of bolts they order when they are used in critical applications where failure could mean injury or death.

This site also has some additional information about bolt failures.


Theran Cochran said...

Were these graded bolts or cheap imports? Without a manufacturer and grade marking on the head, the bolt isn't required to meet any standard of strength or quality. A responsible designer will specify bolts to meet an appropriate standard and accept the increased cost when luck of the draw isn't good enough.

I Heart Robotics said...

These were unmarked imported bolts that came with a set of caster wheels. While this may not be the most critical application, I think people should be aware of the issue.

There is nothing that guarantees that graded bolts are properly manufactured without having a record of the entire supply chain throughout the products life. Imagine trying to figure out who to yell at or sue if the bolts in a product start failing after 2 years when the bolts were supplied by different distributors each of which have an unknown number of sub-vendors.

A manufacturer would need a way of tracking by serial number the source of each products components down to the lot number. This is probably beyond the capabilities of many small businesses which is why I predict that the problem will only get worse.

The other big problem is going to be the difference in mindsets between purchasing and engineering. Engineering should be specifying the minimum strength required, but purchasing may be looking at the cheapest bolt that can be stuck in the hole.

Anecdotally, I have noticed an increasing number of fastener failures in recent years.