The Case of the Unpicked Nuts

DN Staff

September 5, 2005

3 Min Read
The Case of the Unpicked Nuts

"We won't let a soft nut into the plant." These words came between the clenched teeth of the chief engineer at a huge heavy equipment manufacturing firm. I provoked the remark by asking whether a steel nut on one of their machines was hard or soft. The chief engineer's comment indicated that selection errors on hardware had led to calamities. The company's response was to use the more expensive hardware, even when not needed, to avoid the possibility of error.

Scene of the Crime

Cherry pickers are often used to elevate utility workers so that they can work on wiring strung from poles. Usually the pickers help the wiremen do their job in an unobtrusive way. In this case, things went very wrong.

Two electrical workers were at utility pole height when the hydraulic cylinder on the rig blew apart, removing all support. The bucket and workers went into free fall and crashed into the earth, with dire results. I was hired by an investigating firm to determine the reason for the failure of the hydraulic cylinder.

The cylinder was in good order, except for the .50-inch bolts and nuts that had held it together. The threads were stripped in two of the nuts. The other three nuts were still in place, though the bolts were broken. The bolt fractures were of the tensile overload type, with no signs of fatigue.

The Investigation

Hardness testing showed that the bolts were over twice as strong as the nuts. Such a combination makes no sense. The added strength of the hard bolts is wasted. I learned that the nuts had been recently replaced and torqued to a level that would give incipient failure of the threads.

The difference between hard and soft nuts is not small. I once unthinkingly replaced a hard nut with a soft nut on my rototiller. The soft nut stripped within minutes. A hard replacement nut lasted until I junked the machine years later.

The Smoking Gun

The scenario for the fatal accident was now clear. What is called a "picking error" resulted in installation of soft nuts on the hard bolts. The combined forces due to torquing and pressurization caused the threads on two of the nuts to strip. The skewed load then fractured the remaining three bolts.

Picking errors are entirely too common. I had an aggravating, though harmless, experience in a futile attempt to obtain a replacement igniter for my pilot-less gas range. My teen-age son boiled a pot over on an igniter, thereby shorting it out. The igniter ceased to ignite and instead clicked incessantly. I ordered the part from the retailer, and the piece they sent me had little resemblance to the failed igniter. I returned the part and re-ordered, only to get the same part back! I was forced to pay for a service call to do a trivial job. That retailer is at the top of my "do not buy from" list and my new range has pilot lights.

I do not know how this case came out, but suspect the people responsible for installing the soft nuts paid a large settlement. The settlement may have made following the example of the heavy equipment manufacturer economically attractive.

Sign up for the Design News Daily newsletter.

You May Also Like