The Stripped Thread Matter

Forensic Engineering failure analysis, metallurgy and materials science forms a huge part of what we do, and clients from all over Ireland and abroad regularly contact our forensic materials laboratories for advice. Many of these cases involve personal injury and litigation.

Earlier this year, the laboratory was contacted by a Solicitor representing a client who stated that she had been ‘sitting on her office chair and whilst moving the chair out from under her desk, she fell to one side and landed on the ground". In so doing, she received an injury to her head, shoulder, and arm.

We had the chair collected from the employees office. The chair was a swivel 'egg-chair' which rested on a four spoke metal base. At the end of each spoke was a nylon castor. When delivered to the laboratory, one of the castor wheels was not attached to the spoke, rather it had been placed in a plastic bag and affixed to the chair post.

The castor assembly consisted of a plastic wheel (A), into which was a castor stem socket with a male thread at one end (B), with the male thread fitting into the female thread of wheel cover (C).

This was a fatal error as the ensuing union was a friction fit with little to no thread to thread contact. The injured party was awarded a six figure sum.

In addition, we found residues of two lubricants which were differentiated by way of colour. One lubricant was black with a metallic lustre (R1), and the second was clear with a yellow tint (R2). We did suggest to the Solicitor representing the injured party that in order to chemically characterise them, we could carry out Fourier Transform Infrared Spectroscopy on both lubricants.

This was not ultimately necessary, as by contacting the manufacturer of the chairs, they confirmed that the only lubricant which they have ever used is a clear anti-seize compound.

So what could have happened? Given the wear patterns on both the male and female threaded fittings, our Forensic Engineer came to the conclusion that the male thread stem (B), had separated from the female threaded metal housing (C), on more than one occasion as the wear pattern on the male thread stem was inconsistent with it coming loose in one event.

What was the result ? Following submission of our Technical Report, a without prejudice meeting took place between the legal parties and the insurers.

The owner of the company in which this incident took place, did agree that one more than one occasion, the castor assembly had separated. Finding it more difficult to thread the male end into the female end, the company owner decided to use a graphite lubricant to ease the union. This was a fatal error as the ensuing union was a friction fit with little to no thread to thread contact. The injured party was awarded a six figure sum.

This is an example of the one of the many cases which our laboratory and forensic specialists are asked to investigate. In all areas of materials investigation, we use cutting edge instruments and decades of practical expertise, to elucidate the facts. Contact us today and let us take the if's, but's and maybe's out of your materials questions.