Never use thread-lockers on electrical fixings

May 29, 2018 // By Keith Armstrong
‘Thread-locking’ means preventing threaded fixings from loosening due to vibration. There are many mechanical ways to do this, including split pins, locking nuts, special washers, etc. but the term ‘thread-lockers’ is usually applied to chemical products that glue a thread in place. 

Liquid thread-lockers have become quite popular, because they reduce the parts inventory, are easy to apply, and low-cost. But they can be used carelessly, creating big problems for safety and EMC. Examples of liquid thread-lockers include ‘red’ and ‘blue’ products from Loctite and it has become quite common for people to use the word ‘Loctite’ to mean a chemical thread-locker. It’s like calling all vacuum cleaners ‘hoovers’.  

The problem is that all liquid chemical thread-lockers are insulators, and because they have low viscosity and cover most or all of the thread, they ruin any electrical or RF bonding that a fixing might be supposed to provide.

A few years ago, I was working on a product that used a 4mm diameter screw thread as protective conductor (“safety earth”) connector. The entire protective conductor circuit is supposed to measure less than 0.1 Ohms, but this one screw connection alone was 8 Ohms! I noticed that it was coated with something red, which I presumed was a liquid thread-locker. After I scraped it all off, the fixing measured under one milliohm, instead of 8 Ohms!

Clearly, using liquid thread-locker on any screwed/bolted electrical connections is very bad, and quite possibly illegal and unsafe too.

It’s worth noting that there is a conductive liquid product made by Loctite, but it is an epoxy adhesive – not a thread-locker. I often hear people suggest using ‘conductive Loctite’, when what they really mean is conductive thread-locker, but as far as I have ever been able to discover no-one makes (or ever has made) conductive liquid thread-locking products. I imagine the addition of the necessary conductive fillers would make the viscosity too high for a thread-locking product.

Of course, a high resistance in any RF-bond will ruin it, just as surely as it will ruin an electrical safety connection.  

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