'Threadlocking' means preventing threaded fixings from loosening due to vibration. There are many mechanical ways to do this, including split pins, locking nuts and special washers, but the term 'threadlockers' usually refers to chemical products that retain a threaded joint. Liquid threadlockers have become popular because they reduce the parts inventory, are easy to apply and are low-cost. However, as global EMC specialist Keith Armstrong explains, if used incorrectly they can create significant problems for safety and EMC.
Examples of liquid threadlockers include 'red' and 'blue' products from the Loctite-branded range and it has become quite common for people to use the word 'Loctite' to mean any chemical threadlocker; it is like calling all vacuum cleaners 'hoovers.'
The problem is that liquid chemical threadlockers are insulators and, because they have low viscosity and cover most or all of the thread, they ruin any electrical or radio frequency (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 a 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 threadlocker. After I scraped it all off, the fixing measured under one milliohm instead of 8 Ohms.
Clearly, using liquid threadlocker on any screwed/bolted electrical connections is very bad, and quite possibly illegal and unsafe too.
It is worth noting that there is a conductive liquid product made by Loctite, but it is an epoxy adhesive – not a threadlocker. I often hear people suggest using 'conductive Loctite' when what they really mean is conductive threadlocker - but as far as I have ever been able to discover, nobody makes (or ever has made) conductive liquid threadlocking products. I imagine the addition of the necessary conductive fillers would make the viscosity too high for a threadlocking product.
Of course, a high resistance in any RF-bond will ruin it, just as surely as it will ruin an electrical safety connection.
Likelihood of corrosion
For both electrical safety and RF bonding, I always recommend not relying on screw threads because of the likelihood of corrosion. Instead, the screw fixing should be used to press two conductive surfaces together to create the electrical, electrical safety or RF bond.
Unfortunately, unless the amount of liquid threadlocker is precisely metered every time it is applied to an individual screw thread, there is the risk of it not providing sufficient protection from vibration – so people always over-apply it, to make sure that they get reliable antivibration performance. The result is that the excess insulating liquid just gets everywhere – including in between the conductive surfaces that are being clamped together by the fixing.
In a recent military project, I found that the over-liberal use of liquid threadlocker had caused it to squirt into a joint in a shielded enclosure at every screw fixing (and there were a lot of them!). It not only prevented the screw threads from providing RF-bonds across the shielding joint, but also prevented the two metal enclosure halves from making direct contact with each other. The shielding that was achieved by the enclosure was useless.
As conductive liquid threadlockers do not exist, what can we use for locking threads without having to use precisely metered doses or extra parts?
Well, nyloc nuts and bolts use a solid ring of nylon that is cut into by the thread to provide the thread locking function. They have been readily available for over 40 years to my knowledge and are available compliant with a huge range of military, avionics and aerospace specifications.
Loctite offers a solid threadlocker called Dri-Loc. Unlike standard off-the-shelf nyloc fixings, Dri-Loc is applied by fixing suppliers as a custom product (I assume under licence from Loctite or Henkel).
Another alternative is Stanley's Spiralock that achieves about 15 per cent more torque than regular screw threads or coil inserts and provides antivibration threadlocking without any additional materials.
Spiralock products are reusable, whereas liquid or solid threadlockers can only be used once. Nyloc products are sometimes reused, although they should not be. I understand that in the UK military during wartime, reusing a nyloc nut or bolt is a court-martial offence!
So far I have only been discussing threadlocking for antivibration. But all threaded fixings (screws, nuts, bolts, etc) allow air, liquids, fine dusts and RF surface currents to leak spirally along their screw threads and ruin environmental sealing and RF shielding.
Also, you may be surprised to learn that all regular thread types do not properly centre in their hole, so their screw heads all sit at a slight angle to the surface they are fixing. So their heads all leak too!
We can prevent this environmental and RF leakage by screwing fixings into blind threaded holes, so that the screw thread does not penetrate all the way through the joint. Or we can use washers made of a tough conductive and environmental gasket material under the screw heads. But we would prefer to achieve antivibration, environmental sealing and RF shielding without having to add any other parts, even washers.
Spiralock screws are self-centring, so their screw heads do sit flat on the surface they are fixing. We still cannot rely on their screw heads to make such a perfect fit, every time, to ensure environmental and RF sealing – but an interesting possibility is to use Spiralock screws with an 87 degree underhead feature that guarantees a continuous metal–ring contact between the screw's head and the surface it is fixing. Spiralock fasteners can therefore offer reusable threadlocking (antivibration), some degree of waterproofing (I have seen IP7 quoted) and RF shielding – all without any additional parts or materials.
For more insight and information from Keith Armstrong and other experts in the field of EMC, visit the EMC Standards website at www.emcstandards.co.uk.