Never use thread-lockers on electrical fixings: Page 3 of 3

May 29, 2018 //By Keith Armstrong
Never use thread-lockers on electrical fixings
‘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. 

Spiralock products are re-usable, whereas liquid or solid thread-lockers can only be used once. Nylok products are sometimes re-used, although they shouldn’t be. I understand that in the UK military during wartime, re-using a Nylok nut or bolt is a court-martial offence!

So far, I have only been discussing thread-locking for anti-vibration. 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 doesn’t 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 anti-vibration, 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 can’t 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°underhead feature that guarantees a continuous-metal-ring contact between the screw’s head and the surface it is fixing. Re-usable thread-locking (anti-vibration), some degree of waterproofing (I have seen IP7 quoted), and RF shielding – all without any additional parts or materials!

 

About the author: 

Keith Armstrong is global EMC specialist for EMC Standards - www.emcstandards.co.uk

Design category: 

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