MechChem Africa August 2019

Nuts of the threaded variety

T here is no mystery about nuts and bolts, or to give them their proper name, threaded fasteners. Threaded fasteners are socommon, even those who claim zero knowledge of engineering know what they are. Yet threaded fasteners are perhaps the most misunderstood engineering compo- nents. I have lost count of the times when a broken bolt has been brought to me that has clearlyfailedinfatigue,withthecommentthat ‘it must have been over-tightened’. A popular mis-conception, and sadly nothing could be further from the truth. When the tightening operation is carefully considered, it involves nothing more com- plicated than driving one inclined plane up another. That the planes arewrapped around inaspiralontheboltandinsidethenutdoesn’t change anything. It’s probablywell known, or shouldbewell In his August column, Tim Carter talks about nuts, not the sort you get in a little packet on cheap airlines with a printed ‘may contain nuts’ warning, but the ones that, together with their respective bolts, quite literally hold our engineered world together.

Three examples of fatigue failures on bolts. Only under-tightened fasteners can be exposed to the cyclic loads that produce a fatigue failure.

known, that a threaded fastener works by clamping two surfaces together. To do that, the bolt has to be in tension, exerting the needed balancing compressive force on the joint. What isn’t so well known is that only about 10% of the applied torque is used in generating that force. The other 90% is used in overcoming friction, about 40% on the threads themselves and about 50% on the under-head, or under-nut, surfaces. That iswhymost specified torque settings mandate ‘dry threads’. Any kind of lubricant will change the friction coefficients – and fiddlingwith the friction coefficients changes everything, including the clamping force generated, which has the ability to spoil somebody’s day when it comes loose and the nut falls off. As supplied, most fasteners have a coatingof some formof lubricant, even if it’s only an anti-rust treatment for storage. The other day I was treated to the sight of a wheel bounding down the road on its own. Since my car will flash warning lights and shout at me if one tyre is down on pressure, let alone going its own way, I knew it wasn’t one of mine, so I got out of the way of whoever had lost it, before pulling off the road at a safe distance to give whatever help I could. Looking at thewheel hub, I noted it

A fastener that failed when the joint came loose.

had two nutless studs and two stud stumps. The driver, a young lady, (well younger than me, she didn’t have much in the way of grey hair) told me she had recently had the tyre checked for a slow puncture. Both stumps were clearly fatigue failures. I thought this a little strange, sincewhen I have tyres changed, I take the car home, loosen the wheel nuts (often with a 20-inch long breaker bar), and then re-tighten them with a torque wrench (45-50 ft-lb) so I know I can take themoffwith the spanner in the car anddon’t needGodzilla fromthe tyre shopwho tightened them in the first place, probably with a pneumatic power wrench. I know that if I have to, it will be at

Tim J Carter is a consulting physical metallurgist previously in private practice and now with ImpLabs in Benoni: timjcarterconsulting@gmail.com.

It’s impossible for an over-tightened threaded fastener to fail in fatigue, which is the most common cause of failure.

10 ¦ MechChem Africa • August 2019