What are Torque To Yield bolts?

QUESTION

Dear John,

I enjoy your channel and wonder if you would like to sink your teeth into an engineering question that I can’t get my head around.

I notice that a lot of bolts on modern engines (head bolts etc.) specify that you give them a particular torque setting and then you add X number of degrees of rotation after that torque has been reached.

Surely once you add the X degrees to the original torque value you would end up at a new higher torque value than the original.

So why not just give the final value and be done with it rather than complicating the matter?

I am guessing it may only apply to these single use torque to yield bolts, however I still don’t understand the reason.

And if it is relevant, what’s the deal with torque to yield bolts anyway ?

Regards,

Peter

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ANSWER

Hi Peter,

Let’s go back to basics.

When you’ve got a particular amount of stretch in the bolt you get a certain amount of clamping force which holds the head onto your cylinder block.

When you consider the millions of engines that start up every hour, and that cylinder head bolts, bolted conrod joints, big end bearing caps, they all rarely fail - clearly something’s going right out there in that regard.

And there are two ways to fasten bolts, either using The Force like a mechanical Jedi by feeling it, or there’s the torque wrench way.

When you apply your angled force on the wrench, you eventually hear a click which tells you you’re at the required torque. A lot of mechanics think their torque wrench is some lightsabre, philosophically, but in actual fact it’s not that accurate and this has nothing to do with the accuracy of the wrench itself.

It has to do with the nature of the fastener itself.

The torque you’re applying actually relates to a certain amount of clamping force - and that would be fine if you could standardise the bolt’s operating environment - but a great deal of that torque is devoted to just overcoming friction between the mating faces of the threads.

Only about 20 per cent or less of that torque stretches the bolt. And this can vary if there’s material on the surface of the thread (anti-seize grease, surface rust, mill scale etc).

The angle tightening you refer to, Peter, is the technique which ensures you get to the required tension that takes friction out of the equation. It’s been used in industry for ages.

You can dial in the amount of stretch and simply torque the bolt to a light setting. It’s very precise.

Now, a torque to yield (TTY) bolt is a type of fastener, designed to do a particular job.

So, let’s talk about yield. Metallurgists understand it, but not many others do.

When you break a piece of a high-tensile steel faster (bolt, set screw, whatever), think of the graph with the x-axis being strain and the y-axis being stress. Or, to simplify it, stretch (x) versus load (y) - and I know that’s technically not completely accurate, but for the sake of teaching, it’s tantamount.

The first straight region is the elastic region. Past the yield point (YP), is the plastic region which, if you’ve ever folded a coat hanger, you’ll see some of that deformation becomes permanent.

If the load keeps increasing, you’ll get tighter and tighter until you reach the ‘Oh shit’ point - the ultimate tensile strength (UTS) - and suddenly tightening the wrench goes light and it’s all over. Past that point - catastrophic failure.

These are very conservatively loaded components, so with TTY there’s a higher torque spec for the material, you go past the YP and on the road to UTS.

There’s so much bullshit online about greater consistency in clamping force etc.

The main reason for TTY is because you can get greater clamping force out of the fasteners. You can use smaller fasteners to do the same job, or fewer fasteners of the same size to do the same job.

This is important because in applications like engine building, space is at an absolute premium. You need mechanical clamping locations, you need to get air in, water in, fuel in, oil in, you’ve got valve guides, you’re getting exhaust out, and so on.

If you can get away with a smaller cylinder head using smaller fasteners doing the same job, then that’s a victory for design.

And lastly, of course, the main problem with TTY bolts is that once you reach that YP and return back to the elastic region, you’ve changed the crystal properties inside the steel and it’s permanently deformed - and cannot be used again.

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