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Calculating Fastener Torque

Calculating Fastener Torque

Calculating Fastener Torque


I would like to calculate how much a bolt with a long grip length might twist during torque tightening of the nut to a specific torque value.

The hole needs to be interference and it is important that I consider differences between reacting the head of the fastener with a spanner and only tightening from the nut.

Can anybody point me in the direction of an article/book/specific calculations I can read about to enable me to calculate this?

I am not a stress engineer so if anybody can explain the basics to me I would very much appreciate it.

Thanks in advance

RE: Calculating Fastener Torque


Can you upload a sketch? What is it that is interfering? The length between the torqued nut and the interfering faces is important, and should be easily understood by someone with basic mechanics of materials.


RE: Calculating Fastener Torque

This is one of those things where the formula is pretty simple, but the result is 100% dependent on your assumptions.

Best you can do is probably bound the possibilities.

Max twist is the case where 100% of nut torque is reacted by the head of the bolt.

Min twist is essentially zero where you assume that 100% of nut torque is reacted at the end of the bolt immediately next to the nut.

It seems unknowable where the torque will actually be reacted with a bolt going through an interference hole. Possibly knowable for a known bolt in a known hole, but for a population of bolts and holes, no.

RE: Calculating Fastener Torque

"The hole needs to be interference" .

How much interference?

I'm having trouble imagining a situation when significant interference would be required or desired.

For garden variety automotive connecting rods the body of the bolt was knurled to fit the range of con rod holes, and used to align the cap and rod, so the necessarily precise big end machining would be re-created.

Ground body bolts can be a real nightmare to install, requiring multiple tons to seat fully.

Considering the clamp force of a 3/8 -24 is on the order of 7000 lbs, I guess three is a possibility of having a tight bolt "hung up" in the hole after tightening.

RE: Calculating Fastener Torque

Hi, thank you all for the information above. I will explain some more about the reason for this question.

The fasteners in question hold a large heavy plate onto the side of a digger arm (farm equipment) if an Allen key has not been used to react the fastener while torquing then after a couple of weeks use the paint cracks away from the countersunk hole and flakes off all bolt heads... it does not get worse over time and only shows a couple of degrees movement. if I use the Allen key to react the bolt while torquing the nut then the bolts don't move and the paint stays on. I need to show what might be happening theoretically.

Also if I re paint after first movement it never flakes again.

So starting with twist being put into the bolt on installation and settling during use and secondly showing a low tension issue through the joint?

Therefore I need to work out how much twist might be going into this fastener or assume it doesn't bind centrally in the hole and moves after installation. As mintjulep says above bounding the possibilities minimum and maximum twist would be a great start if I can get to how many degrees of movement I might see at the fastener head?

Thanks again.

RE: Calculating Fastener Torque

What is the process required to insert a bolt into the "interference fit" hole?"
During actual assembly, but especially when test fitted into the plate with the interference fit hole.

First I'd quantify the interference fit.

Commercial countersunk screw body diameters have several 0.001s" tolerance. Page 28 here -


Then I'd trial assemble a few bolts with blued underhead and body, and see what the contact between the screw and several countersunk plates.

I'm expecting a low % of contact between the mating angles, and indications of the interference of the machined hole and the slightly radiused transition between body and head angle. Both invite some embedment of the fastener head over time.

How much does the un-restrained screw spin during the torquing process? If it spins much, the mating underhead surfaces may be damaged or even galled, also inviting embedment over time.

RE: Calculating Fastener Torque

The calculation MintJulep explains seems very sensible,

I would like to simply bound the possibilities to calculate the maximum and minimum twist possible in degrees, can somebody help to explain how?

I have purchased a Roark's stress and strain book but exactly what I should be doing isn't clear.


RE: Calculating Fastener Torque


I am not familiar with the term "react the bolt". When you tighten down that nut, you need to constrain the bolt from turning, presumably with an Allen key. Almost certainly, the insertion force of the screw into your press-fit hole is trivial compared to the tensile strength of your bolt, easily achieved by torquing your nut. Sometimes, the friction under your bolt head is enough to allow you to torque your nut, but it is not something I would rely on. This does create some failure modes. Maybe your nut and bolt are actually not clamping!


RE: Calculating Fastener Torque

How much interference are we talking about, and what level of clamp load are you trying to achieve. The interference is going to reduce the ability of the tensile stress created during tightening to pull the fastener head down into the countersink and if there is any axial movement then you will lose clamp force in the joint. Could the paint peeling be due to embedment of the "interference' fit shank as the fastener head fully seats?
An interference fit like this is very hard to control; are you reaming the holes after match drilling them? A couple .001" of hole size change will greatly effect the performance of this joint by causing the insertion force and resistance to go from low to very high. One other thing to keep in mind: socket flat heads are not intended for structural use; the head to shank area will fail through the recess at a lower tensile value than the full strength of the threads. Make sure that you down rate the connection strength to account for this.

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