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Fastener Engineering

Tue Aug 24, 2004 8:33 am

Does anyone know a formula to figure out the amount of holding force produced by a threaded fastener given the thread pitch, diameter, and torque values?

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Joined: Sat Aug 18, 2001 7:10 am

RE: Fastener Engineering

Tue Aug 24, 2004 9:37 am

You can either do it by saying the force is equal to the stress times the tensile stress area,

F = σ x A

where A = (pi/4)(D - 0.9382P)² (D= diameter in mm and P is the thread pitch in mm. Bear in mind that this can be defined differently in critical areas.

But a stress is just Young's modulus times the strain, and strain is the extension over the original length, so you can rearange the above to

F = E.δ.A/L

Where E is Young's modulus, δ is the extension, A is the tensile stress are and L is the effective bolt length. Remember to use the forumla above things need to be in SI.

If you've got a few different sections, just split up the strains:

F = E.d/((L1.A1)+(L2.A2)+...)

If you've got the torque, if I recall correctly you divide the torque by the effective area and something called the nut factor, which is dimensionless and varies with material and lubrication.

F = T/A.P

where P is the nut factor (between .08 and .5ish)
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RE: Fastener Engineering

Tue Aug 24, 2004 9:30 pm

Are you trying to design a bolt or just choose a correct bolt? If it is the latter, 777...ER has it right above. Choose a tension head bolt and nut, and used the tension strength (say 95ksi) and mutiply by the diameter of the shank to figure how much load it can handle. You need to make sure that the bearing area of the bolt head is strong enough to handle the clamp up and tension force. ie, don't use a 1/2 inch bolt in .032 Al sheet!!  Smile

If you are trying to actually design a bolt, that is different story. I think I have seen something like the nut factor above, but I have also seen equations for tread angle and such. Try Liaison, they probably have it up there.
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