Maximum torque before thread damage 1

[robpigg]

I am currently using a 7/16"-20 UN nut to retain a clutch on a shaft. I need to know the max. torque I can apply to the nut before thread damage occurs. I can not seem to find a straight forward calculation. Is there one?
Thanks.

 

[CoryPad]

Assuming that the joint is solid, there is no prevailing torque, and all torque is used to stretch the external threads (and overcome friction), then yes, there is a straightforward way to calculate the torque that will damage threads. You can use the following equation (uses SI units, so you will need to convert):

M = F/1000*(0.16p+mu(0.58d2+((do+di)/4)))

where

M is the applied torque in N m
F is the preload developed in N
mu is the friction coefficient for the materials involved
p is the pitch in mm
d2 is the pitch diameter
do is the nut outer contact diameter
di is the nut inner contact diameter (most likely the hole size)

p, d2, do, and di are all dimensions that will come from your part drawings. Unless you specify mu, you will need to estimate this (your nut supplier can provide this for you). F will need to be calculated using the materials properties for your parts. Your nut will have a materials specification, and perhaps a load requirement (e.g. Proof Load). For the shaft, you can use this equation:

F = s * A

where

F = force in N
s = material yield stress in Pa
A = cross-sectional area in m^2

for a 7/16"-20 fastener (assumed to be a solid threaded region on the shaft), A = 7.66 * 10^-5 m^2.

Good luck.

 

[vonlueke]

In addition (or similar) to the excellent information provided by CoryPad, above, perhaps also try

http://euler9.tripod.com/fasteners/

and select "Bolt Preload Calculation." For a reference table of bolt geometric properties, including tensile stress area, etc., try

http://euler9.tripod.com/bolt-database/23.html

. For bolt material properties, try

http://euler9.tripod.com/bolt-database/

.

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[vonlueke]

robpigg: Do you know the grade of the nut material (such as SAE grade 2, 5, 8, etc.)? (If SAE grade 5, it will have 3 marks. If SAE grade 8, it will have 6 marks.) Do you just want to know the typical, maximum recommended torque that typically guards against damaging the nut? Do you know how much applied tensile force will be applied to the nut during operation (in addition to preload during installation torquing)? What factor of safety is required in your project for this part? Thanks.