## Hex torque strength

## Hex torque strength

(OP)

I have done mechanical testing on a set screw with a 4mm hex and its driver. I now have a set screw with a 3mm hex. Without the time and expense of making parts and testing, I want to know the percentage decrease in maximum torque that the set screw and/or driver will be able to withstand on the 3mm hex. I am getting a torque limiting device and I need it to be used for both set screws and I don't want it to strip the hex on the 3mm driver by being too large of a value.

If the materials and hex depth do not change (only change is 4mm to 3mm hex), what decrease do I expect to see in the 3mm hex?

Thanks,

sbbiomed

If the materials and hex depth do not change (only change is 4mm to 3mm hex), what decrease do I expect to see in the 3mm hex?

Thanks,

sbbiomed

## RE: Hex torque strength

the area of the 3mm divided by the area of the 4mm.

areas being f(d^2)

3^2/4^2 = 9/16 ~ 56%

but you state your limitation is in the key fit

## RE: Hex torque strength

Or you could estimate that for the same force on the corners of the 3mm hex requires 75% of your 4mm test torque. Guessing that failure is the rounding out of the hex socket in the screw or rounding over of the corners on the hex driver. The force torque arm of the hex is reduced from 2 to 1.5mm.

Ted

## RE: Hex torque strength

The resistance of the plain circular section of the screw to

torsion is given by:-

J= 3.142 * d^4/ (32)

where d = root dia of thread

therefore if you have a 3mm dia and a 4mm dia then the ratio

of resistance to torsion is:-

3^4/4^4 = 81/256

desertfox

## RE: Hex torque strength

G. S. Case, "Stresses on Bolts-Nut Dimensions-Wrench Design:, Mechanical Engineer

This reference uses the equation:

T

_{H}= C_{H}· WAF^{2}· WR.H · UTS · NCwhere

T

_{H}is the hex torque capacityC

_{H}is a constant (=0.103)WAF is the width across flats

WR.H is the wrenching height

UTS is the ultimate tensile strength

NC is the number of pairs of driving contacts (NC = 3 for a hexagon)

So for your example, WAF goes from 4 mm to 3 mm, so T

_{H,3}is reduced to 3^{2}/4^{2}, or 56 % of T_{H,4}Regards,

Cory

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.

## RE: Hex torque strength

Corey and Bryjd both came up with 56% reduction in torsion resistance, but by different ways. Hydtools has the correct problem, but simplified it too much as just a linear change. I will use 50-56% and I should not go wrong.

Thanks

## RE: Hex torque strength

My linear approach relates to the load on the hex corners and not the loading across the crosssection of the hex. Of course the stress on the crosssection varies as the square of the hex size as other responders noted. The force on the hex corners varies directly as the size of the hex. The corners transmit the torque from driver to screw socket. Excess force on the hex corners causes hex socket rounding out or hex drive rounding over.

At 50-56% of the 4mm data you should not have the 3mm hex drive break or the hex drive corners rounding over.

Ted