Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.


Hex torque strength

Hex torque strength

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?


RE: Hex torque strength

for the sress in the threaded section

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

If the manufacturer changed the hex from 4 to 3 mm but didn't change the size of the screw, I suspect he expects you to apply the same torque for the recommended screw tightness/load.

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.


RE: Hex torque strength

Hi sbbiomed

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


RE: Hex torque strength

I don't think byrdj and desertfox answered the correct question - they are assuming torsional failure of a cylindrical shaft.  I think hydtools has identified the correct problem (rounding of the hex feature), but he assumes a linear relation, which I think is wrong also.  This is the only reference I have seen for torsional strength of a hexagon feature:

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

This reference uses the equation:

TH = CH · WAF2 · WR.H · UTS · NC


TH is the hex torque capacity
CH 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 TH,3 is reduced to 32/42, or 56 % of TH,4



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

RE: Hex torque strength

Thank you all for your help.  I was not concerned with the threads or the cylinder failing.  What I was concerned with is the hex tip of the hex driver only (the set screw is made of Cobalt Chrome and will not strip, whereas the driver is Stainless Steel).  The manufacturer did not change anything as I tested the 4mm hex and now need a 3mm hex for a different set screw, but don't want to spend the time and money to test the 3mm hex driver, but want to calculate its maximum torque based off the testing of the 4mm hex.

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.


RE: Hex torque strength

sbbiomed, your choice of 50-56% is conservative and should result in not causing your hex drive to fail.  Are you sure that the lower torque will result in the setscrew performance you expect or need?  We don't know what you expect the setscrew to do or what the failure mode you tested the 4mm for.

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.


Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close