stress in the corners of hex head bolt

[ranas1]

Im triying to calculate the max. torque on hex head bolt and its thickness, im confused. i have had a equation

TH= CH*(WAF2)*(WRH)*(UTS)*(NC)

CH.- constat =0.103
WAF .- width across flats
WRH.- wrenching height
UTS.- ultimate tensile strenght
NC.- number pair of contact (for hexagon =3)

but what happend if is not a hexagon?, maybe an squared head???, NC and CH it should change, what value??.
Can anybody help me?? , please

 

[chicopee]

I would think that you would want to calculate the stresses on the threaded part or on the shank of the bolt, not on the head. What is your reason?

 

[tbuelna]

I don't understand what you mean by "max. torque on hex head bolt and its thickness". Do you wish to determine the max wrenching torque that can be applied to a hex head bolt of a given head height and tensile strength without damaging the hex surfaces? Typically, hex bolts will fail in torsional shear before the wrenching flats totally fail.

If you wish to determine what the surface bearing stresses produced on the hex faces from applying a wrenching torque are, that is not a simple thing to do if an accurate result is required. Due to variations in the shape of the bolt hex and the wrenching device, the contact forces are not equally distributed at 6 locations. The large relative radial structural stiffness difference between the bolt head and the wrenching device also has a big influence.

 

[racookpe1978]

6 locations?

Most often case for a damaged nut or bolt head is TWO places caused by a wrong-sized wrench head, a sloppy wrench badly machined in China that touches in only twoplaces, or a worn out old wrench or simply the wrong size or even a metric "almost fits" ANSI or an uncalibrated ANSI-Crescent wrench used on a metric bolt head. 8<)

Also: A hammer drill (or impact wrench) used improperly.

Stupid un-trained un-supervised workers are your worst-case condition here. If you really want to analyze the problem, assume the wrench only touched twice. And is off-center vertically as well.

 

[tbuelna]

I would agree that even a high quality socket on a hex bolt might only provide 2 or 3 contacts. Another thing to remember is that most bolt heads are cold forged in the axial direction, which means they have some draft angle from top to bottom on the wrenching surfaces.

Normally, when a bolt and nut are used, the nut is the part that is turned. And one issue with the forces applied to the wrenching surfaces on hex nuts is that they create a radial inward compressive force that creates additional friction on the screw threads, which can affect the perceived level of installation torque. This was the reason the spline drive type of nut was created, so that the forces produced on the wrenching surfaces was entirely tangential to the screw threads.

 

[ranas1]

Hi,
Thanks for helping me out with my problem

Yes tbuelna ,i wish to determinate the max. wrenching torque that can be applied with a given head height without damage the hex surface, im working with PPS bolts and nut.