# Design of breakaway Bolt

1. Apr 15, 2014

### rsk2mc

I am trying to design a breakaway Bolt, is there an equation to calculate how much torque is required to break the head of the bolt. Typically this bolt has two heads, one that breaks off and the permanent one. These are connected with a small neck that breaks at a given torque. That's what I need to calculate, the diameter needed to break at 40 ft-lbs.

2. Apr 15, 2014

### Seth.T

Sounds like a shear bolt.
So this is what i found

The relationship between the tensile and shear moduli is

E = 2*G*(1+v)

E = Elastic modulus
G = Shear modulus
v = Poisson's ratio

so for common steels assuming E = 30,000,000 psi and v = .33 Then G = 11,278,196 psi

You should also use Mohr's Circle.

There are several failure theories that can be used such as the Von Mises and the Maximum Shear theory.

Remember in shear bolt the failure (breakage) occurs at the ultimate strength not at the yield strength.

3. Apr 16, 2014

### rsk2mc

I saw the same post, however this does not help me figure out the required torque to break the bolt.

4. Apr 16, 2014

### nvn

rsk2mc: In your case, the bolt breakaway head will not be preloaded, which is different from a typical bolt head. Therefore, in your particular case, if your breakaway shaft is a solid circular cross section, you could use the following.

d = 2.04[T/(Rsu*Stu)]^0.3333,​

where d = solid circular breakaway shaft diameter, T = breakaway bolt installation torque, Stu = bolt material tensile ultimate strength, and Rsu = dimensionless factor, such as 1.00 or 1.40.

You would need to test each different bolt alloy, to determine the value of Rsu for that particular alloy. Fortunately, there is probably no test easier than this test. Once you determine Rsu for a particular alloy, then you can plug it into the above formula, for that particular alloy.

For an initial guess at Rsu, I might arbitrarily guess 1.30 or 1.35 (?). Regardless of your initial guess, after you test it, you can then solve for Rsu, using the above formula, to obtain the correct value of Rsu, for that particular bolt alloy.

If you do determine your above Rsu value, then if you want to post your Rsu value, and bolt alloy, here, that would be great.

Here is an example. T = 54 240 N*mm, Stu = 515 MPa, Rsu = 1.45. Therefore, d = 2.04[(54 240 N*mm)/(1.45*515 MPa)]^0.3333 = 8.51 mm.

Last edited: Apr 16, 2014