How Much Should a Bolted Joint be Tightened?

This article was updated April 3, 2023. It was originally published June 7, 2016.

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Workers often tighten bolts to 75% to 80% of the bolt proof load. This works for many joints, but in some cases, external tensile loads reduce bolt clamping forces to zero. Something other than the 75% or 80% rule of thumb is needed.

The external tensile force needed to reduce clamping to zero (within the bolt’s and cramped parts’ elastic limits) is determined by:

(F_e/F_PR)₀ = (F_b/F_PR)₀

                = ((1+r)/r)(F_i/F_PR)

where r = K_p/K_b, K_p is the clamped part’s spring constant (lb/in), and K_b is the bolt’s spring constant (lb/in); F_e is the external load on the joint (lb), F_PR is the bolt proof load (lb), F_b is bolt load (lb); and F_i is bolt preload (lb). The subscript “0” identifies a value at zero clamping load. This equation can be used to calculate the maximum force that can be applied to a specific joint without the parts separating.

For common values of r, the force can determined from the graph below, which is based on the equation.

The bolt preload that produces a given non-zero clamping force can be determined from the above graph in combination with the equation:

F_e/F_PR = (1-b)(F_e/F_PR)₀

Where b =|F_p/F_i|

The corresponding bolt load can be computed from:

F_b/F_PR = (F_e/F_PR)₀ – F_i/F_PR)(b/r)

Here’s an example:

For a bolted joint where r is 10 and F_i/F_PR is 0.75, the graph gives:

(F_e/F_PR)₀ = (F_b/F_PR)₀

For a clamping force of 25% of the preload (b = 0.25):

(F_e/F_PR) = (1-0.25)(0.825)

             =0.619

And then:

 (F_b/F_PR) = 0.825 - (0.75)(0.5)/10

              = 0.806