Leigh Prather/Dreamstime
Bolted joint superimposed over chalkboard

How Much Should a Bolted Joint be Tightened?

April 3, 2023
Engineers often ignore external forces acting on the joint when specifying the amount of torque for installing the bolts. Here’s a way to avoid that.

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


Structural Properties of Bolted Joints

What’s the Difference Between Bearing, Shear, and Tear-Out Stress?

An Introduction to Lean Bolting

The Preload Matters…A Lot

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:

(Fe/FPR)0 = (Fb/FPR)0

                = ((1+r)/r)(Fi/FPR)

where r = Kp/Kb, Kp is the clamped part’s spring constant (lb/in), and Kb is the bolt’s spring constant (lb/in); Fe is the external load on the joint (lb), FPR is the bolt proof load (lb), Fb is bolt load (lb); and Fi 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:

Fe/FPR = (1-b)(Fe/FPR)0

Where b =|Fp/Fi|

The corresponding bolt load can be computed from:

Fb/FPR = (Fe/FPR)0Fi/FPR)(b/r)

Here’s an example:

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

(Fe/FPR)0 = (Fb/FPR)0

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

(Fe/FPR) = (1-0.25)(0.825)


And then:

 (Fb/FPR) = 0.825 - (0.75)(0.5)/10

              = 0.806

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