Bolted joints are often tightened to 75% to 80% of bolt proof load. This amount of tightening, a rule of thumb, is suitable for many joints, but in some cases, external tensile loads can reduce bolt clamping force to zero. Thus, for some joints, values other than 75% or 80% are needed.

The amount of external tensile force that reduces clamping to zero is given, within the elastic limits of the blot and clamped parts, by:

*(F _{e}/F_{PR})_{0 }= (F_{b}/F_{PR})*

_{0}

= *((*1+*r)/r)(F _{i}/F_{PR}*)

where *r = K _{p}/K_{b}*

and *F _{e}* is the external load on the joint (lb),

*F*is the bolt proof load (lb),

_{PR}*F*is bolt load (lb),

_{b}*F*is bolt preload (lb),

_{i}*K*is the spring constant for the clamped members (lb/in),

_{p}*K*is the spring constant for the bolt (lb/in), and the subscript 0 identifies a value at zero clamping load.

_{b}The maximum force that can be applied to a specific joint without the parts separating can be calculated with this equation. For common values of *r*, the force can determined from the accompanying graph, 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}|

And the corresponding bolt load can be computed from:

*F _{b}/F_{PR }= (F_{e}/F_{PR})*

_{0}–

*F*)(

_{i}/F_{PR}*b/r)*

For 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 equal to 25% of the preload (b=0.25),

*(F _{e}/F_{PR})=(*1-0.25)(0.825)

=0.619

And *(F _{b}/F_{PR})*=0.825-(0.75)(0.5)/10

=0.806

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