Enter the values of percentage of yield strength desired, yp (%),the yield strength of the bolt material, S(psi), the nominal diameter of the bolt, d(in) and the tensile stress area of the bolt, A (in²) to determine the value of Preload torque, Tpl(in-lb).

 Enter Percentage of Yield Strength Desired: % Enter Yield Strength of the Bolt Material: psi Enter Tensile Stress area of the Bolt: in2 Enter Nominal Diameter of the Bolt: in Result – Preload Torque: in-lb

Preload torque is a critical concept in bolted joint assembly, referring to the torque required to stretch the bolt to a specific tension before the joint members begin to separate or move. This tension ensures the joint is secure and can handle the applied loads without loosening.

Preload torque, Tpl(in-lb), is calculated by multiplying the yield strength of the bolt material, S(psi) in per square inch, by the tensile stress area of the bolt, A (in²) in inches per square, and by the percentage of yield strength desired, yp (%), divided by 100. This product is then multiplied by the nominal diameter of the bolt, d(in) in inches, and divided by 12 to convert the torque into inch-pounds (in-lb).

Preload Torque, Tpl(in-lb) = ( (yp/100) * S(psi) * A (in²) * d(in) / 12

Tpl(in-lb) = Preload torque in pound inches, in-lb.

yp = percentage of yield strength desired, (%).

S(psi) = yield strength of the bolt material in per square in inches, psi.

A (in²) = tensile stress area of the bolt in inches per square, in2.

d(in) = nominal diameter of the bolt in inches, in.

1. A structural bolt with a tensile stress area of 0.20 in² and a nominal diameter of 1/2 inch is to be tightened to 70% of its 120,000 psi yield strength. Calculate the preload torque.

Given:  yp = 70, S(psi) = 120,000psi, A (in²)  = 0.20in², d(in) = 0.5in.

Preload Torque, Tpl(in-lb) =  (yp/100) * S(psi) * A (in²) * d(in) / 12

Tpl(in-lb) = (70/100 * 120,000 * 0.20) * 0.5 / 12

Tpl(in-lb) = (84,000 * 0.20) * 0.5 / 12

Tpl(in-lb) = 8400in-lb.

1. Assume we aim to achieve a preload torque, Tpl(in-lb) of 700 in-lb, with a desired percentage yp is 75% of the bolt’s yield strength, a tensile stress area, A (in²) is 0.20in2, and a nominal diameter d(in) is 0.5inches.