Standards for Bolt Seating Torques

Bolt Seating torques are very important in any machine design.  In order to maximize the holding power of a bolt it must be properly preloaded into the plastic range to optimize its strength.  This preloading produces useful tensile stress in the bolt which prevents joint separation and ensures that the bolt's endurance limit is not surpassed under future loadings.

Bolt seating torques are dependent on the grade of bolt, bolt diameter, the threads per inch, and bolt condition.  Bolt condition is accounted for by applying a bolt condition factor depending on whether bolt surface is dry, light oiled or plated, or whether anti-seize is used.

For reusable fasteners it is recommended that a preload of 75% of proof load be used when calculating bolt torque.  The value for the bolt torque is a product of proof load, bolt diameter, and bolt condition factor.

It should be noted that the following tabuleted values apply to nut torquing only and for torquing by the head, such as capscrews, an additional 15% should be added to the values shown

Bolt Condition Factors

K = 0.2 - Plain Dry or Zinc-Plated Surface

K = 0.15 - Lightly oiled or Cadium-plated surface

K = 012 - With Anti-seize

Torquing Formula

T = (KDW)/12

T - Torque

K - Friction Factor

D - Dia of bolt

W - Load as fraction of proof load

Bolt Seating Torques (imperial sizes)

Bolt Seating Torques (Metric Sizes)

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