The Bolt Torque & Preload calculator can be used to calculate the torque required to achieve the desired preload on a bolted joint. See the reference section for details on the methodology and the equations used.

(Reference -- a value of 10% is sufficient as a general rule)

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A summary of the installation parameters is given in the table below:

Preload (% Yield)

Preload Force, F_{PL}

Torque Coefficient, K_{T}

Install Torque, T

Warning: The preload in this joint is unusually low. It is recommended that the preload %_{yld} be within the range of 64% - 77%. This ensures that the clamped parts always remain in contact and in compression. A higher preload also allows you to get more utility out of the bolted joint (up to a point), since a higher preload increases the force necessary to separate the joint. See this preload reference for more details.

Warning: The preload in this joint is unusually high. It is recommended that the preload %_{yld} be within the range of 64% - 77%. This ensures that the clamped parts always remain in contact and in compression, while also avoiding yielding of the bolt material. See this preload reference for more details.

The mean thread radius is calculated as half of the mean bolt diameter, which is the average of the minor diameter and nominal diameter:

The collar area is the area of the bearing face of the part being rotated during installation (either the nut or the bolt head). The width across flats of a nut is typically 1.5 times the nominal diameter. In this case, the mean collar radius is calculated as:

Reference Values

d_{nom}

=

d_{m}

=

The lead angle, λ, is calculated as:

°

Reference Values

P

=

r_{t}

=

The torque coefficient, K_{T}, is calculated as:

where α is the thread half angle ( α = 30°, per ASME B1.1, 10.1b).

The nominal preload as a percent of yield strength is calculated as:

%_{yld} = F_{PL} / (S_{ty} A_{t} ) =

Reference Values

F_{PL}

=

S_{ty}

=

A_{t}

=

Preload Uncertainty & Relaxation

Due to preload uncertainty, the actual preload applied to the bolt may be more or less than the nominal value. Due to preload relaxation, there will be some loss in the preload after the joint is installed.

The maximum value of preload accounts for the preload uncertainty, and is calculated as:

F_{PL.max} = (1 + %_{uncrt} ) · F_{PL.nom} =

The minimum value of preload accounts for the preload uncertainty as well as relaxation, and is calculated as:

The table below summarizes the preload values. The nominal value is the design target, and the min and max values account for preload uncertainty and relaxation.

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