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Force Required Strip Bolt Threads Formula and Calculator

Bolt & Screw Torque Charts and Equations

Force Required to Strip the Bolt Threads Formula and Calculator

The amount of tension we can create in a bolt depends not only on the strength of its body but also on the shear strength of its threads. If we’re designing a nut or deciding how deep to make a tapped hole, we’ll want to be sure that the thread engagement length will be great enough to allow us to develop the full ultimate strength of the bolt. A broken bolt is easier to detect than a stripped thread, so we never want the threads to strip.

When thread engagement is equal to thread diameter or when L_e = D

Where

L_e = length of thread engagement (in., mm)
D = nominal diameter of the bolt (in., mm).

Equation 1
F_St = S_u · A_TS

where

S_u = ultimate shear strength (psi, MPa) of the nut or bolt materials
A_TS = cross-sectional area (in.², mm²) through which the shear occurs for a length of engagement equal to D
F_St = the force (lb, N) required to strip that length of threads of a bolt or nut

Tensile force required to break the bolt

Equation 2
F_Ten = UTS · A_S

Thread Stress Area
Metric M Series Thread Stress and Shear Areas

 

Where:

UTS = ultimate tensile strength of the bolt material ( psi, MPa )
A_S = the tensile stress area of the bolt ( in.², mm² )
D = nominal diameter of the bolt ( in., mm )
n = number of threads per inch
F_Ten = the tensile force required to break the bolt ( lb, N )

If the computed F_St is greater than F_Ten your deign is adequate.

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Reference:

Introduction to the Design and Behavior of Bolted Joints Non-Gasketed Joints, Forth Edition
Founding Editor
L. L. Faulkner
Columbus Division, Battelle Memorial Institute,
Department of Mechanical Engineering
The Ohio State University
Columbus, Ohio