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Bolt Elongation Calculator per. MIL-HDBH-60

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Bolt Elongation Stress Formulas and Calculator per. MIL-HDBH-60

Micrometer Bolt Elongation Method: Below the proportional limit. bolt elongation is directly proportional to axial stress, i.e., elongation increases in the same proportion as stress increases. If both ends of the bolt are accessible, a micrometer measurement may be made before adafter tension is applied in order to insure propoer axial stress.

Preview Bolt Elongation Stress Calculator

Note: Variations due to rounding

The following formula applies:

Equation 1
σB = ft LB / E

Equation 2
LB = σB E / ft

Equation 3
ft = σB E / LB

Equation 4
E = ft LB / σB


σB = Bolt elongation, in (mm)
ft = Axial stress based on thread stress area, psi (MPa)
LB = Effective bolt length, in (mm), see Figure 1
E = Modulus of elasticity, psi (MPa)

Note that for more complex bolt geometry, elongation is equal to the summ of the elongation of each section, with allowances made for transitional stresses in bolt head height and nut engagement length.

The micrometer method is most easily applied to a bolt that is hteaded its entire length or has very few threads in the bolt grip area so that elongation will be practically uniform throughout the length. Ends must be parallel. it is not procatical for general use but may be used on a limited basis. Though this mehod is not practical for general use but may be used on a limited basis.

Equation 5
LB = ( dts / d )2 ( LS + 0.5 HB ) + LJ - LS + 0.5 HN

Elongation length application in elongation formula
Elongation length application in elongation formula Figure 1

Military Handbook, Threaded Fasteners - Tightening to Proper Tension MIL-HDBK-60