Bolt Stretch and Clamp Load
Clamp load from F = (stretch * area * E) / grip; cross-check torque.
Example
You enter
- Bolt diameter (in) 0.5
- Grip length (in) 3
- Target stretch (0.001 in) 5
- Fastener material steel
- Torque K-factor 0.18
You get
- Clamp load 7,095 lb
- Cross-check torque 53.2 ft-lb
- Modulus (psi) 30000000
No published worked example is on file for this calculation. It is checked against a hand-worked derivation of its method and an independent reference test.
Details, formula, and sources
Formula or tableClamp load F = (stretch × area × E) / grip. Stretch is residual elongation after torque; E is fastener elastic modulus per material.
Source and editionASTM A325 / A490 / SAE J429 by name; SAE bolt material moduli (steel ~ 30e6 psi, stainless A2/A4 ~ 28e6, Inconel 718 ~ 29.6e6, titanium 6Al-4V ~ 16.5e6, aluminum 2024 ~ 10.6e6). The code uses these except Inconel 31e6 and aluminum 10e6, round figures within the grade-to-grade spread; enter the fastener maker's modulus for a critical joint.
Free accessASTM / SAE licensed; engineering-property values free in Machinery's Handbook (older editions free at archive.org).
What governsEstimate. AHJ and licensed professional govern.
API field namesdiameter_in, grip_length_in, stretch_thou, material, k_factor, clamp_load_lb, cross_check_torque_ft_lb, modulus_psi
- Elastic-region behavior true (stretch < proof-load yield)Hooke's law assumption