mechanical calculator
Bolt Clamp Load Calculator
Recommended clamp load (bolt preload) for a UNC bolt by nominal size and SAE grade — 75% of proof load, the Machinery's Handbook standard.
How it works
clampLoad = 0.75 x proofStrength x tensileStressArea. Grade proof: Gr2 = 55,000 psi, Gr5 = 85,000 psi, Gr8 = 120,000 psi.
Worked example
A 1/2"-13 Grade 5 bolt (At = 0.1419 in2): 0.75 x 85,000 x 0.1419 = 9,046 lb.
When to use it
- 75% of proof load is the standard target for a reusable joint; some specs use 90% for permanent joints.
- Convert clamp load to tightening torque with the bolt-torque calculator (T = K x D x F).
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All results are an engineering estimate — not for custody transfer. Field measurement and certified tables govern.