How Bolt Clamping Force Works
When you tighten a bolt, you convert torque into tension. The bolt stretches like a spring. That stretch pulls the joint together and creates clamping force.
The Torque-Force Relationship
The formula F = T / (K x D) is simple but powerful. T is your applied torque in inch-pounds. K is the friction coefficient. D is the bolt diameter. Most of the torque you apply goes to fighting friction. Only about 10 to 15 percent becomes actual clamping force.
Friction Matters
Friction is the biggest variable. A dry bolt with K = 0.20 gives 25% less clamping force than an oiled bolt with K = 0.15 at the same torque. Always know your friction condition before setting torque values.
Pro tip: Use a consistent lubricant on bolt threads. This makes your clamping force more predictable. Inconsistent friction is the top cause of loose joints and broken bolts.
Proof Load
Proof load is the max force a bolt can take without permanent stretch. Grade 5 bolts handle 85,000 psi. Grade 8 handles 120,000 psi. For reusable joints, stay below 75% of proof load. For critical joints that get torqued once, you can go to 90%.
Multiple Bolts
Total clamping force equals the force per bolt times the number of bolts. Tighten bolts in a star pattern to spread the load evenly. Uneven tightening causes one bolt to carry more than its share.