Bolted Joint Calculator: Preload, Separation Load and Clamp Force

Preload, bolt stress, the load at which the joint separates, the clamp force left under load, and the tightening torque.

Calculated from standard textbook formulas, with every step shown. The formulas are fixed, so the working does not need checking: what matters is the numbers you type in. Use these calculators only if you are competent to choose the inputs (loads, material strength, dimensions, supports) and check your units and assumptions. For learning and checking your own working: design and sign-off are for a qualified engineer. Drafting Central does not store the numbers you type.

How this calculator works

Enter the bolt stress area and proof strength, the preload as a fraction of the proof load, the joint constant C (the share of the outside load that the bolt carries) and the outside load on each bolt. The preload is the fraction times the proof load. The joint separates at the preload divided by 1 minus C.

Under the outside load P the bolt force rises to the preload plus C times P, and the clamp force falls to the preload minus (1 minus C) times P. The calculator checks that the clamp force stays above zero and that the bolt stays below its proof load. The tightening torque is the nut factor times the preload times the bolt diameter.

The joint constant can be entered, or worked out from the bolt and clamped part stiffnesses. The proof strength and nut factor are inputs, and should be taken from the bolt standard and the tightening procedure.

Formulas used

  • F_i = f\,S_p A_s
  • P_{sep} = \frac{F_i}{1 - C}
  • F_b = F_i + CP
  • T = K F_i d

Worked example

With the default numbers in the calculator above:

  1. Proof load of the bolt: 91.06 kN
  2. Preload: 68.30 kN
  3. Bolt stress at preload: 435.00 MPa
  4. Load at which the joint separates: 85.37 kN
  5. Safety factor against separation: 2.13
  6. Bolt force under the outside load: 76.30 kN (stress 485.96 MPa)
  7. Clamp force left in the joint: 36.30 kN
  8. Tightening torque: 218.54 N·m

Change the numbers in the calculator to see the full working, every step, for your own case.

Common questions

What preload should I use?

About 75 percent of the proof load for a joint that is taken apart and reused, and up to 90 percent for a permanent joint.

What is the joint constant?

The bolt stiffness divided by the bolt plus joint stiffness. A stiff clamped joint and a soft bolt gives a small value, so the bolt sees little of the load.

How accurate is the torque?

Torque by the nut factor can be 25 percent out because of friction. Use a controlled method such as turn-of-nut or tension measuring for critical joints.

Related calculators

Got a question about this? Ask an engineering question and get a plain-language answer with references.

Further reading: Shigley, Mechanical Engineering Design (bolted joints, the joint constant); AS 1110 and AS 4291 (bolts); AS 4100 (steel structures).

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