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Bolted Joint Torque Explanation and Calculation

We are often asked what torque value should be used with a specific nut/bolt assembly.

First, it’s helpful to understand that torque is a rotational force that is equal to a linear force times the distance of that force from the center of the rotational axis.

Remarkably, a simple ratio, with an added K factor, also applies to the torque required to tighten bolted joints.

The K is included in the equation to account for the friction between moving surfaces and several other factors in the assembly that absorb the energy created by the input torque. This simple formula can be used but it is helpful to understand the limitations of this calculation and the resulting values as discussed at the bottom of this page.

Want to skip the math? Download an easy-to-use spreadsheet at the bottom of the page.

Clamp Force

Typically, clamp load (also known as clamp force) is specified in pounds (lbs) for UN bolts and Newtons (N) for metric bolts. The calculation used to determine clamp load was explained in our post last month.

Bolt Diameter

For UN bolts, the diameter is specified in inches (in) and usually converted to feet (divide inches by 12) for the calculation. For metric bolts, the diameter is specified in millimeters (mm) and usually converted to meters (divide mm by 1000) for the calculation. 

K Factor (Basic)

The K factor is used to account for the energy lost to friction and other factors during the tightening process. The generally accepted K factors for this calculation are:

  • 0.20 for assemblies with dry threads (no lubrication)
  • 0.15 for assemblies with lubricated threads

Here are a few example calculations for reference:

  • The formula for a 3/4 x 10, grade 8.8 bolt with dry friction surfaces would be:
    (30,100 lbs) x (0.750 in ÷ 12) x (0.20) = 376 ft lbs
  • The formula for a M24 x 3, class 10.9 bolt with dry friction surfaces would be:
    (219,500 N) x (24 mm ÷ 1000) x (0.20) = 1,054 N m

K Factor Limitations

The K factor can vary significantly from the basic values shown above. Under certain conditions values can be as low as 0.05 or more than 0.30 based on the variables shown below. Actual K factor can only be determined experimentally, and it must be redetermined for each new application and condition change. Even when known you can expect to get plus-minus 30% in application.

Friction may absorb 90% or more of the energy applied to the fastener when torque is applied. This only leaves 10% for generating fastener tension (common friction loss areas shown in illustration).

Other Variables that Affect Tension

  • Condition of the threads (deformation, dirt, dust, or rust for example)
  • Lubrication (type, amount, how applied, where applied, contamination)
  • Surface finish (threads, nut face, clamp surface)
  • Plating
  • Material (type and hardness)
  • Clamp surface alignment

In the next post we’ll examine bolt stretch.

Are you having troubles with bolted joints? Give us a call. We’re here to help.
(847) 970-4050