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Bolt Torque Calculator

Calculate required bolt torque

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The relation between applied torque and axial force - or load - in a bolt can be calculated as

T = K F d                      (1)

where

T = wrench torque (Nm, in lb)

K = constant that depends on the bolt material and size

d = nominal bolt diameter (m, in)

F = axial bolt force (N, lb)

Typical values for K with mild-steel bolts in range 1/4" to 1":

  • normal dry: K = 0.2
  • nonplated black finish: K = 0.3
  • zink-plated: K = 0.2
  • slightly lubricated: K = 0.18
  • cadmium-plated: K = 0.16

Note! - be aware that this is a rough calculation - in addition the accuracy of a torque wrench is normally no better than +-25%.

Bolt Torque Calculator

The calculator below can be used to calculate the torque required to achieve a given axial bolt force or load. The calculator is generic an can used for imperial and metric units as long as the use of units are consistent.

K - constant

d - diameter of bolt (m, in)

F - axial bolt force or load (N, lb)

torque reduction due to lubrication (%)

Note that standard dry torques are normally calculated to produce a tensile stress - or axial force or clamp load - in the bolt that equals to 70% of minimum tensile strength or 75% of proof strength.

Example - Required torque for tightening a Imperial bolt

The required bolt clamping force for a joint is 20000 lbs. The torque required for a 3/4" dry steel bolt to achieve this tension can be calculated as

Tdry = (0.2) (20000 lb) (0.75 in)

     = 3000 (in lb)

     = 3000 (in lb) / 12 (in/ft)

     = 250 (ft lb) 

Example - Required torque for tightening a Metric bolt to proof load

The proof load for a M30 metric bolt grad 8.8 is 373000 N. The torque required to achieve this tension with a dry bolt can be calculated as

Tdry = (0.2) (373000 N) (30 mm) (10-3 m/mm)

     = 2238 (Nm)

Lubricating the bolt with SAE 30 oil reduces the torque required to achieve the same tension with approximately 40%. The reduced torque can be calculated

TSAE30 = (2238 Nm) (1 - (40%) / (100%))

   = 1343 Nm

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  • en: bolt torque load calculator tension
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