Bolt axial force vs torque

Bolt axial force vs torque

The torque required to tighten the bolt:

T=Kcdot Fcdot d

Where:

T:            Torque (N.m)

K:            Torque coefficient

F:            Axial preload force (N)

d:            Nominal bolt diameter (mm)

The torque coefficient:

K=0.5cdot left ( frac{lcdot cosalpha +pi cdot dcdot mu }{pi cdot dcdot cosalpha -lcdot mu } right )+0.625 cdot mu _{c}

Where:

l:             Lead (mm)

α:           Half angle of the screw thread (°)

d:            Nominal bolt diameter (mm)

µ:            Thread friction coefficient

µc:          Collar friction coefficient

Common torque coefficient values for µ=µc=0.15:

Bolt condition    K
Non plated black finish steel bolts 0.3
Mild steel bolts 0.2
Zinc plated steel bolts 0.2
Lubricated steel bolts 0.18
Cadmium plated steel bolts 0.16
With bowman anti-seize 0.12
With bowman-grip nuts 0.09

 

It is estimated that roughly 90% of the input energy is lost in overcoming the mating friction under the head (collar) and between the thread or nut and its mating threads.  Consequently only the remaining 10% of input energy is turned into bolt stretch.

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