Finnveden Bulten - FastCalc

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Tightening torque

Tightening torque information

 

In this app tightening torque is calculated with the following formula:

Where dm is thread friction diameter, µ1 is thread friction, Dk is bearing surface friction diameter, µ2 is bearing surface friction and P is pitch. There are three terms within the brackets, two describe the part of applied torque that is required to overcome the friction and one how the pitch transforms torque to preload.

D = Bearing surface outer diameter

d = Bearing surface inner diameter

Thread friction diameter, dm, is considered equivalent to pitch diameter.

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Preload

Preload information

In this app preload is calculated with the following formula:

Where Dk is the bearing surface friction diameter, dm is the thread friction diameter, µ1 is the thread friction and µ2 is the bearing surface friction. The torque is devided by three parts: Pitch, thread friction and bearing surface friction.

Thread friction diameter, dm, is considered equivalent to pitch diameter.

Bearing surface friction diameter, Dk, is the mean diameter of the screw head surface in contact with the underlying surface.

 

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Torqued tensile load

Ultimate tensile load information

Ultimate tensile load = Stress area*Tensile strength

The tensile strength is given by the fasteners property class. The stress area is calculated with the formula stated in ISO 8908-1:

 

Where d2 is the basic pitch diameter of the thread and d3 is the minor diameter of the thread.

In which d1 is the basic minor diameter and H is the height of the fundamental triangle of the thread.

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Surface pressure

Surface pressure information

In this app bearing surface pressure is calculated with the following formula stated in VDI 2230:

Where P is bearing surface pressure, F is clamp load, D is bearing surface outer diameter and d is beraing surface inner diameter. D is usually equivalent to dw in fastener drawings. The most accurate way to determine D and d is to tighten and untighten the fastener which will give a measurable marking on the bearing surface.

 

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Engagement length information

An approximation of minimum* thread engagement can be calculated with the following formula:

*Minimum engagement length is often less than fastener guidelines recommend. Use safety factor to get appropriate engagement length for your design.

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