Assembly preload

The assembly preload force is the required force in the axial direction that is necessary for the installation of a screw to ensure safe operation. Knowledge of the force is necessary for the appropriate selection of the material or strength class of screw and nut .

The assembly preload is the sum of all forces required to tighten a screw . A distinction is made between minimum and maximum assembly preload. To calculate a screw connection, 3 forces acting on the screw must be taken into account:

1. The clamping force required to connect the parts to be joined. ${\displaystyle F_{\mathrm {K\ erf} }}$
2. The loss of prestressing force as a result of settling , which is caused by the flattening of the roughness of the material surface through pressure. This force must also be applied so that the screw does not loosen due to the setting process. ${\displaystyle F_{\mathrm {Z} }}$
3. The operating force that acts on the connection through dynamic or static loads. E.g. the static pressure in a pressure vessel, or the dynamic load in a coupling. ${\displaystyle F_{\mathrm {A} }}$

Minimal assembly preload

The minimum assembly preload is calculated as follows:

${\displaystyle F_{\mathrm {M\ min} }=F_{\mathrm {K\ erf} }+F_{\mathrm {Z} }+(1-n\cdot \Phi _{\mathrm {K} })F_{\mathrm {A} }}$

In words: The minimum assembly preload force is equal to the sum of the required clamping force , the loss of preload force due to setting and the product of the operating force with the sum of 1 minus the product of the clamping length factor (force application factor) and the force ratio . ${\displaystyle F_{\mathrm {M\ min} }}$ ${\displaystyle F_{\mathrm {K\ erf} }}$ ${\displaystyle F_{\mathrm {Z} }}$ ${\displaystyle F_{\mathrm {A} }}$ ${\displaystyle n}$ ${\displaystyle \Phi _{\mathrm {K} }}$

n can be calculated as follows:

Distance between the force application points divided by the grip length .

Power ratio

The force ratio results from the flexibility of screw and workpiece: ${\displaystyle \Phi _{\mathrm {K} }}$

${\displaystyle \Phi _{\mathrm {K} }={\frac {\delta _{\mathrm {P} }}{\delta _{\mathrm {S} }+\delta _{\mathrm {P} }}}}$

Maximum assembly preload

The maximum assembly preload follows from the minimum:

${\displaystyle F_{\mathrm {M\ max} }=\alpha _{\mathrm {A} }\cdot F_{\mathrm {M\ min} }}$

The tightening factor takes into account the tolerance required by the tightening process. The type of tightening of the screw must therefore be known in the calculation, or a high factor must be expected. ${\displaystyle \alpha _{\mathrm {A} }}$

Assembly tightening process (AV)

${\displaystyle \alpha _{\mathrm {A} }}$	Tightening process
1	Yield point controlled AV
1	Rotation angle controlled AV
1.2	AV with length measurement
1.4-1.8 *	Manual torque controlled AV
1.4-2.5 *	Motorized torque-controlled AV
* (the mean value is often used here)

See also

• Joining (manufacturing technology)
• Clamping force
• Loss of prestressing force as a result of settling
• Operating force
• Expansion screw
• Tension triangle

literature

• Ulrich Fischer: Metal table book . 36., rework. Edition. Verlag Europa-Lehrmittel, Wuppertal 1987, ISBN 3-8085-1086-2 .