Clamping force

The clamping force is the force that acts in a screw or rivet connection between the workpieces lying on top of one another after assembly and during operation. It should be noted that the clamping force required for a secure and permanent hold does not correspond to the necessary axial force in the screw (see assembly preload ).

The required clamping force F _k depends on the axial and / or tangential load in the components to be connected. The clamping force is also required to calculate the assembly preload .

Axial load

Axial load

The screw should ensure a form fit . This means that the joined workpieces must not lift from one another. This also names the second function: The workpieces pressed together should have a sealing effect at the same time . Since containers are often closed in which an operating force (e.g. overpressure ) prevails, the clamping force depends on this. The operating force of the entire workpiece is distributed over the number of screws. If there are more screws available, less force has to be used by each screw. Therefore it is divided by the number of screws.

${\displaystyle F_{\mathrm {k} }(F_{\mathrm {A} })={\frac {S\cdot F_{\mathrm {A} }}{z}}}$

It is south the to be guaranteed safety, d. H. dimensionless redundancy ; F _{A is} the maximum operating force and z is the number of screws.

Tangential load

Tangential load

The screw is intended to prevent the workpieces from shifting against each other, which is achieved through high static friction between the plates. This is called a force fit (or friction fit). The static friction results from the product of the orthogonally occurring normal force (the pressure force) with the coefficient of static friction. By increasing the normal force (here the clamping force of the screw, since it is orthogonal to the friction surface), the friction effect is also increased. The force that the screw exerts on the plates should be so great that it is only subjected to tensile stress and not to shear stress . This case must not occur and must be avoided by using correspondingly high clamping forces.

Clamping force as a function of the transverse force:

${\displaystyle F_{\mathrm {k} }(F_{\mathrm {t} })={\frac {S\cdot F_{\mathrm {t} }}{\mu \cdot z}}}$

It is south the to be guaranteed safety; F _{t is} the tangentially occurring force (also transverse force ), the coefficient of friction and z the number of screws. ${\displaystyle \mu }$

Resulting clamping force required

The required clamping force is the sum of the axial and tangential force components:

${\displaystyle F_{\mathrm {kerf} }=F_{\mathrm {k} }(F_{\mathrm {A} })+F_{\mathrm {k} }(F_{\mathrm {t} })\,}$

See also

• Joining (manufacturing technology)
• Assembly preload