Screw calculation

Screw verifications can be divided into prestressed and non-prestressed connections. The decisive factor is that the shear force is transferred to the connection either via the bolt shaft or via the friction of the connected plates.

In the case of transmission via the screw shank, also known as a shear-hole friction connection, the force is transmitted via the shear stress (shear force) across the cross-section of the screw. This is also the case with bolts or rivets. A typical standard for the verification of these connections is Eurocode 3, DIN EN 1993-1-8. This also includes the verification of the connected plates against punching (due to axial force) and against perforation (due to shear force). The forces occurring are input variables in the calculation, and care must be taken to ensure that the partial safety factors on the loads are taken into account. The forces can be calculated using FEA, for example.

In the case of slip-resistant pretensioned screw connections, the principle is that the pretensioning of the screw ensures a sufficiently large frictional force so that sliding resistance is given. In Eurocode 3, this verification is carried out in a “simplified” way and checks whether the shear force that occurs can be transferred.

The bolt calculation according to VDI 2230 is essentially based on the principle that the necessary preload force is less than the permissible preload force. In addition to the shear force transmission, the necessary clamping force for sealing functions is also taken into account. In the case of the permissible preload force, the tightening process in particular is taken into account, as the achievable preload forces have a relevant scatter as a result. Other points are, for example, the loss of preload force due to setting and thermal load. VDI 2230 also takes other points into account, such as the minimum screw-in depth, the cyclical load on the screw and the surface pressure.