Montagetechnik für
Kunsstoffverbin-dungsschrauben

Assembly technology for
plastic connecting screws

To ensure secure screw connections and smooth assembly, many influences must be taken into account. A sufficiently large difference (Δ) between the tightening and overtorque is just as important here as a suitable screwdriver with torque and / or rotation angle cut-off.

The screwdriver speed should be between 400 and 800 min^-1 Basically, when screwing thermoplastic components, important design points such as application temperature, load time and applied elongation or stress must be observed, as on the one hand plastics can withstand significantly lower surface pressures compared to steel or similar and on the other hand the creep behavior causes a drop in stress in the screw connection, which in certain cases, such as under alternating thermal loads, can lead to loosening of the connection.

In the case of direct screw connections of plastics, large variations in the screw-in torque and overtorque are generally observed, which are caused by geometric influences or varying consistency of the material.

• The tightening torque MA must be set low in order to minimize the stress on the material.
• The following applies to MA: MA=ME + k(MÜ-ME) where the factor k is between 0.3 and 0.5
• The factor k covers the fluctuation range of the screwdrivers. Modern precision screwdrivers, for example, work with scatter ranges of approx. 5% and allow a factor of 0.3.
• A practical rule requires that MAmax= 0.6 x MÜmin. This should be the upper limit of the tightening torque.
• The screw-in torque ME is divided into a thread-forming component MForm and a friction component MReib.
• ME=MShape+MFriction
• The tightening torque is determined as a function of the required clamping force and set on the screwdriver. The repeat accuracy of the screwdriver and the actual preload force achieved should be checked in component tests in order to be able to take into account any influences that have not yet been recorded.
• Under normal design conditions, it is possible to repeat the tightening process several times on the same component. The original tightening torque should be selected.

Determining the tightening torque MA: For a secure screw connection, it is necessary to achieve the greatest possible difference between the screw-in torque ME and the overtorque MÜ (or breakage of the screw).