Screw Tightening Process

When selecting the assembly and tightening technique, one must remember that passenger car engines are manufactured in large numbers; utility vehicle engines, by contrast, are built in short production runs or even individually.

Torque-Controlled Tightening. Torque-controlled tightening is normally used only for secondary applications (minimum preload force need not be exactly defined). It is employed only in demanding applications (such as mounting the belt pulley) in automated assembly lines. It continues to be used in service work.

The problem is that the preload value applied under torque control has to be selected so that in the worst case (i.e., smaller actual coefficient of friction than what was estimated when establishing the torque level) the 0.2% offset limit is not exceeded as otherwise the screw would be stretched. Preload is the force that is present in the threaded connection after the completion of the assembly.

At a very high actual coefficient of friction (higher than what had been assumed), the preload value is very low. Consequently, the properties of the screw cannot be fully exploited with this technique. Screw and bolt manufacturers and the automotive industry have agreed upon the coefficients of friction to be expected. They lie between μ_Total = 0.08 and 0.14. They are a component in the quality agreement in each case and are spot checked for each batch of screws at a friction value test device.

A special form of torque-controlled tightening is the combination with “snugging down”; once the tightening phase is completed, the connection is retightened with a torque wrench (Fig. 7-331). This technique is used in mass production for all critical connector elements at those manual assembly stations that are still found in short production runs.

Fig. 7-331. Subassembly installation using a handheld power screwdriver with integrated torque and rotation angle transducers (Atlas-Copco)

When manual connection is used, a torque-controlled pneumatic screwdriver is employed to tighten the screw or bolt down to the specified moment; then a torque wrench is used to retighten. The final position is normally marked with paint. The torque required to restart rotation is the snugging moment. Experience has shown that snugging usually goes beyond the adjustment value for the wrench so that an indirect, rotation-angle-controlled tightening process is often used.