Notes On Threaded Connections Fastened Using A Tightening Angle-Based Tightening Process - AH00.00-P-0020-01A

Model all (4xWD, BUS, CAR, Light transporter, TRUCK, UNIMOG, smart) 

In the angle-controlled tightening method, the torque is referred to first, then the tightening angle of the bolted connection as a set variable. In other words, the bolt or the nut is tightened up to a threshold torque, and then turned further from this point onward to a specified retightening angle.

This method is used, in particular, for locking bolt connections in which the bolt has to be tightened in the plastic range. In this plastic range any further turning of the bolt will not lead to any practical, appreciable increase in torque rise, so that the torque cannot then be used here as a set variable.

Generally, bolts designed for the angle-controlled tightening method have a wide plastic range. As they are permanently deformed while being tightened, they can no longer be reused.

If a bolt is tightened beyond the specified range (angle), this can led to the maximum load for the bolt being exceeded and the bolt being destroyed.

By comparison: In the torque-controlled tightening method, the threaded connections are only tightened in the elastic deformation range. In other words, the deformation recedes when the load is relieved (preload force/torque), and the fasteners can be used again.

Bolted connections manufactured in accordance with the angle-controlled tightening method are therefore subject to the following rules:

• Use fasteners are damaged when unfastened and they are a danger to road safety. After unfastening the bolted connection all used bolts, nuts and other fasteners must be disposed of. New fasteners must always be used when making a new bolted connection.
• If the retightening angle is exceeded the threaded connection will be destroyed. In this case, the bolted connection must be renewed and the procedure repeated. This is the only way of guaranteeing a correct bolted connection as well as avoiding any damage, malfunctions and consequential damage.

Exceptions 

• Model 963, 964:

  All screws for fastening the steering bracket to the frame can be reused. The tightening angle was reduced from the outset so that the bolts were tightened into the plastic range, but because of the flat and elongated plastic range they could be used up to a total of 5 times.

• Stretch bolts:

  Stretch bolts are used where bolted connections are exposed to high operating forces and fluctuating temperatures, and where an endurance limit for the bolted connection can be assured.

  Stretch bolts can be tightened according to both the torque-controlled and the angle-controlled tightening methods. In contrast to conventional bolts they are however not tightened in the plastic range. This means that stretch bolts can be reused, as long as they do not exceed the permissible shank length.

  The use of stretch bolts is documented in the corresponding work instructions. The permissible shank lengths are also available there.