Belt Pulley Bolt

The belt pulley is secured with a bolt at its center. Often mounted on the crankshaft in addition to the belt pulley are a gear for the oil pump drive and possibly the vibration damper. The inside bore of the belt pulley is mounted on the crankshaft end journal.

The large bore diameter in the belt pulley makes it necessary to create a positive connection between the bolt and the pulley with a large washer or a large-diameter bolt collar. Often an M 12 bolt is fitted with a washer or collar diameter of up to 38 mm (in gasoline engines) or an M 18 bolt with a collar diameter of up to 65 mm (in a diesel engine with 2.5 liter displacement, for instance).

The pulley is press-mounted separately on the crankshaft journal or is pulled onto the crankshaft by the screws at a previously defined tightening torque. In utility vehicle engines sizes of up to M 24 X 1.5 are used and the washer is positioned just before assembly. In large utility vehicle engines, the pulley is seated on the vibration damper and, passing through oversize bores, is bolted directly to the crankshaft with six or eight screws or lug bolts (e.g., M 10).

In the past, belt pulley bolts were tightened down with only a torque wrench. Today, the rotation angle technique has become more or less standard. Tightening takes place through a snug-tight fit until all mating planes are seated firmly one on the other. The bolt is then turned down further, the amount based on a measurement of the rotation angle.

Extremely high ultimate tightening torques are achieved in this way. When using an M 12 X 1.5 - 10.9 bolt, torques of up to 260 Nm can be achieved, while the ultimate torque calculated theoretically lies between 120 and 150 Nm. The great spread in the ultimate torque results from the large head contact area, which causes “seizure” if there is even the slightest misalignment.

A tightening technique based on the 0.2% offset limit cannot be applied for the pulley bolts when there is an extremely large screw head collar diameter or where several components have to be bolted together so that there is a large number of mating planes between the parts to be joined. Manufacturing inaccuracies and unavoidable grime results in greater setting and a connection that is so flexible that the 0.2% offset limit point is sensed not only for the bolt but also for the connection.