<b>Shaft Coupling</b> The propeller shaft is rarely connected directly to the gearbox or the crankshaft of the engine. As a rule, the shaft systems are divided, especially for reasons of ease of assembly. The connection of the individual shafts is made structurally by shaft couplings. Both fixed (rigid) and flexible couplings can be used. Furthermore, a distinction is made between switchable clutches (which enable the drive train to the engine to be switched on and off) and non-switchable clutches. Shiftable clutches work on the principle of frictional engagement. There are also hydraulic and electromagnetic clutches. When using fixed or rigid couplings, ensure that the shaft components are precisely aligned. Elastic couplings, on the other hand, are able to compensate for deviations in the longitudinal direction caused by temperature changes or changing shear forces. They compensate for wave movements in a radial direction, caused by bending moments (dead weight, torsional vibration). They can also compensate for slight angular deviations between two shafts. In addition, elastic shaft couplings compensate for abrupt torque fluctuations (e.g. through rapid speed changes); the flexible couplings can alleviate such shock loads by temporarily storing mechanical work.