Shaft alignment is the process of aligning two or more shafts with each other to within a tolerated margin. It is an absolute requirement for machinery before the machinery is put in service.
When a driver like an electric motor or a turbine is coupled to a pump, a generator, or any other piece of equipment, it is essential that the shafts of the two pieces are aligned. Any misalignment between the two increases the stress on the shafts and will almost certainly result in excessive wear and premature breakdown of the equipment which can be very costly. When the equipment is down, production might be down. Also bearings or mechanical seals may be damaged and need to be replaced. Flexible couplings are designed to allow a driver (electric motor, engine, turbine, hydraulic motor) to be connected to the driven equipment. Flexible couplings use an elastomeric insert to allow a slight degree of misalignment. Flexible couplings can also use shim packs. These couplings are called disc couplings. Tools used to achieve alignment may be mechanical or optical, like the Laser Shaft Alignment method, or they are gyroscope based. The gyroscope based systems can be operated very time efficient and can also be even used if the shafts have a large distance (e.g. on marine vessels).
Before such a shaft alignment can be done, it is also essential that the foundations for the driver and the driven piece are designed and installed correctly. If that is the case, then shaft alignment can be started.
The resulting fault if alignment is not achieved within the demanded specifications is shaft misalignment, which may be parallel, angular, or both. Misalignment can cause increased vibration and loads on the machine parts for which they have not been designed (i.e. improper operation)