Laser Alignment

What is shaft alignment?
For machinery installation, only the rotating shaft centrelines of different machines are aligned. Not the feet, not the coupling, not the shaft surfaces, not the machine housings, not the bearings; only the rotating shaft centrelines. It is important to understand that alignment refers to the positions of two centrelines of rotation or two rotational axes. Note that the shaft’s rotational centreline may be different than its machined centreline.

Shaft alignment means: Positioning two or more machines so that their rotational centrelines are collinear at the coupling point under operating conditions. Collinear means two lines that are positioned as if they were one line. Collinear as used in alignment means two or more centrelines of rotation with no offset or angularity between them.

Rotating axes must collinear during operation

The phrase "coupling point" in the definition of shaft alignment is an acknowledgement that vibration due to misalignment originates at the point of power transmission, the coupling. It does not mean that the couplings are being aligned. The shafts are being aligned, and the coupling centre is just the measuring point.
"At operating conditions" is an acknowledgement that machines often move after start-up due to wear, thermal growth, dynamic load shifts, or support structure shifts.

Besides the above considerations, the term shaft alignment also implies that the bearings and shafts are free from preloads. In properly installed equipment, there are no forces or strains on the bearings and shafts, except those the designers intended. If the machine is installed with the frame distorted because of uneven or imperfect base plates, bent feet, pipe stresses, or whatever, then machine life will be shortened, often significantly.