March 8, 2012
By Stan Riddle
We are sometimes asked, “Does your laser alignment equipment measure axial alignment?” While it sometimes raises some eyebrows, the answer most frequently given is, “There’s no such thing.” What is sometimes referred to by maintenance personnel as “axial alignment” is really axial spacing, or coupling gap.
Shaft alignment refers to orienting two shafts, connected by a coupling, to where they are collinear, or in the same line in space. The reason the term axial alignment is used is because many mechanics learned to correct angular misalignment by measuring the faces of the two coupling flanges. But what is actually measured during the shaft alignment process is angular misalignment, not the coupling gap. They are different.
Coupling gap refers to the distance between the two coupling flanges, and is normally done to allow the coupling to compensate for slight misalignment, to allow the coupling insert to “float,” or have some degree of freedom of movement, to minimize coupling binding, and to prevent unwanted axial or thrust loading of the shaft bearings.
If your couplings can be moved on the shafts, it is preferable to complete the shaft alignment, and then set the coupling gap. If the coupling flanges are interference fit, sometimes spacer blocks are employed to keep the coupling flanges at the proper spacing.