Does the z axis affect alignment?
In short, no it does not. There are aspects of the z axis that could affect the running of a machine, but rotational shaft alignment is not of them.
What is the z axis?
The X and Y axes are references to horizontal and vertical planes respectively. With regard to precision shaft alignment, this refers to the vertical and horizontal correction(s) necessary to adjust the position of two or more shafts when aligning them to be collinear. The Z axis is the axial plane. This is the movement of the shafts along the rotational axis of the shafts typically referred to as thrust. Thrust does not necessarily affect the machines alignment position.
Why should you be concerned about thrust when aligning a machine?
The primary concern regarding thrust is to allow the proper clearances between shaft ends or coupling hubs. Clearances are required to allow axial movement caused by any force acting upon the machines. Examples are: Thermal growth, bearing play, gear lash or magnetic centering. If the gap is too small, shaft ends or coupling hub faces may come into contact with each other causing damage to the machine components. If no damage occurs, the least would be increased vibration in the machine.
If the gap is too large, unforeseen stress could be applied to the flexible portion of the coupling causing premature failure and downtime. Coupling manufacturers will typically insert the proper “axial gap” information in the coupling installation guide. Different coupling types require different gasp. It’s best to read the instructions. Over greasing a coupling is detrimental also in this regard. Too much grease can cause suction binding or pressure and not allow for axial movement.