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The way to prevent misalignment is to follow a set sequence: first the base and foot seating, then rough alignment, then fine alignment with a measuring instrument, and finally a repeat measurement after the bolts are tightened. A connection left "by eye" or set with a straightedge may look fine in the first days, but it noticeably shortens bearing and coupling life.
There are two kinds of misalignment
Parallel misalignment means the two shaft axes are parallel but offset. Angular misalignment means the axes meet at an angle. In real installations both occur together, in both the vertical and horizontal planes; that is why measurement is done in two planes.
Recommended working sequence
- Clean the base surface; leave no paint blisters, burrs or old rust flakes.
- Set the motor in place and fit the bolts by hand, then loosen each foot in turn to check for a gap underneath (soft foot).
- Carry out rough alignment with a straightedge and feeler gauge.
- Correct in the vertical plane, then in the horizontal plane, using a dial indicator or a laser alignment tool.
- Tighten the bolts in a cross pattern and repeat the measurement; readings often change during tightening.
Factors that are easily overlooked
- Thermal growth: A pump or fan casing that runs hot may rise at operating temperature; if the machine maker recommends a cold-alignment offset, follow it.
- Pipe strain: If the pipework connected to a pump pushes on it, alignment is lost without the motor ever being removed.
- Base rigidity: A thin sheet-metal base flexes in operation and changes the alignment.
The motor dimensions must be right from the start
If the new motor's shaft height differs from the old one, alignment will require a large number of shims. Comparing shaft height and foot dimensions before ordering therefore makes the job easier. For heavy drives you can review general-purpose industrial motors and the shop model list with its dimension tables.