Alignment: Two Shafts That Look In Line Are Not In Line

SOFT FOOT is a machine that rocks on its feet — one foot not touching, or touching only when a bolt is tightened. Align a machine with a soft foot and the alignment changes every time the bolts are torqued. Check it by loosening each foot bolt in turn with a dial indicator on the housing. If the machine lifts more than a few hundredths of a millimetre when a bolt is released, that foot needs shimming. Check the baseplate is flat and clean too. Rust, paint and old shims under a foot compress over the following weeks, and a machine aligned on top of them goes out of alignment by itself.

Clamp an indicator to one coupling half reading the other, and turn both shafts together through four positions — twelve, three, six and nine o'clock. The readings give you the offset and the angle in both planes. Turning both together is what makes it a measurement of ALIGNMENT rather than of the coupling's own runout. Rotate only one and you are measuring how round that half is, which is a different and much less interesting question. Take the readings twice, all the way round, and check the first reading repeats when you get back to it. A set that does not close means something moved — a loose bracket, a soft foot, or the indicator itself — and the numbers are worth nothing until it does.

Correct the vertical error by shimming the feet, then the horizontal by sliding the machine sideways. That order matters: shimming changes the horizontal position slightly, and sliding does not change the vertical. Use as few shims as possible, in the largest sizes available. A stack of ten thin shims compresses over time and springs when the bolts are torqued — so the machine you carefully aligned is out again by the next morning. Allow for thermal growth. A pump that runs hot rises on its feet as it warms, so it is deliberately aligned LOW when cold. Cold alignment is not the target; alignment at operating temperature is, and the cold offset that achieves it comes from the manufacturer or from measuring the machine hot.