When trouble-shooting alignment problems are encountered when using a laser system it is imperative to distinguish between lack of ‘Repeatability’ and lack of ‘Response to Corrections’.

Repeatability: The consistency of alignment condition results between two or more consecutive sets of readings, without any intervening adjustments or changes in field conditions.

Response to Corrections: How accurately the machines respond to corrections made (shimming or moves).
On well-built machinery, a good laser alignment system should be able to repeat readings within 0.5 to 3 mils Offset and 0.5 to 3 mils/10 inches Angularity depending on the magnitude of the misalignment. There is little sense in proceeding with the alignment until repeatability is established. If repeatability is poor, seek for and correct the underlying reasons for it first, before attempting to correct the misalignment. Some of these reasons may include mechanical looseness (worn bearings allowing the shafts to rotate inconsistently within them, loose brackets, loose support posts, loose anchor bolts, mounting to a coupling hub that is not solid to the shafts, brushing against obstructions as you rotate, external forces acting on the machinery while readings are being taken (welding going on, pipe strain), changing field conditions (external vibration, machines cooling off or heating up), significant torsional play (coupling backlash), etc.

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, by Pedro Casanova CRL