Loosening the Coupling Bolts before Measuring for Misalignment

Loosening the Coupling Bolts before Measuring for Misalignment

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Loosening the Coupling Bolts before Measuring for Misalignment

Author: Stan Riddle, VibrAlign

VibrAlign recently had a webinar on the importance of roughing in alignment before measuring with the laser. Just a couple of weeks after our webinar, I got a call from a customer who was having repeatability issues with his laser. Since this customer lives near me, I rode by to see if I could help.

Aligning a motor to a gearbox
Aligning a motor to a gearbox

The customer was trying to align a motor to a gearbox for an overhead hoist, which they manufacture. Their hoists are extremely well made. All components of the hoist to be aligned were clean, straight, and solid. Soft foot had been reduced to 1 mil or less. The coupling between the motor and gearbox is a single spline “half-coupling”. This type of coupling uses one rigid hub (mounted on the gearbox side), and a single splined element on the motor side. This coupling design can transmit large amounts of horsepower in a fairly small size.

The customer performed a few alignment measurements, which had gotten the alignment close. But when performing the final shim, the laser calculated adding 2 mils under the inboard, and 7 mils under the outboard, which they added exactly as specified.

laser align motor to gearbox
After shimming, the laser recommended removing 7 mils from the inboard, and 11 from the outboard.

laser align motor to gearbox

This is very typical of the errors encountered when looseness is present somewhere in the system-either in the bolting, or the laser brackets. All brackets were tight, the sensors were clean, and there was zero backlash.

I recommended loosening the coupling bolts, and re-measuring, which showed the motor 11 mils high at the coupling. Using the Fixturlaser Foot Lock™ app told them to add 5 mils under inboard side of the gearbox. This achieved a final alignment value of 1 mil or less at the coupling, and was repeatable.

laser align motor to gear box

When aligning semi-flexible (or rigid) couplings, you must either rough in the alignment closely, remove the coupling insert, or as we did in this case, loosen the coupling bolts a bit to minimize radial forces on the shafts. Your alignments will be faster and repeatable, and should not cause you to question the accuracy of your laser.

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