March 30, 2016
If It Moves Your Laser Will See It
By Brad Case
Laser Shaft Alignment tools are a great asset for today’s maintenance professional, enabling them to perform precision shaft alignments to very tight tolerances; and do so quickly when using the VibrAlign Verti-Zontal Process. Laser alignment systems are a tool and as such show the user what is occurring “mechanically” during the alignment process, including if components are moving during said shaft alignment process. “If it moves, the laser will see it”.
There have been numerous blogs on the VibrAlign Blog (formerly TheAlignmentBlog.com) concerning controlling unintentional movement (soft foot, cupped washers, excessive bearing clearances, coupling backlash, etc.). Sometimes you cannot correct the cause of the unintentional movement so you have to control it with your alignment process, the most common example being coupling backlash. Othertimes the source of unintentional movement can be more difficult to identify.
The OJT portion of a recent training class was to check the alignment of a 125HP, 1785 RPM electric motor to a gearbox on a conveyor drive. The motor was mounted to the gearbox by a “sugar scoop” style mount.
SUGAR SCOOP MOUNT
The initial alignment results revealed the vertical angle was 10.5 times the maximum allowable tolerance and the vertical offset 3 times tolerance. The horizontal angle was in tolerance with the horizontal offset 6 times tolerance.
After making a substantial shim correction and adjusting the horizontal position the mechanics noticed the left front motor foot now had Obvious Soft Foot which took 30 mils (.030”) of shims to correct. No other obvious soft foot was detected however as the motor feet hold down bolts were tightened (in 3 passes) movement was observed with the alignment moving out of tolerance.
The mechanics utilized the soft foot app of the XA which showed 14 mil of movement at the right front foot when the hold down bolt was loosened. The other 3 feet also showed some movement when checked.
Further investigation found there really wasn’t soft foot. So was the laser alignment system incorrect? It wasn’t, the unintentional movement was due to other factors.
It was determined the replacement electric motor was slightly larger than the original as the rear feet pads overhung the end of the sugar scoop mount by approximately 2 inches. Also, the right side of the sugar scoop had an angled brace welded to. There was not a brace on the left side and this conveyor drive was mounted several stories above ground on steel girders.
It was concluded that as the motor feet hold bolts where loosened and tightened (using the recommended 3 passes) the approx. 1800 lb. motor was causing the sugar scoop base to flex resulting in the unintentional movement.
In this case the mechanics adapted their procedure by tightening the hold down bolts in multiple passes, one turn of each foot bolt each pass. It took about 10 passes, however there was very little movement and the alignment stayed within tolerance.
After re-measuring the vertical and horizontal coupling values were well within tolerance, almost meeting 3600 RPM tolerances.
If there is movement your laser shaft alignment system will “see it”. The key to a successful alignment is to explore the possible causes of the movement, determine how best to correct or control it and not get frustrated by thinking that your laser system is giving you “incorrect information”.