Pumps & Systems, February 2008
One of the first steps in shaft alignment preparation is to make sure that the machines are locked out and tagged out. If equipment is accidentally started while an alignment is being performed, serious injuries could occur.
Listed below are other issues, in no particular order, which could prevent the attainment and retention of good alignment.
Foundations
A solid, rigid foundation is essential for a successful alignment. Look over the entire base and be on the lookout for possible effects of elastic deformation of the foundation or machine frame. If the base has any defects, it should be replaced or repaired prior to any attempts at aligning the machinery. Any grout should be of suitable material, in good condition, with no voids beneath the baseplate. The baseplate must be rigid so that no part of any machine foot moves more than 0.001-in out of plane with the other feet of the same machine.
Machine Mobility
It is advisable to set both machines with about 0.050-in of shims underneath the feet in case one machine needs to be lowered. When installing shims, be sure to measure the actual thickness of each shim. Some shims may not be the exact thickness stamped on the shim (i.e., a 0.050-in may measure 0.046-in). Jacking bolts provide a safer and more accurate way to make horizontal moves.
Positional Change
After start-up, machines grow warmer or colder, undergo thermal gradients and may suffer dynamic load shifts. This may cause their shaft centerlines to move from the position they were in when stopped. A good shaft alignment done when the machine is cold and stopped may result in a poor alignment when the machines are running and under load. Some laser alignment systems on the market offer special brackets to measure machine movement in order to establish good alignment targets.
Rigid Couplings
The bolts must be loosened to achieve accurate readings. Another option-if your alignment tool permits-is to take readings uncoupled.
Coupling Play
This can be avoided by turning the shafts a few degrees before starting measurements and then taking readings in the same direction (see the image below). Some of the more advanced laser alignment systems on the market automatically compensate for any coupling play. Whenever possible, rotate from the bottom to the top to let gravity take care of the backlash.
Bearing Play
Check for excessive bearing play in your machines. Excessive radial play of the shaft within the bearing will lead to poor alignment results (see the image above). Measure with a dial indicator or with the actual coordinate values function of your laser alignment tool.
Machinery
Eliminate rust, dirt and any other contamination from the contact surfaces of the machine feet, shims and frame or foundation. Never insert more than four shims at a time beneath a single machine foot. More than three shims may cause a spring effect, or a "squishy" soft foot.
Soft Foot
Soft foot can severely affect the operating condition of a machine, which will undoubtedly shorten its life expectancy. Here are a few simple tips to help avoid soft foot in your machines:
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When taking individual soft foot readings on a four-footed machine, always do so one foot at a time, with the other three feet tight.
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Eliminate external forces on the machine such as those from connected piping, conduit, auxiliary supports, etc. The piping should be well-fitted, supported and sufficiently flexible, so that no more than 0.002-in soft foot movement occurs when the pipe flanges are tightened.
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Use high quality, clean and uniform shims when shimming is necessary.
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The shims should be free from burrs, bumps, nicks and dents of any kind.
Housekeeping
Always keep the area around your machinery neat and clean. Any dirt, rust, loose paint, etc., will eventually get knocked onto the shimming area by wrenches, jacks and other tools. Likewise, clean any jacking or lifting devices that would be a source of dirt to find its way under the foot.
Rough Alignment
Though not exactly necessary with today's advanced laser alignment systems, a quick rough alignment can usually expedite the alignment process.