What alignment means
When a motor drives a pump, fan or gearbox through a coupling, the two shaft centerlines are meant to form a single straight line when the machine is running. Shaft alignment is the process of measuring how far they depart from that line and moving one machine, usually the motor, to correct it. Poor alignment loads bearings, seals and couplings unevenly, and is a recognized contributor to vibration and early failures. The effects are described in the article on flexible couplings and shaft misalignment.
The misalignment is normally described in two planes, vertical and horizontal, and in two forms: parallel offset (the centerlines are displaced) and angular error (the centerlines meet at an angle). Good alignment reduces both.
Prepare before measuring
Most alignment problems start before any tool is attached. A few steps normally come first:
- Check the foundation and base. A cracked, loose or poorly grouted base cannot hold alignment. Concerns about machine foundations should be settled first.
- Check for soft foot. If one foot of a machine does not sit flat on its base, tightening the bolt distorts the casing and shifts the alignment. Soft foot is normally checked and corrected before alignment.
- Check pipe strain. Pipes forced into position can pull a pump out of line once connected.
- Check coupling and shafts. Look for runout, bent shafts, worn bearings or damaged coupling hubs. Aligning a machine with a bad bearing wastes effort.
- Clean surfaces. Rust, paint or dirt under feet and shims affects readings.
- Consider thermal growth. Some machines grow as they warm up, and the manufacturer may specify an intentional offset when cold.
Straightedge and feeler gauge
The simplest method lays a straightedge across the two coupling halves and uses feeler gauges or a visual check to find gaps. A gauge between coupling faces can estimate angular error. It is quick, needs little equipment, and may be acceptable for low-speed, forgiving equipment and rough initial positioning. It is limited by the accuracy of the coupling faces, which may not be perfectly round or flat, and by the person’s judgment. It is generally not suitable for precise work.
Dial indicator methods
Dial indicators measure small movements. Two common set-ups are:
- Rim and face. One indicator reads on the rim of the other coupling half to measure offset, and another reads on the face to measure angle, while both shafts rotate together.
- Reverse dial. Two indicators read on the rims of both halves. The readings let the technician calculate the corrections at each machine foot. This approach is often considered more accurate for coupled shafts with spacers or large gaps.
Dial methods can deliver good results when done carefully, but they require attention to details such as indicator bar sag, where the hanging bracket flexes under the weight of the indicator, and the arithmetic used to turn readings into shim and move values. Errors in readings or calculation are common, so repeating the measurements is standard practice.
Laser alignment
Laser systems mount a laser emitter on one shaft and a detector on the other. As the shafts rotate, the system records how the beam moves and calculates offset and angle, then shows how much to shim or move each foot. Most can display live results as adjustments are made.
Advantages are speed, clear graphical output, less arithmetic, and often better repeatability. They typically still need a clean setup, rigid brackets, and attention to the same preparation as other methods, including soft foot. A laser tool does not fix a bent shaft or loose base; it reports what it measures. Users should also understand the tolerances in use, not just the numbers on the screen.
Tolerances
Acceptable misalignment depends on speed, coupling type, and manufacturer guidance. Higher speeds generally require tighter tolerances. Coupling makers and machine makers publish limits, and many plants adopt a standard set of targets by speed range. Stated tolerances are limits and not goals. Aligning well within them tends to improve bearing and seal life.
Practical sequence
A typical job, in outline, looks like this:
- Isolate and lock out the equipment using lockout tagout practices.
- Inspect the base, feet, shims, bolts and coupling.
- Check and correct soft foot.
- Mount the tools and take measurements.
- Calculate or read the required vertical shims and horizontal moves.
- Make corrections, retighten, and re-measure.
- Record final values, so future trends can be compared.
- Reinstall the coupling guard before returning to service.
Records are valuable. A drive that repeatedly drifts out of line suggests a base, pipe or thermal problem that needs investigation, perhaps through root cause analysis.
Checking the results in service
Alignment can change over time. Vibration analysis, temperature checks of bearings and the condition of coupling wear parts all give clues about how a drive is running after alignment. Excess vibration at running speed or twice running speed is often associated with misalignment, but it can also have other causes, so findings should be interpreted by trained people.
Safety and limits
Rotating equipment must be fully stopped and locked out before alignment work, and guards replaced afterward. Work should be done by trained personnel following the equipment manual and local regulations. This article describes methods in general terms and is not engineering advice.