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Næxon Learning Center | Millwright Fundamentals

Shaft alignment is one of the most important skills an industrial millwright develops. Motors, pumps, compressors, fans, gearboxes, turbines, and other rotating equipment depend on two connected shafts operating on the same intended centerline.

When those centerlines don’t match, the machine is misaligned.

Two fundamental conditions every millwright needs to understand are offset misalignment and angular misalignment. A machine can have either condition individually, but in the field, it is extremely common to find both at the same time.

What Does Shaft Alignment Actually Mean?

Imagine looking directly through the center of a motor shaft toward the shaft of a pump.

Ideally, the centerline of one shaft would continue directly into the centerline of the other.

DRIVER ───────────── DRIVEN

The shafts don’t necessarily need to physically touch. A coupling connects them. What matters is the relationship between their rotational centerlines.

The driver might be an electric motor while the driven equipment is a pump.

The millwright’s job is to position the movable machine so those shaft centerlines meet the required alignment specification.

What Is Offset Misalignment?

Offset misalignment occurs when the two shaft centerlines are parallel but displaced from each other.

Think of two perfectly straight railroad tracks running beside each other. They’re pointing in exactly the same direction, but they’re not on the same line.

Simplified:

Motor shaft: ─────────────

Pump shaft:     ─────────────

The shafts can have the same angle while one sits higher, lower, left, or right of the other.

Offset can therefore exist in both the vertical and horizontal planes.

Vertical offset is typically corrected by changing the machine’s elevation with properly sized shims. Horizontal offset is normally corrected by moving the machine sideways.

What Is Angular Misalignment?

Angular misalignment occurs when the shaft centerlines are positioned at an angle relative to each other.

Simplified:

Motor shaft: ────────────

Pump shaft:            ╱────────

If those centerlines were extended, they would eventually intersect.

Again, angular misalignment can exist vertically, horizontally, or as a combination of both.

Correcting it requires changing the machine’s orientation—not simply moving the entire machine the same amount.

For example, you may need to add more shim under one pair of feet than another to change the vertical angle.

Offset vs. Angular: The Easy Way to Remember It

Think about the relationship between the shaft centerlines.

OFFSET = parallel, but not on the same centerline.

ANGULAR = centerlines pointing in different directions.

If you understand those two conditions, interpreting alignment measurements becomes much easier.

Most Machines Have Both

Real-world alignment usually isn’t perfectly separated into one condition or the other.

A motor may be:

0.020” too high at the coupling

while simultaneously having an angular condition that places the rear feet farther out of position.

This means simply dropping the entire motor by 0.020” may correct one measurement while leaving the angular problem.

The millwright has to determine how much movement is required at the front feet and rear feet.

That’s where dial indicators or laser alignment systems become extremely useful.

Vertical and Horizontal Alignment

Alignment should be considered in two planes.

The vertical plane deals primarily with the machine being too high, too low, or vertically angled. Corrections are normally made by adding or removing shims beneath the machine feet.

The horizontal plane deals with the machine being too far left, too far right, or horizontally angled. Corrections are normally made by moving the machine sideways using jacking bolts, come-alongs, hydraulic tools, or other approved positioning methods.

A machine can be vertically aligned while still being horizontally misaligned.

Both must be checked.

A Simple Field Example

Suppose you’re aligning a motor to a centrifugal pump.

Measurements show that the motor shaft is parallel with the pump shaft, but the motor centerline is:

0.030” too high.

That primarily indicates a vertical offset condition.

If the required correction truly calls for a parallel vertical move, the motor needs to move downward while maintaining its orientation.

Now imagine the motor is:

0.010” high near the coupling

but the calculated position at the rear feet indicates:

0.040” high.

Now you’re dealing with an angular component as well. The rear of the motor requires a different correction than the front.

The exact foot corrections should come from the alignment method and measurements being used rather than from eyeballing the coupling.

Why Small Misalignment Matters

A few thousandths of an inch may look insignificant when you’re standing beside a large motor and pump.

Rotating equipment sees things differently.

Misalignment can contribute to increased vibration, coupling wear, bearing loading, seal problems, elevated temperatures, looseness, and premature equipment failure.

A machine can even appear to run reasonably well while continuously loading components unnecessarily.

That’s why experienced millwrights don’t judge alignment by appearance.

They measure it.

Don’t Force the Coupling Together

A common mistake is assuming that if the coupling bolts together, the shafts must be aligned.

They aren’t necessarily.

A flexible coupling is designed to accommodate limited movement and operating conditions according to its design—not to compensate for poor machine installation.

Forcing a coupling together can hide an alignment problem rather than correct it.

Alignment Starts Before the Indicators

Good alignment doesn’t begin with a dial indicator or laser.

Before precision alignment, the millwright should verify the equipment foundation and base condition, mounting surfaces, machine feet, fasteners, coupling condition, and obvious mechanical problems.

One particularly important condition is soft foot.

If one machine foot isn’t sitting correctly on the base, tightening the hold-down bolt can distort or move the machine and destroy an otherwise good alignment.

Correct the mechanical condition before chasing thousandths.

Rough Alignment Before Precision Alignment

Machines should normally be brought reasonably close before precision measurements begin.

Check the general equipment position, coupling relationship, elevation, and obvious horizontal displacement.

Then perform the appropriate precision alignment procedure using the equipment, tolerances, and methods specified for that machine.

This keeps you from wasting time taking precision readings while the machine is obviously far out of position.

Common Millwright Mistakes

One of the biggest mistakes is treating every alignment problem as simple offset. Adding or removing the same shim thickness under every foot won’t correct an angular condition.

Another is making large horizontal corrections before understanding what the readings are telling you.

Other problems include dirty shims, excessive shim stacks, burrs beneath machine feet, loose bases, soft foot, incorrect indicator setup, coupling movement, shaft runout, and failing to recheck measurements after tightening the hold-down bolts.

Alignment is a process of measure → correct → tighten → verify.

Never assume the machine stayed where you put it.

Thermal Growth Matters

A machine that is perfectly aligned while cold may not necessarily be correctly aligned at operating temperature.

Motors, pumps, turbines, compressors, piping systems, and their supports can expand as temperatures increase.

For certain equipment, the specified cold alignment position intentionally places the machines slightly misaligned while shut down so thermal growth brings them into the intended operating alignment.

Always follow the equipment manufacturer’s specifications and the project’s alignment requirements.

Field Rule

OFFSET MOVES THE CENTERLINE.

ANGULAR CHANGES THE DIRECTION OF THE CENTERLINE.

And remember:

Most real alignment problems contain some of both.

Knowledge Check

1. Two shaft centerlines are parallel but 0.025” apart. What type of misalignment is present?

Offset misalignment.

2. Two shaft centerlines would intersect if extended. What condition does that indicate?

Angular misalignment.

3. Can a machine have offset and angular misalignment simultaneously?

Yes. This is extremely common.

4. What is normally used to make vertical corrections beneath machine feet?

Properly sized shims.

5. Why should alignment be checked after tightening the machine?

Because tightening hold-down bolts can move or distort the machine and change the alignment readings.

Practical Exercise

Draw two machines labeled MOTOR and PUMP.

First, draw their shaft centerlines parallel but separated vertically. Label this:

OFFSET MISALIGNMENT

Next, draw the motor shaft centerline at a slight angle relative to the pump shaft centerline. Extend both imaginary centerlines until they intersect. Label this:

ANGULAR MISALIGNMENT

Finally, draw a third example where the shafts are both displaced and angled.

Label it:

COMBINATION MISALIGNMENT

If you can look at those three diagrams and immediately recognize what is happening to the shaft centerlines, you’ve learned one of the fundamental concepts behind precision millwright alignment.