Næxon Learning Center | Millwright Fundamentals
Machine alignment can be performed with extremely accurate instruments and still produce a poor result if the machine is sitting incorrectly on its base.
One of the first conditions an industrial millwright should check before aligning rotating equipment is soft foot.
Soft foot occurs when all of the machine’s mounting feet do not sit firmly and evenly on their mounting surfaces. When the hold-down bolts are tightened, the machine frame can be pulled, twisted, or distorted to force those feet against the base.
That movement may be only a few thousandths of an inch—but in precision machinery, a few thousandths matter.
Understanding how to identify and correct soft foot is therefore one of the foundations of good shaft alignment.
What Is Soft Foot?
Imagine placing a four-legged table on an uneven floor.
Three legs touch the floor, but the fourth sits slightly above it. Push down on that corner and the table rocks into position.
A machine can behave the same way.
A motor, pump, compressor, gearbox, or other rotating machine may have four or more mounting feet. Ideally, every foot should make solid contact with the mounting surface before the hold-down bolts are tightened.
If one or more feet do not, tightening the bolts forces the machine into position.
That condition is called soft foot.
The problem isn’t simply the gap underneath the foot. The real concern is what happens to the machine when the mounting bolts are tightened.
The casing can distort.
Bearing housings can move.
The shaft centerline can shift.
And the alignment readings you worked hard to achieve can change.
Why Soft Foot Must Be Corrected Before Shaft Alignment
A common mistake is treating alignment and soft foot as two separate problems.
They are directly connected.
Suppose you align a motor to a pump while one motor foot has a significant soft-foot condition.
You achieve excellent alignment readings.
Then the hold-down bolts are tightened.
As the bad foot is pulled against the base, the motor frame moves slightly. That movement changes the position of the shaft.
Your alignment is no longer what the instrument originally showed.
This is why a proper alignment sequence generally follows this order:
Inspect → Correct Soft Foot → Rough Align → Precision Align → Tighten → Verify
Soft-foot correction establishes a stable foundation for everything that follows.
The Four Main Types of Soft Foot
Not every soft-foot condition looks the same. Understanding the different types helps determine the correct repair.
1. Parallel Soft Foot
Parallel soft foot occurs when the machine foot and mounting surface are essentially parallel, but there is a uniform gap between them.
Think of two flat surfaces separated by a small amount.
For example:
Machine foot
↓
0.010” gap
↓
Baseplate
A properly sized flat shim can usually correct this condition.
If the measured gap is approximately 0.010 inch, the correction may require approximately that amount of additional shim thickness.
But the final condition should always be verified after the correction.
2. Angular Soft Foot
Angular soft foot is different.
Instead of having an even gap underneath the entire foot, the machine foot contacts the base on one side while the opposite side remains elevated.
The surfaces are not parallel.
Imagine a wedge-shaped gap:
0.000” → 0.004” → 0.008”
Simply stacking a flat 0.008-inch shim underneath the entire foot may not correct the actual problem.
Possible causes include:
- bent machine feet
- damaged mounting pads
- poorly machined surfaces
- weld distortion
- debris underneath part of the foot
- excessive paint or coatings
- distorted baseplates
The actual cause should be investigated rather than automatically adding shims.
3. Squishy Soft Foot
Sometimes the foot appears to be properly supported but still moves when the mounting bolt is tightened.
This is sometimes called squishy soft foot.
A common cause is excessive shim stacks.
For example, instead of using one 0.050-inch shim, someone may have built the correction from many thin shims.
The stack can contain:
- dirt
- oil
- rust
- paint
- burrs
- damaged shim edges
- trapped material
The stack compresses differently as the bolt is tightened.
The result is an unstable mounting condition.
A good correction is often to replace multiple thin shims with fewer, properly sized clean shims.
4. Induced Soft Foot
One of the most important soft-foot conditions to understand is induced soft foot.
The foot itself may not actually be the original problem.
Instead, another external force is distorting the machine.
Common causes include:
- pipe strain
- improperly supported piping
- misaligned piping connections
- excessive nozzle loads
- distorted baseplates
- improperly installed grout
- coupling forces
- improperly installed equipment
Consider a pump connected to piping that does not naturally line up with the pump nozzle.
If workers use flange bolts to pull the piping into position, the piping can place significant force on the pump casing.
That force can distort the machine and create what appears to be a mounting problem.
Adding shims under the pump may hide the symptom without correcting the actual cause.
How Millwrights Check Soft Foot
There are several field methods.
The exact procedure may vary depending on the machine, alignment equipment, tolerances, manufacturer requirements, and site procedures.
One common method uses a dial indicator.
The indicator is positioned so movement of the machine foot can be measured.
The hold-down bolts are initially tightened according to the procedure being used.
One mounting bolt is then loosened while movement is observed.
If the machine foot rises when the bolt is loosened, that movement indicates the foot had been pulled downward when tightened.
For example:
Indicator before loosening: 0.000”
Indicator after loosening: +0.009”
That tells the millwright the machine moved approximately nine thousandths at that location.
That does not automatically mean “install a 0.009-inch shim.”
The type and cause of the soft foot still need to be determined.
The Feeler-Gauge Method
Feeler gauges are another extremely useful tool.
With the mounting bolt loosened according to the applicable procedure, carefully check around the perimeter of the machine foot.
Try different thicknesses.
For example:
Front edge: 0.002”
Outside edge: 0.003”
Rear edge: 0.009”
Inside edge: 0.008”
Those readings tell you much more than simply knowing that the foot moved 0.009 inch.
The pattern suggests an angular condition.
If approximately 0.009 inch fits uniformly around most of the foot, the condition is more consistent with parallel soft foot.
This is why experienced millwrights don’t just measure how much movement exists.
They determine how the foot is sitting.
Using a Laser Alignment System
Modern laser alignment systems often include a soft-foot function.
The system monitors shaft movement while individual hold-down bolts are loosened and tightened.
This can make identifying problematic feet much faster.
However, the instrument doesn’t eliminate the need to understand the mechanics.
A laser may tell you:
Foot 3 = excessive movement
The millwright still has to determine why.
Is there a uniform gap?
An angular gap?
A damaged shim stack?
A burr?
Pipe strain?
A warped foot?
A damaged baseplate?
The instrument measures the symptom.
The millwright diagnoses the machine.
A Practical Four-Foot Example
Consider a motor with four mounting feet:
A — Drive End Left
B — Drive End Right
C — Opposite Drive End Left
D — Opposite Drive End Right
Suppose the measured vertical movement is:
A = 0.001”
B = 0.002”
C = 0.011”
D = 0.002”
Foot C immediately deserves attention.
The next step isn’t simply:
Add 0.011” under C.
Instead, inspect underneath C.
Check the existing shims.
Check for burrs.
Check whether the foot is parallel to the base.
Check whether tightening another foot changes C.
That last point matters because machine frames behave as systems.
Changing one foot can affect another.
The Diagonal Soft-Foot Problem
Soft foot sometimes appears diagonally.
For example:
A — high movement
B — low movement
C — low movement
D — high movement
This pattern can indicate that the machine is rocking across the opposite two feet.
Think again about a four-legged table.
If two diagonal corners are supported differently, pressing one corner can lift the opposite corner.
This is why randomly adding shims one foot at a time can become frustrating.
You fix one corner.
Another changes.
You fix that one.
The first changes again.
The machine is telling you that the mounting geometry needs to be evaluated as a whole.
Correcting Parallel Soft Foot
Parallel soft foot is generally the simplest condition to correct.
The basic process is:
Determine the gap.
Select the appropriate shim thickness.
Clean the mounting surfaces.
Insert the shim fully beneath the machine foot.
Retighten the mounting bolts using the required procedure.
Measure again.
The goal is not merely to make the indicator read better.
The goal is to establish stable, repeatable contact between the machine and its mounting surface.
Correcting Angular Soft Foot
Angular soft foot requires more investigation.
First determine why the surfaces are not parallel.
Inspect for:
- burrs
- dents
- damaged shims
- bent feet
- paint buildup
- corrosion
- weld spatter
- damaged mounting pads
A tiny piece of debris can create surprisingly large movement at the opposite side of a machine foot.
If the mounting geometry itself is damaged, the proper repair may involve machining, correcting the mounting pad, or another engineered repair rather than building a complicated shim stack.
Follow equipment-manufacturer and site requirements.
Shim Installation Matters
Shims are precision components.
Treating them like scrap sheet metal defeats the purpose of precision alignment.
Shims should be:
Clean.
Dirt trapped underneath a shim changes its effective thickness.
Flat.
Bent or damaged shims create inconsistent support.
Properly sized.
The shim should adequately support the machine foot.
Free of burrs.
A burr can act like a tiny high point underneath the machine.
Kept to a reasonable stack.
Whenever practical and permitted, fewer appropriately sized shims are preferable to a large stack of thin pieces.
Don’t Forget the Existing Shim Pack
Before adding anything, inspect what is already underneath the machine.
A previous mechanic may have installed:
0.002”
0.003”
0.005”
0.010”
0.020”
0.025”
That is already a substantial stack.
If another 0.020 inch is required, simply adding another shim may not be the best solution.
A cleaner arrangement could replace several thin shims with fewer thicker ones while maintaining the required total thickness.
Always remeasure the final stack rather than assuming the stamped thicknesses perfectly represent the installed condition.
Pipe Strain Can Destroy Good Alignment
For pump installations, this deserves special attention.
The piping should not be used to force the pump into position, and the pump should not be used as an anchor for piping that doesn’t fit.
A pump can be perfectly aligned before piping is connected.
Then the suction and discharge piping are bolted up.
Suddenly:
Soft-foot readings change.
Shaft alignment changes.
The coupling moves.
The casing may distort.
That is a major warning sign.
The problem may be the piping system rather than the machine feet.
Correcting the motor alignment without addressing the pipe strain may only mask the problem.
Soft Foot and Machine Reliability
Why care about movement measured in thousandths of an inch?
Because rotating equipment operates continuously.
Small geometric errors can contribute to:
- vibration
- bearing loading
- coupling wear
- seal problems
- shaft misalignment
- casing distortion
- premature component failure
Soft foot doesn’t guarantee that every one of these failures will occur.
But eliminating soft foot removes an unnecessary source of distortion before precision alignment begins.
That is exactly what good machinery installation is about: removing avoidable variables.
A Better Alignment Workflow
For many industrial machines, the field workflow can be thought of as:
1. Inspect the foundation and baseplate
Look for obvious damage, corrosion, loose components, grout problems, contamination, and mounting-surface defects.
2. Inspect the machine feet
Clean the feet and mounting pads.
3. Rough-position the equipment
Bring the machine reasonably close to its required position.
4. Check soft foot
Measure each mounting location systematically.
5. Diagnose the type
Determine whether the condition is parallel, angular, squishy, induced, or a combination.
6. Correct the cause
Clean, repair, replace, or adjust the shim arrangement as required.
7. Recheck every foot
Corrections at one foot can affect another.
8. Perform precision shaft alignment
Now the machine is sitting on a stable foundation.
9. Tighten the hold-down bolts
Follow the appropriate tightening procedure.
10. Verify alignment again
Never assume the machine remained exactly where it was.
Field Rule: Watch the Machine While Tightening
A valuable habit is observing alignment readings while the hold-down bolts are tightened.
If the shaft position changes significantly as one bolt is tightened, investigate it.
The bolt may be forcing the machine into position.
A properly prepared machine should remain relatively stable as it is secured.
The hold-down bolts are there to hold the machine down.
They should not be used to straighten a poorly supported machine.
Common Soft-Foot Mistakes
One of the biggest mistakes is immediately reaching for another shim.
Measurement comes first.
Another mistake is checking only one foot.
All mounting points interact.
Another is ignoring dirty mounting surfaces. Paint flakes, rust, weld spatter, dirt, and damaged shim edges can create measurable problems.
Another is building huge shim stacks.
And perhaps the most expensive mistake is assuming every apparent soft-foot condition originates underneath the machine.
Sometimes the real problem is somewhere else—especially piping strain.
Knowledge Check
A motor foot rises 0.012 inch when its hold-down bolt is loosened.
Does that automatically mean a 0.012-inch shim should be installed?
No.
The movement confirms that the mounting condition needs investigation, but the millwright must determine whether the problem is parallel, angular, induced, or caused by another condition.
That distinction separates simply taking a measurement from understanding what the machine is telling you.
Practical Exercise
Picture a four-foot motor with these readings:
A = 0.002”
B = 0.003”
C = 0.014”
D = 0.003”
Your first instinct might be to place approximately 0.014 inch underneath C.
Instead, perform the diagnosis.
Check C with feeler gauges around the entire foot.
Inspect the shim pack.
Inspect the mounting pad.
Look for burrs and contamination.
Observe whether loosening or tightening the other feet changes the reading.
Then determine the correction.
That process is the lesson.
Measure → Diagnose → Correct → Recheck
Not:
Measure → Add shims → Hope.
The Millwright Standard
Precision alignment begins before the laser ever measures the shafts.
A skilled millwright understands that the machine must first be mechanically stable.
Soft foot is one of those conditions that can look insignificant—a gap measured in thousandths—but affect an entire alignment job.
Find it.
Determine what is causing it.
Correct the cause.
Recheck the machine.
Then align it.
Because a machine that isn’t sitting correctly cannot be expected to stay aligned correctly.
