I've seen alignment jobs fail within days because techs skipped this step. Perfect readings on the alignment system, then three weeks later, the pump's vibrating like crazy. Pull it open and you find cracked feet or blown bearings.
The problem? Soft foot was there all along.
What Soft Foot Actually Is
Soft foot means one or more mounting feet aren't sitting flat on the baseplate. Think of a wobbly table—same concept, worse consequences.
When you torque the hold-down bolts, the machine frame distorts. That distortion throws off your alignment, stresses the bearings, and shortens equipment life.
There are four types:
Parallel soft foot: Uniform gap under the entire foot. Common on worn or corroded baseplates.
Angular soft foot: The Foot or base is warped. Only one edge makes contact. Usually requires machining.
Squishy soft foot: Debris, rust, or too many shims create a springy base. Most common type I see.
Induced soft foot: Pipe strain or electrical conduit pulls the frame out of position. Check this first on pump installations.
How to Find It
Use feeler gauges. Not a laser system—feeler gauges.
Industry standard: Any gap over 0.002" (2 mils) needs correction.
Here's my process:
Loosen all hold-down bolts (finger tight)
Check each corner of every foot with feeler gauges
Note which feet have gaps and how much
Identify the type (parallel, angular, squishy, induced)
Laser systems can detect soft foot exists, but they won't tell you the type or exact gap. You need feeler gauges for diagnosis.
How to Fix It
Step 1: Clean Everything
Strip the feet and baseplate down to bare metal. Wire brush, solvent, whatever it takes. Paint and rust will cause a squishy, soft foot.
Step 2: Shim Correctly
Start with the foot that has the largest gap.
My shimming rules:
Maximum 3-5 shims per foot (I prefer 3)
Use pre-cut stainless steel shims only
Shim must cover at least 80% of the foot area
Sandwich thin shims between thick ones
Never stack more than 0.125" total shim pack
For angular soft foot, step-shim or machine the surface. Don't try to fill a tapered gap with standard shims—it doesn't work.
Step 3: Torque in Stages
Tighten bolts in a cross pattern:
Stage 1: Hand tight
Stage 2: 50% of final torque
Stage 3: 100% torque per manufacturer spec
Check the soft foot again after each stage. The gap should stay under 2 mils.
What Happens If You Skip This
I've pulled bearings that lasted three months instead of three years. The bearing wasn't bad—the soft foot killed it.
Here's what an uncorrected soft foot causes:
Immediate effects:
Frame distortion when bolts are torqued
Misalignment returns within days
Increased vibration at startup
Long-term damage:
Bearing failures (thrust bearings go first)
Seal leaks from frame deflection
Cracked feet or baseplates
Internal clearance problems in pumps and gearboxes
Motor air gap distortion
On a 500 HP motor driving a centrifugal compressor, I've seen soft foot cause a $40,000 bearing failure in six weeks. The soft foot check would've taken 15 minutes.
Prevention
Make soft foot checks mandatory on:
New installations
After any maintenance that requires unbolting the machine
Before every alignment job
After piping modifications
Also check:
Foundation flatness (use a straightedge)
Pipe strain (disconnect piping and check if soft foot changes)
Shim condition (replace corroded or damaged shims)
Base grouting (repair voids or cracks)
Bottom Line
Soft foot isn't optional to check. It's the foundation of alignment.
You can have the best alignment tools and procedures in the world, but if the machine's not sitting flat, you're wasting your time.
Fifteen minutes with feeler gauges saves weeks of troubleshooting and thousands in repair costs.
What's your experience with soft foot? Ever had a machine fail because someone skipped the check?