Understanding Soft Foot

What is Soft Foot?

Soft foot is a condition where one or more machine feet fail to make proper contact with the baseplate. When the anchor bolts are tightened, the unsupported foot is pulled down, causing the machine frame to distort.

The effect is similar to a table with one leg shorter than the others. The table wobbles until pressure is applied to hold it down, at which point the table top bends slightly. In machinery, this distortion affects the bearing housings and can alter alignment readings.

Why Soft Foot Matters

Soft foot creates internal stress in the machine frame that affects both alignment accuracy and equipment reliability. Even small amounts of frame distortion can have significant consequences.

Types of Soft Foot

1. Parallel Soft Foot (Rocking Soft Foot)

The gap between foot and baseplate is uniform across the entire foot surface, similar to a wobbly restaurant table where not all four legs are on the same plane. This is the simplest form to correct and requires flat shims of the appropriate thickness.

Common causes include undersized legs, non-coplanar baseplates, or missing shims from previous work.

2. Angular Soft Foot (Bent Foot)

The gap varies across the foot, creating a wedge-shaped or tapered space. One edge of the foot contacts the baseplate while the opposite edge is raised.

Angular soft foot may indicate a bent or dropped foot, poorly machined baseplate, welded feet, or foundation settling. The preferred correction is to re-machine the feet or base. Step-shimming is an option but should be avoided where possible.

3. Squishy Foot (Spring Foot)

With squishy foot, a feeler gauge will not detect any gap underneath the foot, yet the laser system still indicates a soft foot condition. The problem is usually that the space between foot and base contains too many shims from previous alignment attempts, or a build-up of other material.

Common causes include dirt, grease, paint or rust accumulation, excessive shims (more than three or four per foot), bent or damaged shims, and burrs from bolt threads. The solution is to clean all surfaces thoroughly and replace old shims with new, flat stainless steel ones.

4. Induced Soft Foot

External forces distort the machine frame rather than a gap at the foot. The laser system will typically indicate soft foot at more than one location, usually on the same side or the same end of the machine.

Common causes include pipe strain, coupling stress, belt or chain loads on pulleys, overly rigid conduit connections, structural bracing, and tight jacking bolts. Induced soft foot cannot be corrected by shimming alone. The external forces must be identified and addressed at their source.

How to Detect Soft Foot

Soft foot is measured by monitoring machine movement as each anchor bolt is loosened in turn. A dial indicator or laser alignment system records the vertical movement at the foot position.

1. Complete a rough alignment first (both vertical and horizontal planes)
2. Tighten all anchor bolts to their normal torque value
3. Position a dial indicator or laser target to measure vertical movement at the first foot
4. Loosen the anchor bolt at that foot and record any vertical movement
5. Retighten the bolt and repeat for each remaining foot

Correction Procedure

1. Perform rough alignment check — This step alone resolves approximately 90% of soft foot conditions by ensuring proper machine positioning.
2. Identify the foot with the largest gap — Check all four feet and note which shows the greatest movement when its bolt is loosened.
3. Insert shim under loosened bolt — Select a shim that just contacts the foot without forcing it upward. The shim should slide in with light resistance.
4. Tighten bolt and re-measure — The movement reading should now be below 0.05mm. If not, adjust the shim thickness.
5. Repeat for remaining feet — Work through each foot in turn, starting with the largest gaps.
6. Final verification — After all adjustments, check every foot again to confirm all readings are below the 0.05mm threshold.

Using Shims Correctly

• Inspect and Clean — Before shimming, inspect the foundation, grout, and baseplate. Remove rust, scale, paint, and dirt from under and around the feet.
• Pre-cut Stainless Steel — Use pre-cut stainless steel shims that are perfectly flat and corrosion-resistant. Avoid homemade or low-quality alternatives.
• Limit Shim Count — Three pre-cut shims should always be enough. In extraordinary situations, four may be needed, but no more. Excessive shims cause squishy foot.
• Sandwich Thin Shims — Insert pre-cut shims under the foot until they touch the bolt, then withdraw slightly. Sandwich thin shims between thicker ones for stability.
• Verify Thickness — Use a micrometer to measure the thickness of shims 1mm and above, as these are nominal values and may vary slightly.
• Avoid Bolt Contact — Position shims so the slot does not contact the anchor bolt threads. Thread contact can damage the shim and make adjustments difficult.