Free Engineering Tool
Soft Foot Calculator
Check for machine frame distortion by entering dial indicator readings at each foot. Identify the worst soft foot, determine shim corrections, and verify acceptable limits.
Results
| Foot | Reading (mm) | Reading (μm) | Status | Shim Needed |
|---|
What is Soft Foot?
Soft foot occurs when one or more machine feet do not make full, flat contact with the baseplate or foundation. When the hold-down bolt at that foot is tightened, the machine frame is distorted, causing internal stresses that misalign bearings and seals.
Acceptance Criteria
| Reading | Status | Action |
|---|---|---|
| < 0.05 mm (50 μm) | PASS | Acceptable — no correction needed |
| 0.05 – 0.10 mm (50–100 μm) | WARNING | Marginal — correction recommended |
| > 0.10 mm (100 μm) | FAIL | Excessive — must correct before alignment |
Types of Soft Foot
- Parallel (Gap) Soft Foot — uniform gap under the foot. Caused by missing or incorrect shims. Corrected by adding flat shims equal to the gap.
- Angular Soft Foot — wedge-shaped gap where the foot rocks on one edge. Caused by non-parallel machining. Corrected with tapered or stepped shims.
- Springing (Reactive/Induced) Soft Foot — caused by external forces such as pipe strain, conduit stress, or dowel pin binding. The foot appears flat but the frame deflects. Must address the external force source.
Measurement Procedure
- Tighten all machine foot bolts to normal torque.
- Mount a dial indicator (or use a laser alignment system) on the machine frame near Foot 1.
- Zero the indicator, then loosen the bolt at Foot 1.
- Record the maximum dial indicator movement — this is the soft foot at Foot 1.
- Re-tighten the bolt at Foot 1.
- Repeat for each remaining foot.
Practical Example
Readings: Foot 1 = 0.02 mm, Foot 2 = 0.08 mm, Foot 3 = 0.03 mm, Foot 4 = 0.15 mm
Foot 4 = 0.15 mm → FAIL (exceeds 0.10 mm limit)
Foot 2 = 0.08 mm → WARNING (between 0.05 and 0.10 mm)
Feet 1 and 3 → PASS (below 0.05 mm)
Correction: Add 0.15 mm shim under Foot 4, then re-check all feet.
💡 Tip: Always correct soft foot before performing shaft alignment. Soft foot will cause alignment readings to be inconsistent and non-repeatable.
⚠️ Note: If loosening a bolt gives a negative reading (foot lifts), this indicates springing/reactive soft foot caused by external forces. Do not add shims — instead, identify and relieve the external force (pipe strain, conduit, etc.).
Soft foot is a condition where one or more machine feet do not make complete, flat contact with the baseplate or foundation. When the hold-down bolt at that foot is tightened, it pulls the machine frame down, causing internal distortion that misaligns bearings and seals.
The term comes from the analogy of a table with one short leg — it rocks and is unstable.
Soft foot is critical because it:
- Distorts the machine frame — causing internal bearing misalignment
- Makes alignment non-repeatable — readings change each time bolts are tightened
- Increases vibration — due to internal stresses and bearing preload
- Causes premature bearing failure — from uneven load distribution
- Wastes alignment time — if not corrected first, alignment will never converge
Always check and correct soft foot before performing shaft alignment.
Follow this procedure:
- Tighten all hold-down bolts to normal torque
- Mount a dial indicator on the machine frame near the first foot
- Zero the indicator
- Loosen the bolt at that foot (about 1/4 turn)
- Record the maximum indicator movement (positive = gap)
- Re-tighten the bolt
- Move to the next foot and repeat
Alternatively, modern laser alignment systems have a built-in soft foot check function that measures all feet quickly.
There are three main types:
- Parallel (gap) soft foot — A uniform gap exists under the foot, typically due to missing, incorrect, or worn shims. Corrected by adding flat shims equal to the measured gap.
- Angular soft foot — The foot has a wedge-shaped gap and rocks on one edge. Caused by non-parallel machining of the foot or baseplate. Corrected with tapered or stepped shims, or by machining the foot.
- Springing (reactive/induced) soft foot — No gap exists, but external forces (pipe strain, conduit stress, dowel pin binding) distort the machine when the bolt is tightened. Cannot be corrected with shims — must address the external force.
Parallel soft foot: Add flat shims equal to the dial indicator reading under the affected foot. Use quality pre-cut stainless steel shims. Avoid using more than 3–4 shims in a stack.
Angular soft foot: Use tapered shims, stepped shims, or machine the foot surface to achieve full contact. Sometimes a thin coat of epoxy or polymer can fill small angular gaps.
Springing soft foot: Identify and relieve the external force. Common causes include pipe strain, electrical conduit stress, tight dowel pins, and foundation settling. Do not add shims for springing soft foot — it will make the problem worse.
After correction, always re-check all feet to verify the fix.
The procedures described on this page were developed and field-tested by the Vibromera team using the Balanset-1A, a portable dual-channel balancer and vibration analyzer for on-site rotor balancing.
Professional alignment and vibration analysis instruments with built-in soft foot detection. Achieve precision alignment on-site. Used in 50+ countries.
Nikolai Shelkovenko
Nikolai Shelkovenko is a vibration analysis engineer and the founder and CEO of Vibromera. For more than 15 years he has balanced rotating equipment in the field rather than on a test bench: mulchers, industrial fans, crushers, centrifuges, shafts and spindles. That work is what the Balanset instruments grew out of — they were designed as a tool a specialist can carry to the machine and use alone, on site, not as laboratory equipment. Vibromera was founded in 2017 and has been based in Porto, Portugal, since 2023. Development, assembly and support of the Balanset line all happen here. The flagship instrument is the Balanset-1A, a portable analyser for single- and two-plane balancing and for vibration diagnostics. Nikolai is personally involved in customer support, in working through difficult balancing cases and in the development of the software. He works with customers worldwide, in any language.
