Soft Foot: Causes, Diagnostics and Correction
Soft foot is one of the most common yet underestimated causes of excessive vibration in rotating equipment. According to field service statistics, up to 80% of machines at industrial plants operate with uncorrected soft foot. This article provides a detailed look at the physics of the phenomenon, its classification, detection methods — from feeler gauges to cross-phase vibration analysis — and practical correction techniques.
1. Definition and Physical Nature
Soft foot is a condition in which one or more machine feet do not have full contact with the foundation frame (sole plate, baseplate) before the hold-down bolts are tightened. When such a bolt is tightened, the machine casing deforms, the bearing bore geometry is distorted, and the rotor axis deviates from its designed position.
Physically, the following occurs: the tightening force of a bolt on a foot with incomplete contact creates a bending moment in the casing. This deformation is transmitted to the bearing supports, causing:
- Misalignment of rolling bearing inner rings
- Uneven load distribution in plain bearings
- Angular misalignment of coupled machine shafts
- Dynamic unbalance due to rotor deflection
As a result, vibration increases at the rotational frequency (1×), and in severe cases, at harmonic multiples as well.
There are documented cases where correcting soft foot on a single bolt reduced the vibration velocity (RMS) from 12 mm/s to 2 mm/s — a sixfold reduction.
2. Soft Foot Classification
International practice distinguishes four types of soft foot. Each requires a different approach to identification and correction.
Parallel (Air-Gap) Soft Foot
A uniform air gap is present under the foot across the entire bearing surface. Causes include: a short foot, non-flatness of the sole plate, or incorrect shim thickness.
✓ Flat calibrated shimsAngular Soft Foot
The foot contacts the frame along only one edge or corner. When the bolt is tightened, the opposite side lifts, distorting the casing. Occurs when the foot is not perpendicular to the bolt axis or when the surface has wedge-shaped wear.
✓ Tapered / stepped shimsSquishy (Springy) Soft Foot
The surface formally contacts the frame, but compressible material is present: excessive thin shims, paint, dirt, corrosion, or gasket residue. Alignment "drifts" over time as it settles. Identified by unstable repeated measurements.
✓ Clean surfaces, ≤3 shimsInduced Soft Foot
The foot and frame have correct geometry, but external forces — pipe strain, cable tray loads, guard forces, jacking bolt pressure — pull the casing out of the support plane. Most insidious: static measurements may not reveal it.
✓ Pipe strain correctionGapExternal forceCorrection First determine the type of soft foot by the nature of the contact, then select the correction method (shims, surface machining, removal of external loads).
3. Impact on Machine Vibration Condition
Soft foot has a complex negative effect on machine condition across multiple parameters:
| Parameter | Mechanism of Impact |
|---|---|
| Vibration velocity (RMS, mm/s) | Amplitude increase at 1× rotational frequency due to rotor deflection and misalignment |
| Vibration phase | Phase angle difference between supports can reach 180° — a characteristic sign of soft foot |
| Spectrum | Elevated 1× with possible presence of 2× and line frequency (for electric motors) |
| Bearing life | Ring misalignment causes point overload on rolling elements, drastically reducing service life |
| Shaft alignment | Unstable alignment: values "drift" from target after bolt tightening |
| Seals | Casing deformation disrupts geometry of mechanical seal seats |
If vibration remains elevated after performing quality shaft alignment, the first thing to check is soft foot.
4. Diagnostic Methods
4.1. Static Detection (Feeler Gauges and Dial Indicators)
The most common method during scheduled alignment work.
- Loosen all machine hold-down bolts.
- Insert a feeler gauge set between each foot and the frame. Record the gaps.
- For each foot with a gap exceeding 0.05 mm, select calibrated shims.
- Tighten all bolts with a torque wrench.
- Repeat the measurement with a dial indicator: mount the base on the frame, position the indicator tip on the foot, and loosen the bolt. Allowable displacement is no more than 0.05 mm (50 µm).
This method does not detect induced soft foot that occurs under operating load (temperature, pressure, pipe strain).
4.2. Dynamic Detection (Bolt Loosening on a Running Machine)
This method detects soft foot directly under operating conditions — at temperature, pressure, and pipe strain.
- Mount a vibration sensor (accelerometer) on the machine casing near the support.
- Connect the instrument in real-time vibration velocity RMS monitoring mode. A portable dual-channel vibrometer such as the Balanset-1A can be used, enabling simultaneous monitoring of vibration level and phase angle at the rotational frequency.
- Sequentially loosen each hold-down bolt (to finger-tight), observing the change in RMS.
- Immediately retighten the bolt after checking and move to the next one.
- The bolt whose loosening results in a significant reduction in vibration indicates soft foot at that location.
A reduction in vibration velocity RMS of more than 20% when loosening a single bolt is conclusive evidence of soft foot.
Working with fasteners on running equipment involves elevated risk. Strict compliance with occupational safety requirements is mandatory, including the use of non-sparking tools in hazardous areas and proper authorization for work on live equipment.
4.3. Cross-Phase Vibration Analysis
The most informative instrumental method, enabling soft foot identification without loosening fasteners on running equipment.
Required Equipment
- Dual-channel vibration analyzer with cross-phase function
- Two accelerometers
- Phase reference sensor (tachometer) and a reflective marker on the rotor
The dual-channel vibrometer Balanset-1A provides simultaneous measurement of vibration amplitude at 1× and the phase angle on two channels with ±2° accuracy, making it suitable for cross-phase analysis in the field. A photoelectric phase reference sensor (0–360° range) is included as standard equipment.
- Mount accelerometers on two machine supports in the same direction (e.g., vertical).
- Attach the marker to the rotor and aim the tachometer sensor at the marker.
- Perform the cross-phase measurement: the instrument determines the vibration phase angle difference between two points at the 1× rotational frequency.
If the phase difference is approximately 180° with a simultaneously significant amplitude difference between the two supports, this is a characteristic sign of soft foot. The support with the higher amplitude indicates the problem location.
Differential Diagnostics
| Defect | Phase Difference Between Supports | Amplitude |
|---|---|---|
| Soft foot | ≈ 180° | Significant difference between supports |
| Unbalance | ≈ 0° (in-phase) | Comparable levels |
| Misalignment | 0° or 180° | Depends on misalignment type |
CH1 / CH2Δφ ≈ 0°Δφ ≈ 180° In-phase signals typically indicate unbalance; anti-phase signals point to soft foot. For a definitive conclusion, verify amplitudes, the 1×/2× spectrum, and the bolt loosening test.
The advantage of the cross-phase method is that it works during normal machine operation and does not require loosening any fasteners.
5. Pipe-Induced Soft Foot
Pipe strain on pump or compressor equipment is one of the key — yet most frequently overlooked — causes of excessive vibration and unstable alignment.
5.1. Mechanism of Occurrence
If piping is connected to a machine flange under strain (without a free fit), the pipe force is constantly applied to the machine casing. Under operating pressure and temperature, this force increases due to thermal expansion. The pipe "rocks" the machine, leading to:
- Periodic changes in shaft alignment
- Increased vibration at 1× and 2× rotational frequency
- Premature wear of bearings and mechanical seals
- Unstable readings when attempting alignment
Strain forceDeformation Red arrows show the pipe strain force that pulls the machine out of its geometry. The 12–3–6–9 circle shows the order for measuring flange gaps at four points before alignment.
5.2. Piping Condition Inspection
Before shaft alignment, inspection of flange angularity and offset is mandatory.
- Disconnect the piping from the machine flange.
- Measure the gaps between the pipe flange and machine flange at four points: 12, 3, 6, and 9 o'clock.
- Determine the angularity (gap difference at opposite points) and offset (parallel mismatch of flange centerlines).
Tolerances
- Ideal angularity and offset value: 0 mm
- Practically achievable with careful fitting: 0.01–0.02 mm
- Values exceeding 0.05 mm require mandatory correction before alignment
5.3. Pipe Fitting
The goal is to achieve a stress-free flange connection without applying external forces. Methods include:
- Adjusting pipe supports and hangers
- Trimming or extending spool pieces
- Using expansion joints
- Correcting intermediate support positions
According to field practice data, up to 80% of operating organizations neglect pipe strain verification, continuing to search for the vibration cause elsewhere. This work is labor-intensive, but without it any alignment — even precision alignment — will be unstable.
6. Foot Contact Area Requirements
The minimum contact area of the machine foot with the sole plate (foundation frame) must be at least 80% of the foot sole area.
When the contact area is less than 80%:
- The load is distributed unevenly, creating local stress concentrations
- Shims deform and are indented at point contact zones
- Bolt tightening does not provide stable fixation — alignment "drifts" over time
- The risk of fatigue failure of the foot or sole plate increases
Inspection Methods
- Visual inspection: contact marks, oxidation, scoring on foot and frame surfaces
- Prussian Blue (marking paste): apply a thin layer to the sole plate, press the foot down, evaluate the contact pattern
- Feeler gauge set: measure around the foot perimeter with the bolt loosened
If contact is found to be less than 80%, the flatness of the bearing surfaces must be restored: scraping, milling, or grinding of the sole plate and/or foot sole.
7. Soft Foot Correction Procedure
Recommended sequence of work when soft foot is detected:
Prepare Bearing Surfaces
- Clean sole plates and foot surfaces of dirt, paint, rust, and old gasket material
- Check flatness with a straightedge and feeler gauge set
- Machine the surfaces if necessary (grinding, scraping)
Verify Contact Area
- Ensure foot-to-sole plate contact is at least 80%
- Eliminate any compressible (springy) materials in the contact zone
Measure Gaps
- Loosen all hold-down bolts
- Measure gaps with feeler gauges or a dial indicator at each foot
- Select calibrated stainless steel shims. No more than 3 shims per foot (to avoid "squishy" effect)
Check Pipe Strain
- Disconnect the piping
- Measure flange angularity and offset at four points
- If tolerances are exceeded, correct to achieve a stress-free connection
Final Tightening & Verification
- Tighten all bolts with a torque wrench in a cross pattern
- Dial indicator check: displacement ≤ 0.05 mm when loosening any bolt
- Test run and verify vibration levels
Perform Shaft Alignment
Shaft alignment should be performed only after soft foot has been fully corrected and piping has been fitted. Otherwise, alignment results will be unstable.
8. Instrumentation
8.1. Tools for Static Diagnostics
- Calibrated feeler gauge set (from 0.02 mm)
- Dial indicator on a magnetic base (graduation 0.01 mm)
- Straightedge
- Marking paste (Prussian Blue) for contact area assessment
- Calibrated torque wrench
8.2. Tools for Dynamic Diagnostics
Dynamic soft foot detection and cross-phase analysis require a portable vibration analyzer with simultaneous dual-channel measurement and phase analysis capabilities.
The Balanset-1A (manufactured by VibroMera) is a portable dual-channel vibrometer-balancer suitable for these tasks. Key specifications relevant to soft foot diagnostics:
The dual-channel architecture of the Balanset-1A enables simultaneous amplitude and phase vibration measurement at two supports, which is a prerequisite for cross-phase soft foot diagnostics. After soft foot correction, the same instrument is used for rotor balancing in its own bearings — in one or two correction planes — without equipment disassembly.
9. Normative References
- GOST R ISO 20816-1-2021 — Vibration. Measurement and evaluation of machine vibration. Part 1. General guidelines.
- GOST R ISO 18436-2-2005 — Condition monitoring and diagnostics of machines. Vibration condition monitoring and diagnostics. Part 2. Requirements for training and certification of personnel.
- ISO 1940-1:2003 — Mechanical vibration. Balance quality requirements for rotors in a constant (rigid) state. Part 1: Specification and verification of balance tolerances.
- ISO 10816 / ISO 20816 — Series of standards for evaluating machine vibration condition.
10. Conclusion
Key Takeaway
Soft foot is a systemic installation defect whose correction is a mandatory prerequisite for successful shaft alignment and vibration reduction in rotating equipment. Ignoring soft foot renders any subsequent commissioning work pointless: alignment will be unstable, vibration will remain elevated, and bearing and seal service life will be reduced.
Modern portable dual-channel vibrometers such as the Balanset-1A provide a complete diagnostic cycle — from soft foot detection via cross-phase analysis to subsequent on-site rotor balancing. Using instrumental diagnostic methods instead of visual inspection greatly increases defect detection reliability and reduces commissioning time.
Recommended Commissioning Workflow
Work logic"Yes" branchFinal check Key rule: proceed to alignment only after confirmed soft foot correction. The practical criterion: foot displacement ≤ 0.05 mm during control bolt loosening and absence of anti-phase vibration.
Following this sequence is the foundation for reliable and long-term operation of rotating equipment.
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