Soft foot is a common problem in rotating machinery where one or more machine feet do not sit correctly on the baseplate.
Although it may appear minor, soft foot can distort the machine frame, affect shaft alignment measurements and contribute to vibration, bearing wear, seal problems and premature equipment failure.
For maintenance teams carrying out pump and motor alignment, identifying and correcting soft foot should be completed before final shaft alignment.
What Is Soft Foot?
Soft foot occurs when all of the machine feet do not make consistent contact with the mounting surface. When the hold-down bolts are tightened, the machine frame can distort as the unsupported foot is pulled down to the baseplate. This can affect the shaft position and make accurate alignment difficult.
Common causes include:
- Uneven foundations or baseplates
- Bent, damaged or distorted machine feet
- Incorrect or excessive shimming
- Dirt, corrosion or debris beneath the feet
- Stressed pipework or external forces acting on the machine
- Poorly machined or stepped mounting surfaces
Why Is Soft Foot a Problem?
Ignoring soft foot can create several problems for rotating equipment.
1. Inaccurate Shaft Alignment
Machine-frame distortion can change the shaft position as the mounting bolts are tightened, resulting in inconsistent alignment readings.
2. Increased Wear
Soft foot can contribute to additional loading on bearings, seals, couplings and other rotating components.
3. Increased Vibration
Frame distortion and alignment changes can contribute to vibration and harmonic problems.
4. Reduced Reliability
If the underlying mounting problem is not corrected, alignment and vibration problems may return even after other components have been repaired or replaced.
How Is Soft Foot Identified?
Soft foot can be checked using several methods.
Dial Indicators
Traditional dial indicators can be used to measure movement as individual machine feet are loosened and tightened.
Laser Alignment Systems
Modern laser shaft alignment systems can measure movement at each machine foot during the soft-foot checking process.
Quantum uses specialist Prüftechnik laser alignment equipment that allows soft foot to be measured and corrected before final shaft alignment.
What Are the Different Types of Soft Foot?
Soft foot generally falls into several common categories.
1. Parallel Soft Foot
Parallel soft foot occurs when the machine foot sits flat but there is a consistent gap between the foot and the mounting surface. This can usually be corrected using suitable shims.
2. Angular Soft Foot
Angular soft foot occurs when the machine foot and mounting surface are not parallel.
Adding flat shims alone may not fully correct the problem because only part of the foot is making contact.
3. Squishy Soft Foot
Squishy soft foot can occur when excessive, damaged or contaminated shims compress or move as the hold-down bolt is tightened.
4. Induced Soft Foot
External forces can distort the machine frame even when the feet and mounting surface are otherwise correct.
Common causes include stressed pipework, connected ductwork or other equipment imposing load on the machine.
What Is a Soft Foot Indication?
A soft foot indication is the measured movement at a machine foot when the hold-down bolt is loosened.
A significant change in position indicates that the machine is being distorted when the foot is tightened.
The measured values across all feet can then be used to determine the type of soft foot present and the appropriate corrective action.
What Is the Maximum Permissible Soft Foot?
The acceptable soft-foot value depends on the equipment, operating speed, machine construction and alignment requirements.
The aim should always be to reduce soft foot as much as reasonably practical before final alignment.
While ideally there should be no soft foot, the industry standard generally allows a maximum of 2 thousandths of an inch (0.051 mm).
How Is Soft Foot Corrected?
The method used depends on the type and cause of the soft foot.
Typical corrective actions include:
- Adding or adjusting stainless-steel shims
- Removing damaged or excessive shims
- Cleaning the machine foot and baseplate
- Correcting stepped or uneven mounting surfaces
- Machining or correcting angular feet where required
- Relieving external pipe strain
- Repairing or correcting the supporting base
The machine should then be rechecked before final shaft alignment is completed.
Soft Foot Correction Formula
For some diagonal soft-foot conditions, a calculation may be used to estimate the shim correction required across the affected feet.
However, the correct approach depends on the measured values and type of soft foot present.
Example:
Measurements:
- Rear left: 0.024″
- Front left: 0.003″
- Rear right: 0.001″
- Front right: 0.015″
Working:
- Add the two highest soft-foot readings: 0.024″ + 0.015″ = 0.039″
- Add the two lowest readings: 0.003″ + 0.001″ = 0.004″
Subtract the lower total from the higher total: 0.039″ − 0.004″ = 0.035″ - Multiply the result by 0.8: 0.035″ × 0.8 = 0.028″
- Divide by two: 0.028″ ÷ 2 = 0.014″
This gives a calculated shim correction of 0.014″ under each of the two high feet.
Soft Foot vs Mechanical Looseness
Soft foot relates specifically to how the machine sits on its mounting surface.
Mechanical looseness is a broader condition that can involve loose mounting bolts, bearing clearances, structural movement, damaged bases or looseness within the equipment itself.
Both conditions can contribute to vibration, but the corrective action is different.
Soft Foot & Laser Shaft Alignment
Checking soft foot is an important part of precision shaft alignment.
Correcting alignment without first addressing soft foot can produce misleading measurements and may allow vibration or alignment problems to return after the machine is placed back into service.
Quantum Plant Maintenance provides laser shaft alignment for pumps, motors, fans, compressors and other rotating equipment across greater Sydney and NSW.
Using Prüftechnik laser alignment systems, our technicians can measure soft foot, correct the mounting condition and complete the final shaft alignment.
Need Help With Soft Foot or Shaft Alignment?
If you are experiencing recurring vibration, inconsistent alignment readings, premature bearing or seal failures, or suspected soft foot, Quantum can inspect the equipment onsite.
Contact Quantum Plant Maintenance on 1300 308 717 to discuss laser shaft alignment and rotating equipment maintenance across Sydney.
Soft foot, a seemingly simple term, can cause significant headaches for industrial maintenance professionals. While often associated with motors, it poses a major challenge during shaft alignment, leading to inconsistencies and inaccurate results. This article delves into the intricacies of soft foot, equipping you with the knowledge to identify, address, and prevent this common issue.
Why Is Soft Foot a Problem?
Imagine a motor wobbling precariously on its baseplate, resembling a child’s seesaw. This uneven support, where one or more feet lack proper contact with the base, is precisely what defines soft foot. It arises from various factors, including:
- Uneven foundation or baseplate: Faulty construction or settling over time can create an uneven surface.
- Twisted or damaged feet: Bent or cracked motor feet compromise proper contact.
- Improper shimming: Over Reliance on shims to correct alignment can mask underlying soft foot problems.
Consequences of Soft Foot
Ignoring soft foot can have detrimental consequences for your equipment:
- Inaccurate shaft alignment: The distorted motor frame throws off alignment measurements, leading to incorrect adjustments and persistent vibration.
- Increased wear and tear: Uneven load distribution accelerates bearing wear, shaft misalignment, and potential seal failures.
- Reduced pump efficiency: The additional energy wasted in overcoming the wobble translates to higher operating costs.
Identifying Soft Foot
Several methods can help identify soft foot, including:
- Dial gauges: These traditional tools measure the gap between the motor foot and the baseplate under loosened and tightened bolt conditions.
- Laser alignment tools: Advanced methods like laser shaft alignment can indirectly detect soft foot by analysing shaft movement during the loosening and tightening process.
Common Soft Foot Questions Answered
1. What is a soft foot indication?
Soft foot typically manifests as two diagonally opposed feet rocking on the base, creating an air gap under one foot when the other three are tightened.
2. What is the formula for soft foot correction?
This formula applies specifically to diagonal soft foot correction. While not universally applicable, it provides a general approach:
- Add the soft foot values of the highest feet: (Highest value 1 + Highest value 2)
- Subtract the lowest value from the sum: (Sum – Lowest value)
- Multiply by 0.8: (Result x 0.8)
- Divide by 2: (Result / 2)
- The resulting value represents the shim thickness required under each of the high soft foot feet.
Example Calculation:
Where:
- Rear left foot is 0.24″.
- Front Left foot is 0.003″.
- Rear right foot is 0.001″.
- Front right foot is 0.015″.
- Add the soft foot values of the highest feet: 0.024″ + 0.015″ = 0.039″
- Subtract the lowest, from the highest: 0.039 – 0.004″ = 0.035″
- Multiply by 0.8: 0.035 x 0. = 0.028″
- Divide these results by 2: 0.028 / 2 = 0.014″ shim under each of the feet with the high soft foot values.
3. What is the maximum permissible soft foot?
While ideally there should be no soft foot, the industry standard allows a maximum of 2 thousandths of an inch (0.025 mm).
4. What are the different types of soft foot?
There are 4 different types of soft foot:
- Parallel: Two diagonally opposed feet have equal soft foot or a stepped base issue.
- Angular: One foot or the base has an angular face.
- Squishy: The shim or base under one foot is distorted, offering resistance when loosened.
- Induced (external force): Stressed pipes connected to the equipment distort the frame when feet are loosened.
5. What is the difference between soft foot and looseness?
Soft foot is an issue with the mounting of an item to a base, while looseness could be a moment or structural issue with the base itself or the equipment frame.
Addressing Soft Foot
The primary solution for soft foot involves shimming the affected foot to eliminate or reduce the gap. However, in some cases, addressing the base or stressed pipework might be necessary.
Quantum Plant Maintenance: Your Soft Foot Elimination Partner
Identifying and resolving soft foot is one of many problems we check for through our Laser Alignment services.
A regular maintenance regime is vital for ensuring the optimal performance, longevity, and smooth operation of your industrial equipment. Regular inspections and proactive maintenance are key to preventing soft foot and its associated problems.
Contact QPM to book an on-site inspection.