Imbalance, Misalignment, Looseness and Soft Foot: Four Faults, Four Vibration Signatures

• Each fault has a signature in vibration analysis: imbalance at 1× running speed, misalignment at 1× and 2× with high axial readings, and looseness as many harmonics.
• Imbalance force grows with the square of speed. A 20 percent speed increase adds 44 percent more force, and a 50 percent increase more than doubles it.
• ISO 20816-3 sorts vibration into four zones. For a medium pump on a rigid base, 4.5 mm/s (0.18 in/s) marks the start of the damage zone.
• A trend from the same point beats any single reading. A jump from 1.0 to 2.4 mm/s is a real change, even inside the acceptable zone.

Every rotating machine vibrates a little, because no rotor is perfectly balanced and no two shafts are perfectly in line. The question is whether the vibration is steady and low, or rising and pointing at a fault.

Too much is defined two ways. ISO 20816-3 sets absolute zones by machine size and mounting, covered in section 07. The machine's own history sets the second test: a reading that has changed since last month matters even inside its zone.

The four faults in this guide are the most common mechanical causes. Each adds a force the machine was never designed to carry, and that force goes straight into the bearings covered in Part 1 of this series.

• Imbalance: a heavy spot on the rotor pulls outward once per turn.
• Misalignment: the motor and pump shafts do not form one straight line.
• Mechanical looseness: something that should be tight, from a base bolt to a bearing in its housing, can move.
• Soft foot: a machine foot does not sit flat, so bolting it down bends the frame.

Imbalance looks like a strong vibration at exactly once per revolution, called 1× running speed, mainly in the radial direction. A heavy spot on the rotor creates a force that turns with the shaft and pulls the bearings outward on every revolution.

The force follows a simple law: mass times radius times angular speed squared. Because speed is squared, small speed changes have large effects.

• 20 percent faster: 1.2 × 1.2 = 1.44, so 44 percent more force from the same heavy spot.
• 50 percent faster: 1.5 squared is 2.25, so the force more than doubles.

This is why a VFD speed change can make a quiet fan rough overnight. The balance was fine at the old speed and is not fine at the new one.

What causes imbalance in heating, ventilation and air conditioning (HVAC) equipment

• Dirt on fan blades and wheels: build up collects unevenly, especially on the forward curved fan wheels common in air handling units (AHUs).
• Erosion and corrosion: blades lose metal unevenly over time.
• Lost balance weights: a clip or weld weight falls off and the original heavy spot returns.
• Water in hollow blades: cooling tower fan blades can absorb water and become heavy on one side.
• Ice: uneven ice on tower fans in winter, covered in Part 8.

Balance quality is graded under ISO 21940-11, formerly ISO 1940-1. Fans and pump impellers are commonly balanced to grade G6.3. When a fan is rebuilt or cleaned, ask what grade it was balanced to and at what speed.

Misalignment looks like vibration at 1× and 2× running speed, with unusually high readings along the shaft axis, and heat at the coupling. It means the motor and pump shafts meet at an angle, sit offset from each other, or both.

• Angular misalignment: the shafts meet at an angle. It shows high axial vibration at 1× and 2×.
• Parallel, or offset, misalignment: the shafts are parallel but displaced. It shows strong radial vibration at 2×.

Phase confirms it. Readings taken on either side of the coupling are typically about 180 degrees out of phase, moving opposite to each other. Heat concentrates at the coupling and the bearings nearest it, which thermal imaging shows clearly.

How pumps fall out of line

• Pipe strain: piping that is not properly supported pulls a pump out of line, often after maintenance.
• Thermal growth: hot pumps grow as they warm up, so an alignment made cold can be wrong at running temperature.
• Base settling: the foundation shifts slowly over years.
• A motor replaced without laser alignment: the new motor goes in by eye and runs out of line from day one.

Laser alignment after every motor or seal change is the simplest defense. The record belongs in the maintenance management system, so the next technician knows the starting point.

Mechanical looseness is any joint that should be rigid but has room to move. Analysts commonly sort it into three types, because each sits in a different place and leaves a different pattern.

• Structural looseness: a weak base, cracked grout or a loose frame. It shows at 1× in the direction of the looseness, often far stronger in one direction than the others.
• Loose bolts or a cracked frame or bearing pedestal: a strong 2× reading, because the part lifts and lands twice each turn.
• Rotating looseness: a bearing loose in its housing or an impeller loose on its shaft. It fills the spectrum with many harmonics of running speed, sometimes with half order peaks such as 0.5× and 1.5×.

Rotating looseness is the most serious of the three. A loose part keeps hammering against its seat, and the gap grows with every turn.

Structural looseness is the easiest to see. Cracked grout, missing hold down bolts and a base that moves when the machine starts are all visible on a walk past.

Soft foot means one or more machine feet do not sit flat in the same plane. Tightening the hold down bolts then bends the frame, which distorts the bearing bores and the motor's air gap.

The machine looks fine standing still, because the distortion is hidden inside the frame. Once it runs, the twisted bearing bores and uneven air gap turn into vibration and heat.

How it is checked

Set a dial indicator at a foot, loosen its bolt and watch how far the foot rises. A common field threshold is about 0.002 inch (0.05 mm) of movement. Check every foot, one at a time, before any alignment work.

Pipe strain can create the same effect without any gap under the foot. This is called induced soft foot, and the fix is to support the piping properly, not to add shims.

The electrical tell

An uneven motor air gap shows up as vibration at twice line frequency: 120 hertz (Hz) in 60 Hz countries. The giveaway is that it disappears the instant power is cut, while mechanical vibration fades as the machine coasts down.

The US Department of Energy (DOE) also lists 120 Hz vibration as a sign of voltage unbalance (DOE). So a 120 Hz reading can point to soft foot or to the power supply, which is covered in Part 6.

You tell them apart by where the vibration sits in frequency, which direction it is strongest, and how it behaves when the machine changes speed or stops. A quick reference:

• Imbalance: 1× running speed, mainly radial, rising with the square of speed.
• Angular misalignment: 1× and 2×, high in the axial direction, about 180 degrees out of phase across the coupling.
• Parallel misalignment: strong radial 2×, with heat at the coupling.
• Structural looseness: 1× strongly in one direction.
• Loose bolts or cracked pedestal: strong 2×.
• Rotating looseness: many harmonics, sometimes 0.5× and 1.5×.
• Soft foot: distortion when bolted, often with 120 Hz on motors that vanishes when power is cut.

In practice, faults stack. A misaligned pump often loosens its own base, and a soft foot makes alignment impossible. Analysts usually check looseness and soft foot first, then alignment, then balance.

This is why no single sensor is enough. A spectrum, a phase reading, a thermal image of the coupling and acoustic listening each rule out different suspects. Airborne ultrasound shows whether the bearings have started to suffer.

ISO 20816-3 judges industrial machines above 15 kilowatts (kW) by broadband vibration velocity on the bearing housings. It uses the root mean square (RMS), typically from 10 to 1,000 Hz. It replaced ISO 10816-3 and sorts readings into four zones (Fabrico).

• Zone A: typical of a newly commissioned machine.
• Zone B: acceptable for long term running.
• Zone C: limited running only; plan a repair.
• Zone D: damage is likely.

Group 2: machines from 15 to 300 kW

• Rigid support: A/B at 1.4 mm/s (0.055 in/s), B/C at 2.8 mm/s (0.11 in/s), C/D at 4.5 mm/s (0.18 in/s).
• Flexible support: A/B at 2.3 mm/s (0.09 in/s), B/C at 4.5 mm/s (0.18 in/s), C/D at 7.1 mm/s (0.28 in/s).

Group 1: machines above 300 kW

• Rigid support: A/B at 2.3 mm/s (0.09 in/s), B/C at 4.5 mm/s (0.18 in/s), C/D at 7.1 mm/s (0.28 in/s).
• Flexible support: A/B at 3.5 mm/s (0.14 in/s), B/C at 7.1 mm/s (0.28 in/s), C/D at 11.0 mm/s (0.43 in/s) (Oxmaint).

One inch per second equals 25.4 mm/s. Most chilled water and condenser water pump motors fall in Group 2, while large chiller compressor motors fall in Group 1. Pumps also have their own allowable vibration guidance from the Hydraulic Institute, ANSI/HI 9.6.4.

A trend matters more because the zones are wide, and a machine can change a great deal without leaving the acceptable zone. The change is the early warning; the zone is the late one.

Take a machine that ran at 1.0 mm/s (0.04 in/s) for three years and jumps to 2.4 mm/s (0.09 in/s). It still sits in Zone B, yet its vibration more than doubled (Acoem). Something happened, and it is worth finding before it reaches Zone C.

Trends only work when readings are taken at the same point, in the same direction, under similar load. That discipline is the core of predictive maintenance and of a sound preventive maintenance program.

Speed belongs in every record. On a VFD driven machine, note the speed with each reading from the drive panel readouts, or a speed change will look like a new fault.

Left alone, all four faults shorten the life of bearings and seals by adding load, and misalignment also wears out flexible coupling elements. The damage spreads from the fault to the most delicate part nearby.

• Imbalance: loads both bearings on every revolution, and the load climbs steeply with speed.
• Misalignment: loads the bearings and mechanical seal nearest the coupling, and chews through elastomer coupling inserts.
• Looseness: lets parts hammer against their seats, so the gap and the vibration grow together.
• Soft foot: distorts bearing bores and the motor air gap, adding heat and electrical vibration.

In an office building

AHU fans and condenser water pumps are the usual suspects, and their vibration travels through the structure into occupied floors as noise. A pump that keeps destroying seals also creates water damage risk. Routine checks are covered in our guide to HVAC maintenance in office buildings.

In a data center

Chilled water and coolant distribution unit (CDU) pumps carry the risk. One degraded pump taken out of service spends the plant's redundancy. That problem is covered in cooling loads nobody owns and, for liquid cooled halls, in liquid cooling inspection.

A machine that keeps coming back with the same fault is a root cause problem, not bad luck. Our guide to recurring HVAC failures covers how to break the cycle before it becomes deferred maintenance.

Several other problems produce vibration near running speed, and fixing the wrong one wastes a shutdown.

• Bearing faults: peaks at frequencies that are not whole multiples of running speed. Covered in Part 1.
• Resonance: vibration that peaks at one speed and drops away on either side. Covered in Part 5.
• Belt problems: belt frequency sits below running speed. Covered in Part 4.
• Electrical faults and voltage unbalance: vibration at 120 Hz that vanishes when power is cut. Covered in Part 6.
• Cavitation: broadband noise that changes with flow. Covered in Part 3.

A walk past can prove that something is loose, dirty or rubbing, and it can flag heat or motion that has changed. It cannot say which of the four faults is driving the vibration. Each finding carries a label for how much it shows.

• Directly observable: a visibly rocking base, cracked grout, loose or missing hold down bolts, dirt caked on fan blades, and rubber dust under a coupling.
• Strong indicator: heat concentrated at the coupling, overall vibration trending up, and visible motion in the base or piping.
• Suspected: which of the four faults it is. Telling them apart reliably needs a vibration spectrum and phase readings from a contact measurement.

That is the limit of any route based check: it finds the change and the physical evidence, and a contact measurement names the fault. A walkthrough record with photos makes the handoff fast.

A thermal reading with a reference in the same frame shows a hot coupling beside the cooler free end of the same motor.

• Hold down bolts all present and tight, with no cracked grout under the baseplate.
• Shims: a tall stack of thin shims under one foot, a common sign of rushed alignment.
• Coupling: rubber crumbs or dust under an elastomer coupling.
• Heat: the coupling end of the motor compared with its free end.
• Fan blades: dirt build up on any blades you can see.
• Piping: supports near the pump in place, with nothing hanging on the pump flanges.
• Records: an alignment record or sticker with a date.

Coming Soon

1. 1. Which machine group and support type does each critical pump fall into under ISO 20816-3?
2. 2. Was each pump laser aligned after its last motor or seal change?
3. 3. Has any fan's speed been raised by its VFD since it was last balanced?
4. 4. Do you have a vibration baseline for each critical pump and fan?
5. 5. Who checks pipe supports after work near a pump?

What causes vibration at 1× running speed?

The most common cause is imbalance, a heavy spot on the rotor that pulls outward once per turn. Structural looseness and angular misalignment can also show at 1×, so direction and phase readings are used to tell them apart.

How do you tell imbalance from misalignment?

Imbalance shows mainly at 1× running speed in the radial direction. Misalignment shows at 1× and 2×, with high axial readings for angular misalignment, and readings about 180 degrees out of phase across the coupling.

What is soft foot on a motor?

Soft foot is a motor foot that does not sit flat in the same plane as the others, so tightening its bolt bends the frame. A common field threshold is about 0.002 inch (0.05 mm) of movement when the bolt is loosened, checked with a dial indicator.

What vibration level is acceptable for a pump?

Under ISO 20816-3, a medium pump on a rigid base is fine for long term running up to 2.8 mm/s (0.11 in/s) RMS. Above 4.5 mm/s (0.18 in/s) it enters the damage zone. The Hydraulic Institute's ANSI/HI 9.6.4 also gives pump specific guidance, and a rising trend matters even inside the acceptable zone.

Why did vibration increase after a VFD speed change?

Imbalance force rises with the square of speed. A 20 percent speed increase adds 44 percent more force from the same heavy spot. A fan balanced at its old speed can run rough at the new one.

What is mechanical looseness?

Mechanical looseness is a joint that should be rigid but can move. It comes in three common types: structural looseness at the base, loose bolts or a cracked pedestal, and rotating looseness inside the machine.

Can misalignment cause bearing failure?

Yes. Misalignment adds load to the bearings and seal nearest the coupling, which shortens their life. It also wears out flexible coupling elements.

Can dirt on fan blades cause imbalance?

Yes. Dirt builds up unevenly on fan blades and wheels, especially forward curved AHU fan wheels, and creates a heavy spot. Cleaning the wheel should be followed by a balance check.

What does 120 Hz vibration mean?

In 60 Hz countries, 120 Hz is twice line frequency. It points to an electrical cause, such as an uneven motor air gap from soft foot or voltage unbalance. It disappears the instant power is cut, which separates it from mechanical vibration.

• Bearing Failure, Stage by Stage: Why Ultrasound Hears It Before Vibration Feels It and Heat Shows It
• Vibration Monitoring in Data Centers: Catching a Failing Fan or Pump Bearing Early
• Predictive vs Preventive Maintenance: Which Building Systems Should Move First

• Fabrico: ISO 20816 vibration severity zones
• Oxmaint: vibration analysis zone boundaries, including flexible support
• Acoem: why the trend matters more than the zone
• US Department of Energy: voltage unbalance tip sheet