Resonance, Vibration Isolators and Equipment Foundations: When the Base Is the Fault

• Resonance shows as vibration that peaks at one speed and falls away on either side. Variable frequency drives (VFDs) make it more common because they sweep through speeds.
• A spring's natural frequency in hertz is about 3.13 divided by the square root of its static deflection in inches. Isolation only starts when the running frequency is more than about 1.41 times that.
• A 1 inch (25 mm) spring under a 1,750 rpm pump isolates roughly 99 percent of the vibration. A 0.1 inch (2.5 mm) pad under a fan slowed to 600 rpm amplifies it instead.
• Most isolator failures are visible: bottomed springs, locked limit stops, rigid conduit bridging the isolator, and stretched flexible connectors.

Resonance is a match between a forcing frequency and a natural frequency. Every structure, from a pump base to a steel platform, has natural frequencies at which it vibrates freely once disturbed.

When something pushes at one of those frequencies, a small force produces a large vibration, limited only by damping. The forcing can be running speed, blade pass, a belt frequency or any other repeating push from the machine.

The tell is vibration that peaks at one speed and falls away on either side. A machine with imbalance gets steadily rougher as it speeds up; a resonant machine gets rough at one speed, then quiets down again as it passes through.

That difference matters because the fixes are opposite. Imbalance, covered in Part 2, is fixed on the rotor. Resonance is fixed by changing the structure or keeping the machine away from the problem speed.

A VFD driven pump or fan sweeps through a range of speeds. That makes it far more likely to land on a resonance than a constant speed machine. A constant speed fan either sits on a natural frequency or it does not; a VFD fan visits dozens of speeds every day.

The worst case is a VFD that settles at the problem speed for long periods. The machine runs rough for hours, wearing its bearings and loosening its base, while nothing in the controls looks wrong.

This is why the speed belongs in every vibration record. A reading taken without the speed from the drive panel readout can make a resonance look like a mechanical fault that comes and goes.

A skip frequency is a VFD setting that jumps over a band of speeds, so the drive never settles where the machine resonates. Most VFDs offer it, sometimes called critical frequency avoidance.

The drive still passes through the band when it speeds up or slows down, but it will not run inside it. The machine spends seconds at the problem speed instead of hours.

Finding the band is straightforward. Slowly ramp the drive through its speed range while taking vibration readings, or record the vibration as the machine coasts down. The speeds where vibration peaks are the ones to skip.

Skip frequencies should be set from a test, not a guess. Each one should be recorded with the reason it exists, so a later technician does not remove it after a drive replacement.

A spring isolator works by putting a soft support with a low natural frequency between the machine and the floor. Above that frequency, the spring absorbs most of the machine's vibration instead of passing it into the structure (Fabrico).

The isolator's natural frequency can be estimated from how far the spring compresses under the machine's weight, called static deflection:

• In hertz (Hz): about 3.13 divided by the square root of static deflection in inches.
• In metric: about 15.76 divided by the square root of deflection in millimeters.
• In cycles per minute: about 188 divided by the square root of deflection in inches (Eaton).

Two main types are used. Steel springs compress a long way and suit large pumps, fans and chillers. Elastomer pads, made of rubber or neoprene, compress very little and suit small equipment running at high speed.

More deflection means a lower natural frequency and better isolation. That is why spring isolators, which compress far more than rubber pads, are used under the larger pumps and fans in a plant.

An isolator needs enough deflection that the machine's running frequency sits well above the isolator's natural frequency. Isolation only begins when the forcing frequency is more than √2, about 1.41 times, the isolator's natural frequency. The further above that ratio the machine runs, the more of its vibration the isolator absorbs.

Worked example 1: a pump on springs

A spring with 1 inch (25 mm) of static deflection has a natural frequency of about 3.1 Hz. A 1,750 rpm pump turns at about 29 Hz, a ratio of about 9. At that ratio the spring isolates roughly 99 percent of the vibration.

Adding mass

An inertia base, a heavy concrete block under the pump, lowers the system's natural frequency without changing spring stiffness. That improves the ratio and steadies the pump against piping forces.

The ASHRAE Handbook, HVAC Applications volume, chapter on noise and vibration control, gives deflection guidance by equipment type and floor span. Longer, more flexible floor spans call for more deflection, because the floor itself can resonate.

The wrong isolator makes vibration worse because below a ratio of about 1.41 it transmits as much as a rigid mount or more. At a ratio of 1, it resonates and amplifies the vibration it was bought to stop.

Worked example 2: a fan on pads

An elastomer pad with 0.1 inch (2.5 mm) of deflection has a natural frequency of about 10 Hz. A VFD fan slowed to 600 rpm turns at 10 Hz. It sits right on the pad's natural frequency, and the pad amplifies instead of isolating.

The same pad works well at full speed. This is the VFD trap. An isolator chosen for design speed can become an amplifier at the low speeds the drive now runs at most of the time.

Anyone retrofitting a VFD onto an existing fan or pump should check the isolator against the lowest speed the drive will run, not just the design speed.

A failed isolator usually looks wrong from a few feet away. The common failures are visible on a walk past:

• Bottomed springs: coils touching each other, so the spring is now a solid block.
• Leaning or over extended springs: the load has shifted or the spring was never sized for it.
• Leveling bolts never adjusted: the machine sits unevenly on its springs.
• Shipping bolts or limit stops left locked: the machine is tied rigidly to the floor, and the springs do nothing.
• Damaged pads: elastomer pads crushed, cracked or softened by oil.
• Snubbers in contact: seismic snubbers touching the machine during normal running.

Bridging, or short circuiting

Bridging is any rigid path that carries vibration straight past the isolators. Rigid conduit, pipe, drain lines or ductwork tied directly to isolated equipment all do it.

A perfectly chosen spring does nothing if a rigid conduit runs from the motor to the wall. Every connection to an isolated machine should be flexible.

These faults are often created by later work, not the original installer. A new conduit run, a replaced drain line or a pipe support added during a repair can bridge an isolator years after commissioning. Record isolator checks in the maintenance management system so changes are caught on the next round.

Flexible pipe connectors absorb vibration and small movement between an isolated pump and its rigid piping. They let the pump move on its springs without shaking the pipe system.

They are not meant to correct misalignment. A connector that is stretched, compressed or offset to make two pipes meet is already carrying a load it was never designed for.

Over extension, compression or offset shortens a connector's life and creates a leak risk. In a mechanical room that leak can become water damage; near critical equipment it is a far bigger problem, covered in our guide to liquid cooling inspection.

Pipe that pulls on a pump also pulls it out of line, the pipe strain problem covered in Part 2.

Foundations fail slowly, through cracking, loosening and corrosion, until the machine is no longer held rigidly to its base. The signs build up over years and are easy to walk past.

• Cracked or hollow grout: the grout under the baseplate breaks up or loses contact, leaving voids.
• Loose or corroded anchor bolts: the bolts that tie the base to the pad stop holding.
• Rust weeping at anchors: a stain spreading from an anchor is a sign of corrosion underneath.
• Cracked concrete pads: the pad itself breaks up under years of vibration.
• A rocking baseplate: the base visibly moves when the machine starts.

Foundation repairs are rarely urgent until they suddenly are. A pump that keeps losing alignment or eating seals after every repair is often sitting on a failing base. That is a classic recurring failure pattern, and the fix often ends up as deferred maintenance until it is planned.

A loose foundation shows as structural looseness in a vibration reading, covered in Part 2. A cracked pump base can also pull the pump out of line, turning a foundation problem into a bearing and seal problem.

Mechanical room vibration reaches tenants and IT equipment through the structure and the piping, whenever isolators fail or are bridged. The building's frame carries it far from the room where it starts.

In an office building

Rooftop and upper floor mechanical rooms sit directly above occupied space. Vibration and low frequency noise carried into those floors bring tenant complaints, and accelerated wear on everything bolted to the structure. Routine checks are covered in our guide to plant maintenance in office buildings.

In a data center

Vibration transmitted through structure and piping can reach the raised floor and racks. Flexible connector leaks near critical equipment and cracked pump bases leading to misalignment add their own risks.

The Gregg video is the extreme case, but the principle holds: spinning disks are sensitive to vibration. Keeping the plant's vibration in the plant is part of protecting the IT load, a point covered in cooling loads nobody owns.

Resonance is often confused with faults that also make a machine rough. The difference is how vibration behaves with speed.

• Imbalance: rises steadily with speed instead of peaking at one speed. Covered in Part 2.
• Structural looseness: a loose base or cracked grout, strongest in one direction. Also covered in Part 2.
• Water hammer: impact noise during valve or pump transients, covered in the Cooling Plant Performance series.

Other faults sit on top of a resonance and get amplified by it. Examples are a bearing defect from Part 1, cavitation from Part 3 or a belt problem from Part 4. This is why no single sensor is enough: vibration analysis, acoustic listening and the speed reading each rule out different suspects.

A walk past can prove that an isolator has failed or been bridged, and that a foundation is cracking. It cannot prove that a structure is truly resonant. Each finding carries a label for how much it shows.

• Directly observable: bottomed or leaning springs, locked limit stops, rigid conduit to isolated equipment, damaged flexible connectors, cracked grout and rust at anchors.
• Strong indicator: vibration that peaks at one VFD speed, visible base motion, and noise traced to a specific machine.
• Suspected: whether a structure is truly resonant. Proving it needs a run up or coast down test, or an impact (bump) test.

That is the limit of any route based check. It finds the failed isolator and the problem speed; a run up or bump test proves the resonance.

A camera records spring gaps and limit stops, and thermal imaging shows bearing heat on a machine that has been running rough.

Isolator and foundation checks belong in the preventive maintenance schedule, and vibration trends turn them into predictive maintenance.

Changes over time matter most. A digital twin that records what changed can show a spring that has settled or a conduit added since the last survey. A walkthrough record keeps the history.

• Spring isolators with a visible gap between coils, standing straight.
• Limit stop nuts backed off, not tight.
• Flexible conduit, not rigid, at every isolated machine.
• Flexible pipe connectors straight, not stretched, compressed or offset.
• Grout intact under baseplates, with no rust weeping at anchors.
• Speed: note the VFD speed whenever a machine sounds rough.

Coming Soon

1. 1. Were shipping bolts and limit stops released at commissioning, and is there a record?
2. 2. Do any VFD driven machines have skip frequencies set, and why?
3. 3. Which isolated machines have rigid conduit or pipe attached?
4. 4. When were flexible connectors last inspected or replaced?
5. 5. Do vibration complaints from tenants or the IT floor trace back to one machine?

What is mechanical resonance?

Mechanical resonance is a match between a forcing frequency, such as running speed, and a natural frequency of the machine or its structure. A small force then produces a large vibration, limited only by damping.

Why does a fan vibrate more at certain speeds?

The fan is passing through a natural frequency of its base, structure or isolators. Vibration that peaks at one speed and drops on either side is the classic sign of resonance, which is common on VFD driven fans.

What is a VFD skip frequency?

A skip frequency is a variable frequency drive setting that jumps over a band of speeds, so the drive never settles where the machine resonates. It is also called critical frequency avoidance.

How do spring vibration isolators work?

A spring isolator supports the machine on a soft spring with a low natural frequency. When the machine runs well above that frequency, the spring absorbs most of the vibration instead of passing it into the floor.

How much deflection should a vibration isolator have?

Enough that the running frequency is well above the isolator's natural frequency, which is about 3.13 divided by the square root of the static deflection in inches. Isolation only begins above a ratio of about 1.41, and the ASHRAE Handbook gives guidance by equipment type and floor span.

Why isn't my spring isolator working?

The most common causes are springs that have bottomed out, shipping bolts or limit stops left locked, and rigid conduit or pipe bridging past the isolator. A spring chosen for full speed can also resonate when a VFD runs the machine much slower.

What is a short circuited isolator?

A short circuited, or bridged, isolator has a rigid path running around it, such as rigid conduit, pipe, drain lines or ductwork tied directly to the machine. Vibration travels straight through that path into the structure.

What are flexible pipe connectors for?

Flexible pipe connectors absorb vibration and small movement between an isolated pump and its piping. They are not meant to correct misalignment, and stretching, compressing or offsetting them shortens their life and creates a leak risk.

Can cracked grout cause pump vibration?

Yes. Cracked or hollow grout lets the baseplate move, which shows as structural looseness and can pull the pump out of alignment. That in turn adds load to the bearings and mechanical seal.

Can mechanical room vibration affect servers?

Spinning disks are sensitive to vibration, and vibration can travel from the mechanical room through the structure and piping to the racks. Working isolators and flexible connections keep the plant's vibration in the plant.

• Imbalance, Misalignment, Looseness and Soft Foot: Four Faults, Four Vibration Signatures
• Vibration Monitoring in Data Centers: Catching a Failing Fan or Pump Bearing Early
• Digital Twins in Data Centers: How Lidar Maps the Floor and Shows What Changed

• Fabrico: vibration isolation, natural frequency and the √2 rule
• Eaton: isolator efficiency formulas
• Data Center Knowledge: why you shouldn't shout in the data center