High vibration after commissioning: how to find the real cause

A fan that ran smoothly on the test bed and shakes on your foundation is not a mystery. It is a machine whose support, alignment, fit or duty has changed since it was last measured. This is the field procedure our service engineers use to find out which — on any make of centrifugal fan or blower.

What you're seeing

• Vibration magnitude. Broadband velocity in mm/s r.m.s. at the bearings. Anything from mild roughness to 8–13 mm/s on a 22 kW / 2,925 rpm induced-draught fan is common in the field.
• Directionality. A vertical reading far above horizontal and axial (say 8–9 mm/s vertical against 4–5 mm/s in the other axes) is the classic soft-foot / weak-base signature. Radial 1× vibration, roughly equal H and V, points at unbalance. A strong 2× component points at misalignment or looseness.
• Noise. "Abnormal noise on restart", or a growl that was not there at handover. In these cases noise is almost always mechanical — a coupling rub, a worn flexible element, a distressed bearing — not aerodynamic.
• Motor current above the permissible limit. That flags rubbing, over-speed, or an operating point far from the design duty. It is not a balance symptom.
• Visual and physical. Coupling cover touching the rubber spider; pin-and-bush rubber chewed up; inlet cone-to-impeller gap several times the drawing value; in the worst cases a cracked casing, a bent shaft, or material liberated from the wheel.
• Temperature. Bearing housing temperature rise is the secondary confirmer of misalignment or bearing preload. Record it every time.

The thresholds to reach for — and state the mounting. ISO 14694 for a BV-3 industrial fan:

A number without a mounting condition is meaningless. On a rigid mount, 7.1 mm/s is already alarm and 9.0 mm/s is stop-territory — do not let a rigidly mounted fan drift up toward 11 mm/s because "the limit is eleven-something". Those figures (6.4 / 11.2 mm/s) belong to the ISO 10816 / 20816 zone boundaries — a different standard with different assumptions. Keep them separate.

What it usually means

A fan balanced (typically to ISO 21940 grade G6.3) and run-tested at works leaves the factory within limits. When vibration appears afterwards, the higher prior is a lifecycle event — the support, the alignment, the fit or the duty changed, not the rotor. Ranked by how often they actually turn up:

1. Weak, un-grouted or uneven base / soft foot — the most common single site cause.
2. Fasteners loosened and coupling shifted in transport.
3. Coupling and cone-gap set-up errors during site assembly.
4. Impeller hub not properly fitted or seated on the shaft.
5. In-service wear — coupling elements, bearings, dust build-up on the wheel.
6. Off-curve operation — near stall, over-speed, or a throttled system that was never the design duty.

That ranking is an opening hypothesis, not a verdict. An out-of-drawing built dimension, or a bought-out component that arrived defective, is a live possibility and should be checked on its merits rather than argued away. Work the mount, alignment, coupling and wear first because that is where the evidence usually is — then follow the evidence wherever it goes.

How to diagnose it

Safety first. Isolate and lock out the motor. Confirm zero energy at the terminal box, confirm the impeller has coasted to rest, and only then open guards. Nothing below is done on a live machine.

1. Look before you run. Photograph every side of the fan, the impeller, the coupling, the foundation and the duct connections before anyone touches a spanner.
2. Confirm the mounting. Torque-check every foundation and pedestal bolt — road transport backs them off. Then a soft-foot check: dial indicator on the bearing housing, loosen one foot at a time, watch the movement. More than about 0.05 mm of lift is a soft foot. Confirm the base is grouted, flat and rigid — not a steel frame resonating with the fan.
3. Measure properly. Handheld vibration meter at both bearings, in vertical, horizontal and axial. Log 1× and 2× if you have an analyser. Compare against the works run-test report for that serial.
4. Check alignment and coupling. Dial-gauge for angular and parallel misalignment. Inspect the flexible element (pin-and-bush set or rubber spider) for wear and backlash. Confirm the coupling cover cannot touch any rotating part — cover-to-spider contact is a real and frequently missed cause of both noise and element destruction. Check the shaft is not bent.
5. Check the impeller. Hub-to-shaft fit (excess bore clearance gives an eccentric running mass and a textbook 1× unbalance), key seated, locking bolt torqued, no deposit build-up or erosion, and the inlet cone-to-impeller gap measured against the drawing — 6–8 mm is typical; 20 mm is not a commissioning setting, it is a built dimension and must be logged as such. Turn the wheel by hand and listen for rub.
6. Bearings. Feel and listen for roughness; check housing temperature, lubrication and locking-collar tightness.
7. Operating point. Read current, and pressure and flow if you can. A fan run far off its curve, or on the unstable side of stall, will pulsate and over-draw however well it is balanced.

Decision points. 1× radial, roughly equal H and V → unbalance; trim-balance in situ. High 2×, vertical-dominant, soft foot confirmed → re-shim, re-align, re-torque. Worn element or rough bearing → replace. Crack, bent shaft or liberated wheel material → stop the fan and find the driver before you rebuild.

One caution: the vertical-high signature narrows the hypotheses, it does not close the diagnosis. We have seen that exact pattern turn out to be a loose impeller hub, not a soft foot. Confirm the mechanism before you reach for balance weights.

The usual root causes

Transport and handling. Foundation and coupling fasteners backed off by road shock → looseness and misalignment; confirm by torque and soft-foot check. Balance disturbed by rough handling or lifting the fan by the wheel; confirm against the works run-test baseline.

Installation. Weak, un-grouted or uneven base — a non-rigid mount amplifies even acceptable residual unbalance and reads vertical-dominant. Coupling misaligned, or its cover set so close it fouls the element. Inlet cone gap wrong: too wide costs performance and can excite flow-induced vibration, too tight rubs. Impeller hub not seated, or an over-clearance fit on the shaft. Hard-ducted, unsupported ductwork pulling on the casing — confirm by loosening the flexible connector and re-reading.

Commissioning. No baseline reading taken, so nobody knows what "normal" was; bolts never torque-marked, so nobody knows what has moved since.

Operation and process. Running off-curve, near stall or over-speed — pulsation plus high current. Progressive unbalance from dust deposits or erosion on the wheel: the norm rather than the exception in dust-collection and scrubber duty.

Maintenance and wear. Aged coupling flexible element — backlash, noise, effective misalignment. Bearing wear or under-lubrication — vibration and temperature rising together. Restart after a long idle period without a re-check, which is when a settled or seized element announces itself.

Eliminate a defective bought-out component (motor, bearing, coupling) early too — an incoming motor with a cracked foot will not be fixed by balancing anything.

How to fix it

• Unbalance: in-situ trim-balance the assembled rotor to ISO 21940 G6.3. If the wheel is deposit-loaded, clean it first — balancing a dirty wheel buys you a few weeks.
• Soft foot / weak base: re-shim the feet flat, grout the base properly, re-torque foundation bolts to spec, re-measure. On a rigid mount the target is ≤ 4.5 mm/s.
• Misalignment / coupling: re-align to the coupling manufacturer's tolerance, replace the worn flexible element as a complete set, reset the cover clearance so it cannot contact the spider.
• Impeller fit / cone gap: correct the hub-to-shaft fit or re-key; reset the inlet cone gap to the drawing.
• Bearings: replace, re-lubricate, set the locking collars correctly.
• Structural failure (broken impeller, bent shaft, cracked casing): stop the fan. Replace impeller, shaft, plummer block, bearing and drive as required, repair the casing — then find and eliminate the unbalance or misalignment that fatigued it. Rebuilding into the same fault simply repeats the failure.

How to stop it coming back

Design and detailing. Coupling-cover geometry that cannot foul the element. Correct specified hub-to-shaft fit. Flexible duct connectors at inlet and outlet to break system-effect strain into the casing.

Commissioning discipline — where most of the value is: rigid grouted foundation; soft foot verified with a dial indicator; every bolt torqued and torque-marked; coupling aligned and cover clearance set; cone gap checked against the drawing; and a signed commissioning sheet carrying a baseline vibration reading (V, H, axial, both bearings). A fan without a baseline is a fan you can only argue about later.

Handover. Written mounting and alignment guidance plus the operating envelope, so the fan is not quietly run off-curve for two years.

Maintenance intervals. Periodic coupling-element inspection; bearing condition and lubrication checks; wheel cleaning in dusty and scrubber duty, before progressive unbalance becomes structural damage. Repeat the vibration reading against the baseline — the trend tells you more than any single number.

Traceability. Keep the works run-test report with the serial. When vibration appears three years later, that report is the only honest reference point you have.

When to call a specialist

Jitamitra services fans of any make, not only our own — on-site trim balancing, vibration diagnosis, soft-foot and alignment correction, bearing and coupling replacement, and re-rating where the duty has moved away from what the fan was built for. If a fan is sitting above its ISO 14694 alarm band and the checks above have not settled the cause, it is worth a call before it becomes a bent shaft.

Contact: sales@jitamitrablowers.com · Jitamitra Help Desk +91 83291 72325.

Jitamitra Electro Engineering Private Limited. Quality management system ISO 9001:2015 certified. Fan performance tested to IS 4894 / ISO 5801 / AMCA 210 method. CE and ATEX (Zone 2/22) conformity is self-declared.

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