One worn bearing — vibration cut to a fifth

The Setup

A combustion-and-thermal-systems integrator was commissioning a new furnace-air package at their end-customer's plant in western India. Two of the fans in that package — a forced-draught and an induced-draught centrifugal set — were ours. Commissioning is the worst possible time for a fan to misbehave: the whole line waits on it, a demanding end-user is watching every reading, and nothing can be handed over until the machines run clean. We'll call them the Integrator.

The Complication

During commissioning, the ID and FD fans ran with more vibration than the Integrator's end-user would accept — and the two parties were even measuring against different vibration standards, so the acceptance target itself was contested. Foundation bolts had been tightened, the coupling checked, the ducting confirmed clear — the usual first moves — and vibration came down, but not all the way. Then, in the first year of operation, the ID fan developed severe vibration during routine maintenance — vertical readings around 19–20 mm/s on the fan and over 24 mm/s at the motor — and the plant shut it down. A fault that keeps coming back is not a spare-parts problem; it is a root-cause problem, and that is exactly what it turned out to be.

The Diagnosis & Fix

Rather than simply re-balance and hope, our engineers ran an as-found vibration survey — horizontal, vertical and axial readings at the motor and fan, drive and non-drive ends — and benchmarked them to the in-situ fan-vibration standard (ISO 14694, in-service limits) at balance grade G6.3. The vertical-dominant signature and a rough bearing pointed the investigation at the machine, not the aerodynamics.

The fix eliminated the cause on both fronts. During commissioning we added the correct pattern of structural stiffeners to the casing and base (and corrected the base-frame) so the structure no longer amplified vibration; when the bearing later failed, we replaced both bearings and the adaptor sleeve and re-commissioned with a full as-found → as-left survey. Jammed dampers were re-engineered with a wider bush tolerance and new dampers supplied.

The Result

• Root-caused, not guessed: a worn bearing on an under-stiffened structure — proven on a signed on-site report, not asserted.
• Measured: ID-fan vertical vibration fell ~74%, from ~19–20 mm/s to ~5 mm/s (motor peak likewise from ~24 to under 6 mm/s), back within the accepted in-situ limit (ISO 14694).
• Stayed fixed on this unit: bearings and adaptor sleeve replaced; the fan returned to service and the complaint was technically closed.
• Won't repeat across the fleet: we froze a standard stiffener specification and a mandatory design-change-note step, so the structural cause is engineered out of future fans of every drive type — not just this one.
• Fast to acknowledge: every complaint was acknowledged the same working day (our commitment is within 4 working hours), with a written corrective-action plan issued up front; the physical fix followed once the replacement parts were manufactured.

The Takeaway + Call to Action

A fan fault that keeps returning after balancing is almost always a structural-or-bearing problem in disguise. Running an FD/ID or process fan that vibrates high and won't settle? Ask us for a root-cause vibration survey — you'll get a written corrective action benchmarked to the in-situ standard, not another balance pass.

— Jitamitra Electro Engineering · Technical Services

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