G6.3 is fine for general service. For API 673 critical service we hold G2.5 — here is the arithmetic that makes the difference, and the balancing report that proves it.
Reviewed by Jitamitra application engineering
Share on LinkedIn“Dynamically balanced” on a fan quotation can mean a measured, recorded number — or it can mean nothing at all. The word only becomes a commitment when it carries a grade, a speed, and a residual-unbalance figure you can read off a test record. For general industrial fans we balance to G6.3. For API 673 critical service we hold G2.5 as our house balancing standard — and the reason is arithmetic, not marketing.
ISO 1940-1 (now ISO 21940-11) defines balance quality as a G-grade: the product of a rotor’s permissible specific unbalance and its angular velocity, expressed in mm/s. Lower G means finer balance. The grade on its own is abstract — what the balancing machine actually measures, and what the report actually states, is a permissible residual unbalance in gram-millimetres. That number falls out of the grade, the rotor mass and the service speed:
One rule governs the whole calculation: N is the maximum speed the wheel can see in service — the VFD ceiling frequency, not the nominal motor speed. Plug in a lower N and you silently inflate the tolerance.
Take a shrouded impeller of 120 kg running at a maximum 1,480 rpm. At the general-service grade G6.3, e_per = 9549 × 6.3 / 1480 = 40.6 g·mm/kg, so U_per = 40.6 × 120 ≈ 4,877 g·mm total. Re-run the same wheel at G2.5 and the tolerance collapses:
| 120 kg wheel · 1,480 rpm max | G6.3 (general) | G2.5 (critical) |
|---|---|---|
| Specific unbalance e_per (g·mm/kg) | 40.6 | 16.1 |
| Total residual U_per (g·mm) | 4,877 | 1,936 |
| Per-plane share (g·mm) | ~2,439 | ~968 |
| Correction mass at 600 mm radius (g) | ~8.1 | ~3.2 |
G2.5 is roughly 2.5× tighter than G6.3 (the ratio of the grades themselves), and the residual you are allowed to leave in the wheel drops from about 4,877 to about 1,936 g·mm. At a 600 mm correction radius that is the difference between chasing an 8 g imbalance and a 3 g one — well inside the resolution of a properly calibrated soft-bearing machine, but only if you set out to hold it.
A real impeller distributes its mass along the shaft axis, so correcting in a single plane can leave a couple that still shakes the bearings. Production balancing is therefore two-plane: measure and correct at both faces, and the per-plane residual — roughly half of U_per for a symmetric wheel between bearings — must sit at or below its share before the wheel passes. Correction weights are welded or mechanically fixed per the impeller material rules; nothing is tacked on where it could detach into the airstream.
API 673 (3rd Ed.) covers special-purpose fans for petroleum, chemical and gas service, and it does not invent its own balance limit — it invokes ISO 1940-1 directly. That is the honest chain: the standard points at the residual-unbalance method above, and critical-service fans are held to a finer grade because the vibration they can tolerate over years of continuous duty is far lower. To be precise about what we are claiming: G2.5 is our balancing house standard for API-class and critical-service wheels, verified on the shop machine and recorded — it is not, and we do not present it as, a witnessed API 673 mechanical run-test. Two different assurances; we are careful not to conflate them.
None of this counts unless it is written down. Our balancing report travels with the fan and states: impeller mass, service speed, specified grade, the U_per calculation, initial and final residual unbalance per plane, correction masses and their positions, the balancing machine ID and its calibration-due date, and operator plus QA signatures. A wheel that will not come inside tolerance is quarantined, not shipped “close enough.” If you are specifying a critical-service fan, the three questions worth asking any supplier are: what grade, at what maximum speed, and may I see the residual on the record?
Balance is a state, not a permanent property — it is lost to erosion, to one-sided deposit, and to a cleaning crew that knocks off a load-bearing weight. That is why we re-balance after any rotor work before restart, and why field trim-balancing is judged against ISO 14694 vibration limits (the fan-specific standard). If a fan in service is shaking, first prove it is imbalance and not resonance — see our notes on field vibration diagnostics and resonance versus imbalance. And because N drives the whole tolerance, it pays to nail the service speed down first when specifying the duty point.
Talk to us about balancing to G2.5 →
Jitamitra Electro Engineering · Fan-engineering notes, written for the engineer.
Sources & basis. This note applies the ISO 1940-1 / ISO 21940-11 residual-unbalance method (e_per = 9549 × G / N; U_per = e_per × mass), worked through the 120 kg / 1480 rpm example, together with two-plane correction practice, our balancing-report format, and the grade convention we hold: G6.3 for general service, G2.5 for API 673 / critical service, where API 673 invokes ISO 1940-1. Customer and job details are withheld for confidentiality. Standards are named as methods; G2.5 is our balancing house standard, stated with no claim of a witnessed API 673 mechanical run-test.
Not on its own. The word only becomes a commitment when it carries a grade, a speed, and a residual-unbalance figure you can read off a test record. Ask any supplier three questions: what grade, at what maximum speed, and may I see the residual on the record? Without those, "dynamically balanced" can mean nothing at all.
Two lines of arithmetic. Permissible specific unbalance is e_per = 9549 × G / N in g·mm/kg, where G is the grade in mm/s and N is speed in rpm. Multiply by rotor mass for U_per, the total tolerance, which is then split between the two correction planes. That gram-millimetre figure is what the machine measures.
The VFD ceiling frequency, not the nominal motor speed. One rule governs the whole calculation: N is the maximum speed the wheel can see in service. Plug in a lower N and you silently inflate the tolerance. Because N drives everything, it pays to nail the service speed down first when specifying the duty point.
Take a 120 kg shrouded impeller at a maximum 1,480 rpm. At G6.3, e_per = 9549 × 6.3 / 1480 = 40.6 g·mm/kg, so U_per is about 4,877 g·mm. At G2.5 the residual you may leave drops to about 1,936 g·mm. At a 600 mm correction radius that is the difference between chasing an 8 g imbalance and a 3 g one.
No, and we are careful not to conflate the two. API 673 does not invent its own balance limit — it invokes ISO 1940-1 directly, which is the residual-unbalance method above. G2.5 is our balancing house standard for API-class and critical-service wheels, verified on the shop machine and recorded. It is not a witnessed API 673 mechanical run-test.
Balance is a state, not a permanent property. It is lost to erosion, to one-sided deposit, and to a cleaning crew that knocks off a load-bearing weight. That is why we re-balance after any rotor work before restart, and why field trim-balancing is judged against ISO 14694 vibration limits. First prove it is imbalance and not resonance.
Flow, static, gas temperature, application — or attach a spec, GA drawing or a multi-fan schedule. Engineer to engineer.
ISO 9001:2015 quality system · performance-tested to IS 4894 / ISO 5801 / AMCA 210 method · witnessed FAT on request.
*For our standard range. ATEX and special projects need 5 to 7 working days.
This site sets no cookies of its own. Google Analytics loads only if you accept. It sets two cookies, so we can see which fan topics people actually read. Decline and no analytics loads — the site works exactly the same. What we collect →
