Technical note

Rotor balance grades, in microns

G6.3 and G2.5 are not adjectives. They are a permitted residual eccentricity at a given speed, and the arithmetic fits on one line. Here it is, with the numbers for the speeds industrial blowers actually run at.

The rule

A grade is an eccentricity, not a quality

ISO 1940-1 expresses balance quality as the permissible velocity of the rotor centre of mass, in millimetres per second. G2.5 and G6.3 are those velocities. Convert them to a permitted displacement – what a balancing machine actually corrects to – and the formula is short.

Formula
e per in micrometres = 9549 x G divided by n, where n is speed in rpm
Where it comes from
e = U divided by omega, and omega is 2 pi n over 60
What it means
The faster the wheel, the tighter the tolerance for the same grade

That last line is the whole argument about speed. A wheel that runs at twice the speed has to be balanced to half the residual eccentricity for the same grade – and a hot rebuild that skips the balance machine is where a plant finds that out.

Worked

The numbers at real speeds

G6.3 at 740 rpm
about 81 micrometres
G6.3 at 1,450 rpm
9549 x 6.3 / 1450 = about 41 micrometres
G6.3 at 2,900 rpm
about 21 micrometres
G2.5 at 1,450 rpm
9549 x 2.5 / 1450 = about 16 micrometres
G2.5 at 2,900 rpm
about 8 micrometres

Most of what we build sits comfortably at G6.3, and the grade is a build specification rather than a boast: a high temperature wheel with a heavy shroud, a hard-faced radial exhauster, or anything turning quickly enough that 40 microns of residual eccentricity would put an audible tone into the discharge, gets balanced at G2.5. Our published literature states the family basis as ISO 1940-1 G6.3, and G2.5 for higher speed builds – the specific grade for a specific machine is confirmed on the drawing.

Site

Workshop balance is not site vibration

ISO 14694

The fan standard that sets balance and vibration quality requirements for industrial fans, including the workshop balance correction plane practice and how the residual unbalance is expressed. It is the document to name when the purchase order wants a balance requirement rather than a wish.

Machine vibration at the bearing

Once the rotor is on its bearings in its casing, the measurement being made is a vibration severity on non-rotating parts, which belongs to the machine-vibration family: ISO 10816-3 with its Indian adoption IS 14817-3:2004, and the newer ISO 20816 series. Different question, different instrument, different numbers – and the zone limit is an agreement, not a constant.

What a balance grade cannot fix

A badly grouted baseplate, a resonant support, a shaft with a bend, a coupling out of alignment, a wheel with build-up on one blade. Every one of those produces the same symptom the balance was blamed for, and a field balance machine will happily make the residual worse while the plant stays broken.

Diagnosis

Reading the symptom before the instrument

One per rev

Amplitude tracking with rotational speed points at residual unbalance. That is a balance answer, or a bent shaft masquerading as one.

Two per rev

Misalignment, and often a soft foot underneath it. Balancing a misaligned machine moves the problem, it does not remove it.

Blade pass and its harmonics

Aerodynamic, or build-up on the blades. Check the wheel before the balance weight.

A frequency that does not move with speed

Structure or electrical. Bearing defect frequencies are their own conversation, and none of them are answered by the balance machine.

Questions

Balance questions

Is G2.5 always better than G6.3?

It is tighter, and tighter costs money: more correction passes, more weight holes, sometimes a machined rather than a welded correction. For a moderate speed exhauster, G6.3 is achieved easily and gives no trouble. For a high speed wheel, G2.5 is the correct answer and the extra hour in the balance machine is the cheapest bearing order you will ever place.

Why does a rebuilt fan vibrate after it comes back from the field?

Usually because it was balanced as a rotor when it needed balancing as an assembly, or because the fit between wheel and shaft changed. Fan rebuild is a two-plane exercise that ends with the impeller on the shaft.

How often should a fan be re-balanced?

When the vibration tells you to, not on a calendar. Dust build-up is the usual mechanism on abrasive duty: a wheel that was balanced in a clean shop loses its balance the first week it moves cement dust.

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