What Fan Balancing Actually Corrects

Every fan rotor has a mass axis and a geometric axis. When the mass axis does not coincide with the shaft axis, the rotor is unbalanced. At standstill this is invisible. Once the rotor turns, the offset mass generates a centrifugal force that grows with the square of rotational speed. Doubling the speed quadruples the force, which is why a rotor that runs acceptably at low speed can vibrate severely when driven directly by a 2-pole motor at roughly 2930 rpm.

Fan balancing is the process of measuring that offset and correcting it — by adding or removing material — until the residual unbalance falls within an agreed limit. The limit is not a matter of opinion; it is defined by ISO 1940-1.

What the ISO 1940-1 Balance Grades Mean

ISO 1940-1 assigns balance quality grades identified by a G number. The number is the permissible eccentricity of the centre of gravity multiplied by the angular velocity, expressed in mm/s. Two consequences follow directly from that definition:

  • A higher G number is a looser tolerance. G6.3 is the grade typically applied to fans; G4.0 is used for rotors running at higher speed or requiring better running quality.
  • The permitted eccentricity shrinks as speed rises. The same grade becomes harder to meet on a faster machine.

Using the standard relation between grade, speed and permissible eccentricity, a rotor turning at 2930 rpm sits at about 20 µm of permitted eccentricity under G6.3, and about 13 µm under G4.0 — roughly 50 percent tighter. The permissible residual unbalance in g·mm is then that eccentricity multiplied by rotor mass, which is why the allowance is quoted in mass-and-speed terms rather than as a single fixed number.

Why G6.3 Is Often Not Enough on a Direct-Driven Fan

G6.3 was written for general rotating machinery and remains a reasonable starting point for many fans. It becomes marginal in three common situations:

  • Direct drive at 2-pole or 4-pole motor speed, where the small permitted eccentricity is consumed quickly.
  • Long shafts or rotors with significant axial length, where the unbalance is not confined to a single plane and must be corrected in two planes.
  • Continuous duty, where even a small residual force acts on the bearings for thousands of hours.

At 2930 rpm the difference between G6.3 and G4.0 is the difference between an allowance of roughly 20 µm and 13 µm. It is a small number, and it is the number that determines how the fan behaves on site.

What Poor Balance Costs You

Unbalance is a force, and the machine has to carry that force every revolution:

  • Vibration at 1× running speed, transmitted into the bearing housings, base frame and ductwork.
  • Bearing wear as the dynamic load cycles through the rolling elements or sleeve clearance.
  • Motor load as the drive absorbs the additional energy required to move the eccentric mass.
  • Noise radiating from casing panels and connected ducting.
  • Secondary damage — loosened fasteners, fatigue at welds, and in process applications, disturbed airflow.

None of these are visible on a drawing. They appear after commissioning, and they are far more expensive to correct on site than during manufacture.

How SINDFAN Balances Rotors

  • Every rotor is balanced to ISO 1940-1 G4.0 as standard practice. G6.3 is available where the duty permits.
  • Balancing is carried out on our own balancing machine inside the factory, on the assembled rotor rather than on individual components.
  • A balance report is included with delivery, so the residual unbalance achieved is documented, not simply asserted.

What to Include in Your Inquiry

To match the balance grade and the balancing method to your application, send us the operating speed, rotor mass and dimensions, drive arrangement (direct or belt), and the balance grade you require. If you have a vibration limit to meet at the bearing housings, state it — the balance grade can be selected to support it.

Contact SINDFAN with your duty and we will confirm the balance grade, the balancing method and the documentation supplied with the fan. WhatsApp +852 9524 8434 or email info@sind-tech.com.