MD-02Machinery Diagnostics

Balance Quality Grade

Permissible residual unbalance per ISO 21940-11 (formerly ISO 1940-1).

Rotor

Rotor mass

Maximum service speed

rpm

Balance grade

Correction planes

Correction radius

Permissible U_per

3,059g·mm

Eccentricity e_per

20.39µm

Per plane

1,530g·mm

Equivalent mass at r

10.2g

per plane

G 6.3: Fans, pumps, electric motor armatures, general machinery, flywheels.

ISO 21940-11 permissible eccentricity charte_per vs speed

§ Theory of operation

Balance quality grades

No rotor is perfectly balanced. ISO 21940-11 (formerly ISO 1940-1) defines how much residual unbalance is acceptable for a rigid rotor, based on experience with each machine type. The grade G is the product of the permissible mass-centre eccentricity and the angular speed, expressed in mm/s:

\begin{aligned} G &= e_\mathrm{per} \cdot \omega \qquad \text{(mm/s)} \\ e_\mathrm{per}\,[\mu\mathrm{m}] &= 1000 \cdot G\,/\,\omega \qquad \omega = 2\pi\,n/60 \\ U_\mathrm{per}\,[\text{g·mm}] &= e_\mathrm{per}\,[\mu\mathrm{m}] \cdot m\,[\text{kg}] \approx 9549 \cdot G \cdot m\,/\,n \end{aligned}

Because G is constant within a grade, the permissible eccentricity falls as speed rises, the straight lines on the chart. Grades step by a factor of 2.5. G6.3 is the default for most industrial machinery; G2.5 for high-speed turbomachinery.

Allocating to correction planes

For a two-plane rotor the permissible unbalance is split between the planes. For a symmetric rotor with the centre of mass midway, each plane gets half. Asymmetric rotors and overhung designs need the allocation rules in the standard.

Correction mass

m_\mathrm{correction}\,[\mathrm{g}] = U\,[\mathrm{g{\cdot}mm}]\,/\,\,r\,[\mathrm{mm}]

This is the mass that, placed at radius r, would produce exactly the permissible unbalance, a useful sense of how small the tolerance is. It also tells you the resolution your balancing machine and trial weights need.

Flexible rotors operating above their first bending critical speed are covered by ISO 21940-12 and require modal balancing.