Balancing Tolerances

The balancing of a rotating body has different goals:

1) Reduced load on the bearings (low centrifugal forces)
2) Long bearings life
3) Acceptable vibration levels (a good vibration level does not create any problems to the comfort or to component life.

From previous point 3 , it is clear that the optimum value for the residual unbalance can be evaluated in an experimental mode, by considering that:

a) The inertia force generated by the unbalance can be calculated using the formula reported on paragrath 1.15

b) On service vibrations, levels can be easily measured with a simple vibrometer.
For each application an acceptable value for the admitted residual unbalance (which grants good performances ) can be defined.

ISO 1940 standards gives a rule in order to calculate an acceptable residual unbalance, having following features:

1) Gross unbalance deficiencies are avoided,
2) Useless and excessive balancing works are avoided

Where:    E = Mass eccentricity [microns]
                U = Unbalance [gr•mm]
                M = Rotor mass [kg]

According to ISO 1940 standards, all rotors are classified (grouped), depending on their balancing requirements (look at following table). Balancing quality G is a number which defines the balancing accuracy required; for instance G = 2,5 means that a fine balancing is required, G = 6,3 means that a normal balancing is accepted.

Please note that the measuring unit for  G is mm/s, because this value represents the vibration speed assumed by the body rotating freely in the space at the real service speed.

The same value of vibration speed ( G=mm/s) is achieved by the rotor, when it rotates mounted on a soft bearing machine at service speed.

Balancing can be a very complex subject and I would be delighted to hear from anyone experiencing unbalance who is in need of technical advice.

Cemb Hofmann is the UK's leading specialist in balancing - from balancing machine sales to a world-class sub contract dynamic balancing service.

Visit www.cembhofmann.co.uk today for more information or call us on 0161 872 3123 to speak to a member of our team today.