Why Balance impellers

Unbalance of an impeller can be a source of vibrations. These vibrations can lead to a reduced lifetime of the impeller, as well as excessive noise, and possible contact to other non-moving parts of the assembly. The vibrations will add extra stresses to all components of the impeller and possibly other components in the system that interact with the impeller.

An impeller or any rotating component will never be in perfect balance and residual unbalance will always occur. The question is therefore “when is the balance of an impeller acceptable” or when will the impeller not create any excessive vibrations due to the residual unbalance.

Static or dynamic balancing

A rotating object can either be balanced in one or two planes. When a rotating object is balanced in one plane, it means that all the corrective weight is placed in the same plane. Single plane balancing is also referred to as static balancing. If a rotating object is balanced in two planes, the corrective weight is used in two planes and this type of balancing is called dynamic balancing. It is important to emphasise that static or dynamic balancing has nothing to do with the physical method being used in the balancing process.

According to the ISO 1940/1 standard  “Mechanical vibrations – Balance quality requirements of rigid rotors – Part1: Determination of permissible residual unbalance”, the use of one correction plan is sufficient for a disc shaped rotor. Because of the relative narrow axial depth compared with the overall diameter, single plane balancing is considered to be sufficient for an axial impeller. The standard is not specific as to when you need to perform a two-plane balance, however as a guideline, a single plane balance should be sufficient when the axial depth is less than 20% of the impeller diameter.

Caclulating the permissible unbalance

Based on empirical data, the ISO 1940/1 standard defines a balancing quality grade system. The roughest grade applies for equipment such as crankshafts and drives of rigidly mounted slow marine diesel engines with uneven numbers of cylinders. The finest grade applies for spindles, discs, and armatures of precision grinders and gyroscopes.

Between these two is the balancing grade G6.3 that applies for fans, i.e. our products. The standard is quite explicit when it comes to unbalance tolerance for an assembly (impeller and motor), as the complete assembly has to meet the balancing tolerance. If this cannot be achieved by balancing each component separately, the assembly shall be balanced as a unit. Therefore both the motor and the impeller have to meet G6.3 and unbalance must not be added in the fitting of the components.

Permissibile Unbalance

The more mass in the impeller, the greater the permissible residual unbalance. The permissible residual specific unbalance value (e_per) is given by the following formula:

Where U_per [mm · s] is the permissible residual unbalance and m is the rotor mass.

The relationship between the upper limit of e_per and the maximum service speed are shown in figure 1 below.