balancing types

Balancing can be classified into three types depending upon the number of correction planes used for balancing:

• Single plane balancing
• Two plane balancing
• Multi plane balancing

Single plane balancing:

As the name implies, single plane balancing is the correction of unbalance in one plane to achieve good balance. This method is always applicable for thin disks and can also be applicable to long rotors also if the unbalance is mostly in one plane. The one plane does not need be through the center of gravity. The single plane balancing corrects for static unbalance only. It can do nothing for couple unbalance. Generally "thin" overhung rotors, single impellers, motors and other long rigid rotors with unbalance vibration substantially higher at one end, are balanced with this method.

Two plane balancing:

The primary reason for doing two plane balancing is to correct for a couple unbalance or combination of static and couple unbalance, which are usually the case. This type of unbalance generates a rocking force. The correction is to apply two weights in two planes separated by some axial distance. This creates a counterrocking force; i.e. a couple unbalance is corrected by another couple. For rigid rotors, any combination of static, couple or dynamic unbalance can be corrected by two plane balancing and two planes are selected arbitrarily depending upon the accessibility and feasibility.

Multi plane balancing:

When we balance rotors externally by making corrections in planes other than where the original unbalance exists, these cause internal bending moments in the rotors. For the rotors operating above their critical speeds these internal bending moments cause deflection and change in shape of the rotors at different critical speeds as explained earlier. If unbalance is corrected in the plane where it exists, there would be no internal bending moments and the rotors would not deflect at different critical speeds. This necessitates balancing in more than two planes for rotors, which operate above their critical speeds. Rotors like high speed multistage compressors, large steam turbines, gas turbines, rolls of paper making machines etc come under this class of rotors.