Static and Dynamic Balancing of Rotating Machines

Balancing Introduction

Balanced rotors are essential for most kinds of rotating machinery. Unbalance will create high vibrations causing material defects and reducing the lifetime of a material. Rotor unbalance is the major problem of vibration and it is related to the first order (rotational frequency).

The goal of balancing is to minimize vibrations related to the first order. Basically, it works like this: We measure the initial state, then we add a trial weight of the known mass, calculate the position and mass of a counterweight, remove the trial weight, and put the calculated weight on the opposite side, to cancel out the imbalance.

When an unbalance exists, the first order (rotational frequency) can be seen clearly. As shown in the example below, on the rotor exists an uneven distribution of mass. A correction weight is added (or material is removed) on the opposite side, which cancels out the major part. This procedure can then be repeated until satisfaction.

Depending on the machinery, single or dual plane balancing is used. Selecting one plane or two plane balancing generally depends on two factors. One of the factors is the ratio of the length of the rotor (L) to the diameter of the rotor (D). The other factor is the operating speed of the rotor. As a general rule of thumb, we can refer to the table shown below.

Single or Dual Plane Rotor Balancing

Depending on the machinery, single or dual plane balancing is needed. Selecting one plane or two plane balancing generally depends on two factors:

• One of the factors is the ratio of the length of the rotor to the diameter of the rotor.
• The other factor is the operating speed of the rotor.

Weight Splitting

When you have a rotor/plane with a certain number of slots/blades/holes, where the weights can be mounted, it would be much easier just to know the position number and split the weights instead of the absolute angle.

This can be done by selecting “Divide plane xx to xx” from the properties. 

Initialize with System Characteristics

If balancing was already done on a particular shaft, and the system characteristic is known, a trial weight run is not necessary once again; the system characteristics parameters can be entered manually instead, to get the correction mass calculated immediately.

This could be used if a shaft is balanced multiple times at a certain interval. The system characteristics describe the relation between mass and vibration.