A rotor is usually designed to be axially symmetric with respect to its axis of rotation.But for some technical reasons

Due to the prime influence, the rotor which is finally assembled cannot guarantee the complete axial symmetry of the power, and there is a certain amount of partial balance.

This kind of imbalance is commonly called initial unbalance.The main factors leading to initial unbalance are:

1. Uneven rotor material:

2. Imbalance of coupling:

3. Imbalance caused by asymmetric keyway.

4, the rotor processing will always produce some roundness deviation and eccentricity, etc.;

5. Unbalance effect of impeller;

The unbalance caused by these factors is of random nature and cannot be calculated. Therefore, gravity test (static balance) and rotary test (dynamic balance) should be used to measure and correct the unbalance and reduce it to the allowable level.

FIG. 1 unbalance of a wafer

If the thickness of a rotor is divided into glass for Δ z, then each wafer (shown in figure 1) there is an uneven Measure.Where the identified unbalanced mass of G exists at a radius of r and a circumferential reference Angle of alpha, the centrifugal force generated is:

Omega is angular velocity.It can be seen that the centrifugal force F increases with increasing omega squared.Therefore, the product of G and r is called unbalance, which is generally expressed as:

The unit is usually g mm, then:

Thus the centrifugal force is proportional to the unbalance.

Generally speaking, the unbalanced rotor will do great harm to the normal operation of mechanical equipment.