Theory

The apparatus  consists of a steel shaft mounted in a stiff rectangular main frame. 

A set of five blocks of different weights is provided and may be clamped in any position on the shaft, and also be easily detached from the shaft. 

When several masses rotate in different planes the centrifugal forces in addition to being out of balance also form couples. A system of rotating masses is in dynamic balance when there does not exist any resultant centrifugal force as well as resultant couple on the system. 

Thus in general, two planes are needed to balance a system of rotating masses.