Dynamic balancing

What is dynamic balancing?

A system is in dynamic balance when there does not exist any resultant centrifugal force and also a resultant couple. 

For example Several rotating masses 

When several masses rotate in different planes, the centrifugal forces in addition to being out of balance also from the couple. 

Dynamic balancing

As shown in the figure above the product of mr an mrl usually have been referred to as force and couple respectively as it is more convenient to draw force and couple polygons with these quantities. 

If m1 and m2 are two masses revolving diametrically opposite to each other in different planes such that m1r1 = m2r2.

The centrifugal forces are balanced, but an unbalanced couple of magnitude m1r1l1 = m2r2l2 is introduced. 

The couple acts in a plane that contains the axis of rotation and the two masses. Thus, the couple is of constant magnitude but variable direction.