Information on Balancing

  To illustrate the between static and dynamic balance, see figure 1, a disc on a shaft resting on knife edges. The unbalance, W, acted upon by the force of gravity causes the assembly to roll until the weight is at the bottom. This is static, or force, unbalance.

  The disc can be brought into balance (figure 2) by removing weight W, or by adding a similar weight at a point opposite the original unbalance and equidistant from the center of the shaft.

  In figure 3 there are two discs on the shaft. The unbalance in one disc is exactly compensated for in the other, so the assembly is in static balance...but dynamically (rotating) it is badly out of balance.

  The centrifugal forces acting on this rotating assembly will tend to displace the axis of rotation as shown in figure 4. The ends of the shaft, unrestrained, would describe cones. Held fast by bearings the moment unbalance would cause vibration and noise and undue bearing wear. This form of dynamic balance is called couple.

  Changes in weight and in the planes where required or where practical, will eliminate both force and couple in any motor (figure 5). Knowing exactly where to make the weight changes is the precise art of balancing.

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

In-Place, Two-Plane Balancing
Cross effect must be taken into consideration when balancing in two planes. In the past, it has been very difficult to perform a precision two-plane balance in the field. Overhung rotors are the most difficult. With our vast experience and the technology of our vibration software and hardware, in-place two plane balancing has become routine.