Unbalance is the most common cause of excessive vibration in high-speed turbomachinery. High vibration can cause bearing, seal, and rotor damage, support system damage, increased noise, etc.
To control vibration, rotors are dynamically balanced. Although there are many balance standards, there is no practical method to relate vibration and balance. Flexible rotors present additional challenges.
The purpose of this article is to provide a general introduction to balancing, discuss the various balance standards, and present some of the issues with balancing flexible rotors. A rotor with a single disc between bearings will be used as an example throughout this article. See Figure 1.
Figure 1: Single disc between bearing
Unbalance
An unbalance condition exists when the mass centerline does not coincide with the rotating centerline. Referring to Figure 2, if there were a heavy spot at one place on the disc, the mass centerline would be slightly displaced from the rotating centerline. If the rotor were placed in vee-blocks, the rotor would turn until the heavy spot was on the bottom.
Figure 2: Static unbalance
This is an example of a simple-static unbalance. This rotor could be statically balanced by attaching a weight equal to the original heavy spot on the opposite side of the rotor at the same distance from the geometric centerline.
A couple unbalance exists when there are heavy spots at each end of the rotor, and the mass centerline intersects the rotating centerline at the rotor center of gravity. See Figure 3. Unlike a simple-static unbalance, a couple unbalance can only be detected by spinning the rotor and measuring the resultant unbalance forces or the vibration.
Figure 3: Couple unbalance
The most common unbalance condition is a random combination of static and couple unbalances. This is called a dynamic unbalance. In this case, the mass centerline is both tilted and displaced from the rotating centerline. See Figure 4. As with a couple unbalance, a dynamic unbalance can only be detected by spinning the rotor and measuring the resultant unbalance forces or vibration.
Figure 4: Dynamic unbalance
If a rotor is made from several components, like an impeller mounted on a shaft, the assembly process might produce changes in geometry that could cause an unbalance. It is typically recommended that each part of the rotor, including the shaft, be component balanced before assembling the rotor.
It is typically necessary to make a mandrel to balance components like an impeller. If the unbalance is contained within one transverse plane, then all the balance corrections can be made in that plane. This is called a single-plane balance. If the component unbalance is contained in two transverse planes, then the balance corrections need to be made in two planes. This is called a two-plane balance. See Figure 5.
Figure 5: Two-plane balance
Once the individual components are balanced, the rotor assembly can begin. If multiple components are mounted on a shaft, like a multi-impeller rotor, it is normally necessary that the rotor be balanced after every 1-2 components are assembled on the rotor. The balance corrections are made on the last components assembled on the rotor.
