Practical Solutions to Machinery and Maintenance Vibration Problems
Chapter 5, Unbalance
Section 10, Single Plane versus Two Plane Balancing
The most common rule of thumb offered is that a narrow disc-shaped rotating part can usually be balanced in one correction plane only, whereas parts that have appreciable width require two plane balancing. Some guides indicate that the proportion of large diameter to relatively narrow face width suggests single plane balancing. However, the distance between the two planes should be the only guide, not the diameter to width ratio. It is true that the narrower the rotor, the less the chance for a large couple component and, therefore, the greater the possibility of getting by with a single plane balance. For rotors over 4 or 5 inches (approximately 100+ mm) in width, it is best to check for two plane unbalance (or for couple unbalance).
The writer used to balance narrow automobile flywheels on a balancer that separated static from couple unbalance. Although most of the flywheels, at that time, were only 1 inch to 1 ½ inches (approximately 25 to 40 mm) wide, more than half had enough couple unbalance to cause excessive vibration. This was not due to the large distance between planes, but due to the fact that the flywheel's mounting surfaces caused it to be slightly cocked or tilted. Instead of the flywheel being 90° to the shaft axis, it was perhaps 88° to 93°, causing a large couple that still had to be removed, despite its narrow width.
This situation is very common with industrial narrow and disc-shaped rotors, such as single stage turbine wheels, narrow fans, and pump impellers. The manufacturer often accepts the guides supplied by others and performs a single plane balance only. Similar situations exist for relatively narrow rotors that have their centers of gravity very close to one bearing and at a relatively large distance from the other bearing, as well as for narrow or even wide rotors that are mounted overhung from both bearings.
Very modern balancing machines are capable of separating the effect of unbalance in one plane from the effect of unbalance in the other correction plane. With such balancing machines, true dynamic balancing may be performed in two planes without difficulty, even when the rotors are very narrow or overhung. But many balancing machines still in use do not have this capability for very narrow or overhung rotors; therefore, such rotor types would require separate static and couple balancing. Although modern balancing machines may be capable of accomplishing good dynamic balancing on narrow rotors, static and couple balancing usually results in less correction and is therefore faster on narrow rotors.
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