THEORY: Propeller and Rotor Balancing
By Mark Lester
The purpose of balancing
One of the most important applications of vibration analysis is the solution of balancing problems. An unbalanced propeller, rotor or driveshaft will cause vibration and stress in the rotating part and in its supporting structure. Balancing of a rotating part is therefore highly advisable in order to accomplish one or more of the following:
Imbalance in just one rotating component of an aircraft may cause the entire machine to vibrate. This induced vibration in turn may cause excessive wear in bearings, bushings, shafts, gears, cabling, hoses, cowlings and exhaust systems substantially reducing their service life. Vibrations set up highly undesirable alternating stresses in structures which may eventually lead to structural failure. Aircraft performance is decreased because of the absorption of energy by the supporting structure.
A rotating body will not exert any variable disturbing force on its supports when the axis of rotation coincides with one of the principal axes of inertia of the body. This condition is quite difficult to achieve in the normal process of manufacturing since due to errors in geometrical dimensions and non-homogeneity of the material, some irregularities in the mass distribution are always present.
As a result of the above, variable disturbing forces occur which produce vibrations. To remove these vibrations and establish safe and quiet operation, balancing becomes necessary. The importance of balancing becomes especially great in the case of high speed machines. In such cases the slightest unbalance may produce a very large disturbing force.
Vibrations due to irregular mass distribution occur at a frequency that is related to the rotating machine's speed of operation and therefore measuring such a vibration requires that the balancer utilize a filter that isolates the vibration that occurs at the machine's speed of operation.
A rotating body having an uneven mass distribution or unbalance will vibrate due to the excess centrifugal force exerted during rotation by the heavier side of the rotor. This unbalance causes centrifugal force, which in turn causes vibration. When at rest, the unbalance exerts no centrifugal force and does not cause vibration to occur. Yet, the actual unbalance is still present. Unbalance, therefore, is independent of rotational speed and remains the same, whether the rotor is at rest or is rotating (provided that the part does not deform during rotation).
Centrifugal force, on the other hand, varies with speed. When rotation begins, the unbalance will exert centrifugal force tending to vibrate the rotor and its supporting structure. The higher the speed, the greater the centrifugal force exerted by the unbalance and the more violent the vibration. Centrifugal force increases proportionally to the square of the increase in speed. If the speed doubles, the centrifugal force is quadrupled, etc.
Measuring unbalance with a vibration analyzer
As mentioned above, the relationship between unbalance of a rotating body and the vibration produced is highly dependent upon rotor speed and other operating conditions. When making vibration measurements for the purpose of determining and correcting unbalance, the operator must operate the machine in a consistent and repeatable manner to insure that repeatable measurements can be made.
To measure the vibration due to rotor unbalance, a vibration transducer (sensor) is attached to the vibrating body when measuring imbalance. The vibration sensor converts this mechanical motion into an electrical signal that corresponds to the body's motion in space. The vibration analyzer is then used to sample this electrical signal and make various calculations based on the electrical signal's properties.
In addition to the vibration measurement, a tachometer (tach) signal is collected. A tach sensor such as a photo tach or magnetic pickup is used to detect the position of the rotating body with respect to time. As in the case of the vibration transducer, the tach sensor converts this information into an electrical signal which can then be sampled by the vibration analyzer and used in various calculations.
The operator will use the Analyzer to collect a series of narrowband vibration readings called Peak Phase measurements since each reading is composed of a peak value and a phase reading. The peak reading (amplitude) is proportional to the amount of mass imbalance in the rotating machine. The phase reading (phase angle) provides information about the location of the mass imbalance.
A Balance Solution (corrective weight) is computed by the Analyzer based on the amplitude and phase angle of the vibration reading. The corrective weight is then applied to the machine and the measurement process is repeated. The balancing job is finished when the vibration is reduced below an acceptable level.
A complete balancing job will usually consist of a number of Peak Phase readings along with the weight changes made which were made. For more information see the section on Balance History.
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