Balancing machines are used to measure and correct the unbalance of rotating components (rotors). Dynamic balancing machines spin components at a fixed speed in order to measure the dynamic unbalance via force measuring pickups.
By filtering the vibration signal and performing calculations, the measuring system of the balancing machine determines the position and amount at which the rotor must be corrected in order to reduce the unbalance. Correction of unbalance can be performed by adding or removing material, or by moving mass.
Depending on the part shape that needs to be balanced, either a horizontal or a vertical balancing machine is required.
In addition to the classification between horizontal and vertical balancing machines, a distinction is also made between hard-bearing and soft-bearing balancing machines.
Hard-Bearing machines (sometimes also called Force measuring machines) operate at frequencies lower than the resonance frequency of the suspension. To measure the unbalance, hard-bearing machines measure centrifugal forces created by the unbalance of the rotating part.
One of the many advantages of a force measuring balancing machine is the wide range of applications available with the ability to cover many different component sizes and weights. However, a decent fundament is required to use the full performance of the machine.
Through a permanent calibration, a wide range of components can be balanced quickly and precisely. After entering the geometry data of the rotor, the balancing machine displays the unbalance amount and angle directly. To define the limits of the permanent calibration of Hard-bearing balancing machines, the manufacturer provides a Wn2 value with the machine specification. The Wn2 can be used by the operator to calculate the maximum balancing speed of a certain rotor to stay within the permanent calibration limits.
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Soft-bearing balancing machines (sometimes also called displacement measuring machines) operate at frequencies higher than the resonance frequency of the suspension. The system derived its name from the fact that the work supports that hold the rotor have a very low stiffness in the measuring direction. This permits the rotor to move freely in a direction perpendicular to the shaft axis. To measure the unbalance, the machines determine the horizontal movement caused by the unbalance of the rotating part.
A soft-bearing machine is limited in terms of different rotor weights compared to a force measuring machine. Due to the lack of permanent calibration, setting up a soft-bearing machine for individual rotor types is more time consuming. Each individual rotor type must be calibrated at the appropriate speed. A new calibration is necessary every time the rotor weight or the balancing speed changes.
The following features characterize a good balancing machine:
Dynamic balancing is required when the axis of rotation does not coincide with the inertial axis of the rotor, but is tilted at the center of gravity relative to the principal inertial axes.
Dynamic unbalances only occur during operation and can be only measured with dynamic balancing machines. They manifest themselves in a bending moment, the so-called unbalance moment on the axis of rotation. They cause circular vibrations shifted by 180 degrees at the ends of the axis. The center of gravity of the rotating body remains at rest, while the axis wobbles due to the opposing circular motions.
As we just have seen, unbalance is caused by an uneven distribution of mass about the axis of rotation of a rotor. Therefore, balancing is the process of reducing uneven mass distribution in rotating elements by adding or removing compensating mass.
A major problem of an unbalanced rotor is the generated vibration and stress in the supporting structure due to the unbalance. This vibration results in excessive/premature wear or failure of bearings, bushings, gears, shafts, spindles, etc. thus reducing the service life of the component.
In addition to the negative impact on component lifetime, further disadvantages and quality issues caused by vibration need to be considered.
The centrifugal forces reduce performance as they increase friction forces in bearings. This leads to a decreased performance as the energy is absorbed by the supporting structure.
Vibrations reduce reliability, and can seriously impair the ability of a product to function properly.
Now going back to the question why a balancing machine is required. Using a balancing machine to measure and correct the unbalance of a rotor will:
A balancing machine is an investment in the quality of your products.
The price of the balancing machine has only a small influence on the profitability of the investment. Therefore, the following points should be included in the calculation for a return of Investment:
Richard Hoer
Product Manager Universal and Aerospace
Phone: +1.434.522.0348
Mail: rhoer2(at)hofmann-global.com
