Imbalance is caused by the displacement of the center of mass from the rotational axis which is a function of eccentricity in the distribution of the rotating mass. It is measured in units of mass and distance, e.g., gram-millimeters or gmm. Imbalance is considered in single plane modes and dual plane modes.
Balancing is the correction of this mass displacement by the removal, addition or adjustment of mass to the component to compensate for mass distribution error. Most high speed spindles are balanced by the appropriate removal of mass. Some larger machine tool spindles have pre-drilled and tapped holes for the addition of set screws which facilitates the addition of mass to compensate imbalance.
There are two general forms of balancing: “Static” and “Dynamic”. Static balancing is more correctly referred to as “Single Plane” balancing and is adequate for rotating bodies with a low length to diameter ratio (l/d) i.e. thin disc like objects.
Dynamic or “Dual-Plane” balancing is required for components or assemblies of significant length. Rotors with some axial length can have mass imbalances that can be resolved into two “heavy” points at opposing ends of the component, acting independently on the mass center line. In order to balance the component, both planes must be corrected for center line error. Dynamic balancing is required for components such as shafts and multi-rotor assemblies.
Dynamic balancing is performed on a balancing machine. A balancing machine will have two instrumented supports for the ends of the shaft. A small sliver of reflective tape is placed on the shaft that is sensed by a photo tachometer. This enables the machine to determine the rotational angle of the shaft in reference to any mass imbalance when the shaft is spun.
When the shaft is rotated the machine calculates the imbalance and displays the results.
A common display format is what is referred to as a polar balance diagram. Typically the results from both ends of the shaft are displayed side by side.
The display will show the angle of the imbalance referenced to the reflective tape marker and the amount of imbalance. The technician will make the corrections and then repeat the process until the desired balance is achieved.