The term balancing refers to the reduction or elimination of an unbalance.
Each rigid body rotating around a fixed axis has a unbalance which can lead to vibrations (oscillations), noises and increased wear, and can even lead to destruction with high speeds. When the manufacturing tolerance results in an excessive unbalance, the mass distribution must be balanced individually on this body. The balance can be positive or negative:
- In the case of a positive balance, levelling compounds are applied, e.g. by welding, gluing or screwing on weights.
- In the case of a negative balance, leveling compounds are removed, e.g. by drilling, grinding or milling.
A mixed form is the adjustment by screwing or unscrewing. Instead of changing the object, the rotation axis can be changed so that unbalance is minimized. This balancing technique is called mass centering. The tolerances for balancing are standardized in DIN ISO 1940 – 1.
Rotating machines and machine parts
Rotor of a high-speed electric motor with balancing bore holes in the short-circuit ring
Runners or rotors and armatures of electric motors are often balanced by removing the laminated sheet cores of the finished rotor in the form of bore holes, surface abrasion or notches. They are also usually dynamically balanced, i.e. material may have to be removed at both ends of the rotor. Contrary to the terms “static” and “dynamic”, which emanate from stationary or moving parts, balancing with “static” means balancing in a reference plane, in contrast to dynamic balancing, which emanates from 2 planes. Ideally, these should be as far apart from each other as possible.
In order to be able to operate them in any position, the rotary coils of instruments must be balanced. For this purpose, they have balance weights opposite the pointer that can be shifted or bent. For a similar reason, the balance wheel of a clock must be carefully balanced. Otherwise, the clock drift depends on the clock alignment. Vibration and bearing wear are not relevant here.
The rotating masses of spinning washing machines, spin dryers and centrifuges for test tubes cannot be balanced. Therefore, their axes of rotation are mounted movably in a resilient and damping suspension in order to reduce the forces on the bearings and the surroundings. Modern washing machines often perform a spin cycle at low speed first and then try to redistribute the laundry through a forward and backward motion before the spin cycle starts at full speed. They have an acceleration sensor on the drum suspension to monitor the unbalance.
Residual unbalances lead to a so-called critical speed where the forces stimulate the vibratory overall system (spring´ mass system consisting of rotor mass and shaft or total mass and suspension/foundation) to resonance . The critical speed represents a hazard on high-speed machines (turbines, centrifuges, etc.); it is reduced by accurate balancing, by a spring, dampening suspension or by particularly quickly passing through the critical speed during acceleration.
Source: German Wikipedia