Balan­cing

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