* Unbalancing, balancing and the methods of balancing
Unbalancing, balancing and the methods of balancing
- There are only unbalanced forces: These cause static unbalance (where the mass axis is displaced only parallel to the shaft axis) .
- There are only unbalanced couples: These cause rotary unbalance.
- There are unbalanced forces as well as unbalanced couples: These cause static as well as rotary unbalance.
Unbalanced forces or unbalanced couples or both cause vibrations. These unbalanced forces/couples are transmitted to the bearings as well as to the structures and the adjoining machines/personnel. These vibrations cause reversal of forces and will cause fatigue. It will thus reduce the strength of the bearing and other materials alike. These will ultimately cause the failure of the bearing as well as of the structure. Thus unbalance must be avoided at all costs or get removed.
Effects of unbalance
(a) Causes vibrations
(b) Causes noise
(c) Increases maintenance
(d) Causes unsafe working conditions
(e) Decreases life of bearings
(f) Decreases machine life
Units in which unbalance is expressed
(i) In terms of mass ‘m’: g, mg, kg …
(ii) In terms of eccentricity of mass ‘e’ m, mm…………..
(iii) In terms of mass x radius= moment, g.mm, mg.mm. g.cm…………………
Methods of correcting unbalance
(a) Mass removal
(b) Mass addition
(c) Mass shifting
(d) Mass centering
There are balancing machines to measure the vibrations (unbalance) and suggest the way to remove the unbalance.
- to eliminate partially or completely the resultant unbalanced forces to reduce or eliminate vibrations.
- to eliminate partially or completely the resultant unbalanced couples to reduce or eliminate vibration
- to eliminate partially or completely the resultant unbalanced forces and couples to reduce or eliminate vibrations.
- Principle of Rigid Rotor Balancing
- (I) Method of Static Balancing—-Single plane balancing-Balancing by a single mass in the same plane of unbalancing mass- thus its accuracy is limited because only one plane balancing.
- (ii) Method of Dynamic balancing—-Two plane balancing–Two balancing masses in different planes
Balancing of Practical Rigid Rotor
- Two Plane balancing (by Cradle Machine Method) only for the rigid rotors having no deflection and no deformation
- Multi-plane balancing for the flexible rotors having deflection or deformation or both