Causes of unbalance
; Distortion from stress: Routine manufacturing processes can cause stress on metal components. Without stress relief, the rotor will distort itself to adjust. ; Thermal distortion: Thermal distortion often occurs with parts exposed to increased temperatures. Metals are able to expand when in contact with heat, so exposure to warmer temperatures can cause either the entire piece of machinery to expand, or just certain parts, causing distortion. ; Buildups and deposits: Rotating parts involved in material handling almost always accumulate buildup. Moreover, when exposed to oil, these parts can be very easily distorted. Without adhering to a maintenance routine or implementing an inspection process, oil can seep into the parts, causing unbalance. ; By design: In some cases, vibration is desired, and a rotor is ''deliberately'' unbalanced to serve as a vibrator. An example of this is an aircraft'sEffects of unbalance
*Units used to express unbalance
* In terms of the mass eccentricity : µm, mm, cm, ...; µin, mil, in, ... * In terms of mass at a given radius: µg, mg, g, kg, ...; moz, oz, ... * In terms of mass × radius moment (mR): mg-mm, g-mm, mg-cm, g-cm, kg-mm, ...; oz-in, g-in, ...Types of balance
Static balance
A static balance (sometimes called a force balance) occurs when the inertial axis of a rotating mass is displaced from and parallel to the axis of rotation. Static unbalances can occur more frequently in disk-shaped rotors because the thin geometric profile of the disk allows for an uneven distribution of mass with an inertial axis that is nearly parallel to the axis of rotation. Only one plane receives balance correction. : where U = balance, m = mass, r = distance between unbalance and the centre of the objectCouple balance
A couple balance occurs when a rotating mass has two equal unbalance forces that are situated 180° opposite each other. A system that is statically balanced may still have a couple unbalance. Couple unbalance occurs frequently in elongated cylindrical rotors. : where d = distance between the two unbalance forces along the rotation axis.Dynamic balance
In rotation an unbalance when the mass/inertia axis does not intersect with shaft axis then it is called dynamic balance. Combination of static and couple balance is dynamic unbalance. It occurs in virtually all rotors and is the most common kind of unbalance. It can be fixed by correcting the weight on at least two planes.How to correct or compensate balance
* Mass addition. * Mass removal. * Mass shifting. * Mass centering. The measurement of existing vibration and calculation of the change of mass required is typically carried out using some form ofGrades
Important formulas
: : : Where F = force due to unbalance, U = unbalance. ω = angular frequency. e = specific unbalance. m = mass. r = distance between unbalance and the axis of rotation of the object.References
{{Authority control Rotation Mechanical vibrations