Most people’s knowledge of balancing is limited to car wheels and tyres that must be balanced before fitting to cars; otherwise they will experience steering wheel shudder, uneven tyre wear, and so on.
Unbalance exists when the centre of mass does not coincide with the centre of rotation, when the mass centre axis is different to its running centre axis. The forces which result in vibration are primarily due to minor imperfections in the rotating components. Practically all newly machined parts are non symmetrical due to blow holes in castings, uneven number/position of bolt holes, parts fitted off centre, machined diameters eccentric to the bearing locations, etc.
“Balancing is the procedure where the mass distribution of a product (roller, impellor, etc) is checked and adjusted in order to ensure that the forces on the bearings, at a frequency corresponding to the devices operational speed, are within specified limits as specified in International Standards”.
An uneven distribution of the mass is called unbalance. Because the bearings restrict this movement, the centrifugal force due to the unbalance causes the device to vibrate. This vibration causes wear to the bearings, creates unnecessary noise, and can result in complete failure.
Balancing is the process of improving this mass distribution of a body so that it rotates in its bearings without unbalanced centrifugal forces. The process of “balancing” is the removal or addition of weight to the unit such that this effective mass centre line approaches the true axis adding or removing weight so as to make the two centres coincide.
There are two forms of balancing: Static and Dynamic
STATIC out of balance is unevenly distributed mass on the rotating objects axis causing the heavier side of the object to fall to the lowest position when it settles at the point of rest. A form of unbalance that can be corrected by adding a single correction weight directly opposite the heavy spot on the rotor. (180 degrees from the unbalancing weight). This type of balance can be corrected by removing the weight or by adding an equal weight directly opposite. Either measure would move the center of mass back to the centerline of the part.
Static unbalance can be detected by setting the rotor on knife edges and letting the heavy spot bottom out. Dynamic unbalance cannot be detected in this manner.
DYNAMIC out of balance can only be detected when the object is rotating. It is the result of unevenly distributed masses in two or more planes of rotation. This type of unbalance requires at least two different planes along the shaft to correct. To create a true dynamic unbalance on a centrifugal fan wheel, first add a weight on the inlet side of the fan wheel. At the same time add an equal weight (at the same radius) on the back side of the wheel with the second weight rotated 180 degrees from the first weight. This is also called “couple unbalance.” To correct this type of unbalance, one must compensate for both eccentricity (caused by static unbalance) and wobble (caused by couple unbalance). In practice, any dynamic unbalance can be corrected by making adjustments in two axially separated planes.