Causes of Unbalance
Unbalance exists when the mass centre axis of a rotating component is different to its running centre axis. There are a number of reasons why this happens: machining tolerances, blow holes in castings, uneven number of holes, inaccurate position of holes, parts fitted off-centre, machined diameters eccentric to the bearing locations to name just a few.
Effects of Unbalance
An unbalanced rotor, when rotated, wants to revolve around its mass centre axis. Bearings restrict this movement and the centrifugal force causes the rotor to vibrate due to the unbalance. This can cause wear to the bearings, unwanted noise, part degradation, uneven finishes, wear on other parts and so on. For manufacturers this can result in a product that is noisy, has a short operational lifespan, suffers repeat breakdowns and simply says poor quality.
There are balance limits, just like machining limits, where the unbalance is acceptable. This is largely determined by manufacturers testing and specifying acceptable tolerances however there are international and national standards which can be used as a guide, for example: car wheels are typically balanced to a limit of Grade 40 and small electrical armatures are typically balanced to Grade 2.5. The grades are converted to unbalance units, depending on the rotational speed of the rotor as per ISO 1940 standards.
Balancing machines are used to determine the angle and amount of unbalance. Sophisticated software then takes the measurements and display clear and simple data to enable the unbalance to be corrected. These machines are so sensitive that they can easily and accurately identify any mass axis 0.001mm off the running axis.
To correct unbalance you either remove material from the "heavy" side for example by drilling or milling, or you add material to the 'light' side such as welding weights or adding putty.