Rotor Balancing Is No Longer A¡®Black Art'

To the average person, rotor balancing is often called a 'black art'. 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 judder, uneven wear on tyres etc. I hope this article will give you a little insight into the balancing world and take the 'black art' out of it. * An unbalanced rotor - unbalance exists in a rotor when the mass centre axis is different to its running centre axis. Practically all newly machined parts are non symmetrical due to blow holes in castings, uneven number and position of bolt holes, parts fitted off-centre, machined diameters eccentric to the bearing locations etc An unbalanced rotor,when rotating, would want to revolve around its mass centre axis. Because the bearings, restrict this movement, the centrifugal force due to the unbalance, causes the rotor to vibrate. This vibration causes wear to the bearings, noise, and in extreme cases, disintegration of the rotor. It is therefore necessary to reduce the unbalance to an acceptable limit. * Balancing limits - there are balance limits, just like machining limits, where the unbalance is acceptable. International and national standards are quoted for rotors. e g, car wheels are balanced to a limit of grade 40, small electrical armatures are 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. * Units of unbalance - the units of unbalance are mass times radius, i e, a weight added to a certain position on the part being balanced would shift the mass axis into the running axis and therefore be in balance. The weight of correction multiplied by the applied radius will give an unbalance unit. For metric measurement the units will be gram millimetres (gmm) or for large rotors, gram centimetres. The imperial equivelent will be dram inches or ounce inches. This weight ( mass) would be applied at a radius from the running centre at the light position. * Types of rotors - rotors fall into two groups, one where the rotor is rigid, i e, does not deflect up to and including the operating speed. The other group are flexible rotors that 'bow' up to the operating speed. The first deflection seen is a 'skipping rope effect', which means the centre of the rotor at speed moves out from it rotational axis causing high 'static' unbalance. * Types of unbalance - there are three types of balance, one where the mass axis is displaced only parallel to the shaft axis. This causes static unbalance and is so called. The unbalance is corrected only in one axial plane. The second type is called couple unbalance where the mass axis intersects the running axis. For example - a disk that has swash runout with no static unbalance. The third is where the mass axis is not coincident with the rotational axis and is called dynamic balance. This unbalance is corrected in two or more axial planes. This unbalance is usually a combination of static and couple unbalance. * Methods of correcting unbalance - removal of material from the heavy position on the part for is used to correct the unbalance by drilling, milling etc or adding material to the 'light' position of the parts by i e,.bolting or welding balance weights to reduce unbalance. * Balancing machines - to identify the position and amount of unbalance, balancing machines are used by a rotor manufacture to correct any unbalance that exists. These machines are so sensitive that they can easily and accurately identify any mass axis 0.001mm off the running axis. One type of machine will only identify static unbalance. These are used for balancing disk shaped parts. Another type of machine will identify unbalances in two axial planes, e g, for balancing rotors where its length is proportional greater than its diameter. These machines are available which balance the rotor in a horizontal or vertical axis. With the use of 'modern day' electronics, the accuracy will easily meet the National and International standards. The set up of the machine is very simple, just typing measurements into a computer. * Balancing rigid rotors - because unbalance exists in a component even when stationary, Rigid rotors can be balanced at a low speed, just enough to produce a centrifugal force to register the unbalance. Generally, this type of rotor will only require two planes to correct the unbalance. * Balancing flexible rotors - this type of rotor is balanced at a low speed where the rotor does not flex, correction for unbalance is made, Then the speed is gradually increased, correcting the unbalance in stages until the rotors operating speed is reached. * Modern techniques - with the high cost of replacing damaged rotors, the airline industry has required parts, or sections, of rotors to be changed while still keeping an acceptable balance. The technique involves using dummy adjacent parts. For example, balancing a compressor module with a dummy turbine module, replacing compressor and turbine blades without any further balance. These techniques are available to general industry if it becomes a economical customer requirement. * Summary - the latest production methods help or eliminate the need for balancing in low speed applications, but with ever increasing speeds used on rotating machinery makes dynamic balancing a necessary process for the foreseeable future. An understanding of dynamic balancing streamlines the complete production process.