Rotor Balancing. Causes of Unbalance

There are many reasons that unbalance may be present in a rotor. The most common causes are described briefly in the following paragraphs as described by API RP 684 “Paragraphs Rotodynamic Tutorial: Lateral Critical Speeds, Unbalance Response, Stability, Train Torsionals and Rotor Balancing”:

• Blow holes in castings: on occasion, cast rotors such as pump impellers or large sheaves will have blow holes or sand traps which result from the casting process. Blow holes may be present within the material undectable through normal visual inspection. Nevertheless, the void created may represent a truly significant unbalance.
• Eccentricity: eccentricity exists when the geometric centerline of a part does not coincide with its rotating centerline. The rotor itself may be perfectly round; however, for one reason or another, the center of rotation has been located “off-center”.
• Addition of keys and keyways: unfortunately, there are few industrywide standards regarding the addition of keys when component balancing. A motor manufacturer may balance his product with a full key, a half key or perhaps no key at all. Thus, if a pulley manufacturer balances a pulley without a key, and a motor manufacturer balances his motor without a key, when the two components are assembled with a key, unbalance will result. Similarly, if both were to balance their products with a full key, the assembled units would be unbalanced.
• Distortion: although a part may be reasonable well balanced following manufacture, there are many influences which may serve to distort or otherwise change the shape of a rotor to alter its original balance. Common causes of such distortion include stress relief and thermal distortion.
• Stress relieving is sometimes a problem with rotors which have been fabricated by welding. Actually, any part that has bene shaped by pressing, drawing, bending, extruding, etc., will naturally have high internal stresses. If the rotor or component parts are not stress relieved during manufacture, this may undergo this process naturally over a period of time, and as a result, the rotor may distort slightly to take a new shape.
• Distortion which occurs with a change in temperature is termed thermal distortion. It is natural for metal to expand when it is heated; however, most rotors, due to minor imperfections and uneven heating, will expand unevenly causing distortion. This thermal distortion quite common on machines that operate at elevated temperatures including electrical motors, fans, blowers, compressors, expanders, turbines, etc. thermal distortion may require that the rotor be balanced at its normal operating temperature, even though it may have been well balanced when it was cold.
• Clearance tolerances: one of the most common sources of unbalance is the stack-up-of-tolerances possible in the assembly of a machine. The bore in the pulley is necessarily larger than the shaft diameter, and when a key or setscrew is used, the take-up in clearance shifts the weight of the pulley to one side of the shaft rotating centerline.
• Corrosion and wear. Many rotors, particularly fan, blower, compressor and pump rotors, as well as other rotors involved in material handling processes, are subject to corrosion, abrasion or wear. If the corrosion or wear does not occur uniformly, unbalance will result.
• Deposit built-up. Rotors used in material handling may become unbalanced due to the unequal build-up of deposits (dirt, lime, etc.) on the rotor. The resultant gradual increase in unbalance can quickly become a serious problem when portions of the deposits begin to break away. As small deposits break off, this increases the vibration to break off even more deposits, which can quickly create a serious unbalance vibration problem.

In summary, all of the above causes of unbalance can exist to some degree in a rotor. However, the vector summation of all unbalance can be considered as a concentration at a point termed the “heavy spot”. Balancing, then, is the technique for determining the amount and location of this heavy spot so that an equal amount of weight can be removed at this location, or an equal amount of weight added directly opposite.

(adapted from API RP 684 Paragraphs Rotodynamic Tutorial: Lateral Critical Speeds, Unbalance Response, Stability, Train Torsionals and Rotor Balancing)