Causes of an Unbalanced Rotor

Posted by IRD® LLC on Dec 3rd 2021

Unbalance is often defined as simply the unequal distribution of the weight of a rotor about its rotating centerline.

When your rotor gets unbalanced it can vibrate, causing damage to itself and its supports. In severe cases, it can render the rotor unusable. In order for the manufacturing process not to be slowed down, it's important to investigate the cause of vibration or unbalance. Identifying and correcting the mass distribution and thus minimizing the
force and resultant vibration is the technique known as dynamic balancing.

So, what are the causes of unbalance? Causes of unbalance could occur due to:

    Blow Holes in Castings

    • Occasionally, cast rotors such as pump impellers or large sheaves have blow holes or sand traps which result from the casting process. While undetectable through normal visual inspection, blow holes may be present within the material and create a significant source of 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 is off centered.

    Addition of Keys and Keyways

    • Unfortunately, there are few industry-wide standards regarding keys when component balancing. Any component manufacturer may balance their product with a full key, a half key, or no key at all. Thus, if both a pulley and a motor manufacturer were to balance their components without keys, the final assembly would be unbalanced with the added weight of a key.

    Distortion

    • Following manufacture, a rotor can have its weight distribution altered by distortion.Distortion is a change in shape, commonly caused by stress relief or thermal distortion. Stress relieving, if not performed during manufacture, is sometimes a problem with rotors which have been fabricated by welding. Any part that has been shaped by pressing, drawing, bending, extruding, etc., will naturally have high internal stresses. Over time, they may distort to relieve this stress. Thermal distortion occurs with a change in temperature. Most metals expand when heated, but any minor imperfections in a rotor may lead to uneven heating, causing uneven distortion. Thermal distortion is common on machines that operate at elevated temperatures including electric motors, fans, blowers, compressors, expanders, turbines, etc. This Affect can sometimes require the rotor to be balanced at its normal operating temperature

    Clearance of Tolerances

    • The most common source of unbalance is the accumulation of tolerances in the assembly process of a machine. An example of when this occurs is when the bore in a pulley is larger than the shaft diameter. A key or setscrew would be required to fill the gap, thus pushing the shaft to one side of the shaft rotating centerline.

    Corrosion or Wear

    • Many rotors, particularly fan, blower, compressor, pump rotors, or any other rotors involved in the material handling processes, are subject to corrosion, abrasion, or wear. If the corrosion or wear does not occur uniformly, unbalance will result.
    Deposit Build-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, the vibration increases, which in turn, breaks off even more deposits, thus, creating a serious unbalance. Routine inspection and cleaning can minimize the effect, but usually the rotor will need to be removed and balanced eventually.

    Manufactured Unsymmetrical Configurations

    • Many rotors are manufactured in ways that produce dissymmetry. Examples of these include: rough surfaces on forgings, core shifts in castings, uneven number or position of bolt holes, and unsymmetrical parts, such as crankshafts, etc.

    Hydraulic or Aerodynamic Unbalance

    • Oil trapped in oil galleries, oil trapped in grinding wheels and cavitaion or turbulence can sometimes produce unbalance forces.

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 is the technique for determining the magnitude 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.