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Balancing of Rotating Parts such as Rotor Assemblies


This article has been written to provide customers with a basic understanding on how rotating components can be balanced by MCE, the specifications that MCE uses, and the information that MCE will require so as to meet the customer's objectives.


This applies to all components that the customer wishes MCE to provide in a balanced state.


Understanding Balancing

MCE has substantial experience with supplying magnet assemblies that will be utilized in high speed rotation systems such as rotors for turbo-machinery, high speed magnetic couplings, rotors for high speed motors, and specific rotor assemblies that are required for applications where vibrations are undesirable.

A rotating component, if not correctly balanced, will have unbalanced centrifugal forces which will cause noise, component wear, excessive vibration, a reduction of bearing life, and subsequently a premature component failure.  The faster a component rotates, the more critical balancing becomes.

During the manufacturing of a rotor assembly, MCE does take care to ensure that components of the assembly are located as close to their intended locations as practical.  However, one must realize that even with this care, it is not possible to control all factors that affect the location of the parts that make up the assembly.  Very small variations in location eventually add up, thereby creating an imbalance of the rotor assembly; the imbalance may be very small, however, it may still exist. 

As the rotor assembly is rotated, the imbalance, due to centrifugal forces, will start causing the assembly to vibrate.  As the rotational speed is increased, this vibration increases rapidly (by the square of the rotational speed).  This means that a small increase in rotational speed can lead to a much larger increase in vibration which will increase the rate at which failure can be expected.

By balancing the rotor assembly, the imbalance condition is either minimized or removed.  This allows for the rotor to run smoothly, without undue vibration, hence allows the system to work for extended periods increasing the MTBF (mean time between failure).

Types of Balancing

There are two forms of balancing: static and dynamic.

Static balancing is done by holding the component at its axis, then compensating (by removal or addition of mass) for the "heavy" side of the component. During Static balancing, the component is not rotating, hence "static".  Static balancing is typically done on "flat" parts, or parts that have a large diameter to axial length ratio (pancake like parts, such as fans, pulleys, wheels). 

Dynamic balancing is done on parts that are long compared to their diameters such as rotor assemblies.  These parts require balancing to be done in two planes since the actual imbalance will intersect the centerline/axis.  Unless both ends of the part are balanced, mass imbalance will continue to exist.  Rotor assemblies that MCE manufactures are balanced dynamically (in two planes).

Balancing can be achieved by the addition or removal of mass in certain locations.  MCE only provides balancing by use of mass removal which is achieved by abrasive material removal or by drilling/machining.  Note that when designing your part, take into account that material removal will be required and allow for extra material such as a balancing ring or thicker flanges than required by design to achieve mechanical structural integrity. 

Standards Utilized

Several standards have been developed to address balancing issues.  The most common standard utilized today was developed by the International Organization for Standardization (ISO) which is a worldwide federation of national standards bodies (ISO member bodies).   This standard is available as document number ISO 1940-1, titled "Mechanical Vibration - Balance quality requirements for rotors in a constant (rigid) state."  This standard outlines the "Specification and Verification of balance tolerances."

This specification is copyrighted material (please note that MCE cannot send you a copy of this specification, you must obtain your own copy).  The specification can be purchased from several sources, such as, the specific document that relates to balancing is the ISO 1940-1:2003.

Stating Requirements

At a minimum, you must state the following -

  1. RPM at which you wish the unit to be balanced (this should be, at a minimum, the maximum service RPM).
  2. The balance "grade".
  3. The drawing/specifications submitted must indicate the location where mass removal is permissible.  If there is a specific zone that allows the mass removal, such as a diameter band, and a maximum depth, this must be stated.  Note that when such zones are identified, the more restrictive your requirement, the more expensive the cost, and in some cases, it may not be possible to achieve the required balance by remaining in the specified zone.  The closer this zone is to the rotational axis, the less effect it has on the out of balance condition.