Unbalance occurs when the center-of-gravity of a rotating object is not aligned with its center-of-rotation.

Unbalance is caused when the center of gravity is out of alignment with the center of rotation. To correct the problem, a weight of mass m must be attached to the opposite side of the rotating object. Given a rotor mass M and radius R, the following relationship applies.

◇Units Used to Express Unbalance◇

In Terms of the Mass Eccentricity e ： µm、mm、cm

In Terms of Mass m ： µg、mg、g、kg・・・

In Terms of Mass x Radius (mR) ： mg・mm、mg・cm、g・mm、g・cm、kg・cm・・・

Dynamic balancing machines measure the size (magnitude) and angle (direction) of vibration.

There is no limit to the number of products that rotate. Using cars as an example, they consist of tires, shafts, flywheels, gears, and crankshafts, to name just a few. In our homes, we have a variety of equipment, including fans, vacuum cleaners, refrigerator compressors, video heads, DVD and CD systems, and computer hard drives. It would be impossible to count all the types of rotating objects and how many exist.

When these rotating parts or materials rotate, they generate centrifugal forces. Normally, the sum of these centrifugal forces equals zero, however, if they do not, that rotating object will generate vibration and noise. Dynamic balancing machines measure the amount and angle of this vibration.

It means to reduce the amount of unbalance.

Unbalance refers to the center-of-gravity of the rotor being out of alignment with its center-of-rotation (eccentricity). If unbalanced, centrifugal forces will generate vibration and noise during rotation. The world in which we live is overflowing with rotating objects. In most homes, looking around many rooms you can find air conditioners, forced air heaters, or fans, and sometimes even dehumidifiers, humidifiers, or a video, CD or DVD player under the television. In the bathroom you can find washing machine, drier, electric razor, n the kitchen ventilation fans, refrigerators, microwave ovens and in a different room a computer and printer. All of these consumer electronic products include internal rotating parts (motors). Outside the home you see automobiles, which are a dense concentration of multiple rotating objects, as are trains, ships, and aircraft. In manufacturing plants, you can find all types of rotating machinery on a production line. At power plants there are rotating turbines and generators. If all these items were not balanced properly, our lives would undoubtedly be extremely noisy. Furthermore, generating vibration and noise consumes energy, meaning energy is being wasted, which is counterproductive to current efforts to save energy. Not only does vibration waste energy, it causes bearings to wear more quickly and shortens the operating life of products.

The process of reducing the amount of unbalance in rotating items like these is called balancing.

Unbalance created by motion

Dynamic Unbalance
In general, unbalance can occur at any lateral (axial) position along the rotating body and with any magnitude. This unbalance is a combination of static unbalance and couple unbalance and is called dynamic unbalance. The couple unbalance component only appears when the object is rotated, so measuring dynamic unbalance requires rotating the object. Dynamic unbalance must be corrected at two locations in the axial direction (two-plane correction for two-plane unbalance). Put simply, unbalance apparent when the object is perfectly still (no rotation) is called static unbalance and unbalance that becomes apparent after an object begins to rotate is called dynamic imbalance.

 Unbalance Present With Static

Static Unbalance
When a rotor is placed on smooth rails, the heavy side will turn to face downward. The unbalance experienced when an object is static (not moving) is called static unbalance. Static unbalance can be corrected using only one location (one-plane correction for one-plane unbalance).

In terms of eccentric distance, in the range of thousandths of a micron.

Assuming a cylindrical rotor 50 mm in diameter and weighing 300 grams, for example, our standard model could measure a 0.5 mg weight attached to the rotor perimeter.

In comparison, one grain of rice weighs about 20 mg.

This means it can detect an unbalance equivalent to 1/33 of a grain of rice on the surface of a 300 g workpiece roughly the weight of a filled small-beverage can.

The primary cause is an accumulation of error.

There are many factors than can cause dynamic unbalance.

• For formed or machined parts, potential causes include:
  • Variations in the specific weight of materials
  • Shape is not symmetric with respect to the center axis
  • Variations in forming, machining or other processes
• For part assemblies, potential causes include:
  • Variations in assembly process
  • Variations in the mass of individual parts
  • Variations in placement