Unbalance is the uneven distribution of mass around the rotor’s axis of symmetry. In the ISO standard, unbalance is defined as: A condition in the rotor caused by the transmission of vibrations resulting from centrifugal forces to the bearings. Correcting this asymmetric mass distribution in the rotor is called balancing. In fact, balancing covers errors caused by faulty manufacturing.
One of the causes of vibration in rotating machines is unbalance. Unbalance is a condition where the rotational axis of the shaft and the weight distribution axis (mass centerline) are not coincident due to uneven mass distribution around the rotating shaft’s centerline.
1. Static Unbalance
2. Couple Unbalance
3. Dynamic Unbalance (Combination of 1 and 2)
Static unbalance occurs when a heavy point on the rotor is located at the midpoint between the bearings supporting the rotor. In practice, static unbalance is seen in rotors that are short relative to their diameter. Static unbalance is characterized by the weight distribution axis being parallel to the rotational axis.
Couple unbalance occurs when the center point of the weight distribution axis coincides with the center point of the rotational axis. This condition causes the rotor to create a rocking motion from end to end during rotation. Couple unbalance differs from static unbalance: If supported between two frictionless centers, a rotor with static unbalance will always rest with the heavy point at the lowest point, whereas a rotor with couple unbalance rests at random points. Couple unbalance is characterized by the rotational axis intersecting the center of gravity on the weight distribution axis.
Dynamic unbalance occurs when the rotational axis and the weight distribution axis do not intersect at all. This is a combination of static and couple unbalance and is the most common unbalance observed in rotating machines. Dynamic unbalance is characterized by the weight distribution axis and the geometric axis not intersecting. It can be said that all rotors possess dynamic unbalance, and pure static or couple unbalance does not exist in reality.
You can always determine the system’s unbalance status by measuring the phase angle at two bearings in the horizontal or vertical direction to determine what percentage of the system’s unbalance is couple and what percentage is static. (This helps in deciding between single-plane or two-plane balancing.)
These are several reasons that cause a machine to be unbalanced:
- Corrosion or erosion of rotors
- Asymmetric mass distribution in the rotor (material anisotropy)
- Deposit of dirt on rotors
- Manufacturing defects
- Material defects due to voids, blowholes, foreign matter, etc.
- Eccentric rotors
- Cracked fans / Damaged fans
- Incorrect and asymmetrical drilling
- Machining errors