What is an unbalance

A rotor has axes around which it “naturally prefers” to rotate usually called the principal axes. Intuitively, they are axes which pass through the center of mass and the mass distribution around these axes are uniform and symmetrical. A physical rotor has a physical axis of rotation,i.e. engineered axis obtained through machining processes. However due to the non uniformity in mass distribution of the rotor, the physical axis of rotation and the principal axis do not coincide. Because of the non co-incident axes, now the mass distribution around the real-life axis of rotation is non uniform. This non uniformity in mass is quantified and shows up as an unbalance of the rotor.

When the mass distribution is non uniform purely in the radial direction, but uniform axially, only the center of mass and the principal axes is shifted parallel to the axis. of rotation. This is a purely static unbalance

Representation of Pure Static Unbalance

When mass distribution is such that the axis of rotation and principal axes intersect at the center of mass, this is a purely couple unbalance (dynamic unbalance）. This typically happens when the mass distribution is symmetric about the center of mass axially, but equal and opposite radially on either side of the center of mass.

Representation of Pure Couple Unbalance （Dynamic Unbalance)）

Real rotors have a combination of static and couple unbalance. The relative effect of each type of unbalance dependent on the shape and size of the rotor.