Unbalanced vibration occurs when the mass of a rotating object is not evenly distributed around its axis of rotation. This causes a centrifugal force during rotation that vibrates the bearings or mounting. There are three main types of unbalance: static, couple, and dynamic. Static unbalance causes the heavy side to always drop down, couple unbalance exists when two unbalances are 180 degrees apart in different planes, and dynamic unbalance has an axis that is both tilted and displaced from the centerline. Unbalance can be caused by issues like blow holes in castings, wear over time, or accumulation of tolerances during assembly. It is diagnosed using vibration spectrum analysis of the 1x amplitude and phase readings across sensors.