A lot of widely used process equipment, such as pumps, compressors and mixers, rely on rotation of an element (impeller, blade, etc.) attached to a rotor. In a perfectly balanced machine, all rotors or rotating parts turn true on their centerline and all forces are equal. However, imperfections always exist and lead to some unbalance, which can cause vibration. Other effects such as misalignment, forces from working fluids, etc., also may create vibration.
The combination of all applied forces (including those from unbalance, working media, misalignment, etc.) and the stiffness and damping of the rotor-support system (including bearings and bearing pedestals) determine the vibration of a piece of equipment. Rotor-support stiffness is important because forces from different sources (unbalance, etc.) can deflect rotating elements from their true centerline; the stiffness resists the deflection.
Keeping all dynamic forces under control and practically in balance requires a clear understanding of the mechanical movement of the machinery and its components, as well as all involved forces. These are the foundations for vibration analysis and rotor-dynamics.