Vibration excited by residual rotor unbalance is always present in all rotors at all operating speeds, because it is of course impossible to make any rotor perfectly mass balanced. Therefore, the objective concerning unbalanceexcited vibration is its minimization, not total elimination. Chapter 8 addresses the fundamental question of whether the residual vibration of a machine is within its acceptable limits or is excessive. When vibration levels are deemed excessive and it has been established that the excitation is unbalance (see Section 9.8), the proper corrective course of action is often simply to rebalance the rotor. This is especially typical in some machinery types that are inherently susceptible to going out-of-balance in normal operation, like large fans in power plants and steel mills where crud collects nonuniformly on fan blades. Many machines are designed with externally accessible rotor balance planes where balance correction weights can be added. Thus, in-service rebalancing of the rotor does not typically require opening up the machine and is considered a relatively routine procedure.