TABLE OF CONTENTS Vibration problems in present-day turbomachinery are as pressing and important as those encountered in their design, manufacture, and general maintenance. Considerable amounts of precious energy go unused during machinery breakdowns, and the associated costs of machine downtime add to unproductive overheads. The modern trend of building high-speed engines requires new, dependable techniques to reduce vibrations.
Of the several factors that can cause vibrations in turbomachines, an unbalanced rotor stands at the top of the list. The lack of balance in a rotor may be caused by internal nonhomogeneity and/or external action. The general sources which can cause this problem are classified in the following categories:
Dissymmetry
Nonhomogeneous material
Eccentricity
Bearing misalignment
Shifting of parts due to plastic deformation of rotor parts
Hydraulic or aerodynamic unbalance
Thermal gradients
A certain amount of the unbalance from factors such as misalignment, aerodynamic coupling, and thermal gradients may be corrected at running speeds using modern balancing techniques; however, in most cases they are basic problems that must be initially corrected before any balancing can be done. Rotor mass unbalance from dissymmetry, nonhomogeneous material, distortion, and eccentricity can be corrected so that the rotor can run without exerting undue forces on the bearing housings. In balancing procedures only the synchronous vibrations (vibration in which the frequency is the same as the rotor rotating speed) are considered.
In a real rotor system the amount and location of unbalances cannot always be found. The only way to...
