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Practical Solutions to Machinery and Maintenance Vibration Problems

Chapter 5, Unbalance

Section 24, Slightly Cocked Rotors

If a rotor is cocked on a shaft differently than the position in which is was originally balanced, this will also result in an unbalanced assembly. For very narrow rotors, such as a disc-shaped pump impeller or pulley, the distance between the centrifugal forces of each half of the rotor may be very small, but the amount of centrifugal force is the result of half the rotor weight times the radial distance from the axis of rotation to the center of gravity of that half (at any one time, half of the rotor is pulling against the other half). This results in a very high centrifugal force which is mostly counterbalanced by its counterpart in the other half of the rotor. However, with the rotor cocked slightly, the small axial distance between the two very large centrifugal forces causes a couple unbalance, often several times greater than the allowable tolerance.


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