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Entry type: FAQ Entry ID: 38711638, Entry date: 10/02/2009

Types of unbalance

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What types of unbalance are available?







Static unbalance

Example: Eccentrically positioned bearing pins.


Effect: The center of gravity is beyond the shaft axis, as a result of the static unbalance the centrifugal force approaches the center of gravity and generates rectified rotating bearing forces in both bearings. These are only of the same size, however, when the center of gravity lies equidistantly from the bearings.






A) shaft axis, S) center of gravity of the rotor, T) axis of inertia of the rotor,
FU) centrifugal force [N], FA, FB) bearing forces

e) referenced unbalance (= distance of axis of inertia – shaft axis)


Torque unbalance

Example: Bearing pins aligned obliquely to the rotor

Effect: The rotor's center of gravity remains on the shaft axis. The centrifugal force torque generates equally sized bearing forces aligned in opposite directions in both bearings when the center of gravity is directly in between the two bearings.


A) shaft axis, S) center of gravity of the rotor, T) axis of inertia of the rotor,
FU) centrifugal force [N], FA, FB) bearing forces

MU = FU x a = Unbalance torque [Nm]


Dynamic unbalance

This is the entire unbalance state of a rotor which consists of the static unbalance and the torque unbalance.




Measured variable of the unbalance

The unbalance is measured in gmm and produces the following results:


U = u x r
U = Unbalance amount ingmm
u = Unbalance mass ing
r = Distance between the center of gravity of the unbalance mass and the shaft axis inmm


During the balancing, the balance state of the rotor is corrected by adding (positive balancing) or removing (negative balancing) the corresponding unbalance mass on the specified balance radius.


Residual unbalance:

A residual unbalance remains after the balancing, this may not exceed a permitted value, the greater the unbalance may be, the greater the rotor mass is. The unbalance thus relates to the rotor mass. The ratio corresponds to the center of gravity's eccentricity in the case of purely static unbalance.

ezul = Uzul / m
ezul = referenced, permitted residual unbalance inµm
Uzul = permitted residual unbalance ingmm
m = Rotor massin kg
    The value of ezul results from the balance quality by means of the angular velocity






ezul = Uzul / m = balance quality / ω






The effects of unbalances essentially depend on the distribution of masses within the rotor and on the rigidity and damping of the bearing and foundation. This is the reason why the smooth running of the unit cannot be guaranteed by the residual unbalance and vice versa.

For this reason, agreements should relate to the vibration severity of the entire motor.

Only in a few individual cases, is an agreement for a residual unbalance advisable (e.g. if motors are supplied without bearings or only with one bearing, rigidly coupled drives)




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