Two unbalances (shown as arrows in the adjacent sketch) may have the same
direction and angular position. The same state of unbalance would be generated
by a single unbalance with a magnitude which is twice as high, acting at the
centre of gravity, i.e. for example at the centre of the rotor .
if such
a rotor is supported on two knife-edges, it will roll off on the knife-edges
until its "heaviest point" points to the bottom. This type of unbalance is
making itself felt even without rotation, and is therefore referred to as
"static unbalance". Static unbalance causes a shift of the mass centre away from
the geometrical centre, which causes the rotor to vibrate during
operation.
Static unbalance can be corrected in the centre of gravity
plane, either by removing the appropriate amount of material, or by adding a
weight at the opposite side.
Static unbalance occurs particularly on
disc-shaped rotors. Therefore, a vertical balancing machine is usually the best
choice for correcting static unbalance
Couple unbalance
Two unbalances (shown as arrows in the illustration) may have the same
magnitude, but their direction may be offset by 180° relative to each other.
This state of unbalance can no longer be determined by rolling a component on
knife-edges, as the rotor will not settle in a clearly defined
position.
The rotating component will wobble about its vertical axis
(perpendicular to the axis of rotation), as the two unbalances generate a moment
in the workpiece. This type of unbalance is therefore referred to as couple
unbalance.
To correct a couple unbalance, you need to generate an opposed
moment, e.g. you need two correction weights, arranged at a specific distance
relative to each other. In the balancing planes thus defined, you either have to
remove a specific amount of material, or add an appropriate weight at the
opposite side.
Couple unbalance is usually encountered in long,
cylindrical rotors. Therefore, horizontal balancing machines are usually best
suited to correct dynamic unbalance.
Dynamic unbalance
In actual practice a rotor normally does not have a single unbalance, but an
infinite number of unbalances distributed at random along its axis of rotation.
These can be expressed as two resultant unbalances (shown as arrows in the
illustration) acting in two arbitrary balancing planes. They differ in
magnitude, and do not have a clearly defined angular position.
As this
state of unbalance can only be determined comprehensively when the component is
rotating, it is referred to as dynamic unbalance. It includes both a static
component and a couple unbalance, whereby either of these two components may be
dominant in the workpiece.
Due to the couple unbalance present, you need
two balancing planes to be able to completely correct dynamic unbalance.
Correction is achieved by removing the appropriate amount of material in these
two balancing planes, or by adding weight on the opposite side.
Dynamic
unbalance occurs in nearly all rotors. Therefore either horizontal or vertical
balancing machines may be suitable for balancing.