What is Balancing?
What is Balancing?
To the average person, rotor balancing is often a "black art". Most
people’s knowledge of balancing is limited to car wheels and tires that
must be balanced before fitting to cars; otherwise they will experience
steering wheel judder, uneven wear on tires and so on.
This article will give you an insight into the world of balancing and
help to remove the "black art".
An unbalanced rotor
Unbalance exists in a rotor when the mass center axis is different to
its running center axis. Practically all newly machined parts are non
symmetrical due to blow holes in castings, uneven number and position of
bolt holes, parts fitted off-center, machined diameters eccentric to the
bearing locations etc.
An unbalanced rotor, when rotating, wants to revolve around its mass
Center axis. Because the bearings restrict this movement, the
centrifugal force, due to the unbalance, causes the rotor to vibrate.
This vibration causes wear to the bearings, creates unnecessary noise,
and, in extreme cases disintegration of the rotor itself can be
experienced. It is therefore necessary to reduce the unbalance to an
acceptable limit.
Balancing limits
There are balance limits, just like machining limits, where the
unbalance is acceptable. International and national standards are quoted
for rotors, for example:. car wheels are balanced to a limit of grade 40
and small electrical armatures are balanced to grade 2.5. The grades are
converted to unbalance units, depending on the rotational speed of the
rotor as per ISO standards.
Units of unbalance
The units of unbalance are mass times radius, for example: a weight
added to a certain position on the part being balanced would shift the
mass axis into the running axis and therefore be in balance. The weight
of correction multiplied by the applied radius will give an unbalance unit.
For metric measurement the units will be gram-millimeters’ (gmm) or for
large rotors, gram-centimeters’. The Imperial equivalent will be
gram-inches or ounce-inches. This weight (mass) would be applied at a
radius from the running center at the light position.
Types of rotors
Rotors fall into two groups. One is where the rotor is rigid and does
not deflect up to and including the operating speed.
The other group comprises flexible rotors that “bow” up to the operating
speed. The first deflection seen is a “skipping rope effect” which means
the center of the rotor at speed moves out from its rotational axis,
causing high “static” unbalance.
Types of unbalance
There are three types of unbalance:
1. Static unbalance – is where the mass axis is displaced only
parallel to the shaft axis. The unbalance is corrected only in one
axial plane.
2. Couple unbalance – is where the mass axis intersects the running
axis. For example: a disk that has swash run-out with no static
unbalance. The unbalance is usually corrected in two planes
3. Dynamic unbalance – is where the mass axis is not coincidental
with the rotational axis. This unbalance is usually a combination
of static and couple unbalance and is corrected in two planes
Methods of correcting unbalance
Removal of material by drilling, milling etc from the heavy position on
the component is used to correct the unbalance. Alternatively it can be
corrected by adding material to the “light” position on the component by
bolting or welding balance weights to reduce unbalance.
Balancing machines
To identify the position and amount of unbalance, balancing machines are
used by a rotor manufacture to correct any unbalance that exists. These
machines are so sensitive that they can easily and accurately identify
any mass axis 0.001mm off the running axis.
One type of machine will only identify static unbalance. This is used
for balancing disk shaped parts. Another type of machine will identify
unbalances in two axial planes, e.g. for balancing a rotors whose length
is proportionally greater than its diameter. These machines are
available in versions that balance the rotor in either the horizontal or
vertical axis.
With the use of modern electronics, accuracy easily exceeds national and
international standards. The set-up of the machine is very simple by
just typing measurements into a computer.
Balancing rigid rotors
Because unbalance exists in a component even when stationary, rigid
rotors can be balanced at a low speed, just enough to produce a
centrifugal force to register the unbalance.
Balancing flexible rotors
This type of rotor is balanced at a low speed where the rotor does not
flex. Correction for unbalance is made, then the speed is gradually
increased, and the unbalance is corrected in stages until the rotor’s
operating speed is reached.
Modern techniques
With the high cost of replacing damaged rotors, the airline industry
stipulates that parts or sections of rotors that are changed must
maintain an acceptable balance. The technique involves using dummy
adjacent parts, for example balancing a compressor module with a dummy
turbine module and replacing compressor and turbine blades without any
further balance. These techniques are available to general industry if
customers require them.
Summary
The latest production methods reduce or eliminate the need for balancing
in low speed applications, but with ever increasing speeds used on
rotating machinery, dynamic balancing will be necessary for the
foreseeable future. An understanding of dynamic balancing streamlines
the complete production process.
