Author - Ashley Wass Technical Services Engineer Bearing and Power Transmission
What does ‘dynamic balancing’ mean?
The word dynamic means
“pertaining to force related to motion.” And the word balance means “a state in
which opposing forces harmonize
machine to be out of balance (unbalanced) it must have anuneven distribution of mass about its rotor’s rotating
centreline. The key phrase to note is “rotating centreline,” as opposed
to “geometric centreline.” The rotating centreline is defined as the axis about
which the rotor would rotate if not constrained by its bearings. The geometric
centreline is the physical centreline of the rotor. When the two centrelines
are the same, then the rotor will be in a state of balance. When they are
different, the rotor will be unbalanced.
dynamic balancing means to reduce the uneven
distribution of mass about a rotor’s rotating centreline.
The effect this has on the rotor, as well as on the complete machine, is to
reduce the vibration levels of the machine. When balanced, the vibration levels
will be under an acceptable vibration level.
So what are
the acceptable vibration levels for rotors to be considered to be in balance? International
Organisation for Standardisation (ISO) publishes international standards for
business, government, and society. In industry, the
relevant standard for balance is ISO 1940-1:2003 Mechanical vibration
-- Balance quality requirements for rotors in a constant (rigid) state --
1: Specification and verification of balance tolerances. This standard
gives specifications and acceptable vibration levels for rotors in a constant
(rigid) state according to their machinery type and maximum service speed.
following table is an incomplete copy of ISO 1940/1 for reference only. For all
standards, purchases, and information refer to the International Organisation
*there are other meanings for these words but in
relation to other topics
|Grade ||General Examples |
- Crankshaft/drives of fast
diesel engines with six or more cylinders Complete engines (gasoline or diesel)
for cars, trucks and locomotives.
- Car wheels, wheel rims, wheel sets, drive
- Crankshaft/drives of elastically mounted fast four-cycle engines with six or more cylinders.
- Crankshaft/drives of engines of cars, trucks and locomotives.
- Drive shafts (propeller shafts, cardan
shafts) with special requirements.
- Parts of crushing machines.
- Parts of agricultural machinery.
- Individual components of engines (gasoline
or diesel) for cars, trucks and locomotives.
- Crankshaft/drives of engines with six or more
cylinders under special requirements.
- Parts of process plant machines.
- Marine main turbine gears (merchant service).
- Centrifuge drums.
- Paper machinery rolls; print rolls.
- Assembled aircraft gas turbine rotors.
- Pump impellers.
- Machine-tool and general machinery parts.
- Medium and large electric armatures (of
electric motors having at least 80 mm shaft height) without special
- Small electric armatures, often mass produced,
in vibration insensitive applications and/or with vibration-isolating mountings
Individual components of engines under special requirements.
- Gas and steam turbines, including marine
main turbines (merchant service).
- Rigid turbo-generator rotors.
- Computer memory drums and discs.
- Machine-tool drives.
- Medium and large electric armatures with
- Small electric armatures not qualifying for
one or both of the conditions specified for small electric armatures of balance
quality grade G 6.3.
- Turbine-driven pumps.
- Tape recorder and phonograph (gramophone)
- Grinding-machine drives.
- Small electric armatures with special
- Spindles, discs and armatures of precision
other dynamic balancing service providers or consultants that perform dynamic
balancing, will provide this service in-situ, for example fan balancing.
In-situ fan balancing is where the correction of the
unbalance in the fans rotating component is done while the component is in its
working environment, in other words it is performed in the field.
are many advantages of in-situ balancing over off-site balancing. These
- Costly and time consuming dis-assembly to remove the rotor to a balancing
machine is eliminated.
- Improves product quality.
- Reduces downtime, labour, and material costs.
- Can be planned into scheduled short time outages e.g. process changes.
- Extends bearing and machine life.
- Reduces the possibility of catastrophic failures.
- Increases safety.
- Balancing is performed on the complete assembled machine and compensates for
the assembly tolerances.
- The effects of temperature, pressure, distortion, and other environmental
influences can be incorporated.
- The resultant vibration can be the tolerance applied to the rotor, rather than
the published balance tolerances normally used in a balancing machine. This is
particularly advantageous if the supporting structure is close to a resonance.
The unbalance in the rotor may have to be adjusted to abnormally fine levels to
minimize the resultant resonant structural vibration.
unbalance fixed in-situ? CBC, and most service providers or consultants, use
modern instruments such as vibration analysers, data collectors, and portable
balancers to provide accurate information to assist in the balancing process.
The vibration level measured at the rotating speed frequency is used as an
indicator of the amount of unbalance. The location is determined by measuring
the phase. Phase, the relative motion of one part of a machine to another, is
measured by means of a stroboscopic light or by an indicator in the instrument,
triggered by a photocell.
machine driving the component must be able to be operated and shut down
multiple times to enable data about the rotor’s vibration, in relation to phase
angle, to be measured, and adjustments to be made. Access to the rotor is a
must. This is for weight to be added or removed, i.e. drilling holes, to reduce
the uneven mass distribution around the rotating centreline. Again, in the case
of fan balancing, this means drilling holes in the fan blades etc.
following is a simplified procedure to balance a rotor as in fan balancing:
is initially run and the vibration levels recorded. A trial weight is then
added, or multiple weights added if there are two or more planes, and the new
vibration levels are recorded. The instrumentation performs mathematical
calculations by comparing the two vibration levels and phase angles and
recommends where weight should be added to reduce the unbalance. Adding weight
may need to be done several times in several locations, with the vibration
levels being recorded each time, and the instrumentation recalculating the data
to give further corrections to reduce the unbalance to within acceptable
to providing dynamic balancing services on site, CBC also provides training and
necessary components, such as balance weight kits (CBCAPO13O(A)), data
collectors, and portable balancers. Rotor and fan balancing are common services
provided by CBC.