Term Paper on

Explain about the balancing of machines

Abstract
In recent years, the requirements for reliability of machines that perform different
technological processes and their vibration and noise levels have become much
more strict. In many industries, this necessitates balancing machines in their own
supports. When the amount of machinery is very large at a plant, substantial costs
may be incurred if outside consultants are used to perform the balancing.

To avoid these costs, machines are frequently balanced at manufacturing plants or

at repair shops, but this is often unsatisfactory because of excessive vibration that
appears after the machines are installed. In this situation, it is expedient for the
staff of the plant to perform the balancing which necessitates that the plant staff be
trained in the use of special instruments for balancing. In addition, diagnostic
methods should be applied before and during balancing so that other problems that
can occur be recognized and corrected.

Introduction
A machine is any mechanical or electrical device that transmits or
modifies energy to perform or assist in the performance of tasks. It normally
requires an input as a trigger, transmits the modified energy to an output, which
performs the desired task.

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Mechanical mechanisms and machines have been in steady use to amplify the
abilities of human beings since before written records were available. The primary
difference between simple tools and simple mechanisms or machines is a power
source and a somewhat independent operation.
The term machine generally applies to an assembly of parts operating together to
perform work. Generally these devices increase intensity of applied force,
changing direction of force, or changing one form of motion or energy into
another.
The mechanical advantage of a machine is the ratio between the resistance or load,
and the force required to overcome it, although this ratio is not entirely accurate as
force is required to overcome friction, as well. To compensate for this, mechanical
advantage is calculated as the ratio between the distance moved by the force
applied, and the distance moved by the resistance.
History
The first patent for balancing technology was filed by Henry Martinson of Canada
in 1870, four years after the development of the dynamo by Siemens. Near the turn
of the century, Akimoff and Shohola attempted to develop Martinson's technology
and apply it for industrial use.

However, it was in 1907 when a modified version of the technology was patented
by Dr. Franz Lawaczek, and offered to Carl Scheck, Darmstadt, Germany, for
development. Scheck built the first industrial two-plane balancer, and subsequently
bought exclusive world rights to the dynamic balancing machine in 1915.

Through the years, craftsmanship and quality have been the hallmarks of Scheck
products.  Technology advancements gave way to improved sensitivity, frequency
selectivity and plane separation capability. The development of electronics and

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mechanical/electrical transducers, greatly reduced balancing time and paved the
way for modern balancing technology.

Today Scheck balancing equipment is used with confidence for a wide range of
applications - from the smallest rotors for dental drill instruments to the largest
steam turbines in the world.

What is Balance ?
Balancing operation is an operation applied on the rotating bearings of a body,
which has the purpose of improving the mass balance distribution so as not to
affect the centrifugal forces. It is a fact that the operation shall give results only up
to a certain point and there will be an unbalance in the rotating members also after
the balancing operation. This Standard relates to the amount of the permitted
permanent unbalance
It is possible to decrease unbalance to very low limits using the measurement
instruments today. However, it may not be economical to excessively decrease the
limits. The degree down to which unbalance will be decreased may be determined
through technical and economical comparison and the optimum value can be
determined using wide measurement technique, laboratory and in situ use
accuracy.

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Since usually there is no noticeable relation between the rotor unbalance and the
vibrations under the operational conditions of the machines, it is impossible to
derive a result relating to the permissible permanent unbalance from the available
standards relating to the vibration conditions of the machines. The amplitude of
vibration is affected by many factors such as the masses of the vibrating machine
body and base, frequency of the bearing and the base, operational speeds
converging to various resonance frequencies, etc.

Balancing Machine
Balancing machine is a tool which is used for establishing and maintaining the
proper tension of a rotating machine. It is able to perform the task through the
individual part rotation and also sensor detection. 
Most of the balancing machine has a set of firm pedestals along with the bearings
and the suspension. Most of this machine is
also able to balance some different parts
such as rotors for the electric motors, turbines,
disc drives, fans, pumps, and also
propellers. 

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The use of the balancing machine is quite simple. we have to place the items which
need alignment directly to the bearing either mechanically or manually. Then, the
unit is rotated by using the air-drive, end-drive, and also belt-drive. The item is
then vibrating during the rotation. The vibration will make the attached sensor in
the machine determine the condition of the unbalanced unit. In addition, it can
determine the amount of the shift needed for establishing the equilibrium.
Moreover, it can also pinpoint in which the weight is required for balancing or in
which the weights ought to be placed.
You can also find vertical balancing machine. They are used mainly for calculating
the balance of how far the item can move away from the geometric center in the
standing position. Blade balancing machine is also available. It is mainly used for
preventing the additional corrections for some items such as fans, turbines, and
propellers. 

Furthermore, for the instrument which cannot be disassembled easily, the portable
balancing machine will be ideal to be used. Through the displacement sensors
which are mounted on the part, they are able to measure the vibrations during the
operation. Then, they will identify the parts which needs balancing.
Types of Balancing machine
There are two main types of balancing machines, hard-bearing and soft-bearing.
The difference between them, however, is in the suspension and not the bearings.
 Hard-bearing machine
In a hard-bearing machine, balancing is done at a frequency lower than
the resonance frequency of the suspension. In a soft-bearing machine, balancing is
done at a frequency higher than the resonance frequency of the suspension. Both
types of machines have various advantages and disadvantages.

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 A hard-bearing machine is generally more versatile and can handle pieces with
greatly varying weights, because hard-bearing machines are measuring centrifugal
forces and require only a one-time calibration. Only five geometric dimensions
need to be fed into the measuring unit and the machine is ready for use. Therefore,
it works very well for low- and middle-size volume production and in repair
workshops.
 Soft-bearing machine
A soft-bearing machine is not so versatile with respect to amount of rotor weight to
be balanced. The preparation of a soft-bearing machine for individual rotor types is
more time consuming, because it needs to be calibrated for different part types. It
is very suitable for high-production volume and high-precision balancing tasks.

Hard- and soft-bearing machines can be automated to remove weight

automatically, such as by drilling or milling, but hard-bearing machines are more

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robust and reliable. Both machine principles can be integrated into a production
line and loaded by a robot arm or gantry, requiring very little human control.
Other Types of Balancing Machines
 Static balancing machines 
Static balancing machines differ from hard- and soft-bearing machines in that the
part is not rotated to take a measurement. Rather than resting on its bearings, the
part rests vertically on its geometric center. Once at rest, any movement by the part
away from its geometric center is detected by two perpendicular sensors beneath
the table and returned as unbalance. Static balancers are often used to balance parts
with a diameter much larger than their length, such as fans. The advantages of
using a static balancer are speed and price. However a static balancer can only
correct in one plane, so its accuracy is limited.
 Blade balancing machines
A blade balancing machine attempts to balance a part in assembly, so minimal
correction is required later on. Blade balancers are used on parts such as fans,
propellers, and turbines. On a blade balancer, each blade to be assembled is
weighed and its weight entered into a balancing software package. The software
then sorts the blades and attempts to find the blade arrangement with the least
amount of unbalance.

 Portable balancing machines 

Portable balancing machines are used to balance parts that cannot be taken apart
and put on a balancing machine, usually parts that are currently in operation such
as turbines, pumps, and motors. Portable balancers come with displacement
sensors, such as accelerometers, and a photocell, which are then mounted to the
pedestals or enclosure of the running part. Based on the vibrations detected, they
calculate the parts unbalance. Many times these devices contain a spectrum
analyzer so the part condition can be monitored without the use of a photocell and
non-rotational vibration can be analyzed.
Working of Balancing Machine

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With the rotating part resting on the bearings, a vibration sensor is attached to the
suspension. In most soft-bearing machines, a velocity sensor is used. This sensor
works by moving a magnet in relation to a fixed coil that generates voltage
proportional to the velocity of the vibration. Accelerometers, which measure
acceleration of the vibration, can also be used.
A photocell sometimes called a phased, proximity sensor, or encoder is used to
determine the rotational speed, as well as the relative phase of the rotating part.
This phase information is then used to filter the vibration information to determine
the amount of movement, or force, in one rotation of the part. Also, the time
difference between the phase and the vibration peak gives the angle at which the
unbalance exists. Amount of unbalance and angle of unbalance give an unbalance
vector.
Calibration is performed by adding a known weight at a known angle. In a soft-
bearing machine, trial weights must be added in correction planes for each part.
This is because the location of the correction planes along the rotational axis is
unknown, and therefore it is unknown how much a given amount of weight will
affect the balance. By using trial weights, you are adding a known weight at a
known angle and getting the unbalance vector caused by it. This vector is then
compared to the original unbalance vector to find the resultant vector, which gives
the weight and angles needed to bring the part into balance. In a hard-bearing
machine, the location of the correction plane must be given in advance so that the
machine always knows how much a given amount of weight will affect the
balance.

Necessity of Machine Balancing in the Field

All the rotating elements in a machine are acted on by the inertial forces. The
amplitude of these forces depends on the value of the offset between the shaft
center line of the element and the center of masses of the element cross-sections in
planes perpendicular to the axis of rotation. The presence of such offsets is called
unbalance. The unbalanced rotor of machine is a source of variable forces acting
on the machine elements, particularly on the bearing supports, and these forces can
significantly decrease the service life of the machine. The process of minimizing
these forces is called balancing.

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Balancing is usually done by applying additional balancing weights at the rotating
machine parts to develop inertial forces equal by value and opposite in direction to
the forces created by unbalance. Usually, it is not necessary or practical to place
these weights in each rotor cross-section. Usually, the rotor can be assumed to be
stiff if is a rotor operating below its first critical speed (up to 0.7 of the first critical
speed). In this case, it is sufficient to place two balancing weights in different
planes to compensate the influence of all unbalances. In the same manner, it is
possible to balance the a rotor operating near its first critical speed, but some
restrictions appear on the placement of the cross-section planes of the rotor where
the balancing weights should be mounted. Balancing a rotor operating above its
first critical speed is much more complicated.

Rotor balancing is usually done on balancing machines. But, during assembly of

previously balanced parts, mounting a rotor on its supports and fitting it with other
rotating parts can cause additional sources of unbalance to appear. The causes of
the new unbalance can include:

 assembly tolerances
 accuracy of rotor fitting in the bearing supports
 accuracy of coupled shafts alignment
 influence of operating conditions on the unbalance , which cannot be
reproduced on the balancing machine.

In addition, unbalance can increase during normal machine operation as a result of

materials adhering to the rotating parts of the machine, pitting of the machine
elements, corrosive wear, coupling looseness in complex rotors, and looseness of
supports and bases. Despite the presence of unbalance conditions, the machine may
continue to be used without being removed from service for repair if it is properly
trim balanced, especially if looseness problems are corrected.

If it becomes necessary to balance a machine in the field, the main procedures on

which determine the success of balancing depends:

 vibration parameter measurements

 processing the data acquired

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  optimal calculation of the balancing weights

Preliminary Stage, Selection of the Balancing Conditions

For balancing, a particular rotating speed should be selected. This speed should be
constant (with accuracy about 1%) and should be reproducible from one machine
run to another with about the same accuracy. Missing these requirements can
significantly decrease the balancing efficiency. If the machine has several modes
of operation, it is necessary to choose the modes where the balancing will be
conducted. The modes chosen should ensure constant rotating speed and be very
much like the normal modes for the machine

(The diagram of machine balancing in operational conditions) 

1-machine; 2-places of attachment; 3-vibration transducer; 
4-tachometer transducer; 5-measurement instrument/PC

 First Stage:
Preparation for Balancing

 The first step in balancing is the preparation for vibration parameters

measurements. In balancing, the vibration parameters are the vibration amplitudes
and phases at all measurement points.

 Selection of Instruments

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To conduct measurements of vibration parameters, it is necessary to have an
instrument that can reliably separate the vibration at the machine rotating
frequency from the vibration at other frequencies. The instrument has to measure
accurately the vibration parameters at the rotating frequency with the amplitude
accuracy better than +10% and phase with accuracy better than +5 degrees.

 Vibration Parameters, Measurement Point Selection and Preparation

Usually these points are determined by specification documents. If no such

documents are available, it is better to choose such points near the places where the
vibration energy is transferred from the rotating elements to the fixed ones. In two
latter cases, it is necessary to prepare the place of attachment: to clean the
mounting surface and to make the surface flat for reliable attachment of the
transducer.

 Tachometer Transducer Attachment

To measure the phase of vibration, it is necessary to have a pulse reference

tachometer transducer - a device that gives an electric pulse in corresponding to a
certain position of a rotating machine part. A reference mark should be placed on
a rotating machine part to fix its position. If a machine has several parts that
rotate with different speeds then the balancing can be done only on the part where
the reference mark is placed. The reference mark must be available for the
transducer when the machine is in operation. It should be possible to reattach the
transducer in the same position if a partial machine dismantling is necessary for
placement of trial or balancing weights.

 Second Stage:
Initial Vibration Parameter Measurement

After completing the preparation of places for vibration transducers and trial
weights and attaching the tachometer transducer, you can commence the initial
machine vibration parameters measurements. The measurements are conducted in

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all measurement points in radial direction for all selected modes of machine
operation.

The vibration parameter measurements are one of the main sources of mistakes
that prevent efficient balancing. The following mistakes are the most frequent
ones:

 A loose attachment of the vibration transducer or poor preparation of the

place for attachment.
 A wrong selection of the measurement point (for example, the measurement
might be made in axial direction instead of the radial one).
 An omission of a case when the influence of a random noise signal or a
harmonic component of an external source is very strong.

The last mistake is more common for further stages of balancing as its aim - to
decrease the vibration - leads to comparative increase of the noise level.

 Third Stage:
Trial Weight Attachment and Vibration Parameter Measurement

A trial weight is placed in the first balancing plane and all the vibration parameter
measurements are made for all the measurement points and selected modes of
machine operation. Then, the trial weights are placed in the second, third, and so
fourth balancing planes and the vibration parameters are measured.

The attachment and taking off the trial weights is one of the most common sources
of mistakes. The position of the reference point for the angle reading of weight
attachment, generally speaking, is free. The direction of trial weights attachment
angle increase is determined by the measurement instruments and application
software. Random mistakes often appear in determining the direction of angle
increase. Modern software for balancing weight calculation make it practical to
conduct measurements for further trial weight without taking off the previous
weight. You have to denote the type of weight. But very often, the user forgets to
take off the trial weight before attaching the next one or before attaching the
balancing weight, or informs the computer about his actions incorrectly.

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 Fourth Stage:
Balancing Weight Calculation

After completing the vibration parameter measurement for the last trial weight, you
then calculate the balancing weight parameters for each balancing plane. These
parameters are their values and angles of attachment. Usually the balancing
weights are placed on the same radii as the trial weights. In other cases, the radii of
attachment should be included in the calculation also. Computers are used to
calculate the balancing weights because the addition and multiplication are made
not with numbers but with vectors. To decrease possible mistakes in calculations,
the programmable calculators are recommended.

In this stage of balancing, the values of the machine sensitivity to the attached
weights play significant role. Two types of mistakes can be determined on this
stage:

 The trial weight is very small. In this case the calculations are made on the
basis of the values that may result entirely from measurement errors. The
result of the calculation is unpredictable.
 The trial weight is small. The accuracy of the calculation in this case is also
low and it is in impractical to expect good efficiency of this step of
balancing.

 Fifth Stage:
Balancing Weight Attachment

The calculated weights are attached and the vibration parameters measurements are
conducted at all measurement points and for all selected modes of machine
operation. If the vibration parameters meet the specified requirements, then the
balancing procedure is completed.

 Sixth Stage:
Continuation of Balancing

In practice, it is very rare when the desired results are produced by one step
balancing because there are errors in vibration parameter measurements,

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differences in parameters of attached weights in comparison with the calculated
ones, influences of mechanical machine properties, and non-inertial forces. There
is also a significant influence of mistakes made during measurements and
attachment of trial and balancing weights.

In this case there are three possibilities:

 to interrupt balancing:
 to make a second balance
 to calculate the correction weights that increase the balancing effect using
the results of previous balancing to make a trim balance.

Most of the above mentioned mistakes, if they are detected, can be corrected
during trim balancing. That is why if the application software can enable trim
balancing, even with low balancing efficiency, it is worthwhile to calculate
corrective weights for trim balancing. Otherwise, the correction of mistakes by
trim balancing will significantly decrease your time and financial costs.

The latter is possible only if the inadequate results of balancing are the result of
errors in measurements and weight attachment.

 Last Stage:

Balancing Completion 

A protocol is written in this stage. The figure shows the relative time consumed for
the main procedures of machine balancing with four balancing weight planes. It
should be noted that the time necessary for placing the weights, can reach 90% of
the whole balancing procedure, especially if it necessary to disconnect and connect
the rotor drive to prevent it from self starting during weight attaching. The last
column shows the increase in time if one additional machine start is needed. Such
start-ups are the results of mistakes in measurements or weight placing. For a
machine with two balancing planes, the time for an additional start-up can take up
to 30% of the time necessary for balancing.

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 ( Relative time consumption for balancing operations)
1 - preparing the places for accelerometers, attachment of a probe
2- the trial run/stop of the machine
3- mounting the correction weights
4 - measuring a signal
5 - analysis of measurement results and corrective weight calculation
6 - additional run

When you are working with balancing machine the following

rules apply

 All of the vectors must represent the same value, for example vibration in
mils.
 Each vectors need to be drawn to the same scale. 1 mil = 1 inch, for
example.
 The angles must start with 0 degrees at the same place, vertical, and increase
in the same direction, clockwise.

Use of balancing Machine

The use of the balancing machine is quite simple. You have to place the items
which need alignment directly to the bearing either mechanically or manually.

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Then, the unit is rotated by using the air-drive, end-drive, and also belt-drive. The
item is then vibrating during the rotation.

The vibration will make the attached sensor in the machine determine the condition
of the unbalanced unit. In addition, it can determine the amount of the shift needed
for establishing the equilibrium. Moreover, it can also pinpoint in which the weight
is required for balancing or in which the weights ought to be placed. 

Advantages of Balancing of Machine

 Best suited when rotor configurations change frequently because no

calibration runs are required
 Simple setting of instrumentation by dialing in 5 geometric rotor dimension
no trained operating personnel required
 Universally applicable i.e. large weight range permits large initial
unbalances
 Especially well suited for miniature rotors
 Achieves maximum balance quality.
 Especially well suited for balancing sail assemblies

Disadvantages of Balancing of machine

 Less suited for miniature rotors because sensitivity is restricted

 Calibration runs are required to adjust the instrumentation for a new rotor
type

Bibliography
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