Machine Balancing in Field Conditions

By
Alexander G. Shablinsky
VibroAcoustical Systems and Technologies (VAST,Inc.),
22, Rosenshteina, St. Petersburg, 198095, Russia

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.

Problems that arise in field machine balancing and methods of the solution of these problems are found VAST_BAL, balancing and diagnostic software designed by VibroAcoustical Systems and Technologies, Inc. are discussed in this paper.

Introduction

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.

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. To solve this problem, groups of balancing weights are used that are attached in the rotor planes defined by using special methods.

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:

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 which determine the success of balancing depends:

1. The Problem of Machine Balancing in Operational Conditions

As was mentioned above, the objective of balancing of rotating machines is to compensate for the inertial forces acting in the radial direction caused by unbalanced rotating machine parts. Forces generated at locations close to the machine axis, typically on or very close to the machine shaft, are mainly the result of construction peculiarities and errors. In practice, it is impractical to measure these forces in the presence of unbalance forces produced by larger cross-section rotating elements attached to the shaft. That is why the main premise of field balancing is to decrease the vibration at the rotating frequency of the shaft by attaching the balancing weights in rotor cross-section planes that correspond to the apparent unbalance locations of the rotor sections. The centrifugal forces developed by balancing weights can also compensate for the action of radial rotating components of other forces having different causes, for example magnetic forces. Since not all forces acting at the rotating frequency in a machine have only radial components, vibration at shaft rotating frequency caused by sources other than unbalance from other cannot be completely eliminated by balancing.

In multi-condition machines, that is machines that have different modes and speeds of operation, it is possible to decrease vibration at some speed or speeds and increase vibration at others.

One such example is an electrical machine with an offset between the axis of rotor and axis of its rotation (dynamic eccentricity of the air gap). It is possible in such machines to decrease the vibration of the stator at one mode of operation, but the vibration of rotor will increase. The vibration of both the rotor and stator can increase when switching between different operating modes.

2. The Peculiarities of Machine Balancing in Field

Machine balancing in the field differs significantly from balancing on balancing machines. The primary reason for this is that it is not the inertial forces themselves that are measured or their resultant vibrations on simple balancing machine supports but is, instead, the resultant vibrations on complex mechanical systems, such as the cases and machine supports, base frames and assemblies. In addition, in real machines, almost all rotors are coupled with others by shafts, frames, and base assemblies and their response and coupling characteristics. Furthermore, the machine under real operating conditions is influenced not only by inertial forces, but forces of other nature, for example, electromagnetic, hydro- and aerodynamic forces, and so fourth.

It should be stressed that if we decrease the vibration within the stationary parts of the machine, the loads applied to the bearings can increase with appearance of non-inertial forces. Such a situation may occur if non-inertial forces in the machine are represented by a force couple. For example, all electromagnetic forces are applied between the rotor and stator. In this case, to decrease the vibration of the stator, the inertial forces must produce an additional action applied to the stator through the bearings. This action is an additional load applied to the bearings. In this case, you must choose what is more important - to minimize external vibrations by decreasing the vibration of the stator or to minimize additional loads applied to the bearings. The optimal decision is to make preliminary machine diagnostics and correct the faults that lead to such situations. If the factors that limit balancing are not recognized and and eliminated, satisfactory balancing may be impossible (this occurs in about 30% of cases.

The following are the main problems that limit balancing efficiency:

Additionally, balancing in field is doing usually in respect to some standards. Then the number of accessible plane places can be less than the required measurement points. There efficiency of balancing is limited even theoretically. This case is very common when balancing is conducted simultaneously for several modes of machine operation, which differ from each other, for example by rotating speed, load and so on.

Balancing in field usually cannot be automated because of different balancing conditions for each machine and consequently must be operator controlled. That is why it is important to be able to easily detect operator mistakes. The most common operator mistakes are:

These mistakes dramatically decrease the efficiency of balancing and can lead even to increased vibration. Thus it is necessary to decrease the influence of these factors on machine balancing in field by all possible means means.

3. The Main Faults that Excite Vibrations at the Machine Rotating Frequency

A machine's vibration at rotation speed depends not only on the value of rotor unbalance, but also on the presence and development of a number of possible defects as well as the deviations from technical specifications in the following units:

An analysis of the most common defects that result in an vibration increase at the shaft rotation frequency shows several possibilities:

4. Oscillation Forces in The Presence of Defects

Vibration of a machine at the rotation frequency is defined by the sum of all the oscillation forces including the centrifugal ones excited by the unbalanced rotor combined with the mechanical properties of the machine and its base supports. As both forces and mechanical properties depend on the existing defects of machine or its base supports and their types and severity, it is necessary to define how efficient the procedure of balancing can be in the presence of particular types of defects.

Rotor unbalance excites centrifugal rotation (i.e. tracing out a circle per one revolution) synchronously with the radial (directed with the radius to the rotation axis) forces applied directly to the rotor. Similar rotating forces occur due to defects of flexible couplings or jointed shafts. The magnitude of centrifugal forces depend on the unbalance of the rotor and rotation speed although the force synchronously rotating with the rotor that originated in defects of couplings depend on the defect severity and momentum transferred, i.e. machine load, and is not so dependent on the rotation speed. For this reason, if you attempt to balance a machine with a coupling defect at one load or rotation speed, you may receive a significant increase in vibration at the rotation frequency with another load or rotation speed for the same machine.

Another reason for the appearance of the rotating radial forces is a defect of dynamic eccentricity of air gap in the AC electric machines which coincide in the direction with the inertial forces in only one of two machine parts (rotor or stator). In the other machine part, they are applied in the opposite direction. This is why, when they are compensated by mounting additional weights at the rotor, they cause an increase in the vibration of another machine part and additional large loads, applied to the bearings, appear. Also, these forces are not really dependent on the rotation speed and the change in the rotation speed of AC machine with dynamic eccentricity of the air gap will lead to the increase of its vibration if it was balanced at another speed.

The third practical case when radial rotating forces appear is bending of the shaft line that has more than two distant rotation supports. In this case, the forces applied to the supports do not depend on the rotation speed, but are fully dependent on the degree of bend and the rigidity of the shaft. The amplitudes of the oscillating displacement of the bearing housings also depend on the severity of bend and the rigidity of the housings. To decrease vibration in this case, it is necessary to decrease the shaft bend which is nearly impossible to achieve with centrifugal forces produced by the balancing weights as the shaft is straightened with the forces applied to the bearings on which the centrifugal forces are applied in their turn.

The last of the most frequent practical cases of the appearance of rotating radial forces applied to the supports at the rotation frequency is connected with the appearance of incipient self-sustained oscillation processes. These self-sustained oscillations are the full or partial revolution of the stationary friction surface of the bearing by the rotating friction surface of the rotor. Most frequently, such cases are observed in vertical machines with journal bearings. The appearance and increase of oscillations, as a rule, is nearly independent on the magnitude and angle position of the rotor unbalance. That's why it is impossible to predict whether it is possible to decrease these oscillations in the process of balancing.

A number of friction surface defects in a machine may cause oscillation forces at the rotation frequency, but in this case, the direction of the force vector does not change with rotation. These are the defects of bearing or mechanical transmission wear. So, vertical oscillation forces appear due to non-uniform wear of rotor friction surfaces in the machines with horizontal rotor and horizontal forces when there is wear of the stationary race in the bearings of vertical machines. Also, similar forces occur with gear or pulley wobbling or wear in gear or belt transmissions. These forces are applied in the direction between the shaft axes. None of the above forces can be fully compensated by the centrifugal forces produced by mounting balancing weights.

The most difficult case in rotor balancing occurs when pulsating torque affects the rotor with the rotation frequency. In practice, this case may be observed in multi-bearing rotors and AC electric machines. In multi-bearing rotors, pulsating torque occurs because of combined defects of misalignment of the bearings and bend in the shaft line. In this case, the friction forces depend on the rotation angle and, once per revolution, the shaft sticks in the bearings, thus producing significant pulsating torque. To decrease this torque, you should either correct misalignment or the shaft bend. It is evident that straightening bending of a shaft with high rigidity by mounting balancing weights is practically impossible.

In electric machines, pulsating torque at the rotation frequency can appear with a combined defect of both static and dynamic eccentricities of the air gap. The magnitude of the pulsating torque is proportional to the constant torque of the machine and the product of relative magnitude of each of the eccentricities. This torque can reach very high values, especially in the case of induction motors with small air gaps which produce high relative eccentricities normalized to the normal clearance value. As a rule, pulsating torque in electric machines can be corrected only by repair of the machine or replacement of its bearings.

The forces excited by the rotation of machine parts with hydrodynamic unbalance influence the machine in the same way as a mechanical unbalance. The main peculiarity of these forces is the unknown dependence of hydrodynamic unbalance at the rotation speed of impeller, pressure difference in the inlet and outlet of pump and other factors. This means that changes in operating mode of a well balanced pump may result in a significant increase of its vibration at the rotation frequency.

Many of the defects leading to the increase of vibration at the rotation frequency can be detected prior to balancing by the analysis of machine vibration measured at the bearing housings and at the machine body. Some of the defects are best detected during balancing by the analysis of machine vibration reactions produced by the attachment of the trial weights.

Defects that preferably should be detected before the start of balancing include shaft or coupling wobbling, wear of bearings, self-sustained oscillations of the rotor in the bearings, eccentricity of air gaps in electric machines, transmission defects, collisions or rubs between stationary and rotating parts of the machine. When balancing pumps, it is worth detecting the presence or absence of the hydrodynamic unbalance.

To detect such defects as shaft or coupling wobbling, defects of mechanical transmissions, bearing wear, self-sustained oscillations of the rotor in the bearings, and contact between stationary and rotating parts of the machine, it is necessary to measure an autospectrum of the low frequency vibration and and an envelope spectrum of high frequency vibration at each of the bearing housings. The presence of a series of rotation speed harmonics with similar amplitudes in the autospectrum as well as in the envelope spectrum is a symptom for shaft wobbling, bearing wear or transmission defects (see figure 1). Consider correcting these defects before balancing, especially if their severity is significant.

Figure 1. Envelope spectrum of shaft or coupling wobbling.
Frot
- the rotation frequency of the rotor.

The defect severity can be estimated by the amplitude of the bearing housing oscillations in two different directions orthogonal to each other and to the rotation axis. The defect severity should be considered very high if the amplitude of any of the harmonics with the exception of the first one exceeds the specified level for the vibration at the rotation speed.

Self-sustained oscillations of the shaft in journal bearings can be detected by the appearance of harmonic series with the frequencies k*Frot/2 or k*Frot/3 in the autospectrum or the envelope spectrum of the bearing vibration (see figure 2). Appearance of such spectrum components in envelope spectrum of even one bearing should tell you that there is a high probability for the self-sustained oscillations of the shaft in the other bearings also.

Figure 2. Envelope spectrum of machine with self-sustained oscillations.

In AC electric machines, it is much more important to detect possible dynamic eccentricity of the air gap. It can be detected by the index of slot frequency vibration component modulation by the rotation frequency harmonics. The sidebands Fz+ Frot of the slot frequency component Fz=z*Frot (where z equals the number of slots) and Fz - 2Fm + Frot (where Fm is the mains supply frequency) can be observed in the vibration spectrum in this case (see figure 3).

Figure 3. Autospectrum of AC machine with dynamic eccentricity of the air gap.
Here:
1 - is Frot; 2 - is Fz-2Fm; 3 - is Fz; 4 - is Fz-2Fm-Frot;
5 - is Fz-2Fm+Frot; 6 - is Fz-Frot; 7 - is Fz+Frot

Hydrodynamic unbalance of a pump impeller can also be detected prior to balancing from the envelope spectrum of the high frequency vibration of the connected piping at a significant (up to 3-10 meters) distance from the pump. The cavitation symptoms (k*Fb = k*n*Frot where n equals the number of blades) should be absent in the envelope spectrum. Only components at the rotation frequency of the impeller should be present.

Two more defect types can be detected prior to balancing, soft-foot (fastening looseness between bearings and machine case or machine and the base supports) and the appearance of machine body or base support resonance close to the rotation speed of the machine. To detect these defects, you should compare the levels of rotation frequency components in the spectra measured in two orthogonal directions that are orthogonal to the rotation axis as well. For horizontal machines, these are typically horizontal and vertical directions. If the vibration level at the rotation speed in one direction is 3-4 times greater than in another direction, it should be considered as a symptom for a defect. Note that for machines with belt, gear or other mechanical transmissions, this difference can be natural in the direction connecting the axes of driver and driven shafts in presence of transmission defects. It is defined by significant differences, not only of rigidity, but also of oscillation forces in different directions. This symptom can not be applied to the machines with considerably different rigidity in different directions as well. To distinguish these two defects you can analyze the amplitude and phase-frequency parameters of the machine during run-up or coast-down. The influence of resonance is limited to a narrow band of frequencies close to its central frequency, but a soft-foot produces the same symptoms in a very wide span of frequencies.

During preliminary diagnostics, once you have found any defects in the shaft line or bearings of the machine during diagnostics, you should make a full diagnosis of the machine to determine the volume of maintenance or repair work needed for the machine before balancing. This typically allows not only increasing the efficiency of balancing and reducing the time required, but also increasing the time intervals between periodic balancing as well.

A number of the machine defects that increase the vibration at the rotation speed of the machine can not be reliably detected during preliminary machine diagnostics. Pulsating torque is very hard to detect without using proper diagnostic methods. Similarly, self-sustained rotor oscillations due to severe bearing wear cannot always be detected prior to balancing. For this very reason, it is worthwhile to conduct diagnostics during the balancing process as well. This includes looking for possible self-sustained oscillations in all bearings.

5. Defects Detected During Balancing

Some of the defects discussed above can be missed during the diagnostics of a machine prior to balancing or can appear during balancing or during partial disassembly of the machine for trial and balancing weight mounting and these are the main defects that hamper balancing. First is shaft bending or wobbling. The symptom for this defect is no reaction (changes in vibration within the measurement error) on test actions, i.e. trial weights mounting and centrifugal forces, caused by them. This absence of reaction is usually observed at all measurement points and remains with increase of test action (increase of trial weights). These measurements can not distinguish between shaft bend and wobbling.

The second defect group consists of those which can detected by the calculation from the results of balancing weight changes. By comparison of balancing weights, calculated by measurements conducted in different directions including two radial and one tangential directions (tangential to the machine body and orthogonal to the rotation axis), it is possible to detect the presence of significant pulsating torque at the rotation frequency of the machine. In this case, the values of the balancing weights calculated by the two radial measurements are very close to each other for radial measurements, but are very different from the ones calculated by the results of tangential measurements. An exception is the case when the assembly resonant frequency is close to the rotating frequency. This implies that the forms of oscillations in different radial directions and, hence, the calculated weights significantly differ from each other. This is an additional symptom of having a resonance close to the machine rotating frequency.

Additionally, for the identification of flexible coupling defects, it is possible to use the calculated estimation of vibration after mounting the balancing weights. If the efficiency of balancing is extraordinary (the real efficiency exceeds twice or more times the calculated one) then there is a probability that there is such a defect that excites vibration at the rotating frequency proportional the transmitted torque. When the unbalance is decreased in machines with little load, for example a fan, the load and hence the transferred torque and vibration excited by it also decrease.

6. The Main Sequence of Balancing Procedures and Operator's Mistakes

The procedure for machine balancing under operational conditions begins with vibration measurements at measurement points located at the machine. Such measurements should be made periodically and their results compared with the specification values for the machine. When the measurement results exceed the specified values, a decision to conduct balancing is made. In this section the main mistakes that are usually made by operators during different balancing operations are described in detail.

It should be noted that the inertial forces excite vibration mainly at the rotating frequency of the machine's shaft(s). But each machine has other sources of low frequency vibration that excite vibration both at the rotating frequency and its multiples and at the frequencies lower than the rotating frequency. To make a valid decision about balancing, it is necessary to make a spectral analysis of the vibration signal by using an instrument, typically a narrow band signal analyzer with a vibration transducer, that enables the separation of the various frequencies and evaluation of the vibration at frequency. If vibration at the rotating frequency exceeds the vibration at all other frequencies, then it is necessary to balance the machine. One should remember that unbalance results mainly in radial vibration. The presence of the vibration in axial direction is an evidence of machine defects such as axial misalignment of an electrical machine rotor against its magnetic field. At this stage of balancing, it is necessary to make sure that the vibration is exited primarily by inertial forces. Vibration with the predominant frequency equal to the rotating frequency also can be excited by non-inertial forces, in particular, by a bent shaft. An effort to balance such machine can lead to an emergency situation because the machine can have some special sensitivity to large trial weights that can oscillate the whole machine and its foundation. An additional increase in the size of trial weights for the purpose of getting a perceptible change in vibration for more accurate determination of balancing weights parameters can lead weight attachment failure, catastrophic failure of the machine, and injury to the operator.

Preliminary Stage, Selection of the Balancing Conditions. For balancing, a particular rotating speed (for multi-speed machines - several speeds) 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. It should be noted that for some modes of operation, vibration forces not caused by inertial nature can appear. The effect of some of these forces can be decreased by balancing, but others can hamper balancing. In this case, it is recommended to make diagnostics first and then to search for the defects that cause such forces to appear.

Next, the actual balancing begins. (As was mentioned above, here we shall mainly consider the vibration measurement, analysis of the measurement results, and optimal calculation of balancing weights.) The diagram of machine balancing in field is shown at Fig. 4.

Figure 4. 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. 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. The instrument must have a vibration transducer that can be placed sequentially at each measurement point in the radial direction and a tachometer pick-up that gives a pulse each revolution.

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. As a rule, the measurement points locations are selected on the bearing cases or end shields. The transducer is attached by a stud (most reliable but labor consuming), by cement, or by a special magnet if possible. 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. Multi-channel instruments are available which enable the attachment of several transducers simultaneously which decreases the possibility of poor transducer attachment because there is no need to reattach the transducer during the measurement process. Nevertheless, the necessity to take off the transducer can occur because of trial and balancing weight placement.

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 (i.e. a machine with a gear box), 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.

Usually a photoelectric sensor with a contrast mark is used as a pulse reference tachometer transducer. It is attached to a rotating part of the machine. In the usual practice, the contrast mark is attached to a dirty rotating surface. In this case, the reflection coefficient of this surface can change significantly during one revolution of the shaft. A too sensitive pulse reference tachometer transducer will react to these changes as additional marks and such reaction can be a random one which means that the transducer will produce no definitive impulse with each revolution. A user can be tempted with a transducer with a long distance capability close to the marked surface. It should be noted that this can significantly increase the number of noise signals from the tachometer and it may be necessary to install the transducer far from the mark to eliminate the noise signals. In practice, such a transducer is often impractical to attach in the previous position after removal for weight installation.

Preparation of Places for the Attachment of Weights. The weights and places for their attachment should be prepared. It is quite common, especially for medium and large machines, that such places are made during machine manufacturing (circular slots, holes in the face or side surfaces of the rotor). If the machine does not have special places for weight attachment or the access to them is difficult, then the places of specified mounting (screws, studs) usually on the rotor face or on the couplings are normally used. If there are no specified or previously prepared locations (i.e. on fans), then the weights are attached on accessible rotating surfaces by threaded connections (for low speed rotating machines) or by welding. The trial weights should be prepared beforehand also. Their type depends on the method by which are attached to the machine and their weight.

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 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:

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 attachment (whether it is to be taken off afterwards or not). 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.

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:

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.

At this stage, all the previously mentioned mistakes in weight attachment and vibration parameters are common.

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, 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:

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. Figure 5 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.

Figure 5. 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.

7. Specification Requirements for Measurement Instruments and Application Software for Machine Balancing in Field.

Nowadays, measurement instruments and data processing techniques are being improved very quickly because of the rapid development of computers. Most modern measurement instruments include data processing programs. In this section we shall discuss both the requirements for instruments and application software and those that can be realized only when the process of measurements and balancing are controlled as a whole by an "intellectual" software.

As was already mentioned, balancing is a very complicated technological process. The cost of each machine run can be very high. Balancing is usually conducted by a specially trained team so the chief has management responsibilities and is subject to psychological stresses. It is normal that balancing is conducted with a shortage of time. Very often during balancing the faults in machine assembling, its support and foundations are detected. So, the main requirement for the measurement instruments and their analytical components is it's ability to require a minimum of activity and involvement by the operator who usually has much to do without calculation of the various parameters. To meet this requirement, it is necessary to satisfy the following conditions:

Requirements for the Measurement Instruments:

Additionally, it is desirable that the measurement instruments used for balancing do some auxiliary measurements that can simplify the balancing process. Such measurements, first of all, include the spectral analysis of vibration that is used to substantiate the necessity of balancing and to identify the machine faults. Detection of machine resonance conditions which can hamper the analysis of vibration parameters at the rotating frequency is very much simplified if there is a possibility to measure the machine amplitude-frequency and phase-frequency parameters of the machine during its run-up or coast-down or if other means are available.

Requirements for the Means for Data Analysis and Calculations:

Besides these main requirements, it is necessary that the calculation system include all the accessory units that enable quick and accurate auxiliary calculations of necessary values, for example, the values of trial weights, to calculate the splitting of weight among the possible places of attachment, estimate the value of residual unbalance, and so fourth.

Furthermore, it is desirable that the calculation system helps to make a report about the work done.

It should be noted that repeated practice is most important in field of field balancing. A person should be able to quickly and accurately determine most of the problems that might hamper balancing. Nevertheless, even the most experienced professionals in balancing make mistakes in measurements and weights attachments, and the cost of such mistakes can be very high. This should be taken into account when you choose the hardware and analyzing and calculation software.

8. Measurement Instruments and Application Software, VAST_BAL, an Example of a Systematic Solution of the Complex Problems that Occur During Machine Balancing in the Field.

The application software, VAST_BAL, designed by VibroAcoustical Systems & Technologies, Inc., Saint Petersburg, Russia, is an example of very sophisticated software. This software enables the solution of all the previously mentioned problems that arise in machine balancing in the field including balancing near the critical rotating frequencies. This application software is intended to be used in measurement systems consisting of a portable computer with analog to digital conversion boards, or to be installed in the DC-11 produced by VAST, Inc. These instruments differ from each other mainly by their packaging. Also, VAST_BAL is designed so that enables the use of any measurement instrument which can measure the appropriate vibration parameters. In this case, data is entered into the computer manually and the control of the certainty of measurement results is defined by the features of the measurement instrument.

Below, we shall discuss how VAST_BAL meet the requirements mentioned above.

A High Accuracy of Vibration Parameter Measurement is ensured, first, by algorithms of digital filtration and consequential processing of the signal from the vibration transducer, and second, by monitoring the parameters of the signal from the pulse reference tachometer. This monitoring enables detection and alerting the user about the following signal problems that significantly decrease the accuracy of measurements:

The Measurement Process Automation is closely connected to the requirement of high accuracy. Accuracy improvement is ensured by statistical processing of the results of sequential measurements. The result of the measurement is displayed as an averaged value of the vibration vector at the machine rotating frequency. Other statistical parameters of the measured value, that is the mean square deviation from the mean value, are also used in the automation process. This deviation helps to detect the following reasons that negatively influence the accuracy of measurements:

Considering the standard deviation enables the measurement process to be stopped automatically if the measurements are accurate or to warn the user that noise signals are present during measurements and that they should be taken into account or eliminated.

The Hardware Applicability for Field Conditions is ensured by a high degree of measurement process automation and mechanical reliability of the instrument. From this point of view, it is recommended that the system include a portable computer that is inexpensive compared with special computers intended to be used in industrial environments, but having high reliability and a wide range of necessary features.

Detailed Analysis of Measurement Information Entered Into the Computer. The most important difference of the products, designed by VAST, Inc., from other products on the market is that it makes detailed analysis of the information entered into the computer. On the basis of this analysis, it is possible to detect a number of machine faults and also mistakes of the operator. The list of defects diagnosed is constantly enlarged by the designers.

Mistakes of the Operator Detected During the Balancing Procedure include:

Appropriate messages are displayed when the mistakes are detected.

Machine Faults. The second large group of fault or problem conditions is the category of other machine defects. Before conducting the balancing, the machine must be diagnosed and all its defects should be corrected. But this is usually true only in theory. In practice, excessive low frequency vibration caused, for example, by shaft bending or bowing, or looseness in foundations, and so fourth is can be excited by unbalance. This is the reason why, in VAST_BAL, attention is paid to the identification of other machine defects. The presence of such defects can not only decrease the balancing efficiency, but even seriously hamper the balancing process. The balancing procedure itself can be considered as a type of diagnostics, viz. - the diagnostics of a mechanical system by using the test diagnostic method. The following defects are identified:

When the balancing software is a part of a general VAST, Inc. machine diagnostic application software, the the list of identifiable defects is much greater.

Balancing Planes that Equally Influence All the Vibration Parameter Measurement Points. The analysis of the measurement information enables the detection of the machine balancing planes that equally influence all the vibration parameter measurement points. These planes are interchangeable so the operator given a choice of one of them to use for weight attachment. This enables the reduction of the number of balancing planes and, the reduction of the volume of work for balancing. Also, there is a possibility that the weights, attached in these planes, may partly compensate each other which can lead to the unjustified increase of certain balancing weight.

Balancing Efficiency Estimation Using Known Influence Coefficients. During repeated balancing using influence coefficients (trim balancing), a question usually arises about the reasons of the low balancing efficiency. The reason for this can be the influence of measurement mistakes, significant changes in mechanical machine parameters due to changes in vibration amplitude, and the increased influence of non-inertial forces. The analysis of the measurement information enables the consideration of the applicability of available influence coefficients for further calculation of corrective weights plus the possibility of warning the operator about the possible low efficiency of the next step of trim balancing.

Balancing of Objects of Any Complexity. The application of a portable computer or a vibration information data collector with large memory as a measurement instrument enables balancing of objects of practically any complexity. Thus VAST_BAL is intended to be used for balancing machines that have up to eight modes of operation (modes that differ by revolution speeds, loads, temperature condition, and so fourth), up to 16 balancing planes, and up to 64 points of vibration parameters measurement with the option of removing or not removing the trail weights.

Optimization of the Number of Trial Weights. The algorithm for balancing weight calculation that is used in VAST_BAL enables the estimation of the possible efficiency of balancing with sufficient accuracy for each trail weight (the accuracy is about +20% if there are no mistakes or low vibration sensitivity to the trial weights). This enables the interruption of the process of trial weights attachment and vibration parameters measurements when the necessary balancing result is achieved.

A Large Number of Additional Programs. The use of an instrument with advanced calculation possibilities enables the use a large number of additional programs to calculate the values of trial weights, to calculate a weight that influences equivalently to several attached weights, to split the balancing weight to accommodate available attachment places including the case when the value of the weight for each place of attachment is limited, to calculate the value of residual unbalance that is indicated in the governmental regulations (in these documents the values of vibration parameters are not specified). In this case, the design, editing and printing of necessary documentation can be organized very simply.

The functional features of instruments designed by VAST, Inc. ensure the ability of receiving and analyzing both the narrow band vibration spectra and the amplitude-frequency and phase-frequency parameters of an object.

Advanced features for measurements and vibration signal processing, a high degree of measurement process automation, detailed analysis of measurement results, and a large number of additional functions enable the use the of hardware and application software designed in VAST, Inc., by both an expert in balancing to speed the balancing process and decrease its cost, and also by an operator with limited experience. In the latter case, the software prompts the use of the optimal balancing algorithm and warns the operator against most mistakes during use.

References.

  1. A.V.Barkov, M.A. Barkova, A.G. Shablinsky "Rotor balancing for multi-condition machines", Proceedings of the transport and noise conference, ed. S. Kovinskaya, Saint Petersburg, Russia, October 4-6, 1994, pp 53-56.
  2. A.V.Barkov, M.A. Barkova, A.G. Shablinsky "Rotor balancing for multi-condition machines", Shock and Vibration Digest, Volume 26, No.6. November-December 1994, pp 43-47.