Balancing of rotors Basics principles & balancing process

The theory Why Balance? Rotating components experience significant quality and performance improvements when balanced. Balancing is the process of aligning a principal inertia axis with the geometric axis of rotation through the addition or removal of material. By doing so, the centrifugal forces are reduced, minimizing vibration, noise and associated wear. For a better understanding of balancing, it is necessary to have an understanding of its terminology and its fundamental concepts. For terminology see ISO 1925, Mechanical Vibration. Balancing - Vocabulary

Pulley & gear shaft assemblies Starter armatures Airspace components High speed machine tool spindles flywheels Impellers Centrifuge rotors Electric motor rotors Fan and blowers Compressor rotors Turbochargers Precision shafts crank shafts Grinding wheels Steam & GasTurbine rotors Rotating components for balancing

TYPES OF UNBALANCE The process of reducing the out-of-balance forces that cause vibration in rotating machinery is called "Balancing An unbalanced rotor Unbalance exists in a rotor when the mass centre axis is different to its running centre axis. There are three types of unbalance: a) Static unbalance – is where the mass axis is displaced only parallel to the shaft axis. The unbalance is corrected only in one axial plane. b) 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 c) 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

VIBRATION & BALANCING STANDARTS

According to DIN ISO :2004, balancing is a procedure by which the mass distribution of a rotor is checked and, if necessary, corrected to ensure that the residual imbalance or the rotational vibrations of the bearing journals and/or the bearing forces at operating speeds are restricted to within specific limits In-situ balancing is the process of balancing a rotor in its own bearings and support structure, rather than in a balancing machine

Causes of unbalance Unbalance is a common fault in rotating machinery. There are many causes of unbalance, such as wear, dirt, eccentric axle, corrosion and deformation from material tensions. The unbalance results in large forces causing a lower length of life, increased power consumption, higher maintenance costs and sometimes lower product quality. Our motto is: If it rotates, it can be balanced. We have a lot of experience from balancing, everything from fans to large turbines. We also perform certified balancing training.

They are a number of reasons why a machine may not be balanced: Corrosions or erosion of rotors Uneven mass distribution in electrical windings Deposit of dirt on rotors Manufacturing defects Material imperfections due to cavities, bubbles, foreign materials, etc Eccentric rotors Cracked fans/damaged fans Incorrect key hole Machining errors

Why Balancing May Not Work? Other Causes of Vibration Instead of Unbalance 1. Reciprocating forces 2. Resonance coincident with running speed 3. Looseness 4. Distortion (i.e., bowed or bent shaft) Misalignment Compounding Effects Axial Offset Instability Nonlinear Response

Unbalance distribution In reality, unbalance is made up of an infinite number of unbalance vectors, distributed along the shaft axis of the rotor. If a lumped-mass model is used to represent the rotor, unbalance may be represented by a finite number of unbalance vectors of different magnitude and angular direction as illustrated in Figure: If all unbalance vectors were corrected in their respective planes, then the rotor would be perfectly balanced. In practice, it is not possible to measure these individual unbalances and it is not necessary. A more condensed description is needed, leading to practical balancing procedures. Unbalance distribution in a rotor modelled as 10 elements perpendicular to the z-axis

Balancing considerations In the past, International Standards classified all rotors to be either rigid or flexible, and balancing proceduresfor these two main classes of rotors are given in ISO and ISO However, the simple rigid/flexible classification is a gross simplification, which can lead to a misinterpretation and suggests that the balance classification of the rotor is only dependent on its physical construction. Unbalance is an intrinsic property of the rotor, but the behaviour of the rotor and its response to unbalance in its normal operating environment are affected by the dynamics of the bearings and support structure, and by its operating speed.

Rotors with rigid behaviour An ideal rotor when rotating, with rigid behaviour on elastic supports, will undergo displacements that are combinations of the two dynamic rigid-body modes, as seen in Figure for a simple symmetric rotor with unbalance. There is no flexure of the rotor and all displacements of the rotor arise from movements of the bearings and their support structure Rigid-body modes of a symmetric rotor on a symmetric elastic support structure In reality, no rotor will be totally rigid and will have small flexural deflections in relation to the gross rigid-body motion of the rotor. However, the rotor may be regarded as rigid provided these deflections caused by a given unbalance distribution are below the required tolerances at any speed up to the maximum service speed. The majority of such rotors, and indeed many manufactured rotors, can be balanced as rigid rotors, in accordance with the requirements of ISO This aims at balancing the resultant unbalance with at least a singleplane balance correction, or the dynamic unbalance with a two- plane balance correction.

Rotors with flexible behaviour If the speed is increased or the tolerance reduced for the same rotor described as rigid-body, it may become necessary to take flexible behaviour into account. Here the deflection of the rotor is significant, and rigid-body balancing procedures are not sufficient to achieve a desired balance condition. Figure shows typical flexural mode shapes for a symmetric rotor. For these rotors that exhibit flexural behaviour, the balancing procedures in ISO should be adopted. Schematic representation of the first three flexural modes of a rotor with flexible behaviour on an elastic support structure

Balancing procedures in ISO 11342

Balance quality grades for various groups of representative rigid rotors (From ISO 1940/1)

Maximum permissible residual unbalance, e per (Imperial values adapted from ISO 1940/1)

Guidelines for balancing procedures

BALTECH Balance The software for the multiple-plane balancing with opportunity of selecting the number of balancing planes and measurement points

in-situ balancing (balancing in own bearings)

General For in-situ balancing, correction masses are added to the rotor at a limited number of conveniently engineered and accessible locations along the rotor. By doing this the magnitude of shaft and/or pedestal vibrations and/or unbalance is reduced to within acceptable values, so that the machine can operate safely throughout its whole operating envelope. Reasons for in-situ balancing Although individual rotors may be correctly balanced, as appropriate, in a high- or low-speed balancing machine, in-situ balancing might be required when the rotors are coupled into the complete rotor train. This could be due to a range of differences between the real machine and the isolated environment in the balancing machine, including: a difference in dynamic characteristics of the rotor supports between the balancing facility and the installed machine, assembly errors that occur during the installation of the machine in situ, rotor systems that cannot be balanced prior to assembly, a changing unbalance state of the rotor under full functional operating conditions.

Balancing might also be required to compensate for in-service changes to the rotor, including: wear, loss of components, such as rotor blade erosion shields, repair work, where components could be changed or replaced, and movement of components on the rotor train causing unbalance, such as couplings, gas turbine discs and generator end rings. Additionally in-situ balancing might be necessary due to a range of economic and technical reasons, including: the investment in a balancing machine cannot be justified, when a suitable balancing machine is not available in the correct location or at the required time, and when it is not economic to dismantle the machine and transport the rotor(s) to a suitable balancing facility

Objectives for in-situ balancing The reason for balancing is to reduce the vibration magnitude(s) to acceptable values for long-term operation. For most machines, the overall vibration magnitude(s) limits shall either be based on normal practice or the appropriate part of ISO and ISO 7919 for pedestals and shafts, respectively

Safeguards WARNING In-situ balancing shall only be undertaken by a skilled team, including both customer and supplier, who understand the consequences of adding trial and correction masses and have experience of operating the machine. Failure to do this can place the whole machine and staff at risk. Machines may be quickly run up and run down many times and can have unusual loading conditions during the in-situ balancing exercise, which could be outside the normal operating envelope of a machine. It shall be established that such operations will not be detrimental to the integrity or the life of the whole machine. Integrity and design of the correction masses and their attachments When trial and correction masses are added, it shall be confirmed that they are securely attached and their mountings are capable of carrying the required loads. The correction masses shall not interfere with normal operation, such as coming into contact with stationary components due to shaft expansion. The correction masses should be fitted in accordance with the manufacturers instructions, if available

Levels of reporting for in-situ balancing reports

Precautions and safeguards for specific machine types during in-situ balancing

Report introduction Background Any relevant machine history shall be highlighted, with particular consideration being given to the recent operating regime and maintenance work. Objective Reports for all classes of machine shall clearly outline the objective for the in-situ balancing exercise. Normally the reason for balancing will be to reduce the vibration to acceptable magnitudes but, in special circumstances, it might be necessary to reduce the unbalance to permissible limits. Machine details In some cases, a schematic diagram of the whole machine being balanced should be provided, indicating all the rotors and the location of the thrust and support bearings. All vibration transducer locations and directions shall be clearly shown, plus the position and orientation of the phase reference mark. The direction of shaft rotation shall also be included with respect to the viewing direction along the shaft. Vibration measurement equipment Details of all equipment used for the vibration measurements shall be recorded. Transducers used shall be clearly documented, showing their type, serial numbers, sensitivities, calibration dates, locations and orientations. Measurement units All data provided shall be presented with its measurement units, for example: vibration peak-to-peak displacement μm vibration r.m.s. velocity mm/s correction mass g or kg correction mass radius mm or m

Basic principles of balancing The task: 1. The rotor has dynamically unbalanced masses 2. The arrangement of this masses and their size are unknown, available only the measurement of a vector of vibration (size and a phase) from a total imbalance of all unbalanced masses 3. There is an opportunity to establish on a rotor known masses in any angular situation on a rotor circle As a result of balancing it is necessary: to determine the size and the angular provision of weight which minimizes a total dynamic imbalance of a rotor. The objectives are achieved if are minimized vibration levels at a rotation frequency in controlled points.

Steps of vector construction: a) to construct on a circle a vector of A (Ya) - a vibration vector in an initial condition of a rotor, i.e. with unknown to us unbalanced masses; b) to establish the trial weight Mpr in any angular situation., to measure and construct the vector of vibration of B (Yb) which turned out thus on a circle - it already characterizes total influence on vibration of the unknown to us an initial imbalance plus of the brought not balance known to us from Mpr.; c) by the end of a vector of "B" we will construct a vector of C" of the end of a vector of "A". Follows from vector construction that the vector of C" is a difference between vectors of "B" and "A": C = B - A. Thus, the vector of C" characterizes vibration which arises from installation of the trial weight Mpr. As a result of vector construction there is known an influence of trial weight Mpr. on vibration of a rotor is an influence it is characterized by a vector of C"

Trial mass Р – mass of rotor, kg; А – velocity, mm/c; R – radius of correction plane where the mass established, sm; N – speed rotation, RPM.

In practice installation of settlement weight is usually made not due to production of necessary freight, and due to installation of system of the identical correcting freights, symmetrically or asymmetrically located rather settlement direction of the counterbalancing freight Mass angle correction

New! TWO-CHANNEL PORTABLE SYSTEM for in-situ balancing and vibration control BALTECH VP-3470

The active balancing system TYPES Mechanical, Oil (hydraulic) The electromagnetic

The active balancing system (Mechanical)

The active balancing system (Oil (hydraulic)

The active balancing system (The electromagnetic)

The active balancing system (The electromagnetic)

The active balancing system (The electromagnetic)

Example of phase in-situ balancing of fan Balancing of the fan of system of emergency cooling of the reactor of the first power unit of the Leningrad nuclear power-station

First resulat of measuring Velocity level in the range of frequencies of Hz Vertical V = 8.2 mm/sec. (excess max. admissible) Horizontal V = 7.3 mm/sec. (excess max. admissible) Axial V = 3.7 mm/sec. (norm) Vibration level at a frequency of rotation makes 90% of the general level of vibration. The decision is to balance in one plane on two points of measurements

Preparing works Removal of a box of an air duct Installation of a tag on the fan Installation of an optical tachometer

Trial mass As trial mass is used the piece of a copper wire weighing 2.8 grams

First resulat of measuring

Mounting of the trial mass Trial mass (wire) is established on any blade of the fan and is fixed by means of the passatizhy. Angular counting begins from the middle of the blade against the direction of rotation

The calculation of correction mass Calculation showed that it is necessary to establish mass weighing 7.95 gr. at angle 354 degrees from an installation site of trial mass (against the direction of rotation).

Mounting of the correction mass

Result Result start-up showed the following results: Vv = 1.7 mm / sec Vh = 1.7 mm / sec Va = 1.1 mm / sec Vibration levels in all points after balancing are much lower maximum permissible.

The end of work Fixing screws of masses Painting of masses Cleaning of a workplace and installation of an air duct

Approximate relative expenses of time for carrying out the main operations when balancing 1 - preparations of installation sites of sensors of vibration, installation of the sensor of speed; 2 - car start-up/stop; 3 - installation of masses; 4 - measurement of parameters of vibration; 5 - analysis of results of measurements and calculation of balancing masses; 6 - additional start-up.

BALANCING WITHOUT PHASE

For many years in training center Baltech at the course "Dynamic Balancing in-situ" (course TOR-102) listeners often asked a question of opportunity or impossibility of carrying out dynamic balancing of rotors with elementary vibrometres. Balancing in-situ without phase measurements

Before work it is necessary to be convinced that on the unit really there is an imbalance, or in other words, vibration levels at a frequency of rotation of a rotor are prevailing in size in a working strip of frequencies of the used vibrometer. It is rather simple to check it if the vibrometer measures at least two parameters from: A- Acceleration V- Velocity S- Displacement v = * a / f a – Acceleration, mm/sec 2 s = * v / f v – Velocity, mm/sec s – Displacement, mkm 10-15% to result f – frequency of rotation, Gz

«Rounding of trial mass» Devide plane for several points In each point on a certain radius of R establish trial mass (M). Start-up machine to working speed and measure vibration amplitude (for example, S- Displacement) First measuring (S) without trial mass, mark it as S 0 Making the plot of measuring points, If ok it is like sinus: Looking for minimal (S) – this is the point for correction Mass Calculate the correction mass: M(correction) = M(trial) *So/(So-Smin)

R1 radius equal (proportional) to vibromovement without trial freight, we describe a circle with the center in O point. From points of A and B circles located under a perpendicul arangle, radius of R2 and R3 proportional to displacement at installation of serially trial mass, we do arches. We draw a straight line through the center of a circle of O and a point of intersection of arches of C before crossing with a circle. Point of D - an installation correction mass. Correcrion mass: М (correctiont) = М (trial) *R1/ОС. «Method of Amplitude»

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