DESIGN AND DEVELOPMENT OF DYNAMIC BALANCING MACHINESubmitted by: Mr. Banamali Nepak (214ME1545). Under guidance of Prof S. C. Mohanty

1. INTRODUCTION-A balancing machine is a measuring tool used for balancing rotating machine parts such as rotors for electric motors, fans, turbines, disc brakes, disc drives, propellers and pumps. The machine usually consists of two rigid pedestals, with suspension and bearings on top supporting a mounting platform. The unit under test is bolted to the platform and is rotated either with a belt-, air-, or end-drive. As the part is rotated, the vibration in the suspension is detected with sensors and that information is used to determine the amount of unbalance in the part. Along with phase information, the machine can determine how much and where to add weights to balance the part. Balancing is a process of attempting to improve the mass distribution of a body so that it rotates in its bearings.The balancing of rotors is necessary:-1. To minimize vibrations, noise etc. which in turn improves the machine life and reduces operator fatigue.2. To minimize the dynamic stressing and fatigue, which leads to reduce sizes of machine elements & foundations necessary.3. To reduce power losses, as vibrating systems absorb energy due to the damping factor present in the system.4. To avoid inaccuracy & improper functioning of adjacent machinery due to transmission of vibrations. The balancing machines are meant for evaluating the unbalance in a rotor. In a typical dynamic balancing machine, the rotor is supported by two sets of bearings and run by a suitable drive through a flexible coupling element. The unbalanced centrifugal forces on the bearings are measured by sensors and are related to the unbalance in the rotor by a measuring instrument. This unbalance can be corrected on the machine itself by suitable correction units or outside the balancing set up.2. HOW IT WORKSWith 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 phaser), 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, a known weight at a known angle are added, and getting the unbalance vector caused by it.

3. CLASSIFICATION OF BALANCING MACHINES

1. 2. 3. 3.1. Soft Bearing Machines -These machines are based on displacement measurement principle. The rotor is mounted on the oscillating bodies, which are flexibly suspended. This soft suspension system permits the rotor to vibrate while rotating oscillating displacement due to the vibrations is sensed by electro dynamic pickups and these signals are fed to measuring panel.The amplitude of vibrations depends upon rotor mass, speed, bearing, suspension mass and distance between bearing. Hence the relationship between pick up displacement and unbalance present also changes with these parameters. Therefore, if the machine is to operate on direct reading basis, calibration of machine for each type of rotor, prior to balancing, is necessary.In these machines 'Cross Effect - the effect of unbalance existing in one plane on another - is required to be eliminated. This is also called as plane separation.

3.2. Hard Bearing Machines -These machines are based on 'force measuring' principle. When the rotors rotate on rigidly supported bearings, centrifugal forces are generated because of the unbalance in the rotor. These forces are sensed by pressure sensitive transducers mounted on the balancing stands. These signals are fed to the measuring panel for resolution of the amount and location of unbalance. For a given speed, the forces generated are proportional to the unbalance, independent of rotor weight. Hence, permanent precalibration of measuring panel is facilitated.

3.3. Horizontal Balancing Machines -These types of machines are mainly used for two plane (Dynamic) balancing. Single plane (Static) balancing is also possible. The rotor runs with its axis of spin in a horizontal plane. The rotors with bores on their axis of spin are balanced with the help of suitable mandrel with inboard or outboard fashion.The machine consists of -1. Machine Bed.2. Balancing Stands3. Drive Stands4. Measuring InstrumentsThe machine bed is made out of cast iron. It has 'T' slots for clamping and two flat guide ways for smooth movement of drive stand and balancing stands on it. There is a traverse chain fixed in a slot in the bed for the moving gear on the balancing stands. These beds need proper foundation. The balancing stands made out of cast iron can be moved along bed ways to accommodate the rotor length. The stand houses an isotropic suspension and the factory set piezo electric sensor with its preamplifier. The twin roller carriage, which supports rotor, is adjusted vertically to accommodate various journal dia, of rotors and clamped on the isotropic suspension. The piezoelectric sensor senses the unbalanced centrifugal force and gives an equivalent electrical output signal. The preamplifier conditions this electrical signal for further processing in the measuring instrument.The normal bearings supplied with the machines are twin roller type, the other types of bearings available on request .The standard rotor guards are provided along with machine. The heavy rotor guards are recommended in case of outboard rotors or with unbalance exceeding the weight or rotor.

4. DIFFERENT TYPES OF ROTOR SUPPORTING ARRANGEMENT

Fig. 4.aFig. 4.bFig. 4.cFig. 4.d

Fig. 4.a - Roller Support Most Commonly UsedFig. 4.b - V Supports for small rotorsFig. 4.c - Saddle Bearing Support for balancing rotors in their bearingFig.4.d - Journal Bearing Support with pressure lubrication

Drive stand construction depends upon types of drives.Common drives are - 1) End Drive 2) Belt Drive1. End Drive (EMB, FBM) These drives can be achieved by -- Universal coupling- Cardon ball joint- Band

Universal Coupling Drive - This is suitable for heavy rotors or rotors having a large moment in inertia or high windage. This drive permits the transmission of sufficiently high torque for fast acceleration as well as safe braking. To connect the coupling to the rotors, adapters are required. Band Drives -

These are recommended where starting torque is relatively low and balancing tolerances are not stringent. The rotor has a tendency to move axially while rotating. The advantage of this drive is that the preparation of different adapters can be avoided within a certain diameter range. This drive is suitable for rotors up to 100 kg. weight.

Cardon Ball Joints -

These are recommended for driving light rotors, usually below 10 kg. For smaller rotors, the ratio of Universal coupling weight to the rotor weight becomes very high causing higher inaccuracies, Cardon ball joints, being light, reduces these inaccuracies.

Belt Drive (SBM, FSBM)Different types of Belt Drive Arrangements

The rotor is driven by a belt running directly over or tangentially to the rotor by suitable means. This is a coupling free drive and eliminates the effect of residual unbalance present in the end drive system. These drives render better balancing accuracy so that it is possible to achieve 0.2 to 0.5 gm mm/kg residual specific unbalance. The balancing speed depends upon the rotor diameter where drive is given and the motor pulley diameter.The drive is built on a steel plate called belt bracket and can be rigidly clamped in between the balancing stands or outside by its broad base. The belt arrangement depends upon the weight range of the machine. There are adjustments provided for accommodating the various rotor dia and belt tension adjustments. The braking arrangement provided is either DC Braking of the drive motor or reverse braking.

5. METHODS OF UNBALANCE ANGLE MEASUREMENT:There are 3 methods for getting the angular position of unbalancePhase Reference GeneratorOn end driven machines, a phase reference generator is directly coupled to the machine spindle. The generator generates an electrical reference signal at the same frequency as the speed of the rotating part. The phase difference between the reference signal and the unbalance signal generated by the transducers is interpreted by the measuring instrument, which indicates the angular location of the unbalance.

Stroboscopic MethodThis is used on belt driven machines. Electronic measuring circuit provides a pulse, once in a revolution, to fire a stroboscopic lamp. The flash is directed on a number tape fixed on the periphery of rotor.

The number corresponding to angular location of unbalance appears to be stand still. Although storage of angular location is not possible, this method is preferred as direct indication on rotor is obtained.Photo scanning MethodThis is again used on belt driven machines. A light source from scanning head is focused on to a scanning reference mark on the periphery of rotor. A photo cell senses the interruption at every revolution and generates a reference signal at the same frequency as the rotor speed.

The phase difference between the reference signal and the unbalance signal generated by the transducer is interpreted by the measuring instrument, which indicates the unbalance angular position. This angle is then transferred on the rotor by means of protractor with reference to scanning reference mark on the rotor.Types of Readout SystemsPolar Indication - This indication is suitable for general purpose machines, where corrections on the rotor are to be done on any angular position on the circumferences of the selected planes of rotor. Typical rotors for which polar indication is adequate electric motor armatures, impellers, flywheels etc.Component Indication - This system is suitable where rotors have predetermined locations for correction of unbalance or if only certain sectors on the circumference is available. The unbalance, therefore, must be resolved into components in suitable coordinate system, typically within 30/45/60/90/1200. For this purpose, separate component meters are used with component indication measuring instruments. Typical rotors - Fan blades, textile flyers.Multilane Readout - This is suitable where unbalance resolution is required at more than two correction planes on the rotor with predetermined locations for corrections of unbalance. Typical rotors - 4 throw crank shafts with counter weights at 4 planes, 6 throw crankshafts with counter weights at 6 planes.Although it has been attempted to standardize the measuring instruments depending upon the applications, there still remains a wide variety to cope up with the complexity of typical application. Hence the classification given below is only indicative.

EBM -This type of instrument has only one analogue unbalance amount indicating meter. This meter indicates the amount of unbalance in a selected plane on adjusting the reference generator (by rotating thumb nut) to give maximum reading on meter.The unbalance angle is indicated by the unbalance angle marking on reference generator, which coincides with the fixed reference mark on the machine body. The calibration of this type of instrument for a given rotor needs balancing by trial and error initially, followed by trial runs for plane separation and unbalance amount calibration is done with known weights attached in respective corrections planes. There is no storage facility for the unbalance amount on standard instrument. The unbalance angle is transferred with the help of unbalance angle reference disc, on the machine spindle to the rotor.FBM -The unbalance amount and angle is indicated by a set of two analogue meters for each plane. The machine is calibrated to indicate unbalance amount in grams by feeding the geometric dimensions and selecting appropriate program with the help of 5 dials and a programme switch.No trial runs are required. There is an indication storage facility to retain the reading upto next run. The angle of unbalance is transferred with the help of the unbalance angle reference disc on the machine spindle to the rotor.

6. DIFFERENT TYPES OF ROTOR CONFIGURATION

SBM -This instrument has one unbalance amount indicating analogue meter. The instrument is to be tuned initially to the running speed of the rotor by adjusting a dial. Then all the calibration procedure as per EBM instrument is to be carried out. The angle of unbalance is indicated by the position where the stroboutube flashes on the number strip attached to the rotor for the correction plane, selected. There is no storage facility available on standard instrument.FSBM -This instrument is exactly the same as that of 'FBM'. The unbalance angles are transferred to the corresponding correction planes with reference to the unbalance angle reference marker on the rotor on which sensor scans. These instruments are provided with digital RPM indications for indicating machine operating speed. Microprocessor based Measuring instruments also can be supplied against specific requests.Safety Measures provided -1. Overload protection for drive motor.2. Fuses for measuring instrument.3. Limit switches for universal coupling for models over 100kg capacity.4. Clamp down levers over rotor journals for all models except models upto SBM/FSBM-10.5. Slipping clutch arrangement for smooth torque transmission for models above 1000 kgs. capacity and for vertical machines above 10 kg. capacity.Machine Selection -The range of applications for Dynamic Balancing Machines is extremely wide and it encompasses rotors weighing a few grams (mixer blades) to a few tons (pelton wheels of hydraulic turbines). The following factors primarily influence the choice of balancing machine.1. Type of rotor, its configuration.2. Overall dimensions such as O.D., Length etc. for extreme (Max. and Min. size) rotors to be balanced on the machine.3. Weight of the rotor ( Max. and Min.)4. Service speed.5. Balancing accuracy required.6. Moment of inertia of rotor GD27. Method of unbalance correction.8. Max. initial unbalance.9. Production rate required on balancing machine.In order to arrive at optimum machine selection, maximum data should be furnished. The dimensioned drawings of respective rotors should be sent. Information for both smallest and the largest rotors is essential.

7. REFERENCES:1. http://en.wikipedia.org/wiki/Balancing_machine2. http://www.finemarketingindia.com/metallography_testing.htm3. http://www.productivetechnologies.co.in/machine-configuration-applications.htm4. http://www.balance.net.cn/phe.htm

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