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6 TURBINE SECTION
6.6 DYNAMIC BALANCING PRINCIPLES .1 INTRODUCTION
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6.6 DYNAMIC BALANCING PRINCIPLES 6.6.1 INTRODUCTION
We must all be familiar with the effects of unbalance in one form or another, but perhaps the most common effect is that arising from wheel unbalance in motor cars.
At resonance conditions it causes wobble or bounce, the effects of which are transmitted to the driver through the steering column. This effect may be so violent as to make the car unsafe or at least uncomfortable to ride in, and the continual vibratory movements set up, even outside the resonance range will increase the rate of wear on the various linkages and add to driver and passenger fatigue.
In order to increase passenger comfort, reduce wear and noise levels and also to increase the life of the engine between overhauls, design effort is put into the various aspects of minimising vibration in aero-engines. Design features are also included to permit correction of unbalance forces.
Efforts are made to design engine bearing housings and carcasses with suitable stiffness to avoid resonance in the engine running range. In addition, precise balancing instructions are issued to control the rotating forces on the bearings which could:-
a) be transmitted to other parts of the engine or airframe structure.
b) lead to engine failure in extreme cases.
The loads on the bearings are of three main forms. These are:
a) thrust loads due to the engine doing work.
b) journal loads due to the dead weight of engine parts.
c) unbalance loads.
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6.6.2 CENTRIFUGAL FORCE
Centrifugal force acts on every particle which makes up the mass of the rotating element impelling each particle outwards and away from the axis, about which it is rotating, in a radial direction.
If the mass of the rotating element is EVENLY DISTRIBUTED about the axis of rotation, the part is BALANCED and rotates WITHOUT VIBRATION. However, if there is a greater mass on one side of the rotor than the other, the centrifugal force acting on this heavy side exceeds the centrifugal force on the light side and pulls the entire assembly in the direction of the heavy side.
Centrifugal Forces.
Figure 6.16..
Eccentric Mass.
Figure 6.17.
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The rotor has a heavy mass M on one side. The centrifugal force exerted by M causes the entire rotor to be pulled in the direction of force F.
6.6.3 CAUSES OF UNBALANCE
Unbalance may be caused by a variety of factors occurring singly or in combination with others. These factors include:-
a) Eccentricity
Eccentricity exists when the geometric centreline of a part or assembly does not coincide with its axis of rotation. This may be as a result of locating features (eg.
spigot location, bolt holes, splines, serration’s, couplings), being eccentric to the bearing location.
b) Variation in Wall Thickness
Variation in wall thickness may be as a result of eccentricity between an inner and outer diameter of a cylindrical type feature, or it may be as a result of a difference in thickness between a radial section of a disk type feature and the section diametrically opposite.
Eccentricity.
Figure 6.18.
Variation in Wall Thickness.
Figure 6.19.
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c) Blade Distribution
Unbalance can be caused by an unequal or unsymmetrical arrangement of a set of blades, either by reference to their mass moments or their dead weights depending on the size of the blades. This can be as a result of faulty weighting, inaccurate or illegible recording or assembly errors.
d) Unsymmetrical Features
These may be due to manufacturing processes, such as blow holes in castings or design features such as offset holes, locating dogs, slots, keyways, etc.
e) Distortion
This can be caused by stress relieving, eg. after welding, or by unequal thermal growth during running.
f) Fits and Clearances
Clearance between mating parts allows relative movement of the parts and a consequent shift of the axis of rotation during running (or even during balancing).
Joints incompletely assembled, eg. chamfers fouling radii, abutment faces not pulled together, may cause a ‘bent’ rotor or an unsuitable joint, which may cause a shift during running. It is important to prevent separate locating, or fixing, features from influencing each other eg. bolt holes, spigot locations, serration’s, etc. must be geometrically controlled to prevent ‘fighting’ between more than one feature. See also the section on tooling, adapters, drives, dummy rotors, etc.
Unsymmetrical Features Figure 6.20.
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g) Swash
Swash is caused by out of squareness of abutment faces relative to the bearing diameter, abutment faces not being parallel across the component, eg. spacers, adjusting washers, disks, etc. It is important that the bolted joints are tightened in sequence and in increments according to the torquing instructions.
h) Miscellaneous
Foreign bodies inside assemblies, oil accumulation, carbon deposits, usually found when check balancing after running.
6.6.4 OBJECTIVE OF BALANCING
The objective of balancing is to determine how the unbalanced mass of the rotor must be compensated for in order to keep the bearings free of centrifugal force loading.
6.6.5 DEFINITION OF UNBALANCE
Unbalance can be defined as that condition which exists in a rotor when vibratory force or motion is imparted to its bearings as a result of centrifugal forces.
Unbalance will, in general, be distributed throughout the rotor but can be reduced to:- a) static unbalance
b) couple unbalance c) dynamic unbalance
Swash.
Figure 6.21.
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Static Unbalance
In a gas turbine engine, static unbalance is primarily associated with thin discs such as turbine wheels or single compressor discs. It can be corrected by adding mass to the light side of the rotor. This can be achieved by a single weight DIAMETRICALLY OPPOSITE to the out of balance or by adding a number of smaller distributed weights having the same effect as a single weight. (This distribution can be determined by vectors).
Unbalance in a Long Rotor
If a rotor is checked for static balance using knife edges it is possible to correct an out of balance condition to one end of the rotor by a correction weight at the other end of the rotor. Although in static balance, the rotor may now suffer from other kinds of unbalance. These are couple and dynamic unbalance.
Static Balance.
Figure 6.22.
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Couple Unbalance
This arises when two EQUAL unbalance masses are positioned at opposite ends of a rotor and spaced at 180 from each other. If placed on knife-edges, the rotor would be statically balanced. However, when the rotor is rotated, the out of balance masses will cause a centrifugal force to act at each end and hence each end will vibrate independently as shown in figure 6.23.
Dynamic Unbalance
This occurs when the unbalanced masses may be either unequal in size or positioned at some angle other than 180 to each other, or even both of these conditions. These unbalanced forces now cause the rotor to vibrate.
Couple Unbalance.
Figure 6.23.
JAR 66 CATEGORY B1 requirements of an on-condition maintenance policy is that defects can be detected sufficiently early to permit rectification before secondary damage occurs. The analysis of engine vibration signatures is becoming an increasingly important tool for detecting early failure in mechanical components.
A vibration monitoring system begins with a sensor, which may be a velocity transducer or a peizo electric accelerometer. They both convert the mechanical vibration of the machine into an electrical signal proportional to the vibrations produced and together with the associated electrical circuitry feed signals to either cockpit mounted gauges warning systems or a separate vibration analyser.
Velocity Transducer
This device operates on the principle of a permanent magnet to move within a coil, inducing voltage. Because of the moving parts with all the inherent disadvantages of wear, friction, etc. they have been superseded by the peizo electric principle.
Peizo Electric Accelerometer
In this device, vibrating forces are transmitted to a peizo electric disc the resultant deformation of the disc produces an electrical charge. Accelerometers have a greater frequency range than velocity transducers and their lack of moving parts makes them a much more stable and reliable means of collecting the basic vibration signal.
Many different specifications for accelerometers and transducers are available and some of the considerations which govern their choice are:-
(1) DYNAMIC RANGE. The amplitude range over which the device is required to perform.
(2) SENSITIVITY. The severity of the vibration liable to be encountered.
(3) FREQUENCY RESPONSE. The full operating frequency range required.
(4) TEMPERATURE RANGE. The upper and lower temperature extremities to which the device will be subjected and also any heat soak conditions.
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Peizo Electric Transducer.
Figure 6.24.
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Figure 6.24. shows a schematic diagram of a typical peizo electric accelerometer.
The top nut is torque loaded to give the correct starting datum on the peizo crystal.
When subjected to a force (caused by engine vibration) the piezo electric crystal produces an electric charge on its opposite faces. The output is fed to a charge amplifier, which produces the voltage required for the cockpit indicator or frequency analyser. Most modern transducers employ a synthetic piezo electric such as lead zirconate in preference to natural quartz crystal because of the higher sensitivity for the same force. In many cases, however, the choice of transducer will be dictated by the operating temperature. The maximum allowable temperature for transducers is typically 260C so they have to be sited on fan casings or in the by-pass ducting.
Transducers may be fitted in more than one plane or more than one location. The analyser can then be used to select a ‘broadband’ or overall vibration measurement, which will give a quick guide to the condition of the engine.
Vibration monitoring varies greatly from aircraft to aircraft. The operator’s requirements and the technology of the aircraft will dictate the equipment fitted.
Large commercial aircraft will have fitted a flight deck indication of the vibration levels of engine spools, N1, N2, N3. Their main function is to warn the crew of a malfunction, ie. shed blade. The sensitivity of the vibration sensors may not be good enough for detailed condition monitoring or fan balancing. Extra vibration sensors are fitted to enable these functions to be carried. There are some modern aircraft, which will carry as a permanent fixture, eg. equipment that can carry out all vibration analysis requirement.
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7 EXHAUST
7.1 INTRODUCTION
Aero gas turbine engines have an exhaust system which passes the turbine discharge gases to atmosphere at a velocity, and in the required direction, to provide the resultant thrust. The velocity and pressure of the exhaust gases create the thrust in the turbo-jet engine, but in the turbo-propeller engine only a small amount of thrust is contributed by the exhaust gases, because most of the energy has been absorbed by the turbine for driving the propeller. The design of the exhaust system therefore, exerts a considerable influence on the performance of the engine. The areas of the jet pipe and propelling or outlet nozzle affect the turbine entry temperature, the mass airflow and the velocity and pressure of the exhaust jet.
The temperature of the gas entering the exhaust system is between 550 and 850 deg.C. according to the type of engine and with the use of afterburning can be 1,500 deg.C. or higher. Therefore, it is necessary to use materials and a form of construction that will resist distortion and cracking, and prevent heat conduction to the aircraft structure.
A basic exhaust system is shown in fig. 7.1. The use of a thrust reverser, noise suppressor and a two position propelling nozzle entails a more complicated system as shown in fig. 7.2. The low by-pass engine may also include a mixer unit to encourage a thorough mixing of the hot and cold gas streams.
A Basic Exhaust System.
Figure 7.1.
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An Exhaust System with a Thrust Reverser and Variable area propelling nozzle.
Figure 7.2.
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