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.

JAR 66 CATEGORY B1 MODULE 15 GAS TURBINE ENGINES

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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.

JAR 66 CATEGORY B1 MODULE 15 GAS TURBINE ENGINES

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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 MODULE 15 GAS TURBINE ENGINES

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Before we look at fan balancing we must first look at vibration analysis techniques adopted on modern gas turbines and the reason for doing it. One of the 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.

JAR 66 CATEGORY B1 MODULE 15 GAS TURBINE ENGINES

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Peizo Electric Transducer. Figure 6.24.

JAR 66 CATEGORY B1 MODULE 15 GAS TURBINE ENGINES

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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 260°C 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.

JAR 66 CATEGORY B1 MODULE 15 GAS TURBINE ENGINES

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7

EXHAUST

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.

JAR 66 CATEGORY B1 MODULE 15 GAS TURBINE ENGINES

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