Gas turbine engines have extremely low levels of vibration compared to piston engines. Changes in vibration levels could occur therefore without being noticed. To assist the operator in identifying increasing vibration level, most engines are fitted with vibration indicators that continually monitor the vibration level of the engine. The indication is normally a milliammeter that receives its signals from an engine mounted transmitter via an amplifier. Analysis of engine vibration signals is an important tool for the detection of early failure in mechanical components.
Engine Vibration Monitoring (EVM) System.
This may take the form of a solid state circuit device utilising the piezoelectric effect. The device consists of quartz discs with a metallic pattern deposited on them and, arranged such that they serve as a flexible diaphragm. When subjected to pressure changes the resultant flexing sets up an electrical polarisation in the discs, so that electrical charges are produced relative to the amount of flexing. The electrical charges are routed, via an amplifier to the flightdeck indicator. This is calibrated in inches per second (IPS). On some engines there will be more than one sensor, enabling switching if one fails. Yet another useful variation is the wide and narrow band which means the readings can be either taken from over the whole range of vibrations from the engine or by one or two major rotational assemblies such as N1 and N2 spools. An example of this type is fitted to the RR Tay engine.