A
Accelerometer
Is a specific type of vibration transducer which converts mechanical vibration into electrical signals using a piezoelectric crystal. There are a number of different accelerometer designs each with their own characteristics, advantages and disadvantages.
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Amplitude
Is a measurement of the magnitude, intensity or size of vibration and may be measured in displacement, velocity or acceleration.
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Acceleration
Acceleration is the change of velocity over a period of time, Newtons second law stated that Force was the product of mass and acceleration or put another way Acceleration = Mass/Force.
B
Band Pass Filter
A band pass filter defines a range of vibration frequencies – the filter bandwidth - within which all frequencies are allowed to pass, thus rejecting or attenuating all frequencies above and below the band.
Beat Frequency
A beat frequency will occur when two very close frequencies combine together at a particular location to form a single frequency. The amplitude will vary from a maximum to a minimum over a period of time – the beat frequency - which is related to their closeness. The closer the original frequencies are together the lower the beat frequency. It may be observed on a turboprop aircraft when the speeds of the two engines are not perfectly matched and will detected as ‘vibration/noise’ which rises and falls over a period of a few seconds.
Bins and Binwidth
When an FFT is created the energy in the total signal bandwidth is measured at descrete frequency intervals, the resulting information ‘lines’ are placed in their appropriate ‘bins’. The greater the resolution of the FFT the narrower will be the ‘binwidth’ and the greater the detail which may be obtained, typically we see a resolution of 400, 800 or 1600 lines or bins.
Bump Test
If an object or component, with an accelerometer attached, is struck, bumped or rapped either by hand or using a special hammer at an irregular interval whilst acquiring a asynchronous vibration signature. The resulting spectral line will provide a good estimation of its natural frequency.
C
Compression Type Accelerometer
The Helitune H-901 is a compression accelerometer.
Cycle
A cycle is one complete oscillation, radio engineers often refer to this as the wavelength -l (lambda).
D
Damping
A vibration of an object at its natural frequency, like the plucked string of a guitar, or the swing of a pendulum, will decay to zero over a given period of time due to air resistance, internal molecular resistance etc. Critical damping is the term given to the amount of damping required to ‘kill’ a vibration before the completion of one full cycle.
E
Excitation
A complex mechanical drive train system, such as that found in a helicopter, generates a wide range of vibration forces at their fundamental rotational frequencies and harmonics. These are termed excitation forces which can be imparted into the airframe structure.
F
Frequency
The frequency of a Sine Wave is the count of the number of cycles performed in a defined period of time. It is commonly measured in ‘Hertz’ (Hz) which are ’cycles per second’ or ‘cycles per minute’ (CPM).
FFT
The Fast Fourier Transformation is an algorithm named after Jean Baptiste Joseph Fourier (Born: 21 March 1768 in Auxerre, Bourgogne, and died on 16 May 1830 in Paris, France). In essence, it decomposes or separates a complex waveform or function into sine waves of different frequency which sum to the original waveform. It identifies or distinguishes the different frequency sine waves and their respective amplitudes.
Filter
A filter is an electrical circuit which is specifically designed to attenuate or eliminate unwanted frequencies from a broad range of frequencies. They are usually described as Low Pass, Band Pass and High Pass Filters
G
Gear – Mesh Frequencies
In addition to their fundamental rotational frequencies meshing gears produce a vibration which is the product of the number of teeth and their rotatational frequency and is an indication of machine health. For example a 61 tooth gear rotating at 1705.5 rpm meshes with a 17 tooth gear. The 17 tooth gear will rotate at (61/17)*1705.5 = 6120.09 rpm. The dominant frequencies will be 28.4 Hz and 102.0 Hz for the fundamental rotational frequencies and a gear-mesh of 1734.0 Hz.
H
Hertz
One ‘cycle per second’ is often termed one ‘Hertz’ after the German physicist.
Standard scientific prefixes are used so 1000 cycles per second is 1000 Hertz (1000 Hz) or 1 kiloHertz (1kHz).
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Harmonics
In addition to the fundamental frequency of a vibration there are also superimposed additional frequencies at 2x, 3x, 4x etc in theory all the way to infinity! The 2nd harmonic is 1 octave higher at twice the frequency. Middle ‘C’ on a piano is at 261.626 Hz.
I
Integration
In vibration analysis Integration is the mathematical procedure which is used to convert an Acceleration (ins/sec/sec) signal into a Velocity (ins/sec) and inturn Velocity into Displacement (ins). The reverse of this procedure is differentiation.
L
Lines on an FFT
An FFT resolves the input accelerometer signal into a number of descrete lines or bins. The larger the number of lines the greater is the detail and clarity of information that can be extracted. For example a typical 0 to 500Hz FFT analysis, at a resolution of 200 will sample the signal and produce one lines for all frequencies between 0 Hz and 2.5Hz, and another for frequencies between 2.5 Hz and 5.0 Hz etc.( 500 / 200 = 2.5). This may be satisfactory for some circumstances. However if it is known that in the aircraft there are gears, shafts or other components which are generating frequencies closer than 2.5Hz then it may be impossible to identify them correctly. This will necessitate using a highter 400, 800 or even 6,400 'line' resolution.
M
Mils
Mils is a measure of vibration displacement, where 1 Mil is equal to one thousandth of an inch. It should not be confused with metric millimetres.
N
Natural Frequency
An object such as a bell when struck will vibrate at its fundamental or natural frequency, which is governed by its mass and stiffness. Changes to either will change the frequency.
O
Octave
From the Latin Octavos an eighth. Octaves originate from music domain where 1 octave spans 8 notes, the 8th note is twice the frequency of the 1st note and the 9th twice that of the 2nd etc. This may be seen on the standard piano keyboard. It not often used in vibration analysisis.
P
Period
The period of a repetitive signal, such as a sine wave, is the length of time between one point in the cycle to the same point on the next cycle and is the inverse of the frequency.
Phase
Phase is the measure of the angular relationship, measured in degrees between two events. In the context of balancing it is the relationship between the sine wave generated of the out-of-balance forces compared with a one-per-revolution time reference signal.
Peak-Peak
The amplitude of a sine wave measured from the maximum positive to the maximum negative.
Peak
The amplitude of a sine wave measured from neutral or zero to the maximum positive or maximum negative value.
Piezoelectric Effect
Originates from the Greek piezein to ‘press or squeeze’. When certain types of man-made or naturally occurring crystalline structures are deformed by the application of a mechanical force they generate an electrical signal.
R
S
Sidebands
Sidebands appear as ‘family of spikes’ either side of the gear meshing frequency in a spectral analysis – FFT – they are the result of amplitude or more often frequency modulation and are an indicator of potential problems. If a gear in a pair of meshing gears is running slightly eccentrically it will vary the tooth loading during the period of one revolution as it becomes tight and then loose this will cause minor variations in the rotational speed.
Sine Wave
A sine wave is a wave form which is generated by the path of a point moving around on a circle as it rolls along a line.
T
Transducer
Originates from the Latin transducere to ‘lead across’. Transducers are devices which transform variations in a physical quantity, such as pressure, temperature, flow, or vibration into a proportional electrical signal.
V
Velocity
Speed defines how fast something is moving, calculated from the distance travelled over a given period of time. Velocity is the speed of an object in a particular direction.
Vibration
Vibration is simply the oscillating motion of an object about a reference position. A pendulum set swinging to and fro or perhaps striking a thin ruler overhanging a desk, or rapping a bell, Figure 2-1, are all every day examples of vibration. The motion they generate is termed simple harmonic motion.
W
Windowing
To obtain an accurate FFT representation of the time domain sample it is necessary to introduce some compensations due to the FFT process, this is termed windowing.
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