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14ТЕХНИЧЕСКИЕ НАУКИ
A DEVICE THAT MEASURES VIBRATION PARAMETERS
ASHRAFOVA AYNUR SAHiN
Master's student, Azerbaijan State Oil and Industry University. Baku, Azerbaijan
ABSTRACT
Annotation. In a mechanical sense, vibration can be called mechanical oscillation with respect to a reference point. Vibration is a phenomenon that people encounter at every moment of the day, at home, at work and on the road. Stopping, starting and oscillating of a boarded vehicle are the simplest examples of vibration.
Vibrations are often damaging and impressive. But it is necessary for some work to be done. For example, vibration is a sought-after feature in tools such as a mortar whisk and a concrete drill.
In general, it is desirable to identify and monitor vibrations in machines. In this regard, vibration analysis, which is a predictive maintenance type, comes into play. Although vibration analysis is included in the predictive maintenance class, it can be appliedfor the control of a part or area within the scheduled maintenance carried out at certain intervals in rotating machines.
Keywords: Vibration, Vibration analysis, Maintenance, Troubleshooting, Aircraft
INTRODUCTION
The use of accurate and repeatable vibration analysis is important for predictive maintenance programs. The type and quality of sensors are important in vibration analysis. In addition, there are three other key parameters: measuring point, direction and sensor placement technique. It is very important in predictive maintenance to properly record the history of key parameters. The place, measurement point and direction to be measured should always be in a suitable condition. In addition, care should be taken to ensure that the loads and forces on the sensor are the same in each measurement [1, p 20]. Vibration measurement is systematically shown in the diagram in Figure 1.1. As can be seen in Figure 1.1, mechanical vibrations in machines are detected with the help of sensors. The electrical signal from the sensors is processed by an analyzer and converted into a graphical representation.
MECHANICAL VIBRATION
ELECTRI
CAL SIGNAL
SIGNAL PROCESS ING
Figure 1.1. Vibration analysis chart
Vibration Profiles
Theoretically, although a simple vibration curve is shown, the vibration curves obtained as a result of vibration analysis have a very complex structure. This is because there is more than one source of vibration. Each vibration source creates its own curve. These curves affect each other and a new curve is formed. Two formats, time plane and frequency plane, are used to show these curves.
Time Plane
The time plane shows how much the vibration changes with time. Here, time is on the x-axis and velocity, displacement or acceleration is on the y-axis. The number of information in the waveform depends on the resolution and continuity of the graphic, and its continuity depends on the total time period. The resolution of the waveform is determined by the number of data points. For this reason, the more examples, the more detail.
A machine normally has only one operating frequency. However, in addition to the operating frequency and harmonics, the frequencies of the irregularities are also seen in the frequency plane. Thus, the resulting problems show themselves in the form of hills. Usually, the time plane is converted to the frequency plane with the help of mathematical operations. In order to obtain vibration data, a sensor installed in the appropriate place of the machine and an analyzer that can analyze the information from it are needed. The sensors and analyzers are described in the following sections.
Vibration Sensors
Sensors convert mechanical messages into electrical signals. These instruments detect the information needed to monitor machine condition, diagnose fault and set parameters. The sensors are used for magnitude, frequency and phase evaluations between two signals. Attention should be paid to the selection of the appropriate sensor for the place to be used. Sensor selection; The sensitivity depends on the measurement site size, the desired parameters for the measurement, the frequency response and the machine speed [2, p 18]. There are three types of vibration sensors that specifically monitor the mechanical condition of the machine. These; displacement sensors, velocity sensors and acceleration sensors. Each of them has a limited area of use and some areas have their own special uses.
Displacement Sensor
It is a type of sensor that converts mechanical displacement input to electrical output. Data is recorded in units of p-p mils. This value shows the maximum displacement of the machine shaft between the peaks. The appropriate frequency range for displacement sensors is 10 - 1,000 Hz (600 - 60,000 RPM). For frequency values below this limit or above 28, the measurement format may be distorted. This is not safe to determine machine status. The sensors to be used must be mounted on the key points of the equipment to be measured. However, accurate data collection can be achieved with fixed displacement sensors [3, p 33]. It is a disadvantage in itself that a precise calibration of the displacement sensor is difficult. Another disadvantage of this type of sensors is that the vibrating surface is required to be electrically conductive [4, p 56].
Speed Sensor
The speed sensor is not like the displacement sensors. Because instead of the distance of the movement, the speed of the displacement is measured. Typically, it measures in ips units. Its effective frequency range is 10 - 1,000 Hz.
The main limitations of speed sensors are their mechanical and thermal sensitivity. Normal use may result in loss of calibration and therefore precise calibration programs are needed to avoid data loss. At a minimum, vibration sensors should be calibrated every 6 months [9].
Acceleration Sensor
The acceleration sensor is the best type of sensor for detecting force effects in mechanical vibrations. The general usage name of acceleration sensors is accelerometer. In this study, acceleration meter expression will be used generally. Accelerometers use piezoelectric crystals or films to convert mechanical energy into an electrical signal. Piezoelectric material is a material that produces an electrical charge when subjected to force. There are many types of piezoelectric materials with different properties. The unit of measurement for acceleration meters is the gravitational constant g's inch. The effective range of general purpose accelerometers is between 1 - 10,000 Hz. Generally, vibration data above 1,000 Hz is taken and analyzed as acceleration. Basically, acceleration meters are divided into three groups as compression, shear and beam type.
The piezoelectric crystals are at the base of the accelerometers and there is a plate sandwiched between the piezoelectric crystals. When the accelerometer vibrates due to the vibration, the seismic mass crushes the piezoelectric crystals. A current is generated on the intermediate plate. This current collector plate is connected to the case and transmits the currents formed on it to a resistor in the acceleration meter. When this current passes through the resistor, a voltage is formed.
Sensor Selection
The frequency range that the sensors will sense must match the frequencies produced by the parts of the machine. When it is not suitable, another sensor should be selected and the signal should be converted to the appropriate measure.
Each of the important factors in sensor selection is also important in the selection of the process to be applied. For example, sensitivity and mass are two decisive parameters in the selection of piezoelectric sensors. If a large size sensor is selected, there is usually an increase in sensitivity. However, choosing a small size sensor results in a minimization of weight. Therefore, it is advantageous to use a small size sensor. However, as the additional mass begins to be added, changes in the natural frequency of the structure begin to occur. This starts to affect sensitivity. At this point, a high-sensitivity sensor is preferred.
Sensors are used in accelerometer measurements, at high frequencies where acceleration measurements provide high signal outputs, where forces, loads and stresses need to be analyzed, and where small size and mass sensors are desired.
Velocity measurements are used where vibration measurements are related to acoustic measurements, at medium frequencies, they are commonly used in measurements on machines with velocity spectrums that are smoother than both displacement and acceleration spectra, and vibration measurement in resonant structures associated with strain of shape.
Displacement measurements are used where the amplitude of the displacement is important, where the displacement in the measurement area is large, at low frequencies and to measure the relative motion between the rotating body and the machine structure.
For precise vibration measurement, the sensor should be placed at the point most appropriate to the machine's condition. For example, in bearing measurements, a location as close to the bearing as possible should be preferred, although the placement is restricted by some components such as bearings, coupling guards and fan shrouds [5, p 34].
The methods used to locate vibration sensors can be affected by their response frequency. Because the natural frequency of the acceleration meter may decrease depending on the placement method. Therefore, in the selection of the placement method, care should be taken to ensure a flat response frequency throughout the frequency range in which the study was started.
Vibration analyzers
After the sensors convert the mechanical vibrations they feel on the machine into signals, they send them to the analyzer. After the analyzer processes the incoming signal, it displays it graphically on a screen with the appropriate instruments. Generally, portable vibration analyzers are preferred. It is preferred because there is no tool left at the place where the measurement will be made and it is easy to carry.
The latest model analyzers today are in the form of a card reader that can be connected to a computer. Being in this way provides great convenience for the analyst. Carrying this card alone can assist in measuring vibration [3, p 16].
It is desirable that the cable has the ability to stretch and can be easily extended to any place where measurement can be made. The most preferred cable type with connection to sensors is crimped cables.
Crimped cables are not suitable for low speed applications, such as below 300 RPM or where there is a strong electromagnetic field. Because crimped cables, by their nature, will always try to get loose, this can produce low-level frequencies equivalent to the value of the machine during its movement. Thus, this oscillation frequency can obscure the actual frequency of the machine.
Also, a strong electromagnetic field can accelerate the cable oscillation, and the vibration produced by the cable can obscure the actual machine vibration.
Coaxial cables are used when crimped cables are inconvenient. Although it is difficult to use in portable systems, this cable type is definitely preferred in low speed and electromagnetic field applications.
Vibrations Caused by Damages That May Occur in Machines and Their Definitions
In machines, peaks in the spectra show the problem after the vibrations start to show themselves. Faults are generally seen below the motor speed called 1X (Subharmonic Harmonic), multiples of it (Harmonic - Harmonics) and non-harmonic (Nonharmonic) frequencies. Reading and deciding on these peak values requires an important expertise. At this point, malfunctions that may occur for a machine in general and how they can be seen on the spectrum are explained in detail in other sections.
INVESTIGATION OF VIBRATIONS OCCURING IN AIRCRAFT
The most important maintenance management application used in aircraft is scheduled maintenance. Predictive maintenance practices, which are included in scheduled maintenance, have an important place in maintenance planning today.
With predictive maintenance methods, problems occurring in the system are determined before malfunctions occur and preventive measures are taken in this direction. With the development of technology, the use of various predictive maintenance techniques in aircraft maintenance is increasing. The most used predictive maintenance applications are oil analysis and non-destructive control methods.
In aircraft, there is no rigidity when looking at the overall structure. As a result of the interaction between the systems, very high levels of oscillations can occur. Generally speaking, in the flight profile of the aircraft, many loads are placed on the structure. Vibrations caused by these loads are inevitable. For this reason, while an aircraft is in the manufacturing phase, the forces on the structure are calculated on the basis of parts and systems, and parts are selected accordingly.
In this section, vibrations occurring in aircraft will be examined in two classes, their applications in aircraft and helicopters. First of all, after the vibrations on the aircraft and their effects are mentioned, the vibration on aircraft engines, which is a rotating machine, will be examined. Later, vibrations on helicopters will be mentioned.
In this study, a compilation has been made about how to detect malfunctions in rotary machines by vibration analysis method and vibrations that may occur in aircraft. The purpose of the compilation is to provide a thorough understanding of this subject, which should be given as a course in the nursing school, and to form a basis.
Rotary machines are systems with a lot of moving parts due to their structure. While in motion, the irregularities in the structure show themselves as a jolt, that is, a vibration. Regardless of its size, the vibrations that occur have an effect on the structure. This is at least material fatigue. At this point, the important thing is to detect the vibrations well and to interpret them correctly. This can only be achieved with a good vibration analysis method.
When looking at aircraft, vibrations in the structure due to many loads acting on it are normal. Excessive vibrations are prevented during the manufacturing phase. The important thing is to monitor the continuity of low-level vibrations on the structure. These vibrations that occur on the system cause fatigue in the materials after a certain time and as a result, damage. Fatigue caused by vibrations is important. For this reason, the effects of fatigue on materials and systems have been studied extensively. In aircraft, system parts are designed and manufactured to withstand fatigue.
The loads on the aircraft can be analyzed on a system basis. The first of these is the loads on the landing gear that change depending on the aircraft's flight condition and environmental conditions. The second is the loads on the wing, fuselage and tail assembly due to irregular air flow and irregular aerodynamic structure. Finally, it can be said as the loads in the power system, which is itself a rotating machine.
Vibrations of the landing gear caused by these causes cause an aircraft accident. At this point, monitoring the irregularities on the landing gear with the vibration analysis method will help to determine the malfunction in advance.
Another factor on the aircraft is the power group. Irregularity in the rotating parts in the structure causes irregularities and damages in the system. Generally, a simple vibration analysis
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method is used for tracking irregularities in aircraft engines. This method determines the level of vibration and warns ahead of possible vibration changes. The cockpit has a simple vibration level indicator for the pilot. If a value other than the desired values is read, the pilot can stop the engine. Thanks to this simple warning system, conditions such as balance, breakage and bearing failure are detected in the blades on the engine in general. Developing vibration analysis methods are limited to use in aircraft fault detection. For this reason, it is essential to develop and use systems that detect failures at the initial stage of failure in aircraft engines. Current studies are on active vibration analysis.
Helicopters are the aircraft that are most exposed to vibration due to the main rotor and tail rotor structure. For this reason, the vibrations that occur in helicopters are monitored by using sensors specially produced for them. Thus, the irregularities occurring in the rotors are determined in advance and large-scale damages that may occur are prevented.
CONCLUSION
In the future, the most appropriate predictive maintenance method should be applied using methods based on vibration analysis. Thus, the desired uninterrupted flights are provided.
After this basic study, it can be studied how to track damage on a piece-by-piece or system basis. In addition, the changes in the structure can be examined with the help of sensors placed on the engine. This may be in the form of an in-system inspection without complete disassembly and disassembly of the engine prior to maintenance of the aircraft. Thus, only the suspected part is opened in line with the changes in the structure. In addition to all these, it should be studied how the rapidly developing vibration analysis methods in daily life can be applied in parallel with aircraft.
REFERENCES
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