Dönen makinalardaki mekanik titreşimler ve bunların arıza parametresi olarak incelenmesi

Abstract

Sunulan bu çalışmada, dönen makinalardaki mekanik titreşimler, bu titreşimlerin ölçüm metodları, kontrolü ve arındırılması ile ölçümler sonucunda elde edilen frekans spektrum analizlerinin değerlendirilerek arıza nedenlerinin belirlenmesi incelenmiştir. Konunun mekanik titreşimler olması nedeniyle makina ağırlıklı bir literatür araştırması yapılarak günümüzde yaygın olarak kullanılmaya başlanan Erken Uyarıcı Dinamik Bakım (E.U.D.B) programı da inceleme kapsamına alınmıştır. Bölüm 2' de, genel olarak mekanik titreşim kavramı incelenerek mekanik titreşimlerin sebepleri, çeşitleri ve hareket denklemlerinin çıkarılışı açıklanmıştır. Mekanik sistemlerin temel titreşimleri olan serbest ve zorlanmış titreşimler Bölüm 2' de incelenmiştir. Diğer sık rastlanan titreşimlerden kararsızlık titreşimleri titreşim kontrolünün ele alındığı Bölüm 4' te, özellikle dönen makinalarda rastlanan kendiliğinden uyarılan titreşimler ise Bölüm 5' te ele alınmıştır. Bölüm 3' de dönen makinalardaki mekanik titreşimlerin ölçülmesinde kullanılan transdüserler, bu transdüserlerle yapılan ölçümler sonucunda elde edilen bilgilerin değerlendirilmesi (trending) için kullanılan gelişmiş bilgisayar destekli sistemler ve erken uyarıcı dinamik bakım programı tanıtılmıştır. Bölüm 4' te dönen makinalardaki mekanik titreşimlerin kontrolü, titreşim kontrolunda önemli bir yeri olan yerinde dengeleme ve titreşimlerden arındırma ele alınmıştır. Bölüm S' te ise dönen makinalardaki mekanik titreşim ölçümleri sonucunda elde edilen frekans spektrumlarının analizi ile özellikle en sık rastlanan titreşim kaynaklan olan rulmanlı ve kaymalı yataklardaki arızalar ile dengesizlik gibi arızaların tespit edilmesi tablolar halinde özetlenerek ve frekans spektrum örnekleri verilerek incelenmiştir.

In this thesis, mechanical vibrations and their measurement, control, diagnosis and isolation methods in rotating machines and determination of vibration sources through frequency spectrum analysis are investigated. In the classifications of vibrations, degree of freedom of vibrating system is generally used. Degree of freedom is the number of the independent coordinates to determine the displacement of a system at any moment. Vibrations can be investigated in two groups as to being forced or not and both of these two groups can be further studied as to being damped or not. These are as follows; 1. Damped and undamped free vibrations, 2. Damped and undamped forced vibrations. Free vibrations are movements of a vibrating system without any external force. In forced vibrations, the rhythm of vibration is determined by an external force which is called as excitation force. Excitation shows itself not on a parameter but as another term, which is always time dependent and on the right side of differential equations of forced vibrations. The most common occurrence of forced vibrations shows itself when there is an unbalanced weight distribution in the rotating member of a system. In undamped case, while the energy of the system transfers from potential to kinetic and kinetic to potential, the sum of potential and kinetic energies stays constant. In damped case, a certain amount of energy is drawn out of the vibrating system by any means and the amplitude of the vibration attenuates. In vibrations this is called as damping and the damping element is called as a damper. In practice, one of the most widely used damping element is viscous damper. It is called viscous damper because of the fact that every viscous fluid has certain amount of damping capability and more importantly the viscous damping coefficient has a linear relationship with the velocity of the damper. In the case of squeeze film damper, which has a wide practical usage in controlling especially vibration of turbo machines, by coupling the interior metal ring of damper to rotor by roller bearing or journal bearings. The squeezed film produces damping effect in the system, which is exposed to vibrations. Transducers are devices that convert mechanical displacements of vibrations into electrical signals. Transducers used in the measurement of vibrations are based on these criteria, since the intensity of vibrations is expressed in terms of displacement, velocity or acceleration of the matter. One of the most widely used transducers in the industry are piezoelectric acceleration transducers and their advantages are wide frequency band, noise free and hand held operation and small size. The piezoelectric acceleration transducer is known as accelerometer. Accelerometer generates an electric current when a mechanical force applied in an appropriate direction. If there exists an electrical input, then it gives a mechanical output. In the measurements done by the acceleration transducers, type of transducers' mounting and environmental effects are also very important. The accelerometer can be;. Stud mounted.. Attached with beeswax.. Attached with a magnet. Another type of transducer used commonly is displacement transducer, which is especially applied to the journal bearings of rotating machines with a rotational speed of less than 600 rpm. This type of transducers can measure the axial and radial displacements of the shaft inside the journal bearing. Proximity probe is another widely used transducer and its reading method is named as proximity measurement. The proximity measurement method is based on the Eddy current and the probe does not touch the shaft. The distance between the shaft surface and the probe is measured with the electrical current generated in a small coil in the probe. The disadvantage of this measurement method is the distance between the transducer and the shaft must be accurately adjusted otherwise the transducer may be damaged. It does not matter how complicated or non linear any vibration signal could be, it can be expressed as a sum of many harmonic terms that has different amplitudes and frequencies. This technique is called Fourier Analysis. The components of harmonic amplitudes in different frequencies, whose sum forms the vibration signal, can be measured electronically by passing through a bandpass filter. This filter scans its frequency spectrum and it figures out the degree of intensity at the certain frequencies of the amplitude of the vibration. Of the vibration analysis devices, two of the most used by engineers are vibration analyzer and FFT analyzer. It is possible to investigate components of the amplitude of vibration in different frequencies with the help of vibration analyzer. To accelerate the measurements done by this type of devices, an X-Y recording device connected to the analyzer. While the bandpass frequency spectrum automatically scans the frequency region, the amplitude of the vibration is automatically plotted out by the recording device. FFT comes from the first letters of fast fourier transform. FFT analyzer is used to investigate the components of vibration signal in different frequencies obtained during the vibration of a machine. FFT analyzer is the most valuable device from the vibration analysis and diagnostics point of view. The amplitude of vibrations can be calculated by digital methods in terms of displacement, velocity or acceleration and the results displayed on the screen as a function of frequency. The filters pass signals at certain frequency and reject the others. In the vibration analysis of complex systems, this is very useful in determination of the type and the strength of the vibration. Especially when filters used with a vibration analyzer, they form a very capable vibration diagnosis set. Recently another method of vibrations monitoring method entered the vibrations field, which is a computer-aided-monitoring program. This program is called as Predictive Dynamic Maintenance (PDM). In order to analyze the possible malfunction due to the increasing level of the vibrations in a machine by using PDM, the vibration records of the same machine running under normal circumstances are required. Otherwise since PDM cannot analyze the vibration signals without any comparison data, the diagnosis by this method cannot yield the fault. In order to be successful with PDM the section dealing with vibrations and the maintenance section must communicate effectively. Vibration analysis data do not only show that a machine has a problem and has to be repaired, but at the same time yields valuable predictions as to how long further the programmed maintenance periods can be extended in the safety limits. Mainly the PDM programs are composed of these steps;. Investigation of PDM needs of the plant. PDM planning (classifications of machines, determination of the measurement device needs ). Preparing profiles of the machines running without any malfunction. Monitoring vibrations data taken from the critical points. Diagnosis of malfunction indicators. Taking precautions to fix malfunctions. Annual evaluation of PDM program Trending can be put as the continuous vibrations control with the computer support or it also may mean the complementary part of any periodical maintenance program. Even though the continuous monitoring of all machines in the plant seems to be the most ideal solution, the limiting factor here is the high personnel and related costs. Therefore the point must be reached, is to form a system by the optimization of the periodical maintenance costs and the efficiency of the continuous monitoring of the systems. By using on line monitoring programs supported by modern computers, it is possible to collect data from about 2000 points in the production system. These data can be evaluated to come up with a new trend. The user can implement the capabilities of the software system to determine the alarming points and analyze the malfunction at the computer environment without going to the alarming machine. The vibration control in the machines can be done in three different ways; 1. Reducing the amplitude of the forcing function by balancing, 2. By increasing the rigidity of the system to increase the critical velocities and to protect system from resonance, 3. By using an external damper both to protect the resonance regions from forcing functions and to protect the system from the unstable vibrations If a rotating member's center of gravity does not coincide with its rotating axis, centrifugal forces which are proportional with the square of the angular velocity cause an unbalanced system. In the recent years the balance of the rotors is done with very advanced balance devices after installation of the rotor. The most widely used vibration control element is viscous damper. Viscous damper without effecting the critical velocity displacement damps successfully in the critical velocity regions. Dynamic vibration damper is an important vibration control element, which is especially in the resonance regions of the forced vibrations protects the systems from resonance. There are two main methods to isolate machines from vibrations; 1. If the amplitude of the machines are kept under certain limits and if the force exerted on the foundations by the machine is reduced, this isolation method is called as active vibration isolation. In active vibration isolation, the ratio of the force exerted on the foundations with elastic connectors to the force exerted on the foundations with rigid connector is called as transmissibility (TR) Following are the most important points in the active isolation method;. In order to have a good isolation from the transmissibility point of view the natural frequency of the system must be less than operational frequency. That means the mass of the system must be big and the spring constant must be small.. Since the soft springs increases the amplitude, and the large amplitudes may damage the system or the external connections, there is a limit to the softness of the practically employable spring.. In practice the most common application is to increase the mass of the vibrating member. For this reason, the machine is connected to a generally a cement bloc, which has a much greater mass than the machine itself and the cement bloc is connected to the base elastically. 2. If the environmental vibrations from the base are restricted by to protect some sensitive devices and machines, this is called as passive vibration isolation. Vibrations can be used as an early warning parameter to indicate the rotating machine malfunctions. These malfunctions are unbalanced mass distribution, eccentricity, damaged bearing or gear and mechanical looseness. In rotating machines, diagnosis of malfunctions is possible with spectrum analysis acquired by FFT analyzer that has zooming and fine frequency capabilities. Besides the spectrum analysis, cepstrum analysis, which means the spectrum of the frequency spectrum obtained from time signal, has important advantages, too. With cepstrum analysis, the periodical properties in the signal are eliminated and it becomes easier to find the harmonics separated by uniform spaces by the side bands. Fundamentally in spectrum analysis; 1. Mechanical vibrations in the fundamental frequency and the first harmonic, 2. Mechanical vibrations in the higher order harmonics, 3. Mechanical vibrations in lower scale frequencies and the lower order harmonics are investigated. Mechanical vibrations in the fundamental frequency are caused by unbalanced members and the vibrations in the first harmonic is caused by faulty coupling adjustment, which may cause eccentricity. The reason for the simultaneous increase in the amplitude of the fundamental frequency and the first harmonic, is the looseness in one of the fixed parts of the machine. Reasons for the vibrations in higher order harmonics are generally bearing malfunctions and damaged gears in the gearbox. Mechanical vibrations in lower order harmonics at the fundamental frequency can be investigated in two parts, first self excited vibrations and second non-linear system vibrations. The most encountered self excited vibrations sources by self displacement of the machine and unbalanced members of the machine are called as hysterisis whip vibration also named as shaft whirling or internal friction. Another form of the most encountered self excited vibrations is the oil whip vibration, which is a result of losing stability of oil film in case of not coinciding bearing center and the shaft's rotating axis. It is a fact that the most widely used exciting motors in the industrial plants are induction motors. Vibration problems encountered in induction motors can be classified depending upon the source of the vibration in two groups as mechanical and mechanical based magnetic vibrations. In induction motors, as well as mechanical vibration sources such as mechanical looseness, bearing faults, eccentricity, unbalanced members, which can be seen in other rotating machines, there are additional vibration sources such as; the stator and rotor connections or windings, which has short circuit or looseness and rotor bars which are broken, loose or cracked. Meanwhile magnetic vibrations especially caused in the air gap of the motor and other magnetic vibrations caused by mechanical faults are of interest.