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Dişli çark hata ve hasarlarının titreşim analizi ile belirlenmesi

Identifying the faults in gears with vibration analysis

  1. Tez No: 46545
  2. Yazar: VOLKAN SİPAHİ
  3. Danışmanlar: PROF.DR. TUNCER TOPRAK
  4. Tez Türü: Yüksek Lisans
  5. Konular: Makine Mühendisliği, Mechanical Engineering
  6. Anahtar Kelimeler: Belirtilmemiş.
  7. Yıl: 1995
  8. Dil: Türkçe
  9. Üniversite: İstanbul Teknik Üniversitesi
  10. Enstitü: Fen Bilimleri Enstitüsü
  11. Ana Bilim Dalı: Belirtilmemiş.
  12. Bilim Dalı: Belirtilmemiş.
  13. Sayfa Sayısı: 91

Özet

ÖZET Günümüzde büyük ölçekte tesislere ve bu tesislerde yüksek teknoloji ürünü makinalann kullanımına doğru bir yönelim vardır. Bunun yanısıra böyle tesislerde ortaya çıkacak arızaların oluşturacağı büyük zararlar tesislerde bakımın da önem kazanmasına yolaçmıştır. Bu tipte işletmeler makinalann umulmadık arızalar sonucunda durması ve buna bağlı olarak da üretimin durmasını engellemek üzere bakım ve bakım yöntemleri ile ilgili harcamalarına ağırlık vermektedirler. Özellikle son 20 -25 yıl gözönüne alındığında, çok uzun zamandan beri uygulanmakta olun konvansiyonel bakım yöntemlerinin yanında Erken Uyarıcı Dinamik Bakım olarak anılan yeni bir yöntem ortaya çıkmış ve bu zaman zarfında sürekli olarak da geliştirilmiştir. Yöntem artık Batı'da sanayileşmiş ülkelerin birçok sanayi tesisinde etkili biçimde uygulanmakta ve bu uygulamalar sonucunda yöntem sayesinde çok yüksek verim alınmakta ve sonuçta mali açıdan büyük karlar elde edilmektedir. Titreşim analizi ile çalışan bir makinayı ya da sistemi, önceden belirlenmiş kritik noktalarından ölçümler alarak ya yerinde ya da bir kişisel bilgisayar karşısında izlemeye dayanan bu yöntemin amacı hata teşhisi yaparak bunun makina bütünü içerisinde kaynaklandığı elemanı ya da elemanları belirlemektir. Daha sonra bu hatanın kaynağı da E.U.D.B. yöntemi ile saptanmaktadır. Hata tespiti sonrasında hatanın gelişim eğrisi çıkarılarak bunun hasara ve makinanm bozulmasına yolaçacağı süre tahmin edilmekte, bu arada parçanın yedeği temin edilerek üretim akışı açısından uygun bir zaman belirlenip bakım yapılmaktadır. Burada dikkati çeken özellikler ; ölçümler sırasında makinalann durdurtmasına ihtiyaç olmaması, hatalar önceden belirlendiğinden makinalarda ani anza oluşumunun önüne geçilmesi ve tesiste yedek parça stoğunun gereksiz yere şişirilmemesidir. ıx

Özet (Çeviri)

SUMMARY IDENTIFYING THE FAULTS IN GEARS WITH VIBRATION ANALYSIS In any industry efficiency is the key to success. To maximise the efficiency, it is essential that the initial investment and the lead time from plant conception until its first day of production are as small as possible, while productivity and its operational life are as large as possible. These factors can be optimised by various means. It is when one considers the requirement of long operational life that the problem of reducing maintenance costs emerge. Within many industries, an increasing trend in equipment design towards larger, more complex and more productive units is being experienced. As equipments grow in both size and capital cost, so downtime reflects not only in increasing maintenance costs but also in greater loss of production by an idle unit. This has resulted, therefore in an increasing interest in maintenance schemes that allow maximum operating life. With the tendency towards larger plants and a greater degree of sophistication in both production and automation, maintenance has become an increasingly important factor when considering life cycle profitability. The profitability connected to the application of maintenance depends not only on maintenance costs but also on machine availibility and utilization. Combining the heavy penalties paid for losses of production with increasing maintenance costs, capital tied up in excessive stocks of spare parts and the ever growing costs of labour, maintenance managers can not fail to consider the benefits of employing maintenance tools that can help to keep the machine downtime to an absolute minimum. An ideal machine would produce no vibration at all, as all energy would be channelled into the work to be done. But in practice vibration occurs as a by product of normal transmission of cyclic forces through the mechanism. Machine elements react against each other and energy is dissipated through the structure in the form of vibration. A good mechanical design will produce low levels of inherent vibration. As the machine wears, however, foundations settle and parts deform, subtle changes in the dynamic properties of the machine begin to occur. Shafts become misaligned, parts begin to wear, rotors become unbalanced and clearences between parts increase. All of these factors are reflected in an increase in vibration energy, which, as it is dissipated throughout the machine, excites resonances and puts considerableextra dynamic loads on bearings. As it can easily be seen, vibration is normally a destructive by-product of the force transmission through a machine, which provokes wear and accelerates break-down. Machine elements which constrain these forces, for example bearing housings are usually accessible from the outside of machine so that vibration resulting fromthe excitation forces can be measured at these points. When faults begin to develop, the dynamic processes in the machine change and some of the forces acting on machine parts are also changed - thereby influencing the vibration level and the shape of the vibration spectrum. The fact that vibration signals carry much information relating to running condition of machines is the basis for using regular vibration measurement and analysis as an indicator of machine health trends and the need for maintenance. The one viable solution is predictive maintenance - measuring machine condition and repairing when and only when measurements indicate it necessary. Traditional machinery maintenance practice in industry can broadly be categorised in two methods. Run to Break-Down and Time Based Preventive Maintenance. In industries, running many inexpensive machines and having each important process duplicated, machines are usually run until they break down. Loss of production is not significant as spare machines can usually take over. In these kinds of industries to secure every machine to continue operating properly does not always carry great importance. In such conditions machines can be operated till breakdown occurs. This is Run to Break-Down Maintenance. Where important machines are not fully duplicated or where unscheduled production stops can result in large losses, maintenance operations are often performed at fixed intervals such as every 3000 operating hours or once per year. But usually maintenance period is determined statistically and based upon experience. The period between the time that the machine is purchased and the time at most % 2 of the machines have malfunction is stated as the maintenance period. Another important point is that; by changing the worn machine partsd regularly, the breakdown ratio of the machines is thought to decrease but usuallt the opposite of this assumption occurs. As the malfunction type of each individual machine can not be determined Time Based Preventive Maintenance can not be applied very efficiently. But this method is usually used in many industrial plants. The parts which are planned to be changed during the machine stops are always kept stored in the spare parts stock.But experience has shown that in vast majority of cases Time- Based Preventive Maintenance is uneconomical. Being seperate from these conventional methods, there is an other maintenance method called Condition Based Maintenance, which considers each machine individually. By this method fixed interval overhauls are replaced by monitoring vibration level as a health indicator each machine can be followed closely. The axiom of Condition Based Maintenance or in other words On- XICondition Monitoring, is that servicing is permitted only when measurements show it to be necessary. When the comparison of On-Condition Monitoring Techniques costs and savings it is easily concluded that savings by applying this method is much more than its cost. Initial investigations, selection of monitoring points, establishment of limits, selecting and purchase of instrumentation, instruction of staff in taking routine measurements, and instruction of engineers in evaluating measurements are the costs of the system. But on the other hand increase in the average time between overhauls ( increased productivity and reduced maintenance costs ), elimination of component waste, reduction of spare-part stock, reduction in bussiness interruption and damage insurance premiums, reduced repair duration can be given as the saving reasons. It's obvious that savings are much more than the costs. The equipment used for Condition-Based Maintenance ( Predictive Maintenance ) consists of an accelerometer, a tape-recorder, a vibration meter, a vibration signal analyser and a software created especially for that reason. Accelerometer is the vibration sensor device which converts the vibration to a digital signal, to be stored or processed to form the vibration spectrum. Tape- recorder is the data storing device which receives signals from the accelerometer. Later, these signals are transferred to the vibration analyser. Vibration meter is a simple device giving a single digit RMS, max. RMS or Peak Level reading of vibratory acceleration or velocity. Vibration Analyser is an advanced and integrated device, recieving signals via an accelerometer or a tape-recorder, forming a vibration spectrum. With the analyser an intensive frequency analysis can be performed to detect, diagnose and correct faults. The software is used to interpret vibration data, transferred from the analyser. It can perform 3D plots of vibration spectrum and make Trend Analysis, also it supplies measurement routemaps. The vibration analyser has also some special functions. One of these is the Envelope Analysis which is generally used to detect the faults on the roller bearings. While applying the envelope analysis, as an addition to the standard equipment an envelope detector is used. This detector is connected to the analyser and with the button on it, the resonance frequency of each machine element is chosen before the signal is taken from the structure. Also the analyser has the Cestrum Analysis function. Cepstrum Analysis is specially developed to identify the incipient faults on the gears before a damage occurs. Applying the On-Condition Monitoring and Maintenance techniques, a systematic approach should be used. This approach can be interpreted like that; it starts with the plant engineer's decision on which machines and measurement points are to be measured and monitored. This decision is based on the importance of each machine to the facility, the cost of repair and the history of previous break-downs. The portable vibration analyser is used to study each machine's vibration content before deciding the best location for the measurement points and which type of measurement is best suited to monitoring that particular machine.Details on the machines and measurement points are then loaded in a central computer software and the measurement points are arranged into measurement routes. A referance xnspectrum is measured and loaded into the computer for each measurement point as a reference against which to compare the new spectrum. Afterwards, a member of the maintenance staff by taking the analyser with him begins to take measurements in an order defined by the measurement routemap supplied from the computer software. These measurements are executed periodically. The stored data is transferred to the computer in the office. In the office the responsible engineer evaluates the vibration data by spectrum comparison, trend analysis or 3D Plots. If he distinguishes a vibration level exceeding the tolerance limit by using advanced techniques such as envelope analsis for roller bearings and cepstrum analysis for the fault detection in gears and gearboxes, he determines the cause of the fault, so a counter measure can be taken and the machine's maintenance is performed immediately. By utilising a Predictive Maintenance Programme such as vibration monitoring, the condition of vital machinery can be determined continuously. These monitoring systems can give very early warnings of impending failures, they can determine the cause of the fault and can be used to schedule the repair. Such a system can therefore prevent catastrophic failures; reduce forced outage; give maximum utilisation of available assets; increase the life of the plant and reduce the maintenance costs. In this study, the main objective is to see the faults in a gearbox system by the help of On-Condition Monitoring System. An experimental system, consisting of an electric motor, a gearbox, a shaft and a fan was constructed. The driving electric motor (DC) was coupled to the gearbox with an elastic coupling. The output shaft of the gearbox was coupled to the shaft driving the fan again with an elastic coupling. Fan's shaft was fixed by means of two roller bearing housings. Gearbox was changing the direction of motion by its one stage conical gearwheels, 90 degrees. The electric motor was able to maintain a running speed of 820 rpm. and the reduction ratio of the gearbox is 2. At the beginning referance spectra are obtained from the gearbox at pre determined speeds, these are stored in the memory of the analyser. These speeds can be grouped in three sections; the first is the low speed level which is 246 rpm (4.1 Hz. ) the second is the medium speed level at 491 rpm (8.18 Hz. ) and the third is the high speed level which is 690 rpm ( 11.5 Hz ). At these speed levels measurements are taken from eight points. The first point is on the electric motor on the vertical axis. The second is on the input shaft bearing's axial direction, the third point is on the input shaft bearing's vertical axis. The fourth measurement point is on the opposite side of the input shaft from the horizontal axis. The fifth point is on the output shaft bearing of the gearbox on the vertical axis. The sixth is on the output shaft bearing on the axial direction. The seventh point is on the fan shaft's bearing housing in the vertical direction and the last measurement point on the radial direction of the same housing. xinAfter taking referance spectra, a small pit is welded on the meshing surface of the input gear of the gearbox. As a result a fault is formed on that gear. By taking vibration measurements from the system having that fault, new spectra are obtained. Then these spectra are compared with the previous referance spectra and the effect of the weld pit is analysed clearly. Because of the weld pit the vibration levels of the toothmesk frequency of the input gear have increased and this is confirmed by Cestrum Analysis and Envelope Analysis results. xiv

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