Yüzey topografyasının kaymalı yatak performansına etkisinin sayısal olarak incelenmesi
Numerical investigation of the effect of surface texture on the performance of journal bearing
Yazar: EMRE HAS
Danışman: DR. ÖĞR. ÜYESİ ZEYNEP PARLAR
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Makine Mühendisliği Ana Bilim Dalı / Konstrüksiyon Bilim Dalı
Derece: Yüksek Lisans, 2020
Konu: Makine Mühendisliği = Mechanical Engineering
Dizin: Sayısal akışkanlar dinamiği = Computational fluids dynamic ; Yüzey dokusu = Surface texture

Özet:
Günümüz dünyasında tasarım ve imalat teknolojilerinin gelişmesi ile birlikte birçok hareketli parçalardan oluşan karmaşık makinaların sayısı sürekli artmaktadır. Bu hareketli parçalar arasında oluşan kuvvetler sebebiyle sürtünme ve ısı oluşacaktır. Bu sürtünme ve ısı çoğu zaman kayıp olarak adlandırılmakta ve bu kaybın önlenmesi için çeşitli çözümler bulunmaktadır. Bu çözümlerin en yaygınlarından bir tanesi kaymalı yataklardır. İçten yanmalı motorlarda çok yaygın olarak kullanılan bu çözümde çok düşük kayıplar olmasına rağmen hala geliştirmeye açık noktaların olabileceği düşünülmektedir. 
Kaymalı yataklar iki farklı tasarıma sahip olabilmektedir. Eksenel ve radyal kaymalı hidrodinamik yatakların kullanım yerleri tasarıma göre değişmektedir. Bu çalışmada kullanılan dizel motor yatakları radyal kaymalı yatak olarak adlandırılmaktadır.
Bilgisayar destekli mühendislik programlarının kullanımının yaygınlaşması ve bilgisayarların gelişmesi ile birlikte daha önce bir boyutlu olarak çözülen birçok problem artık üç boyutlu olarak çözülmeye başladı. Bununla birlikte imalat sektöründeki gelişmelere bağlı olarak çok daha karmaşık geometriler üretilebilir hale geldi. 
Bağlangıç olarak uygun geometrilerin tespit edilmesi için Literatür taraması yapılarak uygulanacak geometrilere, bu geometrilerin konumlarına ve yoğunluklarına karar verilmiştir. 
Bu çalışmada dizel motorlarda yaygın olarak kullanılan bir kaymalı yatağın öncelikle GT-Suite programı kullanılarak bir boyutlu modeli oluşturulmuştur. Daha sonra Simcenter Star-CCM+ programında geometriler parametrik olarak oluşturularak yapılacak analizlerde kullanılacak tasarımların modellemesi kolaylaştırılmıştır. Aynı programda hesaplamalı akışkanlar dinamiği yöntemi kullanılarak radyal kaymalı yataklar incelenmiş ve geçmişte yapılan bazı deneysel çalışmalar ile de sonuçlar karşılaştırılmıştır. Modellerin doğrulanması sonrasında sürtünme momentiniiyileştirici desenler üzerinde çalışmalar gerçekteştirilmeye başlandı. 
Literatür çalışması sonucunda başlangıç olarak 4 farklı model oluşturulmuştur. Bu modelerde desen uygulanan yüzey 90° olarak belirlenmiştir. Modellerin analizleri yapılarak arasından en iyi olan seçilmiştir. Daha sonra bu model temel alınarak desen uygulama alanı değiştirilmiştir. 180°'lik alana uygulanan desenin daha iyi sonuç vermesi ile birlikte desenin derinliği yerleşim açısı ve yoğunluğu değiştirilerek sonuçlar karşılaştırılmıştır. 
Sunulan bu çalışma ile radyal yatak yüzeylerine uygulanacak desenler ile yatak boyutları ve kullanılan yağ değiştirilmeden sürtünme kayıplarında iyileşme yapılabileceği görülmüştür.

Abstract:
With the development of design and manufacturing technologies in today's world, the number of complex machines consisting of many moving parts is constantly increasing. Due to the forces formed between these moving parts, friction and heat occur. This friction and heat are often referred to as losses and there are various solutions to prevent this loss. One of the most common solutions is plain bearings. Although this solution is very commonly used in internal combustion engines, it is thought that there may still be points open for improvement despite very low losses.
In hydrodynamically lubricated systems, there should be a relative movement between the two surfaces. These systems have a thin film of several micrometers between the two surfaces and this film layer separates the surfaces from each other. The configuration of the surface is such that it forms a narrowing-opening gap in many parts, also called physical compression in the lubricant film. In the case of sliding motion, the surface movement drags the lubricant through the viscous forces into the tapered cavity and exposes it to a pinch. The lubricant is then subjected to an expansion through the tapered gap after it has passed the minimum film thickness point. Due to the change in film thickness, the lubricant's speed profile is adjusted to ensure mass flow continuity. Changes in mechanical energy occur throughout contact, and pressure increases where losses are minimal.
Plain bearings can have two different designs. The usage places of axial and radial sliding hydrodynamic bearings vary according to the design. Diesel engine bearings used in this study are called radial plain bearings.
Nowadays, CFD is used more widely in fluid mechanics as compared to tribology field. The reason for this difference may be the use of more thin liquid film flow applications in the field of tribology. However, CFD has been used in the field of tribology in recent years and is becoming more and more common.
With the widespread use of computer-aided engineering programs and the development of computers, many problems that have previously been solved in one dimension have now begun to be solved in three dimensions. However, due to the developments in the manufacturing sector, much more complex geometries became available.
A surface is never perfectly smooth, roughness is always present. In some cases, surfaces should be as smooth as possible, while in many other applications, various roughness values are vital. Roughness values range from nanometer scale to macro scale. It is very important in boundary and mixed lubrication, as the surface roughness greatly affects friction and wear. In hydrodynamic lubrication, it is thought that the surface topography in nanometer size generally has no effect since the surfaces in the contact area are completely separated and several micron films are formed. In order for the surface topography to have a noticeable effect on values such as fluid pressure and velocity, it must be on the liquid film scale. Surface texture applications are a way to control surface topography with great precision.
Different texture parameters such as shape, size, density, depth and orientation, or a combination of these parameters control the effectiveness of surface tissue on tribological performance. Surface textures can be positive to protrude from the surface or be negative in structures such as pits and holes.These textures can also consist of continuous grooves, channels and fluctuations, or discrete shapes such as circular, square, triangle, or hexagonal distributed evenly or randomly. Positive surface textures are widely used in micro-electrical mechanical systems (MEMS) and magnetic hard disks by reducing friction with the method of reducing contact area. Negative textures that are the focus of this study are used mainly in automotive components and machining tools and staples used for metal forming processes.
Liquid film lubrication is one of the most researched areas. The reason for this is that due to the wide variety of uses, a small improvement will benefit economically. The main area of this research is to examine the hydrodynamic effects of a textured surface. This subject has been subjected to both analytical and experimental research in recent years and various results have been obtained on lubrication performance.
In order to determine suitable geometries as a beginning, the literature was searched and the geometries to be applied, their locations and densities were decided.
In this study, one-dimensional model of a radial journal bearing, which is widely used in diesel engines, was firstly created by using the GT-Suite program. Later, simcenter Star-CCM + program geometries were created parametrically, and the modeling of the designs to be used in the analyzes will be facilitated. Jorunal bearings were examined and evaluated with some experimental studies in the past. Verification of the models started to work on studies to improve friction torque. 
Four different models were initially created after the literature review. In these modes, the surface is available as 90 degrees in the pattern environment. By analyzing the models, the best ones were chosen. Texture area of the model was changed. With the pattern giving better results than the 180 degree area, the depth of the pattern has been changed by changing the settlement angle and density. 
Model 2, which is a combination of patterns of model 1 and model 4, performs better than all other patterns in terms of load bearing capacity and friction moment. Although it carries 1.5% less load than the reference model, it shows the best performance in terms of friction coefficient, since 2% better performance is obtained in terms of friction moment.
Angular 33% reduction in angular patterns causes a 0.5% decrease in friction moment. Angular patterns reduction of 50% causes a 1% decrease on the friction moment. When the friction coefficients are analyzed, the difference between Model 2 and the pattern with 3 ° intervals is 0.7%, while the difference between the pattern placed with 4 ° intervals is 0.4%. Although there is a slight difference in all models, it can be said that the reference model shows the best bearing performance.
When the analysis is analyzed, while the decrease in friction moment is 5.5% in the model with the deep dimples, load bearing capacity decreases by 26%. The decrease in load carrying capacity causes an increase of 28% on the friction coefficient.
Compared with the shallow dimples patterned models, the load is reduced by 6.8% for the shallowest model. The increase in friction moment is 3%. At the same time, the friction coefficient increases by 10%.
In the analysis where the patterns are placed at different angles, the load bearing capacity is improved by 4% compared to the referenceless reference bearing while the friction moment is reduced by 5%. In addition, the friction coefficient decreases by 8.8%. The loss in one bed was calculated to be 450 watts. There are 7 main beds in a 6 cylinder engine and the total loss in the beds reaches the order of 3150 watts.
With this study presented, It has been observed that, with the patterns to be applied on radial journal bearing surfaces, friction losses can be improved without changing the oil used and journal bearing dimensions.