Ev tipi fırınlarda kullanılan hareketli raf mekanizması yataklarının polimer esaslı malzemelerle tasarım ve imalatı

Abstract

Günümüzde, fırınlar yaşamımızın birçok alanında kullanılan önemli eşyalardandır. 200°C gibi yüksek sıcaklıklarda pişirme imkânı sunan fırınlarda kimi zaman gıdanın üstünün veya altının daha iyi pişmesini sağlamak için raf seviyesini değiştirmek isteriz. Pişme esnasında, kapağın açılıp yer değişikliğinin yapılması, sıcaklık değerleri düşünüldüğünde zordur. Bu amaçla insanlara hem kullanım kolaylığı hem de enerji tasarrufu sağlamak adına hareketli raf mekanizmasına sahip bir fırın geliştirilmiştir. Bu sayede, kullanıcı fırın kapağını açmadan tepsi otomatik olarak pişirilecek gıdaya uygun raf seviyesine gelebilmektedir. Geliştirilen hareketli raf mekanizması hareketi için tasarlanan yataklama sistemi üzerinde geliştirmeye ve değişikliğe açık alanlardan biri olan yatak parçaları üzerinde optimizasyon çalışması ihtiyacı doğmuştur. Bu ihtiyaca cevap vermek adına, tez kapsamında metal olarak tasarlanan yataklar yerine polimer esaslı malzemeler kullanılarak yatakların yeniden tasarımı üzerine çalışmalara başlanmıştır. Çalışmalar sırasıyla araştırma, malzeme seçimi, deney, tasarım ve imalat aşamalarını içermektedir. Araştırma kapsamında benzer uygulamalarda kullanılmak üzere çeşitli malzemelerin denendiği belirlenmiştir. Bu malzemelerden PTFE ve PA başta olmak üzere polime esaslı malzemeler farklı yükleme, hız ve sıcaklık koşullarında bazı testlere tabi tutulmuşlardır. Özellikle aşınma ve sürtünme mekanik özellikleri ile ilgili malzemelerin karakterleri ortaya konmuştur. Hazırlanan bu tez de benzer bir çalışmalar bütünü içermektedir. Farklı olarak bir sistem için tasarım kriterleri de göz önüne alınarak çalışma tamamlanmıştır. Çalışma için önce sistem gereksinimleri belirlenmiştir. Malzemesi seçilecek olan yatakların maruz kaldığı sıcaklık, sistemin kaldırabileceği yük miktarı ve mekanizmanın düşey hareket hızı en önemli girdi olarak kullanılmıştır. Sıcaklık olarak 250°C, yük olarak 10kg ve hız olarak 9m/s belirlenmiştir. Bu şartlar sürekli kullanım için değil en zorlanmış şart olarak düşünülmelidir. Ardından firmalardan koşullara uygun olduklarını söyledikleri dört farklı malzeme siparişi verilmiştir. Katkısız PTFE, katkısız poliimid, karbon katkılı poliimid ve son olarak termoplastik poliimid yatak malzemesi olarak tedarik edilmiştir. Bu malzelerden deneyler için yatak parçaları hazırlanmıştır. DSC analizi yapılarak malzemelerin sistemin çalışma sıcaklıklarına uygunluğu sorgulanmıştır. DSC analizi sonucunda her bir malzemenin sıcaklık açısından sorunsuz çalışacağı belirlenmiştir. Deneyler yapılırken yeni bir deney düzeneği hazırlamak yerine mevcut fırın şasisi ve mekanizmalar kullanılmıştır. Üzerlerinde bir takım iyileştirmeler yapılarak, deneye hazır hala getirimiştir. Bu deney düzeneği malzemelerin aşınma karakterini belirlemek için tasarlanmıştır. Mevcut malzemeler incelenmiştir. Bunlar arasından katkısız teflon, katkısız polyimide, karbon katkılı polyimide ve termoplastik polyimide seçilerek deneylere başlanmıştır. Deneyler sonucunda yatakların mekanik karakteri belirlenmiştir. Yatakların iç orta bölgelerinden ziyade uç kısımlarına yakın yerlerde hasar tespit edilmiştir. Deneyler sonucunda ve malzeme özellikleri kıyaslandığında karbon katkılı polyimide tasarım için uygun bulunmuştur. Düşük ısıl genleşme katsayısı bu tercihte etkin rol oynamaktadır. Tolerans değerleri belirlenirken hesaplamalarda önemli farklara sebep olmaktadır. Çalışmanın son safhası olan tasarım ve imalat aşamasına geçilmiştir. Başta mil referans ölçü kabul edilerek ölçülendirme işlemi yapılmıştır. Bunun sonucunda yatak iç çapı 6mm, dış çapı 10mm ve son olarakta boyu 23mm olarak belirlenmiştir. Yatakların plastik enjeksiyon yöntemiyle üretimi önerilmiştir. Çalışmalar kapsamında ayrıca hareketli braket parçasında bazı iyileştirmeler yapılmıştır. Merkezleme çıkıntıları sayesinde yatakların hasar görmesi engellenmiştir. Mil için sıcaklık değişkeni göz önünde bulundurularak zamanla mil yüzeyinde çatlaklar oluşabilir. Bu durum da düşünülerek tasarım tekrar gözden geçirilmelidir. Bu çalışma sayesinde; "ev tipi fırın sıcaklıklarında çalışabilecek polimer esaslı yatak malzemeleri" konusunda bir tez oluşturulmuştur.

Today, ovens are used in many areas of our lives are one of the important things. At temperatures as high as 200°C, offering the possibility of cooking food in the oven, sometimes the top or bottom shelf level to ensure better cook would want to change. It is difficult to open the door of the oven and to change the place of cooked food while cooking at these temperatures. For this purpose, an oven with a moving rack mechanism has been developed for both ease of use and to ensure energy saving. In this way, the user without opening the oven door tray of food to be cooked automatically to the appropriate shelf level can be reached. Using this mechanism, foods will be cooked in a short time. Changing the level of the tray for roasting is caused a loss of time. The user first opens the door of the oven, second take the tray outside of the oven cavity, and lastly puts the tray top or bottom level of the rack. It takes time and the user has to control food to plan the time that is the changing level and stoping the cooking. The movable shelf mechanism will help this need. There will not be any need to open the door while cooking to change the place of food. This mechanism will change the level automatically. The oven has two movable shelf mechanisms. One is on the right side and the other is on the left side of the oven. The front side is the area where users put foods inside the oven cavity from there. Movable shelf mechanism developed bushing systems designed for movement and changes on the development of open space is one of the optimization studies on the bushing parts that were needed. To address this need, this thesis has started to work on the design of the bushings by again using polymer-based material instead of metal material. Respectively research studies, material selection, testing, design, and manufacturing stages are included. There are lots of studies about polymer materials. Especially, PTFE and PA are used as bushing parts. Because of their performance, they are chosen. If the fillers that glass fiber, carbon fiber, bronze, copper, etc. are used in them, their mechanical and chemical performances are increased. Some of them are wear, friction, UV. These properties are defined with some test and test conditions. The important parameters are velocity, load, time, temperature for the test conditions. Some of the researches are taken to use as a reference before beginning the experimental studies of the thesis. It was seen that polymers with filler have higher mechanical performance instead of unfilled polymer, and the effects of velocity, load, temperature are changing depending on the material. In general higher load means higher wear. The high velocity is sometimes a higher coefficient of friction but sometimes is lower friction it depends on the material. After these literature studies, there is given information about what are the needs of movable shelf mechanisms. The most important parameter or need is temperature. The maximum oven temperature is 280°C for the cooking area and it was expected that the area of the mechanism could be a maximum of 250°C. The other parameters are weight and velocity. The maximum weight is 10kg the mechanism is designed to carry this weight. Lastly, 9m/s is the vertical sliding velocity of the mechanism. The other part of the thesis is a selection of the material of bushing. Four materials are chosen randomly, but they are suitable for the bushing of this mechanism and its parameters and its needs. They are pure PTFE, pure polyimide, carbon-reinforced polyimide, and thermoplastic polyimide. They are prepared for the experiment as a bushing part of the mechanism that length is 20mm, inner diameter is 6mm and outer diameter is 10mm. Before the experiment, some calculation was done to define the character of bushing behavior. It was shown that the end portion of bushing can be damaged while the mechanism is working. The experiment setup was prepared but not a special setup. The ovens themselves and the mechanism themselves were used for the experiment. Just some editing was done. For example, the backside of the body of the mechanism was cut, the algorithm of the electronic card was changed, and just the inside body was used instead of the whole body. The four materials were tested at the same time because the bushing system lets us for this. Two of the materials were put right side and two of the materials were on put left side. Difficult experiment conditions were chosen. The velocity and place weight was changed. 10kg was put on the front side of the rack and the vertical sliding velocity was made 11m/s. The mechanism worked 20 hours and it was matching 210m. The bushing system was heated with a ceramic heater. It could reach 150°C. The first experiment was done for material resistance against the temperature. DSC analysis was used for this experiment. It shows that all of the material could work at oven temperature. After this test, the wear test was done and its result supported the calculation of bushing's character. The end portion of bushings was damaged. Pure PTFE and carbon-reinforced polyimide did not lose their weight but all of the materials were damaged under normal forces, which are affected on the end portion of the bushing. The last stage of selection material is comparing other mechanical properties. Most of the mechanical properties are the same but because of the minimum coefficient of linear thermal expansion, carbon-reinforced polyimide was chosen for the design. According to experiments, PTFE has the same performance as a carbon-reinforced polyimide, but it has a five times bigger coefficient of linear thermal expansion than carbon-reinforced polyimide. This is the important comparison for selecting material. The big coefficient of linear thermal expansion means big tolerance for design. The last part of the study is design and manufacturing. After defining the material, some engineering calculations were done for dimensioning. These calculations were done for the 250°C environment temperature, which is the temperature of the mechanism standing area. The mile tolerances, bushing tolerances, and movable bracket tolerances are calculated step by step. Manufacturing tolerances also are added in calculations. The tolerances of bushing and bushing systems were defined as sliding fit. The mile, which the bushing is sliding vertically on it is used with the same dimensions which are chosen for the mechanism before. Another dimension was selected to reference the mile. The inner and outer diameter of the bushing did not change but its length of it became 23mm. because the calculations that show the character of the bushing system are confirmed by experiment. There are other dimensions changing for the bushing system. Besides designing bushing, some solutions were presented about bushing systems. Centering, changing some dimensions are some of them. For the manufacturing process, injection molding was suggested for bushing, and aluminum injection molding was suggested for the moveable brackets, also the design of it was made. Selecting the aluminum injection molding, the weight of the moveable bracket will be decreased. For this system, many materials can experiment with. The best decision will be given after lifetime experiments. There can be other situations while cooking for example oil is one of them. If the mile surface coats with oil, how will be the bushing performance. In addition, the material of mile can be changed with other types of steel and manufacturing processes. The temperature can cause some cracks on chromium mile because of lower and higher temperature changes. Thanks to this study thesis have been created about "polymer-based bushings that run on household oven temperatures".