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İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Makine Mühendisliği Anabilim Dalı / Malzeme ve İmalat Bilim Dalı

Ağır ticari araç kabin denge çubuğu için malzeme seçimi analizi

Material selection analysis for heavy commercial cabin stabilizer bar

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Özet:

Denge çubuğu tüm taşıtlarda kullanılan, aracın devrilme direncini sağlayan en önemli elemandır. Ağır ticari araçların yüksek yük taşıma kapasitesi ve zor yol şartlarında çalışması yüksek rijitlikte bir süspansiyon sistemi ve denge çubuğuna sahip olmasını gerektirmektedir. Süspansiyon sistemi rijitliğinin yüksek olması güvenli sürüş sağlarken buna karşın sürücü konforunu azaltıcı yönde etkilemektedir. Bu nedenle ağır ticari araçların kabinlerinde fazladan bir süspansiyon sistemi tasarımı yapılmaktadır ve kabin süspansiyonunda kullanılan denge çubuğuna kabin denge çubuğu adı verilmektedir. Bu çalışmada ağır ticari araçların kabin süspansiyon sistemlerinde kullanılan bir kabin devrilme çubuğunun genel tasarımı yapılmış, malzeme, ısıl işlem ve kaynak işleminin yorulma ömrüne etkisinin belirlenmesi hedeflenmiştir. Yeniden tasarlanan kabin süspansiyon sisteminde, kabin denge çubuğunun değişen montaj şekline göre analitik hesapları yapılmıştır. İçi dolu ve boş çubuk kullanımı değerlendirilmiş ve boru kullanımına karar verilmiştir. Kabin denge çubuğunun uç bağlantı şekli için rakip ve teknoloji araştırması yapılarak, uygulanabilirlik ve maliyet açısından gaz altı kaynak yönteminin en uygun olduğu belirlenmiştir. Kabin denge çubuğundan beklenen rijitlik değerine göre boyutları hesaplanmıştır. Gerilme analizi sonuçlarına ve piyasadaki kullanım ve bulunabilirlik göz önüne alınarak malzeme alternatifleri değerlendirilmiş ve üç malzeme seçilerek karşılaştırılmaya karar verilmiştir; SAE 4140, 26MnB5, 34MnB5. Borular kondüksiyon yöntemiyle tavlandıktan sonra yağ ve polimerli su ortamlarında sertleştirilmişlerdir. Sertleştirme sonrası farklı sıcaklıklarda temperlenerek farklı sertliklerde borular elde edilmiştir. Kaynak çalışmasında kaynak ön ısıtma derecesi, kaynak parametreleri ve kaynak sonrası uygulanacak işlemler belirlenmiştir. Kaynak ön ısıtma için indüksiyon yöntemiyle kaynak bölgesi ısıtılmıştır. Kaynak sonrası havada soğutma ve battaniye sararak yavaş soğutma yapılmış ve battaniye ile yavaş soğutmanın daha uygun olduğuna karar verilmiştir. Hazırlanan kaynak numunelerinden sertlik ölçümleri alınmış, makro ve mikroyapı incelemesi yapılmıştır. Hazırlanan numuneler sadece burulma zorlanmasına maruz kalacakları ömür testlerine sokulmuşlardır. Ömür testinde numunelere eş gerilme uygulanmış ve çevrim sayıları üzerinden karşılaştırma yapılmıştır. Ömür testlerine göre en uzun çevrim sayısı 34MnB5 malzeme borunun yüksek sertlikte üretilen numunesinde gözlenmiştir. Tüm numunelerde hasar kaynak bölgesinde meydana gelmiştir. Sonuç olarak yüksek yorulma ömrü gerektiren bir parça olan denge çubuğu malzemesi ve imalat yöntemi belirlenmiştir.

Summary:

Because the spring rates of the chassis springs in heavy duty are inevitably high, due to the heavy vehicle loads, unevennesses in the road surface or even vibrations and structure-borne noise, resulting from axles and the drive train transmitted to a considerable extent, to the chassis via the axle spring mounting. In order to minimize the transmission of such continuous shocks and vibrations to the driver's cabin, and thus to driver's workplace in terms of ergonomics and occupational safety, driver's cabin suspensions have been developed in which the drivers's cab is supported by vehicle chassis using a separate suspension system. Cabin suspension system can be designed with lower spring rates and softer shock absorbers than the axle suspension due to the much lower weight of the cabin, which is why unevennesses of the road surface or vibrations originating from the drive train or axles of the vehicle can therefore be isolated or kept away from the driver's workplace. Cabin stabilizer is sub assembly of cab suspension system and it is very important from the aspect of safety and comfort, it transmits the loads from the outside to the inside wheel to minimize the body roll. Stabilizer bar is working under dynamic loads through all its life. Therefore, fatigue life is very important for robust design. When the vehicle is moving on uneven surfaces and vehicle is rolling, torsional stress is applied on stabilizer bar. Despite lateral and horizonltal stress are applied on stabilizer bar, torsional stress is main stress. It is because stabilizer bar is safety part for heavy commercial vehicles, high strength materials are required to be used. Stabilizer bar should be durable under both high stresses and dynamic loads. To achieve these criteria, high yielding stress and toughness are required. For this purpose, instead of conventional steels high strength steels are selected. High strength steels are required to be heat-treated to have good mechanical properties. In order to introduce the forces, torques and motions of the cabin into the stabilizer bar, and for the purpose of the guidance of the cabin in the longitudinal and transverse direction, generally torque levers are attached to both ends of stabilizer bar. Connection between stabilizer bar and arm exposed to very high stresses during service life. Current stabilizer bar ends are assembled with bolt and nut very reliably after forging both ends. Bolt-nut assembly aftere forging is very good opportunity for attaching the ends of stabilizer bar thanks to its solid bar design. However, because of new stabilizer bar is tubular, new attaching strategy is required. After examining benchmark vehicles, patents and supplier's technology, MIG welding is proposed. Welding is very conventional and well-known method also used at stabilizer bar connections for long years. However, using especially MIG welding with high strength steels requires special attention because of heat introduced during welding and with the associated microstructural changes and possible occurrence of notch stresses. For this special case of welding, material selection, manufacturing process and laboratory tests should be examined and pre-specified before final design study. In this study, optimal material selection, dimensions and manufacturing process for new tubular welded cabin stabilizer bar are specified and verified through laboratory torsional fatigue test and some metallurgical analysis like hardness measurement, micro and macro analysis. Stiffness is calculated analytically for previous and new design stabilizer bar and dimensions are proposed for new design. Current stabilizer bar is solid bar, however for new design tubular stabilizer bar is proposed. Weight reduction plays very important role in fuel economy to be competitive in heavy commercial vehicle market. Weight reduction can be achieved primarily by the introduction of better material, design optimization and better manufacturing processes. Through proposing tubular stabilizer bar instead of solid bar, weight advantage is gained. Three different materials is selected from candidates as SAE4140, 26MnB5 and 34MnB5. Different quenchants are used as quenching media for each materials; oil quenchant for SAE 4140, polymer-water mixture for 26MnB5 and 34MnB5. The use of polymer-water mixture is increased in the automotive and heat treatment industry. Polymer-water mixture quenchants provide uniform heat removal during quenching resulting in reduced distortion and thermal gradient compared to water as a quenchant media. Cooling rate can be controlled by polymer concentration. Hardness of quenched part decreases with increasing polymer concentration of quenchant. After quenching, tempering is applied for all specimens. Tempering temperature is main parameter for final hardness of parts. For SAE 4140, three different specimens are manufactured through three different tempering temperature. For 26MnB5 and 34MnB5, two different specimens are manufactured. The reason of manufacturing specimens with different temperature is to examine optimum hardness of material because weldability and strength of material are not proportional to each other. Rockwell hardness is measured from tube surface. Micro-hardness measurement is done from some specimens tubular bar sections. Up to the hardness results SAE 4140 and 34MnB5 tubular bars' hardness values are around 460HV and 26MnB5 tubular bar's hardness is around 420 HV. Microstructures of bars are checked and it is observed that all specimens have martensitic structure as expected. At second step, tubular bars are welded to forged ends. Forged end materials are selected same for all parts as SAE 4140. To eliminate operator factor, welding is done with automated welding machine. Welding company was responsible for setting welding parameters. Pre-heating before welding and wrapping with blanket after weding is applied to all parts to decrease cooling rate during and after welding. Slow cooling also decreased distortion that is very important for automotive parts because of tight assembly tolerances. Micro hardness is measured from welded section. It is observed that all specimens have highest hardness at heat affected zone and lowest hardness value at welded zone. 34MnB5 has more uniform hardness through welded zones compared to other specimens and highest hardness value at HAZ. SAE 4140 hardness values are differ too much between welding zones for example it is measaured 210 HV at welding location and 460 HV at bar. Up to microstructure analysis, 34MnB5 specimen weldability is better than other specimens. All specimens are subjected to torsional fatigue test at loaboratory. Fatigue test results are evaluated for comparision by means of highest fatigue cycle. One side of tubular bar is fixed and other side is pushed and pulled. Torsional stress is calculated by displacement of one arm so from torsional angle. To correlate analytical calculations, stress is measured with 0-90 rosette strain gage during fatigue test. Fatigue test cycle target is set between 10,000 and 100,000 to avoid low-cycle fatigue and to reduce testing time. Test frequency is not increased very much to avoid possible heat generation and set 1.5 Hz. Up to the results best candidate is 34MnB5 with high strength. All fractures are observed at welding location. This proves that welding is weakest location of welded cabin stabilizer part. New design is proposed for welding location of cabin stabilizer part to reduce stress at welding location. As a result, best candidate material with optiumum hardness is proposed with proven torsional fatigue test.