Ağır ticari çekici araçların ön süspansiyonunda kullanılan yaprak yay tasarımı
Design of front leaf spring in front suspension systems of heavy duty tractor vehicles
Yazar: HASAN AY
Danışman: PROF. DR. EKREM TÜFEKCİ
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Makine Mühendisliği Ana Bilim Dalı / Katı Cisimlerin Mekaniği Bilim Dalı
Derece: Yüksek Lisans, 2018
Konu: Makine Mühendisliği = Mechanical Engineering
Dizin: Bilgisayar destekli tasarım = Computer aided design ; Geometrik tasarım = Geometric design ; Mühendislik tasarımı = Engineering design ; Optimum tasarım = Optimum design ; Ürün tasarımı = Product design

Özet:
Bu çalışmada, ağır ticari çekici araçların ön süspansiyonunda kullanılan yaprak yayın tasarımınının beklenen konfor ve dayanım parametrelerine göre doğrulanması anlatılmaktadır. Yaprak yaylar araçların süspansiyon sistemlerini oluşturan en temel elemanların başında gelir. Ağır ticari çekici araç için tasarlanan yayın konfor beklentisi objektif ve subjektif araç dinamiği değerlendirilmeleri sonucu belli bir yay katsayısı belirlenerek, aracın dinamik özellikleri istenen seviyeye getirilmiştir. Araç dinamiği özelliklerinin yanısıra parçanın dayanım beklentisi için parçanın dayanım testleri gerçekleştirilmiştir. Bu çalışmalara ek olarak parçanın seri imalat ve kullanım koşullarında en büyük ömrü sağlaması için gerekli malzeme yapısı, alt parça dayanım testleri, ölçümsel kontroller, korozyon testi, paketleme testi, üretim kabiliyeti parametreleri de kontrol edilerek mühendislik onayı verilmiştir. Buradaki temel amaç maliyet ve ağırlık azaltma çalşmalarının yanında tasarımı yapılarak kullanıcıya sunulan ürünün en fazla ömür ve en az kalite hatası ile müşteri memnuniyetini artırmaktır. Parçanın dayanım beklentisi için bilgisayar destekli analizler ve fiziksel saha testleri gerçekleştirilmiştir. Bilgisayar destekli sonlu elemanlar analizinde, kullanımı süresince parçaya gelebilecek çeşitli yükler ile sınır koşulları verilerek farklı koşullarda analizler yapılmıştır. Analizler esnasında parçanın malzeme özellikleri girilerek ve gerçeğe en yakın şekilde modellenerek gerçeğe en yakın sonuçlar elde edilmeye çalışılmıştır. Parçanın fiziksel test onayları kapsamında yaprak yayın çalışma prensibi gereği laboratuvarda hazırlanan deney düzeneğinde çekme-basma yönünde test uygulanmıştır. Aynı test, sonlu elemanlar analizinde gerçekleştirilmiş olup sonuçlar karşılaştırılarak gelecek çalışmalar için bağıntı yapılmıştır. Parçanın tasarım onayında en önemli yer tutan bir diğer çalışma da  müşterilerin kullanım profilinden ortaya çıkarılmış olan araç dayanım testleridir. Bu testte aracın öngörülen kullanım ömrü belli parkurlar ve yollar ile kısa sürede araç test edilmektedir. Bu sayede aracın ve alt parçalarının test sonunda sorunsuz şekilde testten geçtiğinde aracın kullanım ömründe istisnai durumlar dışında problem yaşamayacağı doğrulanmış olmaktadır. Buradan toplanan 6 eksenli yük ve moment değerleri sonlu elemanlar analizi için kullanılmıştır. Toplanan yük verileri rainflow yöntemi ile çevrimlere çevrilerek analiz programına girdi olarak verilmeye hazır hale getirilmiştir. Araç üzerinden toplanan yük verileri ile parça araç üzerindeki haline uyumlu modellenerek yaprak yay gözünden toplanan veriler analiz programında kullanılarak koşturulmuştur. Analiz programına parçaların malzeme özellikleri ve uygun mesh tipleri girilerek sonucu gerçeğe en yakın hale getirilmesi amaçlanmıştır. Analizin koşması döngüsü sırasında analiz programı birim yüklü analiz yöntemiyle sonuçları ayrı ayrı adımlarla alarak gelen yükleri vektörel çıktıları ile çarparak hesaplamaktadır. Yaprak yayın bilgisayar destekli analizleri, laboratuvar deneyleri, araç dayanım parkur testleri ile dayanım doğrulanması gerçekleştirilmiştir. Fiziki testler ile bilgisayar destekli sonlu elemanlar sonuçları karşılaştırılarak bağıntılar yapılmıştır. Bu sayede gelecek çalışmalardaki doğruluk oranı artırılmıştır. Bu yöntemlerle kalite iyileştirilmesi, yeni proje tasarımı, maliyet azaltma ve ağırlık azaltması amaçlı yapılacak dizaynlarda izlenmesi gereken adımlar net bir şekilde ortaya koyulmuştur. Tüm bu çalışmalar sonucunda tasarımı yapılan ağır ticari araçların ön süspansiyonunda kullanılan yaprak yayın belirlenen kontrol ve  doğrulanma evrelerinden geçerek seri imalata giriş onayı verilmiştir.

Abstract:
Competition in the automotive industry is increasing dy by day. Increasing customer expectations and technology cause to push companies to make better vehicles. Companies work hard to provide an advantage in this rivalry. To be competitive in the industry, companies must make cost reduction, weight reduction, quality increase, generate new models, provide new technologies and to be ensure customer satisfaction. Automotive industry has different vehicle segments like sport cars, passenger cars, off-road vehicles, commercial vehicles, heavy commercial vehicles that causes variety of expectations of each customers. Heavy commercial vehicles are generally used for carrying heavy loads. However, homologation rules, vehicle own weight and strength of vehicle are roadblocks of loading vehicles. These roadblocks and fuel consumption depending weight lead to make weight reduction on vehicles. Total vehicle heavy equals vehicle own weight and goods weight what called payload. In heavy commercial vehicles, as long as payload is higher it is better for customer. Therefore companies need to have good design capabilities for lighter vehicle to survive in the industry. Design and validation are most important topics for new products generating. This study is aimed to explain how new front leaf spring of heavy duty tractor vehicle front suspension system is designed and validated.
Suspension system, what provides safety and comfort drive of vehicle, is one of the important system of vehicle due to heavy duty tractor suspension system carry driver, passenger, goods and driveline system. Suspension system has different parts such as spring, shock absorber, axles, antiroll bar, mounting brackets, bump stopper. Spring is main part of suspension system that store and release energy while articulation of road forces to vehicle. Shock absorber absorbes instantaneous loads, axles carry the system and connect vehicle to the wheels thanks to knuckle-hub-bearing system, antirollbar provides stability of vehicle thanks to torsion of bar, mounting brackets are used for mounting suspension parts to vehicle body, bump stoppers are used to stop suspension movement to prevent damage other systems. Springs have different types like coil spring and leaf spring. Coil spring is generally used for passenger and light commercial vehicles. Leaf spring is generally used for medium commercial and heavy commercial vehicles. In heavy duty tractor front suspension leaf springs are used due to package and strength requirement. Most of heavy duty tractor companies use mono leaf spring on front suspension due to weight concern. In this study, mono leaf spring is developed. Leaf springs carry the load and provide comfort. Vehicle dynamics foundations are depend on suspension system thereby leaf springs for heavy commercial vehicles. 
Leaf springs are produced from high strength spring steels due to exposing high stress. Heavy commercial vehicles are used different zones such as off road construction areas, normal roads, highways hence there are different requirements for different areas. Leaf springs have two types what are conventional springs and parabolic springs. Conventional springs are used for off road usage and parabolic springs are used for comfort and high stress. Production of leaf spring is very hard due to heating, forming, complexity process and heavy weight. 
Designing new part is very hard, time-consuming and expensive process. Know-how and design capabilities specify part quality. Designing new parts starts from theoritical calculations, solid model shaping, prototype production, computer aided analyzing, testing and production controls. Mono leaf spring study is started from package of parts and dynamics expectations. Leaf springs have spring rate like other springs, it has unique package and design need to put it into this package. Dynamic expectations are determined by objective calculations and subjective drivings by expert team. After determining package and dynamic expectations, leaf spring shape has been decided to provide dynamic and strength requirements. Solid model shaped leaf springs put the suspension system on virtual platform and starts validation process. Leaf springs are generally failed by not static loads by dynamics load hence leaf spring design, material selection and validation need to be for dynamic loads.
Validation is one of the most critical aspect of designing new parts. Providing part and vehicle expectations, not to get quality problem after serial production and customer satisfaction up to validation. Validation is started generally from computer aided analyzing programs. Computer aided engineering analysis program use their own calculation method to estimate part stiffness, strength, life and status on vehicle condition. Generally computer analysis results and real results become different. Know-how and experiment on parts lead to close gap between computer analysis and real life. Therefore, modelling and inputs are really importants to get truer results from the analysis program. Road load data and collection load data are really important to estimate forces and displacements of leaf spring. After computer analysis, laboratory bench fatigue test is other step. Simulating leaf spring working conditions on laboratory fatigue test and see the cycle life tells us the parts real life. 
Other validation method is vehicle durability test. Vehicle durability test can be described as it represents vehicle life averagely. Every customer has different usage habits and heavy commercial vehicles are designed for 1.000.000 km. It is not possible to test them 1.000.000 km in terms of time and cost. Therefore, special roads and tracks are designed for only vehicle validation. Collecting usage area data and converting them to vehicle run cycles lead to validate vehicles fast and more easily. Collecting road load data from vehicle durability areas and customer usage areas is important to use it on computer analysis. Collected time based load data are generated by data processing method like rainflow, level-crossing and range pair. Rainflow is generally used for converting time based load data to fatigue cycle. These cycles are also used for semi-vehicle test model for part validation and vehicle validation. In this study, road load data is collected from Lommel proving ground, where vehicle runs durability tests, and put the computer analysis. Also, vehicle data are generating laboratory bench test cycles to estimate part life. After dynamic and strength validation, other tests such as corrosion, dimensional checks, assembly checks, supplier production capabilities, logistic package check with other parts that affect leaf spring life.
In this study, firstly package control is checked. After that, objective calculations and dynamics evaluation drivings are performed. In this step, 25 kgf/mm, 28 kgf/mm, 30 kgf/mm and 32 kgf/mm 4 springs are produced and all are fitted the vehicle. After vehicle dynamics evaluation 32kgf/mm is selected as mono loaf spring rate. Parabolic thickness are determined for strength of parts to provide strength and less weight. Determining dimensions by package and determining spring rate are performed. Theoritical calculations are performed.
For analysing loads, from previous tests road load data is collected from leaf spring eyes and applying computer aided engineering analysis program and laboratory bench fatigue test. 6 axis and channels road load data is converted to cycles for computer analysis. For computer aided analysing abaqus, hypermesh and ncode programs are used. Loads, boundary conditions, system and dimensions are determined. Modelling are performed by the most appropriate mesh types and size to get the best results. System and part inputs are chosen from supplier reports, real materials specifications and calculation by analysis program. Firstly, static analysis, secondly laboratory bench test and thirdly vehicle durability test analysis are performed. Leaf springs are loaded and establish boundary conditions as real vehicle condition. Loads are generated from road load data and vehicle working condition. After that laboratory bench fatigue test is performed and then vehicle run durability test. To compare real life and analysis reports see the results. Firstly, static analysis report is 1434 Mpa and real life result is1171 MPa and that means 17% error. Secondly, laboratory bench test, analysis report is minimum 82.950 cycles and real test minimum result is 73.634 cycles that means 11% error. Thirdly, vehicle durability test, real durability test is ended at 150% without leaf spring failure so there is unknown leaf spring durability life. Computer analysis durability result is 1,907 vehicle that means under 16% error. 
These results that are under 17% is good for FE and real comparison. In addition, other good way is computer analysis results and real life results varify each other in terms of breakage area and red zones in analysis.In brief, thanks to this study designing and validation steps of heavy duty tractor vehicles front suspension system mono leaf spring development steps are decribed clearly. In addition, correlated FE models, describing test models contribute the literature for future design works.