Ağır Ticari Araçların Havalı Fren Sisteminde Kullanılan Körük Bağlantı Braketi Tasarımı

Abstract

Bu çalışma şantiye / arazi kullanımı olan Ağır Ticari Araçlar’da çevresel etkilerden kaynaklanan fren körüğü hasarlanma riskini önlemek adına yapılmıştır. Çalışma ile fren komplesi üzerinde bulunan ve fren körüğünün bağlandığı braketin tasarımı değiştirilerek yeni braket tasarımı , parça dayanım analizi, imalatı ,yorulma testi yapılmıştır. Fren sistemi paket alanı ve çevresel parçalar ile etkileşimi göz önünde bulundurularak braketin tasarım kriterleri belirlenmiştir. Tasarım yapılırken optimizasyon ve sonlu elemanlar analizlerinden faydalanılarak tasarım süreci hızlandırılmıştır. Sonlu elemanlar sınır koşulları araç üzerindeki fiziksel şartların basitleştirilmesi ile elde edilmiş ve modele uygulanmıştır. Paket kısıtlarına göre belirlenen kütük braket üzerinde topoloji optimizasyon analizi yapılarak ham tasarım elde edilmiştir. Ham tasarım üzerinde geometrik düzenlemeler yapılmıştır. Sonrasında civata bağlantısı modele dahil edilmemiş ve dahil edilmiş olmak üzereiki farklı yapısal analiz koşturulmuştur. Civata bağlantısı modele dahil edildiğinde mevcut civataların oluşturdupu ön gerilmelerin yetersiz olduğu görülmüş ve civata bağlantı hesapları yapılarak yeni metrik ve ön gerilme değeri belirlenmiştir. Bu hesaplar ışığında civata adedi ikiden dörde çıkmıştır.Braket üzerinde civata paterni düzenlenmiş ve sonrasında sonlu elemanlar analizi tekrarlanmıştır.Son seviye braket üzerinde sonlu elemanlar analizi mesh hassasiyetini değerlendirmek adına braket için kullanılan mesh boyutları sırası ile 4mm,3mm,2mm olacak şekilde tekrarlanmıştır. Değerlendirme yapılırken kullanılmak üzere 2 mm mesh boyutuna sahip model sonucu uygun görülmüştür. Sonlu elemanlar modelinin korelasyonu ve parçanın yorulma testi için test düzeneği geliştirilmiştir.Braketin araç üzerinde mağruz kaldığı çalışma koşullarının yansıtıldığı bu düzenekte braketin kritik görünen bölgelerine straingage yapıştırılarak ölçümler alınmış ve analiz sonuçları ile karşılaştırıldığında tutarlı olduğu ön görülmüştür. Sonrasında 0,4Hz’lik frekans ile maksimum fren kuvvetine tekabül eden yük uygulanmış ve 1,3 milyon çevrim sonucunda braket üzerinde herhangi bir hasar ve çatlak başlangıcı tespit edilmemiştir. Hedeflenen çevrim başarı ile elde edilmiştir. Sonuç olarak araç testlerinin uzun sürmesi ve maliyetinin yüksek olması sebebi ile tasarlanan bu braketin valide edilmesi için laboratuar test düzeneği geliştirilmiş, strain-gage yardımı ile parça üzerinde oluşan strain değerleri elde edilerek sonlu elemanlar analiz sonuçlarıyla karşılaştırılmış ve korelasyon sağlanmıştır. Sonrasında kurulan bu test düzeneğinde yorulma testi yapılmış ve parça ömrü için hedeflenen çevrim oranı sağlanmıştır.

Due to increasing competition in Automotive sector it becomes very important to reduce costs and shorten the design and verification process to maintain and /or increase market share and competitiveness. The shorten design and verification process let us respond quickly to customer expectations that increases customer satisfaction. Balance between cost, weight and durability of part is also important to keep sustainability of competitiveness. Nowadays computer programs is used to provide this balance during design processes. The ever increasing need of effective transportations puts automobile manufacturers in a non-avoidable statuation of maintaining and improvement of safety systems. The brake system has always been one of the most critical active safety systems. The assesment of the durability of components becomes more improtant in different technical areas like commercial vehicles with regard to the increasing trend towards light-weight constructions, reduction of time-to-market periods and reliability. For those reasons, the automotive manufacturers have to perform the durability tests in order to release the components considering their operational conditions, on the other words; the durability tests of the components must be realized in order to start the serial production.In this study it is aim to shorten design and validation process of a new brake actuator attachment bracket for construction purposed heavy commercial vehicle which will help to avoid brake actuator damage risk on construction area. In this study it is aim to shorten design and validation process of a new brake actuator attachment bracket for construction purposed heavy commercial vehicle which will help to avoid brake actuator damage risk on construction area.With the study a new bracket which is on brake assy and where brake actuator is attached is designed, analysed, manufactured and tested. Brake system package and environmental part interaction is considered while determining the design constraints. Bolts are used to provide bracket to axle attachment. Input force is provided by brake actuator which transforms air pressure to push road force. And this push force generates a reaction force on bracket where brake actuator mounted. Design space is determined according to package and brake actuator size. Chassis frame , differential housing have effect on design space constraints. Optimization and structural analysis tools are used to speed up design process. Boundary conditions are obtained which is used on finite element and optimization analyses via simplifying the physical conditions that is on vehicle and applied to models. After defining design space, a rough bracket volume designed on computer aided design program to be used for optimization analysis. Boundary conditions applied according to brake actuator size and fixing points defined to optimization program. According to topology optimization result efficient design volume obtained. This rough model was geometrically improved on computer aided design program and used as finite element model . Bracket was modelled with hexahedral elements as fixed from bolt holes and input force applied . Stress values were lower than yield point according to finite element analysis result. Later on, attachment bolts were taken in to account and modelled, friction coefficient was defined between axle and bracket mating surfaces to improve accuracy of analysis. Second result showed that clamp force is not sufficient for this joint and bracket rotated around mounting area . Free body diagram was drawn to analyse force and moments. Later on clamp force calculations have been done. According to results bolt quality and metric have been determined. Number of bolt was changed from two to four. Bolt pattern was rearranged on bracket on computer aided design program. New proposal bracket design was modelled for finite element analysis. Axle was modelled as mounting block with hexahedron mesh and material was chosen as spherodial graphite iron which has 170 GPa Young’s modulus and 0.30 Poisson’s ratio, bracket was modelled with second order tetrahedron mesh and material was chosen as spherodial graphite iron which has 170 GPa Young’s modulus and 0.29 Poisson’s ratio also friction coefficient defined as 0.3 µ for this mating surfaces. Mesh sensitivity analysis has been performed via different mesh size of bracket model as 4mm , 3mm and 2mm. Results have been evaluated and 2mm meshed model result and 3 mm meshed model result was not different so it was decided to use 2 mm meshed model results are useful. According to 2mm meshed model results two critical areas determined to measure strain values on rig testing and stress values were calculated as 280 MPa and 175 MPa on finite element analysis program. Many companies experience the pain of expensive recalls, costly product rework and unexcepted delays in product releases. A major contributing factor is the lack of an effective product design verification and validation process. Most of the failure is based on fatigue at vehicles and it is defined as the weakening of a material caused by repeatedly applied loads. It is the progressive and localised structural damage that occurs when a material is subjected to cyclic loading. The nominal maximum stress values that cause such damage may be much less than the strength of the material typically quoted as the ultimate tensile stress limit, or the yield stress limit.Fatigue occurs when a material is subjected to repeated loading and unloading. If the loads are above a certain threshold, microscopic cracks will begin to form at the stress concentrators such as the surface, persistent slip bands, and grain interfaces. Eventually a crack will reach a critical size, the crack will propagate suddenly, and the structure will fracture. The shape of the structure will significantly affect the fatigue life; square holes or sharp corners will lead to elevated local stresses where fatigue cracks can initiate. Round holes and smooth transitions or fillets will therefore increase the fatigue strength of the structure. So it is very important to determine the fatigue life of new designed parts to understand the behavior under working conditions. Fatigue testing in a laboratory is defined as an accelerated test that is specifically designed to replicate fatigue damage and failure modes from proving grounds. The aim using proving ground is to increase the damage gathering by adding extreme events that would match the target usage mileage in short periods. To understand if the vehicle life cycle is equivalent or not, a customer clinic is performed that shows the customer usage and loading statistics. While braking system in operation bracket is subjected to a reaction force which is generated by brake actuator. This force has more effect than road load data on fatigue life of the bracket. For this reason static stroking test rig has been developed to correlate finite element model and done fatigue testing. Strain values has been measured on critical areas of bracket according to finite element analysis. This strain values are transformed to stress values and compared with finite element analysis results also displacement value measured on bracket during force application. It seemed that the results are consistent. Stress values measured as 270 MPa and 170 MPa at critical sections on bracket. Later on maximum brake actuator push road force applied with 0.4 Hz frequency on that test rig . After 1.3 million cycle no crack initiation or damage investigated. The aimed cycle number has been performed successfully. As a result , design and validation process of a new brake actuator attachment bracket for construction purposed heavy commercial vehicle which will help to avoid brake actuator damage risk on construction area have been performed via using computer programs and test rig is developed to validate this new design bracket to reduce development cost and shortened the development process time which helps to maintain competitiveness of product development.