Bir kamyon şasisinin burulma analizi /
Yazar: ATIL DURUTÜRK
Danışman: PROF. DR. ZAHİT MECİTOĞLU
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Uçak Mühendisliği Ana Bilim Dalı
Derece: Yüksek Lisans, 1999
Konu: Uçak Mühendisliği = Aircraft Engineering
Dizin: Burulma = Torsion ; Gerilme = Stress ; Kamyon = Truck ; Sonlu elemanlar analizi = Finite element analysis ; Süspansiyon = Suspension ; Şasi = Chassis

Özet:
Anahtar sözcükler: Merdiven tipi şasi, burulma, çarpılma, gerilme, süspansiyon, taşıt yükleri, sonlu elemanlar analizi Bu çalışmada bir kamyon şasisinin burulması incelenmiştir. Şasi, kamyonun ana taşıyıcı elemanınıdır. Dolayısıyla mukavemetli olması, yol durumunda karşılaşılacak her türlü yüke dayanması gereklidir. Burulma, aracın dinamik hareketi sırasında herhangi bir lastiğin çukura girmesi ya da bir tümseğe çıkması durumunda oluşan anlık statik aşırı yükler sonucu ortaya çıkar. Şasinin burulmaya karşı dayanımı yeterli değilse, şaside kalıcı deformasyonlann söz konusu olacağı söylenebilir. Burulma analizi, iki ayrı bölümde gerçekleştirilmiştir. İlk bölümde basitleştirilmiş elastik model yardımı ile kurulmuş bir araç modelinde burulmaya neden olabilecek bir yol durumunda oluşan burulma momentinin tespiti yapılmıştır. Bulunan bu momentin tüm şasi çerçevesini nasıl etldleyeceğinin analizi ise başka bir yöntem yardımı ile yapılmıştır. Tüm yapı düğüm noktalarından ayrılmış kabul edilerek, ortaya çıkan tüm şasi yan kol ve kirişlere etkiyen burulma yükleri ve gerilmeleri çıkarılmış; bunun dışmda kesit çarpılmasının söz konusu olacağı durumda burulma sabitlerinin nasıl değişeceği incelenmiştir. Ardından örnek bir şasi modeli ele alınarak, aracın olağan bir çukura düşmesi durumunda gelen moment değeri elde edilerek, şasi elemanlarına gelen momentler ve gerilmeler MATHCAD yazılımı yardımıyla sayısal olarak hesaplanmış ve IDEAS 'ta çizilen şasinin HYPERMESH'te modeli kurularak NASTRAN sonlu elemanlar yazılımında, sonlu elemanlar çözümü gerçekleştirilmiştir. XII

Abstract:
Keywords: Ladder type chassis, torsion, warping, tension, suspension, vehicle loads, finite element analysis In this study torsional analysis of a chassis frame is studied analytically and numerically. Chassis can be defined as the skeleton of the complete vehicle. It is the main load-carrying element of a vehicle. That is why, it should be resistible to all kind of loads that exists in variable types of road condition. With heavy commercial vehicles, we expect to carry tons of loads. This makes the chassis more critical for trucks. Chassis frames that have been mostly used in heavy commercial vehicles can be considered structurally as grillages. In another meaning as ladder frames. The endurance is not the only design criteria for chassis. Heavier chassis can achieve more endurance, but because of the commercial reasons, the vehicles have to be lighter in order to increase the payload and greadibility. Doing an optimal design can solve this problem. The only method of the optimal design will be the analysis of the complete frame due to all possible loads. These loads can be listed as below; - Static load of stationary vehicle - Braking and acceleration loads - Cornering loads - Torsional loads - Maximum load on front or rear axle - Drawbar loads - Antisymetric longitudinal loads - Dynamic loads The subject of this study will be the torsional analysis of ladder frame chassis. Torsional load arises from the uneven road conditions, especially when one of the wheels is in the pothole, while the others are on a flat surface. The depth of pothole determines the value of the torsional load. On the other hand, the torsional moment on the chassis frame depends on the torsional stiffness of the suspension (springs and tyres) and the torsional stiffness of the frame itself. In this study, the torsional moment of a simple vehicle model is found out by using a simplified elastic model. xm~^-g>^T p/The figure will help to express the parameters that used in the simplified elastic model. Torsional moment of complete system is calculated as T = Ke(6-v|/) where; (1) /KS BS N /KT BW -v K0 = ( K, B s "s + (KTBW (2) -) + ( Ke is the torsional stiffness of system, 0 is the rotational displacement of chassis, v|/ is the tilt experienced by the axles; Ks, and Kt are the stiffness of suspension and the tyre, respectively. This torsional moment results in the internal torsional moments on the chassis frames. The general formula of the St Venant torsional moment is defined below. MD=GJtd (3) Here, G is the shear modulus, Jt is the St Venant torsion constant and S is the rate of twist. XIVIt is assumed the cross sections of all the chassis members are stationary and the chassis side members are parallel to each other. With these assumptions, the rate of twist for the chassis members can be written as follows SL=2w/bl (4) where 1 is the wheelbase and b is the length of crossmembers. The rate of twist of the chassis crossmembers can be given as follows aQ=2w/bl (5) All the side and crossmembers are supposed to be separated from the connections. If we assume the rate of twist of all chassis members are the same; the torsional and bending moments of each member can be related each other by the following formula MDI =MDn = =MDm_= MD, =MD2 = =MDn.1 J JJ '??? J JJ J tl J ÜI J tm J tl J a J tm-l Here, Mdi, MonMom are the torsional moments of the cross members and Mdi, Md2Mum-i are the torsional moments of the side members. JqJtm are the St Venant torsion constants of cross members and JtiJtm-i are the St Venant torsion constants of side members. On the other hand the internal shear forces of the chassis crossmembers can be written in terms of the moments as follow Qi =Mdi|, Qn=(MD2-MD1)|Qm=(MDm-MDm.ı) \ (7) b b b where OjQm are the internal shear forces due to the bending moments on the cross members and b is the length of the crossmembers. We can write the torsional moments on the chassis members in terms of the moments of the first member as follow MDn=MDIis_, MDm=MDi^f-MDm=MDi^ (8) Mdi =Md,İI-, Md2 =MDi ^-Mom-i =MDi ^± (9) Jfl.'tl ^ tl Now, let us write the moment equilibrium of the first crossmember as Fl = ( Mm + MDn.... Mom ) + Qn li +Qra ( li + h ) +.... + Qm (li + h1 m-i) (10) XVwhere lı, İ21 m-ı are the length of the chassis side members. If we substitute the equations (7)-(9) into the equation (10) we obtain the moment equation of the first crossmember as follows MDI=^^hOD I^+HlVh) h=I D h=l The moments on the other chassis members can be calculated substituting this equation in to the equation (7) and (8). On the other hand, the torsion can cause warping in open cross sections. Therefore, in this study warping effect is taken into account to obtain more accurate results. Warping effect changes the torsion constants of the members. Total torsional moment can be defined as the sum of warping torsional moment Mw and St Venant torsional moment Md as follows MD * = Mw + MD (12) Where MD= GJtcp' and Mw = -EJwcpm. (13) Here Jw is the warping torsion constant. Substituting the equation (13) into the equation (12) we obtain MD*=-EJwcpm +GJtcp' (14) If we define a warping rate constant as follows