Chassis frames - Abdullah Demir

Transkript

MARMARA ÜNİVERSİTESİ
TEKNOLOJİ FAKÜLTESİ
TAŞIT TEKNOLOJİSİ
ŞASİ, ÇERÇEVE ve GÖVDE KAROSER/İ
Şasinin Çalışma Koşulları
ve Traversler
Yrd. Doç. Dr. Abdullah DEMİR
Dr. PaulJ. Aisopoulos, Chassis Design, Powering the Future With Zero Emission and Human Powered Vehicles –Terrassa2011
Dr. PaulJ. Aisopoulos, Chassis Design, Powering the Future With Zero Emission and Human Powered Vehicles –Terrassa2011
The characteristics of body connection
with the chassis is highly relevant:
• structural function,
• insulation and isolation,
• safety and,
• partially, aerodynamics (due to
floor contribution).
• The remaining functions are not
directly affected.
Lorenzo Morello, Lorenzo Rosti Rossini, Giuseppe Pia, Andrea Tonoli, The Automotive Body, Volume I: Components Design, e-ISBN 978-94-007-0513-5
Şasinin Çalışma Koşulları
Chassis Operating Conditions
The design of an automobile chassis requires prior
understanding of the kind of conditions the chassis is likely
to face on the road. The chassis generally experiences four
major loading situations, that include,
• vertical bending,
• longitudinal torsion,
• lateral bending, and
• horizontal lozenging.
http://what-when-how.com/automobile/chassis-frame-sections-automobile/
Five basic load cases:
– Bending case
– Torsion case
– Combined bending and torsion
– Lateral loading
– Fore and aft loading
Automotive design - Chassis* design, web.iitd.ac.in/.../3-Automotive_chassis-design-...
Bir taşıta etki eden kuvvetler, taşıt cinsine göre değişiklik arz
etmekle birlikte temelde iki ana gruba ayrılırlar. Bunlar, Statik ve
Tekrarlı Dinamik kuvvetlerdir. Taşıtın maruz kaldığı kuvvetlerin
büyüklüğü ne kadar önemliyse kuvvetlerin tekrarı da en az o kadar
önemlidir. Zira, kuvvetleri periyodik olarak değiştirmek ve değişim
sayısını yeter derecede arttırmakla bir malzemeyi statik sınırların
çok altında da harap etmek mümkündür. Statik kuvvetler değişken
olmayan kuvvetler ile aracın ömrü boyunca en fazla 5x103 defa
tekrarlanan kuvvetlerdir.
Statik kuvvetler, taşıtın kendi öz ağırlığı ve yükü, fren ve kalkış
kuvvetleri, viraj kuvvetleri, burulma kuvvetleri, münferit darbe
kuvvetleri, çekici ile römork arası bindirme kuvvetleri olarak
sayılabilir.
Tekrarlı dinamik kuvvetler ise 2≈5x106 tekrar sayısından
başlayan, yol pürüzlülüğü, lastik çevresinin düzgünsüzlüğü
gibi sebeplerden dolayı ortaya çıkar.
Murat Ereke, Kubilay Yay, «Çiftkatlı Otobüs Gövdesinin Bilgisayar Destekli Gerilme Analizi»
Taşıt gövdesi yapısal analizinde, işletme şartlarından doğan
kuvvetlere karşı dayanıklılık temel hedeflerden biridir. Hedef,
ağırlık açısından uygun, yeteri kadar mukavim bir yapı (hafif
yapı) elde etmek, malzeme ve enerji tasarrufu sağlamaktır.
Karoserinin karmaşık yapısı gereği, işletme şartlarından doğan
zorlanma sonucu oluşacak gerilmelerin hangi yoğunlukta ve
hangi şiddette olacağının kestirilmesi büyük zorluk arz eder.
Gerilme yığılmalarının olduğu bölgeler kritik bölgelerdir.
Bu bölgelerdeki kesitlerin doğru tasarımı için gerilmelerin
şiddetleri bilinmek zorundadır. Deformasyonların ve gerilme
yığılmalarının tespitinde gövdenin sonlu eleman yöntemiyle
modellenerek bilgisayar ortamında analizi modern tasarım
tekniklerinin başında gelmektedir.
Taşıtın Kendi Öz Ağırlığı ve Yükü: Taşıtın düz
ve yatay bir zeminde durduğu farz edilirse,
şekildeki kuvvetlerin etkisi altında kalacaktır. G
ağırlığı, taşıtın öz ağırlığını ve yükü birlikte ifade
etmektedir. Ön ve arka aksları birer lastikli olan
taşıtlar binek otomobilleri, hafif kamyonlar ve
hafif römorklardır. Ön ve arka aks yükleri arasında
fazla fark oluşmaz, yani hemen hemen birbirine
eşit kabul edilebilir.
Vertical Bending: Considering a chassis
frame is supported at its ends by the wheel
axles and a weight equivalent to the
vehicle’s equipment, passengers and
luggage is concentrated around the middle
of its wheelbase, then the side-members
are subjected to vertical bending causing
them to sag in the central region.
Bending
• Dynamic loading:
– Inertia of the structure contributes in total loading
– Always higher than static loading
– Road vehicles: 2.5 to 3 times static loads
– Off road vehicles: 4 times static loads
• Example:
– Static loads
• Vehicle at rest.
• Moving at a constant velocity on a even road.
• Can be solved using static equilibrium balance.
• Results in set of algebraic equations.
– Dynamic loads
• Vehicle moving on a bumpy road even at constant
velocity.
• Can be solved using dynamic equilibrium balance.
• Generally results in differential equations.
Longitudinal Torsion: When
diagonally opposite front and rear
road-wheels roll over bumps
simultaneously, the two ends of the
chassis are twisted in opposite
directions so that both the side and
the cross-members are subjected to
longitudinal torsion (Fig. 1), which
distorts the chassis.
Fig. 1: Longitudinal torsion.
Burulma Kuvveti
Bir taşıtın ön tekerleklerinden biri bir tümsek veya bir engebenin
üzerine çıkarsa o tekerleğin dinamik tekerlek yükü artmaktadır. Sağ ve
sol tekerleklerin dinamik tekerlek yüklerinin farkı taşıtı uzunlamasına
eksen boyunca burulmaya zorlamaktadır. Tekerlek yüklerinin
birbirlerine göre farkı ve dolayısı ile burulma momentinin büyüklüğü
engebenin yüksekliği, tekerlek iz genişliğinin büyüklüğü, lastiklerin ve
yayların katılığı ve taşıt gövdesinin katılığına bağlıdır.
Taşıtların burulma momenti engebe yüksekliğine bağlı olarak
artmakta olup, her taşıt için aşılabilecek bir engebe yüksekliği sınırı
vardır. Tekerlek iz genişliği ne kadar büyük olursa, belli bir engebe
yüksekliği için meydana gelen burulma momenti o kadar küçük olur.
Lastik ve yayların yumuşaklığı ise, belli bir engel yüksekliğinde
gövdenin daha az dönüp dönmemesinde rol oynamaktadır.
Süspansiyon sistemi ne kadar yumuşak olursa, burulma o kadar az
olur. Taşıt gövdesinin katılığı veya elastikliği de burulma momenti ile
ilgili olup gövde elastik olduğu ölçüde moment düşmektedir.
Burulma Kuvveti (dvm.)
Bir
taşıt
gövdesinin
boyutlandırılmasında değişik
zorlayıcı kuvvetleri dikkate
almak gerekir. Her şeyden önce
statik
kuvvetler
altında
gövdede veya şaside kalıcı
deformasyonlardan kaçınmak
şarttır. Bir otobüs gövdesi için
ilk yaklaşım hesabında taşıtın
öz ağırlığı ve yükü + %30
münferit darbe kuvveti + %50
burulma kuvveti kullanılabilir.
Otobüs
gövdelerinde
kutu
profilden
oluşan
kirişler
kullanılır. Malzemesi ise St
37’dir. Literatüre göre [1,2],
otobüs
gövdesi
imalatında
kullanılan bu malzeme için
ortalama gerilme sınırı (ön
gerilme) σm= 9 kg/mm2 ve
genlik gerilmesi sınırı σg= 8
kg/mm2 değerleri alınmalıdır.
Akma gerilmesi de “σF ” için de
24 kg/mm2 değerinde alınabilir.
Torsion
• When vehicle traverse on an uneven road.
• Front and rear axles experiences a moment.
• Pure simple torsion:
– Torque is applied to one axle and reacted by other axle.
– Front axle: anti clockwise torque (front view)
– Rear axle: balances with clockwise torque
– Results in a torsion moment about x‐ axis.
• In reality torsion is always accompanied by bending due to
gravity.
Combined bending and torsion
• Bending and torsional loads are super imposed.
– Loadings are assumed to be linear
• One wheel of the lightly loaded axle is raised
on a bump result in the other wheel go off ground.
• All loads of lighter axle is applied to one wheel.
• Due to nature of resulting loads, loading symmetry wrt x‐z
plane is lost.
• R’R can be determined from moment balance.
• R’R stabilizes the structure by increasing the reaction force on
the side where the wheel is off ground.
• The marked –
– Side is off ground
– Side takes all load of front axle
– Side’s reaction force increases
– Side’s reaction force decreases
to balance the moment.
super imposed= birleştirilmiş
Combined bending
and torsion
Lateral
Bending:
The
chassis is exposed to lateral
(side) force that may be due
to the camber of the road,
side wind, centrifugal force
while turning a corner, or
collision with some object.
The adhesion reaction of the
road-wheel tyres opposes
these lateral forces. As a net
result a bending moment
(Fig. 2) acts on the chassis
side members so that the
chassis frame tends to bow in
the direction of the force.
Fig. 2. Lateral bending
Note: In modern production cars a
transversal acceleration of 1 g is
reached while turning at limit
conditions, while about 0.5 is
reached
in
longitudinal
accelerations.
G. Genta and L. Morello, The Automotive Chassis, Volume 1:
Components Design, 351 Mechanical Engineering Series, 2009
Viraj kuvveti
Taşıt
viraja
girdiği
zaman,
merkezkaç
kuvvetin
etkisi
altındadır. Kuvvetin yönü dışarı
doğru olduğu için şasi dış putreline
gelen yük artmaktadır. Viraj
kuvveti hesaplanırken şekildeki
gibi ağırlık merkezinin yerden
yüksekliğinin (h), taşıt ağırlığının
(G), viraj ivmesinin (a) ve iki
tekerlek arasındaki mesafenin
bilinmesi yeterlidir. / Ref: Murat Ereke, Kubilay Yay,
«Çiftkatlı Otobüs Gövdesinin Bilgisayar Destekli Gerilme Analizi»
The Automotive Body Volume II: System Design; 2011
For a vehicle travelling on a road with
known grip characteristics, the maximum
lateral force is given by the lowest of the
limit loads given by the Eq.
μy max = maksimum yanal adezyon katsayısı
Maximum lateral force due to the incipient capsizing
In the case of a road with good grip conditions μy max ≈ 0.8÷ 0.9, depending on the
ratio 2h/t between the height h of the center of gravity and the semitrack (t/2), the
limit of the lateral force is given by the tires or by the incipient capsizing. Considering
a vehicle with track t = 1.2 m and a relatively low center of gravity: h = 0.5 m, so the
ratio t/(2h) = 1.2 > μy max. The maximum lateral force is then imposed by μy max.
For a small commercial vehicle with the same track and a higher center of gravity h =
0.8 m t/2h = 0.75, the maximum lateral force is imposed by the capsizing condition.
The Automotive Body Volume II: System Design; 2011
Lateral loading
Lateral loading
• For a modern car t = 1.45 m and h
= 0.51 m.
• Critical lateral acceleration =
1.42 g
• In reality side forces limit lateral
acceleration is limited within 0.75
g.
• Kerb bumping causes high loads
and results in rollover.
• Width of car and reinforcements
provides
sufficient
bending
stiffness to withstand lateral
forces.
• Lateral shock loads assumed to be
twice the static vertical loads on
wheels.
Longitudinal loading
Limiting tractive and
braking
forces
are
decided by coefficient of
friction b/w tires and
road surfaces (note: b/w
= between / with)
Tractive and braking
forces adds bending
through suspension.
Inertia
forces
adds
additional bending.
Longitudinal loading
•
•
When vehicle accelerates and
decelerates inertia forces were
generated.
Acceleration –Weight transferred
from front to back.
– Reaction force on front wheel is
given by (taking moment abt RR)
• Deceleration –Weight transferred
from back to front.
– Reaction force on front wheel is
given by
Fren Kuvveti
Fren kuvvetleri taşıt fren yaptığı zaman ortaya çıkarlar. Düz
yolda ideal fren kuvveti dağılımı ile elde edilecek fren ivmesi
ortalama 4,5 m/s2’dir. Başlangıçta bir anlık maksimum değere
ulaşan fren ivmesi düz ve kuru bir asfalt yolda 8 m/s2’ye kadar
çıkar. Fren hesaplarında fren yolu boyunca muteber olan
ortalama ivmedir. / Ref: Murat Ereke, Kubilay Yay, «Çiftkatlı Otobüs Gövdesinin Bilgisayar Destekli Gerilme Analizi»
ÖRNEK
https://www.flickr.com/photos/bremach/
BREMACH T-Rex Tanıtım Broşürü
Horizontal Lozenging: A
chassis frame if driven
forward or backwards is
continuously subjected to
wheel impact with road
obstacles such as potholes, road joints, surface
humps, and curbs while
other wheels produce the
propelling thrust. These
conditions
cause
the
rectangular chassis frame
to distort to a parallelogram shape, known as
‘lozenging’ (Fig. 3).
Fig. 3: Lozenging
Asymmetric loading
• Results when one wheel strikes
a raised objects or drops into a
pit.
• Resolved as vertical and
horizontal loads.
• Magnitude of force depends
on
 Speed of vehicle
 Suspension stiffness
 Wheel mass
 Body mass
• Applied load is a shock wave
 Which has very less time
duration
 Hence there is no change
in vehicle speed
 Acts through the center of
the wheel.
• Resolved vertical force causes:
– Additional axle load
– Vertical inertia load through CG
– Torsion moment
to maintain dynamic equilibrium.
• Resolved horizontal force causes:
– Bending in x‐z plane
– Horizontal inertia load through CG
– Moment about z axis
to maintain dynamic equilibrium.
• Total loading is the superposition of all four
loads.
Allowable stress
• Vehicle structure is not fully rigid
• Internal resistance or stress is induced to balance external forces
• Stress should be kept to acceptable limits
• Stress due to static load X dynamic factor ≤ yield stress
– Should not exceed 67% of yield stress.
• Safety factor against yield is 1.5
• Fatigue analysis is needed
– At places of stress concentration
– Eg. Suspension mounting points, seat mounting points.
Bending stiffness
• Important in structural stiffness
• Sometimes stiffness is more important than strength
• Determined by acceptable limits of deflection of the side frame door
mechanisms.
– Excessive deflection will not shut door properly
• Local stiffness of floor is important
– Stiffened by swages pressed into panels (swage: baskı kalıbı)
– Second moment of area should be increased
Bending stiffness
• Thin panels separated by honeycomb structure reduced vibration
• Local stiffness has to be increased at:
– Door
– Bonnet
– Suspension attach points
– Seating mounting points
– Achieved by reinforcement plates and brackets.
Torsional stiffness
• Allowable torsion for a medium sized car: 8000 to 10000 N‐m/deg
• Measured over the wheel base
• When torsion stiffness is low:
– Structure move up and down and/or whip
– When parked on uneven ground doors fail to close
– Doors fail to close while jacking if jack points are at a corner
• Torsion stiffness is influenced by windscreens
• TS reduces by 40% when windscreens removed
• Open top cars have poor torsional stiffness
• Handling becomes very difficult when torsional stiffness is low.
Chassis types‐ Ladder frames
Used by early motor cars
Early car’s body frame did not contribute much for vehicle structure
– Mostly made of wood which has low stiffness
Carried all load (bending and torsion)
Advantages:
– Can accommodate large variety of body shapes and types
– Used in flat platforms, box vans, tankers and detachable containers
Still used in light commercial vehicles like pick up
• Side rails frequently have open channel section
• Open or closed section cross beams
• Good bending strength and stiffness
• Flanges contribute large area moment of inertia.
• Flanges carry high stress levels
• Open section: easy access for fixing brackets and components
• Shear center is offset from the web
• Local twisting of side frame is avoided
• Load from vehicle is applied on web
– Avoids holes in highly stresses flanges
• Very low torsional stiffness.
• Torsion in cross member is
reacted by bending of side
frames
• Bending in cross frames are
reacted by torsion of side
frames
• All members are loaded in
torsion
• Open sections are replaced by
closed sections to improve
torsional stiffness
– Strength of joints becomes
critical
– Max bending occurs at
joints
– Attachment of brackets
becomes more complex
Chassis types‐ cruciform frames (cruciform: krusiform, çapraz şekilli)
• Can carry torsional loads, no elements of the frame is subjected to
torsional moment.
• Made of two straight beams
• Have only bending loads
• Has good torsional stiffness when joint in center is satisfactorily
designed
• Max bending moment occurs in joint.
Hogging: belerme, bombe
Combining ladder and cruciform frame
• Combining ladder and cruciform frame provides good bending and good
torsional stiffness
• Cross beams at front and back at suspension points are used to carry
lateral loads
Chassis types‐ Torque
tube back bone frame
Main back bone is a closed
box section
Splayed beams at front and
rear extent to suspension
mounting points
Transverse beams resist
lateral loads Transverse
• Back bone frame:
bending and torsion beam
• Splayed beams: bending
• Transverse beams:
tension or compression
Chassis types‐ Space frames
• In all frames till now length in
one dimension is very less
compared to the other two
dimensions
• Increasing depth increases
bending strength
• Used in race cars
• All planes are fully
triangulated
• Beam elements carry either
tension or compressive loads.
• Ring frames depends on
bending of elements
– Windscreen, back light
– Engine compartment, doors
– Lower shear stiffness
• In diagonal braced frame s
stiffness provided by diagonal
element
Chassis types ‐ Integral
structures
• Modern cars are mass
produced
• Sheet steel pressings and spot
welds used to form an integral
structure
• Components have structural
and other functions
• Side frames + depth + roof
gives good bending and
torsional stiffness
• Geometrically very complicated
• Stress distribution by FEM only
• Stress distribution is function
of applied loads and relative
stiffness between components
• Advantages:
– Stiffer in bending and torsion
– Lower weight
– Less cost
– Quiet operation
Structural analysis by Simple
Structural Surfaces (SSS) method
• Many methods to determine
loads and stresses
• Elementary method is beam
method, FEM is advanced
method and SSS is intermediate
• Developed by Pawlowski in 1964
• Determines loads in main
structural elements
• Elements are assumed to be rigid
in its plane
• Can carry loads in its plane
– Tension, compression, shear and
bending
• Loads normal to plane and
bending out of plane is invalid
and not allowed
Passenger car
• More complex than box type van
• Detailed model vary according to mechanical components
– Front suspensions loads applied to front wing as for strut suspension
– Rear suspension (trailing arm or twist beam) loads to inner longitudinal member
under the boot floor
– SSSs varies with body types
Vehicle structures represented by SSS
Bus or box type vehicle
Van
Passenger car
SSS and Not SSS
Reading Text:
Flexural stiffness Kf is defined as the ratio between a load applied to the middle of the
wheelbase and the deflection of the same point; reaching acceptable values is generally
not difficult, if other structural requirements are satisfied, except in the case of very long
vehicles.
Torsional stiffness Kt is instead the ratio between a roll torque applied to the wheel
hubs of the front axle and the consequent rotation, when the rear axle hubs are fixed to
the reference system. In this ideal scenario, elastic primary and secondary elements of
each suspension are replaced by rigid elements of equal geometry.
Elementary scheme for modelling
the torsional deformation of a
vehicle: kta and ktp represent the
torsional stiffness of the two axle
suspensions, while Ktc and Ktt
represent torsional stiffness of
body and chassis frame in a
hypothetical vehicle with separated
frame.
G. Genta and L. Morello, The Automotive Chassis, Volume 1: Components Design, 351 Mechanical Engineering Series, 2009
Chassis-frame Design
Side-member and Cross-member
Chassis frames
The chassis frame is the commercial vehicle's actual load-bearing element. It is
designed as a ladder-type frame, consisting of side and cross members. The choice
of profiles decides the level of torsional stiffness. Torsionally flexible frames are
preferred in medium- and heavy-duty trucks because they enable the suspension
to comply better with uneven terrain. Torsionally stiff frames are more suitable for
smaller delivery vehicles and vans.
Apart from the force introduction points, critical points in the chassis-frame
design are the side-member and cross-member junctions. Special gusset plates or
pressed cross-member sections form a broad connection basis. The junctions are
riveted, bolted and welded. U- or L-shaped side-member inserts provide increased
framework flexural strength and reinforcement at specific points.
Side member: Şasi boyuna taşıyıcı, şasi (yan) kolu,
Cross member: Ara taşıyıcı, kuşak, travers
Bosch Automotive Handbook
Chassis Frame Sections
During movement of a vehicle over normal road surfaces, the chassis
frame, is subjected to both bending and torsional distortion as
discussed in the previous section. Under such running conditions, the
various chassis-member cross-section shapes, which find application,
include.
• Solid round or rectangular cross-sections,
• Enclosed thin-wall hollow round or rectangular box-sections,
• Open thin-wall rectangular channelling such as ‘C, T, or ‘top-hat’
sections.
Side-member Bending Resistance: The chassis side-members, which
span the wheelbase between the front and rear axles must be able to
take the maximum of the sprung weight. The sprung weight is the
weight of the part of the vehicle supported by the suspension system.
The binding stiffness of these members must resist their natural
tendency to sag. The use of either pressed-out open-channel sections or
enclosed thin-wall hollow round or rectangular box-sections can
provide the maximum possible bending stiffness of chassis members
relative to their weight.
Chassis Frame Sections
A comparison of the bending stiffnesses of
different cross-sections having the same crosssectional area and wall thickness is presented in
Fig. A to F. Considering a stiffness of 1 for the
solid square section, the relative bending
stiffnesses for other sections are,
Practically, a 4 mm thick C-section channel
having a ratio of channel web depth to flange
width of about 3:1 are used as chassis sidemembers. This provides a bending resistance of
15 times greater than that for a solid square
section with the same cross sectional area. For
heavy-duty
applications,
two
C-section
channels may be placed back to back to form a
rigid load-supporting member of I-section (Fig.
H).
To provide additional strength and support for
an existing chassis over a highly loaded region
(for example, part of the side-member spanning
a rear tandem-axle suspension), the sidemembers may have a double-section channel.
This second skin is known as a flitch frame or
plate.
the relative bending stiffnesses
Square bar
Round bar
Round hollow tube
Rectangular C-channel
Square hollow section
1.0
0.95
4.3
6.5
7.2
Side-and Cross-member
Torsional Resistance
The open-channel sections exhibit
excellent resistance to bending, but
have very little resistance to twist.
Therefore, both side and crossmembers of the chassis must be
designed
to
resist
torsional
distortion along their length.
Figure C to F illustrates the relative
torsional stiffness between openchannel sections and closed thinwall box-sections. Comparisons
firstly between the open and closed
circular sections and secondly
between the rectangular sections are
made, considering the open section
has a resistance of 1 in each case.
the relative torsional stiffness
Longitudinal split tube
Enclosed hollow tube
Open rectangular C-channel
Closed rectangular box-section
= 1.0
= 62.0
= 1.0
= 105.0
Fig.: Chassis-member sections.
A. Square solid bar
B. Round solid bar
C. Circular tube with
longitudinal slit
D. Circular closed tube
E. C-section
F. Rectangular box section
G. Top-hat-section
H. I-section
I. Channel flitch plate
This clearly explains the
advantages of using channel
sections over the hollow tube
due to high torsional stiffness.
The chassis frame, however, is
not designed for complete
rigidity,
but
for
the
combination of both strength
and flexibility to some degree.

Chassis frames - Abdullah Demir

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