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3.1
3
CH
AS
SIS
1CH
AS
SIS
3.1F
ram
e3.
1.1F
ram
e de
sign
x
Ladder frame of a semitrailer tractor
1 2 3 4 5 6
10 9 8
7
LEGEND1 Front end2 Side rail (U section) with grid of holes3 Side rail insert (L section)4 Crossrail (top-hat section)5 Main crossrail6 Crossrail (tube section)7 End crossrail (tractor)8 Trailing arm support (with air sus-
pension)9 Shock absorber bracket10 Frontal underride protection
BASIC PRINCIPLES
Chassis frameThe frame of the commercial vehicle forms the basis of the chassis. It accom-modates all the axles,the entire drive train with engine, gearbox and transfer case and supports the driver's cab as well as the superstructure.
As the vehicles have to be suitable for the flexible deployment of a wide variety of su-perstructures, the frame is the major sup-porting element.
The frame geometries and frame cross-sections are geared to the intended use in each case.
Torsional rigidityWhile the frame should have a high level of torsional rigidity for better driving charac-teristics on well-developed roads in distri-bution and long-distance transport, a weaker frame with high torsional elasticity to absorb the suspension movements is required for off-road and construction site deployment.
FUNCTION
Ladder frameThe chassis frames of commercial vehic-les are usually constructed as ladder fra-mes with two side rails (U sections) and crossrails (➜ Fig.). The connecting ele-ments of side rails and crossrails are gus-set plates which are riveted, bolted or wel-ded. The rivet connections that are mainly used exert less of a load on the frame than welded connections, as they are more de-formable and get by without the introduc-tion of heat (tensions). So-called "huck-spin" connections combine the solidity and strength of rivet connections with the detachability of bolted connections.
Side railsFrame side rails nowadays have different cross-sections along their length accor-ding to the local load of the frame. The "fish-belly shape", for example, is created in this way; it has a higher section for gre-ater flexure resistance between the axles. At critical positions with load peaks (e.g. semitrailer supports), positive-engaged in-serts in the form of U or L sections are fit-ted in the side rails as reinforcement. On frames subjected to extreme loads, some of the frame side rails are closed to form a box section.
CrossrailsAs a rule, with the exceptions of the end and front crossrails, the frame crossrails are only mounted on the side rail bridge so
that the frame side rails, which are subjec-ted to high stress, are not weakened. Va-rious section shapes of the crossrails influ-ence the torsional rigidity of the entire chassis. This rigidity rises from the simple U section, through the top-hat section up to the tube section.
Front endThe front end closes the chassis frame to-wards the front. The front driver's cab mounting, the steering gear, the front springs, the cooling system, the front un-derride protection and the bumpers are attached to the front end. A well-concei-ved and versatile structure of the front end is decisive for the design of the driver's cab and the installation options (e.g. radi-ator with large area).
3.2
3
CH
AS
SIS
3.1.
2Des
igns
x
Ladder frame with rear underride protection
BASIC PRINCIPLES
Frame shapesAlongside the ladder frame which is used almost exclusively for commercial vehicles and trailers (➜ Fig.), space frames and supporting braces are also used in the area of commercial vehicles.
Other forms of frame are used above all for other vehicle types. They usually have specific intended uses (tubular space fra-mes for sports cars and racing cars, pas-senger car ladder frames for off-road ve-hicles, SUVs).
Special frames such as X frames, transax-le frames, frame-floor systems and self-supporting bodies are found almost ex-clusively in the area of passenger cars and will not be described here in more detail.
FUNCTION
Body preparationFor a ladder frame, it is decisive that no components of the chassis protrude bey-ond the upper edge of the frame. The desired superstructures can then be mounted without any spatial conflict.
It has also proven effective to arrange pairs of drilled holes in a tight grid on the side rails as securing options for later add-on parts and conversions. The uniform grid of holes means that add-on parts can be moved easily (module system). Fur-thermore, time-consuming drilling beco-mes superfluous and the corrosion pro-tection applied at the plant is preserved.
SubframeTo accommodate auxiliary units or super-structures, subframes or assembly frames are secured. On the one hand, these form a separate supporting framework and on the other hand, geared to the load, they reinforce the main frame. In the case of so-called twin frame, a subframe is con-nected to the main frame.
Space frameThe supporting frames for buses are set up as space frames. By constructing a supporting framework made of hollow sections, a frame structure that is both torsionally and flexurally resistant is achie-ved.
Supporting braceOn some smaller commercial van types, an integral design is used to create a joint supporting brace. This consists of the subassembly made of bevelled metal-pla-te sections and pressed metal-plate parts and the combined body with body and passenger cell.
3.3
3
CH
AS
SIS
3.2A
xle
desi
gns
3.2.
1Des
igns
x
Front axle as dropped stub axle
3
2
1 2
1
LEGEND1 Steering knuckle 2 Steering knuckle eye3 Axle body
BASIC PRINCIPLES
Axle designsAxles are an important part of the wheel suspension and form the supporting ele-ments for the wheels. This means they are among the unsprung masses of the vehic-le (➜ page 3.9). Reducing the weight of the axles and thus increasing the ratio of sprung to unsprung masses is one of the main objectives of modern axle designs.
Distinctions are made between:
Driven and non-driven axles
Steerable and non-steerable axles
Depending on the drive concept, rear ax-les, front axles or both are driven or seve-ral together (e.g. in the case of all-wheel or multi-axle drive systems). The non-driven axles include standard front axle as well as trailing and leading axles.
In the area of commercial vehicles, rigid axles are usual. For economic reasons, the use of independent wheel suspension, as is normally used in passenger cars, has not become standard practice.
FUNCTION
Front axlesTwo designs of rigid, steerable front axles are fitted in commercial vehicles. The axle bodies of rigid axles are die-forged and usually have a double-T section or a square section. The most widespread front axle, the stub axle (➜ Fig.), has only one eye at the fixing points for the steering knuckle.
The fork axle is more difficult to manufac-ture and thus more expensive; its axle ends are fork-shaped.
Rigid front axles are of dropped design to retain a larger design envelope for the en-gine or to lower the chassis (frame).
Driven front axles have a differential gear in the axle body. The wheels are driven in-directly via universal joints or an additional planetary gear set in the wheel hub.
Rear axlesCommercial vehicles usually have rigid driven rear axles with differential gear.
Distinctions are made between:
Banjo axles
Cone or flared axles
One-piece (undivided) rear axles
The differential gear on banjo axles has the advantage that the entire axle head (bevel gear with differential cage) can be mounted and adjusted as an assembly outside the axle housing. On most mo-
dern commercial vehicles, rear axles based on the banjo concept are used.
In the case of cone or flared axles, the axle head is divided transversely into two hal-ves in the area of the differential.
One-piece (undivided) axles cannot be dismantled. This makes later assembly and adjustment work very complex. One-piece axles are mainly used in passenger cars as well as in commercial vans; their only cost benefits lie in manufacturing.
3.4
3
CH
AS
SIS
3.2.
2Driv
e ax
les
x
Drive axle designs
A
B
C
2
2 3
2
5
1
1
1
4
3
LEGENDA Hypoid axleB Planetary drive axleC Planetary drive axle as drive-through
axle1 Axle drive2 Differential gear3 Planetary gear set4 Inter-axle differential 5 Drive-through axle
BASIC PRINCIPLES
Drive axlesThe axles that drive commercial vehicles are termed drive axles. A commercial ve-hicle can have one or more drive axles.
If all axles are driven, this is referred to as all-wheel drive. The number of driven wheels of a commercial vehicle is speci-fied in the wheel formula (➜ page 2.2).
Depending on the design, distinctions are made between:
Hypoid axle
Shifting axle
Planetary drive axle
These can be designed as drive-through or final axles (tandem-axle assembly).
FUNCTION
Hypoid axleThe hypoid axle is a banjo axle (➜ page 3.3) on which the bevel gear differential (➜ page 6.33) is arranged in the middle drive (➜ Fig.).
On the hypoid axle drive, the pinion of the drive bevel gear is not arranged centrally, rather is offset slightly from the centre of the ring gear (➜ Fig. page 6.32). This de-sign enables a greater pinion diameter and a special toothing shape (hypoid too-thing). This distributes the load onto a considerably greater gear area, enabling the transfer of greater drive power.
Low-cost manufacture and greater me-chanical efficiency (few gear ratio stages) are features of this design. Hypoid axles are regarded as very economical, as they have high load-bearing capacities with low dead weight.
Shifting axleIf an engaging and disengaging planetary gear set is integrated in the ring gear of the differential (➜ page 6.33) on the hypoid axle, this is referred to as a shifting axle. This can be shifted e.g. between a slow gear ratio and gear ratio 1:1 (blocked pla-netary gear set). This design is only used very rarely.
Planetary drive axleA planetary drive axle has a gear ratio sta-ge in the wheel hub in the form of a plane-
tary gear set (➜ Fig.). The illustration on the next page shows (in the bottom right) an example of a planetary drive gear set. The gear ratio directly in the wheel hub means the drive shafts are subjected to lower loads, enabling a smaller differential and greater ground clearance of the axle.
Drive-through axleWith two drive axles located one behind the other, a drive-through axle is necessa-ry (for example with wheel formulae 6x4 or 8x4). A second axle is driven by a drive-th-rough mounted on the first axle. This tan-dem-axle assembly is also referred to as a tandem axle.
On the drive-through axle, torque and ro-tational speed are picked up via a spur gear ratio to drive the 1st axle (➜ Fig.). The drive-through also contains an inter-axle differential for speed balancing between the 1st and 2nd axles of the tandem-axle assembly, which as a rule is equipped with an engaging and disengaging diffe-rential lock (inter-axle differential lock).
3.5
3
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AS
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xx
EXAMPLE
A Hypoid axleB Tandem-axle assembly with
planetary drive axle
C Part of a planetary drive axle1 Axle drive2 Differential gear
3 Drive-through4 Planetary gear set5 Wheel hub
Hypoid and planetary drive axle
12
1 2 3 5
5
5
4
B
C
A
3.6
3
CH
AS
SIS
3.2.
3Lea
ding
and
trai
ling
axle
s
x
Rear-axle assembly with trailing axle
1
32
45
LEGEND1 Drive axle2 Steered and lifting trailing axle 3 Lift system4 Hydraulic cylinder RAS5 Steering tie rod
BASIC PRINCIPLES
Supplementary axlesSupplementary axles are fitted to enable high level of maximum payloads without exceeding the legally prescribed maxi-mum permitted axle loads (➜ page 2.6). If supplementary axles have no drive func-tion, they are designed as leading or trai-ling axles.
Leading and trailing axles are frequently designed as lifting axles.
FUNCTION
Leading axleA leading axle is only used as an additional rear axle and it is arranged in the direction of travel in front of the drive axle. It usually has single tyres and air suspension.
Trailing axleA trailing axle as an additional rear axle is arranged after the drive axle in the direc-tion of travel. It usually has single tyres and air suspension.
Steer axleA steer axle is a leading or trailing axle de-signed as steerable (➜ Fig.).
A second, trailing front axle is usually stee-red by means of a linkage system and its own steering gear.
Trailing or leading axles for the rear axle are steered hydraulically. Rear-axle stee-ring RAS is used at MAN. The RAS does not need heavy linkage: the steering mo-vements are transferred hydraulically by the steering cylinder on the steering gear to the hydraulic working cylinder on the trailing axle.
The electronically controlled RASec provi-des the optimised steer angle in every dri-ving situation. An electronic control sys-tem calculates and controls the steering operation.
Lifting axleIf a trailing or leading axle is equipped to be lifted using air bellows, this is referred to as a lifting axle (➜ Fig.).
According to the load of the drive axle, the lifting axle can be lowered automatically or manually. Activation is electropneumatic.
A lifting axle can be used as low-speed traction control if more load is required on the drive axle at low initial speed. Raising the lifting axle provides the drive axle with an additional load and thus better traction.
3.7
3
CH
AS
SIS
3.2.
4Axl
e m
ount
ing
x
Drive axle with HUB UNIT
2
1
3
2
1
3
HUB UNIT wheel bearing unit
321
LEGEND1 Rear axle bridge2 HUB UNIT wheel bearing unit3 Drive shaft (axle drive shaft)
BASIC PRINCIPLES
Wheel bearings on drive axlesThree different types of wheel bearings on drive axles are distinguished:
Semi-floating axle
3/4-floating axle
Full-floating axle
The full-floating axle type is used mainly in commercial vehicles.
Due to the high loads, semi-floating axles are only used on commercial vans and passenger cars (low permitted total weight).
FUNCTION
Semi-floating axleOn the semi-floating axle, the wheel hub is attached directly to the drive shaft. This is mounted in the axle body with a single be-aring. Loads are applied to the shaft by the torsion), wheel loads and cornering forces (flection).
3/4-floating axleThe relatively uncommon 3/4-floating axle represents an intermediate solution bet-ween the semi-floating and full-floating axle.
Full-floating axleOn the full-floating axle, the wheel hubs are usually mounted with two engaged conical bearings. The one-piece drive shaft is easy to remove and install without the wheel having to be removed (➜ Figs.).
The drive shaft of this type is only subjec-ted to a torsion load on transfer of the dri-ving torque.
HUB UNIT wheel bearing unitThe HUB UNIT wheel bearing unit a full-floating wheel bearing for axles with disk brakes enhanced by MAN. Removal of the complete wheel bearing as a unit (HUB UNIT) enables e.g. rapid and thus low-cost replacement of the brake disk, as the wheel bearing does not have to be dis-mantled on removal and does not have to be reset on fitting (➜ Fig.).
The wheel bearing unit HUB UNIT can be used not only for drive wheels, but also for front wheels with disk brakes.
3.8
3
CH
AS
SIS
3.3S
uspe
nsio
n3.
3.1P
hysi
cal c
onte
xt
x
Spring rate
F
F
2500
2000
1500
1000
500
0 0.05 0.1 0.15 0.2
2500
2000
1500
1000
500
0 0.05 0.1 0.15 0.2
F [N]
F [N]
x [m]
x [m]
a
b
c
LEGENDa Linear characteristic curve:
hard suspension with c = 25 000 N/m
b Linear characteristic curve:soft suspension with c = 7 500 N/m
c Progressive characteristic curve
BASIC PRINCIPLES
Vibration systemThe entire vehicle forms a vibration sys-tem that begins to vibrate (is excited) due to irregularities in the road surface.
A vibration is defined as the change in a physical variable according to quantity and direction that is repeated more or less regularly.
The vertical movements of the vibration system vehicle-wheel-road are part of ver-tical dynamics (➜ page 18.1).
Spring elementsThe course of a vibration depends on the features of the transferring elements. In general, there are referred to as springs of spring elements.
Essentially, spring elements are characte-rised by two variables:
Spring rate c
Natural frequency fe of the vibration
FUNCTION
Spring rateThe spring rate is the ratio of the force F on pressing in the spring to the distance covered x:
The greater the spring rate c, the greater the force that has to be applied to press in the spring, and the "harder" the suspensi-on.
In the case of normal so-called linear springs, the spring rate is constant.
In the case of the progressively acting springs frequently used nowadays, or so-called "stage springs", the spring rate de-pends on the travel, i.e., the value of c ri-ses as the impression of the spring incre-ases. The resistance force of the spring in-creases (➜ Fig.).
Natural frequencyThe natural frequency fe refers to the fre-quency of a vibration that occurs on self-energisation. It is calculated as follows:
Self-energisation means an increase in the amplitude and it is also referred to as
resonance. A reduction in the amplitude is referred to as damping (➜ page 3.15).
In order to prevent a resonance-genera-ted build-up, the natural frequency of vib-ration in vehicles should remain as con-stant as possible. In order to ensure this, the spring rate must rise proportionally with the load (payload, spring compressi-on). A progressive spring rate aims to achieve this and also ensures there are adequate reserves of spring travel for high loads.
Depending on the constructive layout of the suspension, different spring rates re-sult, and thus also different natural fre-quencies.
xF
c =
fe1
2cm
= ⋅
π⋅
3.9
3
CH
AS
SIS
3.3.
2Sus
pens
ion
conf
igur
atio
n
x
Vibrations of sprung and unsprung masses
1
3
2
m1
s1
s2
t
tm2
LEGEND1 Suspension2 Damping3 "Tyre spring"m1 Sprung massesm2 Unsprung massess1 Deflection of sprung massess2 Deflection of unsprung massest Course of vibration over time
BASIC PRINCIPLES
Vibration characteristics of a com-mercial vehicleThe layout of the suspension and dam-ping of the chassis fundamentally influ-ences its vibration characteristics, inclu-ding the subsystems driver's cab with dri-ver and drive train as well as body and load.
In order to ensure stable, safe and com-fortable driving characteristics, the wheel suspension must perform the following tasks:
Transferring weights and dynamic inertia forces from the chassis and body movement to the wheels
Converting hard impacts from the road surface into soft vibrations of the chassis and of the body
Ensuring continuous contact bet-ween tyres and road surface
Limiting rolling and pitching motion of the vehicle (➜ page 18.1)
Regulating vehicle level and ground clearance
FUNCTION
Sprung massesThe suspension configuration depends on the total mass of the vehicle, composed of sprung and unsprung proportions of mass.
Sprung masses are all vehicle parts that are spring-mounted, i.e. chassis, body, load, engine and gearbox.
Unsprung massesUnsprung masses are the components of a vehicle that directly absorb impacts to the vehicle. These include the axles, wheels, wheel hubs and parts of the stee-ring. However, even though they are refer-red to as unsprung masses, they do have "springs" due to the tyres. These are refer-red to as "tyre springs" (➜ Fig.).
Suspension and dampingThe unsprung masses are connected to the sprung masses by the components of the suspension and damping. Their vibra-tion is transferred via the springs and shock absorbers to the sprung masses, which means that both masses vibrate at various frequencies.
The high level of frequency of the vibration of the unsprung masses is converted by suspension elements and shock absor-bers into a vibration with lower frequency (➜ Fig.).
The components of the suspension and damping are main components of the
wheel suspension and thus belong to the unsprung masses of the vehicle.
3.10
3
CH
AS
SIS
3.3.
3Sus
pens
ion
type
s3.
3.3.
1Gen
eral
x
Types of spring
r
1 2
LEGEND1 Coil spring2 Torsion-bar springr Lever arm
BASIC PRINCIPLES
Types of springIn vehicle construction, the following types of spring are distinguished in general:
Torsion-bar springs
Coil springs
Leaf springs
Gas springs (air springs)
Hydropneumatic springs
In commercial vehicles, it is mainly leaf springs and air springs that are used.
As a rule, coil springs can be found in pas-senger cars as well as in light trucks and commercial vans. However, they are also fitted on high-altitude off-road commercial vehicles from MAN. Coil springs require little design space and permit extensive spring travel, but they cannot assume wheel control (➜ page 3.17).
Torsion-bar springs and hydropneumatic springs, on the other hand, only rarely found in passenger cars. In commercial vehicles, they are of subordinate signifi-cance and are mainly fitted in caterpillar vehicles. Heavy mobile crane chassis no-wadays almost exclusively use hydro-pneumatic suspension.
FUNCTION
Torsion-bar springA torsion (bar) spring uses the resistance of the material against torsion to build up spring force. The spring force is proporti-onal to the twisting angle and dependent on the diameter and length of the torsion bar. The torsion-bar spring has no intrinsic damping, but a linear spring rate.
A lever arm for the wheel hub interlocks with a torsion bar or a torsion spring pack (➜ Fig.). Between the bearing position of the lever arm and the fixed clamping at the vehicle frame (body), the torsion element is often fitted in a guide tube. Torsion bar suspensions can be fitted in the vehicle longitudinally or transversely.
Coil springThe coil spring is a special form of torsion-bar spring. Here, the torsion bar does not have a linear arrangement, rather is wound in a spiral.
In the suspension movement, all seg-ments of the spiral are applied a torsion load. Coil springs provide a large number of possibilities to influence the spring rate in the desired manner. In a chassis, pro-gressive coil springs have the greatest si-gnificance. Coil springs can be adapted to each use case by changing the parame-ters spring diameter, spring wire diameter, upward incline, shape and number of coils.
Hydropneumatic springThe hydropneumatic spring is a combina-tion of a gas spring and liquid-filled shock absorber. Gas (usually nitrogen) and pres-surised liquid (oil) are separated by a membrane. Only the gas is involved in the spring effect, similar to an air spring. The oil serves only as a transfer element. As it is incompressible in the same way as all li-quids, it cannot function as a spring ele-ment. On suspension movement, the oil flows through valves. This results in a damping force. The height of the spring can be changed by pumping in or letting out oil. Frequently, there is the possibility in the case of hydropneumatic suspensi-on to use valves to decouple the spring gas volume. This enables the spring to be switched to rigid (e.g. for the operation of additional equipment when the vehicle is stationary).
3.11
3
CH
AS
SIS
3.3.
3.2L
eaf-
sprin
g su
spen
sion
x
Leaf-spring suspension on a front axle
5
2
3
1
4
LEGEND1 Safety winding2 Front spring eye3 Spring clips (spring clamps)4 Parabolic spring (spring pack)5 Spring shackle
BASIC PRINCIPLES
Leaf spring suspensionLeaf-spring suspension is used frequently in the area of commercial vehicles. This is because of the simultaneous assumption of wheel control tasks by the suspension, especially in conjunction with the com-monly used rigid axles. The components for wheel suspension can be fully elimina-ted or are limited to a simple version (➜ page 3.17).
In the case of leaf springs, the material of the leaves is subjected to flection. The re-sistance against the flexural load creates the spring force. The spring force is pro-portional to the flexural yield and depends on the cross-section and length of the leaf, as well as on the number of spring leaves in the case of multi-leaf springs.
FUNCTION
Leaf spring packThe individual leaves of the spring pack are centred on the axle by the heart-shaped bolt and secured with spring clips (spring clamps). Spring clips or grooves in the spring leaves prevent the leaves from moving around.
Leaf springs can thus, alongside their function as spring elements, also assume gear or axle control; furthermore, they re-quire very little space in the transverse ve-hicle direction, which is an advantage in commercial vehicle construction due to the maximum permitted vehicle width.
Leaf spring rateLeaf spring packs generate intrinsic dam-ping through friction at the contact points between the spring leaves. This applies in particular to trapezoidal springs.
Depending on their design, leaf springs have a linear or progressive spring rate. Normal leaf springs have a linear identifier (➜ page 3.8). However, if designed as support leaf, rolling or stage springs, two-stage or also progressive characteristic curves can be set up.
Safety windingTo secure it, the top spring leaf is rolled into a spring eye, at least at the front end. The rear end is mounted in moving bea-rings by means of a rear eye and a spring shackle for low wear to balance out the
length of the spring pack on spring com-pression. The second spring leaf is fre-quently wound partially around the front spring eye as a so-called safety winding (➜ Fig.).
3.12
3
CH
AS
SIS
Leaf
-spr
ing
susp
ensi
on
x
Types of leaf spring
21 54-13
21 54-23
LEGEND1 Safety winding2 Front spring eye3 Spring clips (spring clamps)4-1 Trapezoidal stage spring:
Main spring (bottom spring pack) Additional spring (top spring pack)
4-2 Parabolic spring (spring pack)5 Spring shackle
BASIC PRINCIPLES
Types of leaf springWith regard to the arrangement and type of leaf springs, distinctions are made bet-ween:
Symmetrical and asymmetrical leaf springs
Parabolic and trapezoidal springs
FUNCTION
Symmetrical leaf springAs a rule, the spring packs are located symmetrically across the axle (symmetri-cal leaf spring).
If the axle is located outside of the spring centre, this is referred to as an asymmet-rical leaf spring. It is used for technical re-asons (e.g. axle base or available over-hang) where required.
Trapezoidal springThe name of this spring is derived from the fact that the leaves configured side by side result in a trapezoid shape. Trapezo-idal springs are designed as layered leaf spring packs made of steel leaves of diffe-rent lengths and usually of the same thick-ness.
Friction arises between the leaves which are subjected to flection, and this leads to intrinsic damping. The corrosion of the leaves means that the suspension and damping characteristics change over ti-me.
Different cambers (preformed flection) of the individual leaves or additional springs enable progressive spring rates of the en-tire spring pack (trapezoidal stage springs, rolling springs).
Parabolic springIn order to reduce the high weights of tra-pezoidal springs and their losses due to friction, the parabolic spring has been de-
veloped. This consists of relatively few lea-ves, where the material strength is better exploited in that they are rolled out as pa-rabolic. Compared to conventional leaf springs, approximately 50 % of the weight can be saved.
As the individual parabolic layers only con-tact one another at a few points, parabolic springs have low intrinsic damping. Plastic or rubber-bonded metal leaf spacers gua-rantee the uniform suspension properties over the entire service life.
3.13
3
CH
AS
SIS
3.3.
3.3A
ir su
spen
sion
x
Principle of air suspension
2
1
5
3
4
LEGEND1 Bracket on frame2 Air bellows (hose reel bellows)3 Roll-off piston4 Spring bellows support5 Bracket on axle
BASIC PRINCIPLES
Air suspensionAir suspension uses the compression ca-pability of gases as its spring element. The spring force is proportional to the effective spring area and pressure of the air in the bellows.
Pumping in additional air (control) raises the spring rigidity to the same extent as the load. The course of the characteristic curve for air springs is also dependent on the design (hose reel bellows, bellows). Nowadays, only the hose reel bellows (U-bellows for short) is used in most cases.
Air springs have no intrinsic damping, but they do have the desired progressive spring rate.
FUNCTION
Air spring systemComponents of the air spring system are:
Air compressor
Reservoir
Level control valves
Air bellows
Air flows into the individual reservoirs via the multi-circuit / four-circuit protection valve (➜ page 7.4). Depending on the load (spring compression) of the axle, the pull rod mounted on the axle adjusts the cor-responding level control valve on the fra-me.
The flow of compressed air to the spring bellows is regulated and adapted. This ensures a uniform level of the frame that is independent of the payload. Some axles with air suspension are given separate control valves on each side.
U-bellowsIn the meantime, the design of air bellows as U-bellows (hose reel bellows) has pro-ven most effective. During the suspension movement, the bellows unwind on the roll-off piston (➜ Fig.).
The constructive design of the contour of the roll-off piston enables the spring cha-racteristic curve to be influenced by means of various effective areas over the course of the spring travel. By adding more volume, the natural frequency of air
springs can be lowered further. The cont-rolled addition and subtraction of this vo-lume means that air damping can also be achieved.
BellowsBellows change their effective area on spring compression with expansion of the folds. It is only possible to influence the characteristic curve to a limited extent.
3.14
3
CH
AS
SIS
Air
susp
ensi
onx
Front-axle air suspension with adjustable shock absorbers
1
1
2
3
4
5
6
LEGEND1 Bracket on frame2 Air bellows3 Shock absorber4 Control unit5 Bracket on axle6 Air-spring-shock-absorber system
LDS
BASIC PRINCIPLES
Air suspension controlAir springs permit lower natural frequen-cies and thus a configuration of the chas-sis for a high level of driving comfort.
In contrast to the linear steel springs, which have a high, uncomfortable level of natural frequency with a low load and a falling natural frequency with a high load, the natural frequency remains constant over the entire spring travel.
An additional air suspension control can be used to change the air pressure in the spring bellows and thus the vehicle level. Depending on the layout, this can also re-sult in driving safety advantages.
FUNCTION
Air-spring-shock-absorber system LDSThe MAN front axle with air suspension features a simple structure: air springs and shock absorbers are compiled into one unit, the air-spring-shock-absorber system LDS (➜ Fig.). The purely centric load that occurs means that the air bel-lows have a significantly longer service life. The controllable shock absorbers enables the vehicle to be equipped with electronic chassis control. Other advantages of the LDS are low space requirement, the pos-sibility to have a wider air-spring track with greater roll stability and greater lifting paths to accommodate interchangeable bridges and semitrailers.
Electronically Controlled Air Suspen-sion ECASOn MAN commercial vehicles, the electro-nically controlled air suspension (ECAS) enables precise and rapid level control with two adjustable frame heights in the driving position. Memory systems with two storable height settings are used to adapt the level at loading ramps (➜ page 11.10).
Additional advantages are:
Same body and loading height with every load state (controlled-height system; only the tyre deflection chan-ges depending on the load)
Possible to raise and lower the body
Headlight setting always correct
Electronic Chassis Control EFRMAN also offers electronic chassis cont-rol.
This is a combination of the electronic sys-tems ECAS and ESAC (Electronic Shock Absorber Control) in conjunction with controlled shock absorbers. The rolling and pitching motion of the body (e.g. on changing lanes) can be compensated for the most part by ESAC.
Electronic chassis control provides the highest degree of driving comfort and dri-ving safety by means of suspension and shock absorber control adapted to the load state and driving situation.
The electronic chassis control not only gentle on the driver and load: the 10 % to 20 % lower dynamic wheel loads also re-duce stress on the tyres, wheel suspensi-on components and electronic compon-ents. Electronic chassis control has redu-ced damage to roads by more than 50 %.
3.15
3
CH
AS
SIS
3.4V
ibra
tion
dam
ping
3.4.
1Phy
sica
l con
text
x
Shock-absorber configuration with degressive damping characteristics
FD
Fcomprmax
Frebmax
Frebmax ≈ 9 . Fcomprmax
v
Rebound stage
Rebound rate
Compression stage
Pressure rate
LEGENDFD Damping forcev Speed (suspension movement)
BASIC PRINCIPLES
Vibration dampingVibration dampers (shock absorbers) are used to damp the vibrations occurring at the suspension. Damping of the commer-cial vehicle vibration system is necessary for the following reasons:
The vibration damping of unsprung masses to minimum amplitudes enhances driving safety, as it ensures road grip of the tyres.
Damping the vibrations of the sprung body limits its vibration amplitude, i.e. its vertical vibration path, to a comfor-table degree that is also gentle to the loaded cargo (shock absorbing).
Resonance-generated build-up and long post-vibration periods of the commercial vehicle are prevented.
FUNCTION
Shock absorberA shock absorber uses friction to trans-form the vibration energy into heat. The damping force generated limits the vibra-tion path (amplitude). Here, the scale of the damping force is proportional to the speed of the suspension movement.
As a general principle, the following ap-plies to the damping force FD depending on the speed v of the suspension move-ment:
FD = k ⋅ vn
The damping constant k and the damping exponent n depend on the engineering design of the shock absorber and of the medium used (liquid).
The exponential relationship between the shock absorber force and the spring speed enables effective gearing of the damping components to the vibration system commercial vehicle.
Degressive damping characteristicsDegressive damping characteristics (n < 1) lead to high damping forces at low spring speeds. This leads to a lower rolling and pitching tendency but poorer absorption capability of the suspension and poorer road grip at higher driving speeds.
Progressive damping characteristicsProgressive damping characteristics (n > 1) increase the rolling and pitching
tendency, but lead to greater safety (road grip) at higher driving speeds.
Shock-absorber configurationThe compression (compression move-ment) and rebound (rebound movement) of hydraulic vibration dampers are confi-gured independently of one another be-cause kreb is greater than kcompr (➜ Fig.) by the factor 2.5 … 9.
The rebound movement is dampened more strongly than the compression mo-vement to counteract excessive load re-duction on the tyres (loss of road grip). At the same time, the compression move-ment should not be too hard.
3.16
3
CH
AS
SIS
3.4.
2Typ
es o
f sho
ck a
bsor
ber
x
Single-tube telescopic shock absorber
15
4
3
2
Twin-tube telescopic shock absorber
2
6
1
5
4
7
1
LEGEND1 Oil2 Gas3 Dividing piston4 Piston valve5 Working piston6 Bottom valve7 Compensating chamber
FUNCTION
Telescopic shock absorbersIn the entire field of vehicle construction, the aim is to achieve the best possible compromise between comfort (low dam-ping force) and driving safety (high level of damping force) in the configuration of damping. Here, hydraulic telescopic shock absorbers are used as vibration dampers in the commercial vehicles sec-tor. In MAN commercial vehicles, the elec-tronic chassis control ensures optimised harmonisation of the damping and sus-pension (➜ page 3.14).
In general, two types of telescopic shock absorber are distinguished:
Single-tube telescopic shock absor-bers
Twin-tube telescopic shock absorbers
In the meantime, telescopic shock absor-bers with gas pressure are being used to an increasing extent, as in comparison with shock absorbers without gas volume these achieve better response and more exact damping, and they develop less noi-se.
EXAMPLE
Single-tube telescopic shock absor-berIn the case of the single-tube telescopic shock absorber, the gas-pressurised damping fluid (oil of a certain viscosity) is displaced by a piston with valves for com-pression and rebound movement (com-pression and rebound). The piston move-ment is inhibited by flow resistances at the valves. The damping force counteracts the suspension movement.
The gas volume with a pressure between 25 and 40 bar prevents the fast-flowing li-quid from foaming and, depending on the design, it is separated from this by an im-pact plate (open) or a dividing piston (➜ Fig.). The gas also enables balancing of the volume change in the damping fluid caused by the piston rod moving in and out. The high pressure safety supports the damping forces and ensures rapid re-sponse of the shock absorber.
The single-tube telescopic shock absor-bers with dividing pistons can be fitted in any position. However, their manufacture is more expensive than that of twin-tube telescopic shock absorbers and the criti-cal piston-rod seal restricts their service life.
Twin-tube telescopic shock absorberWith this telescopic shock absorber, when suspension movements occur the oil volume is balanced out via a bottom
valve between the working cylinder (work chamber) and container pipe (reservoir) (➜ Fig.). This balancing of the volume change in the upper work chamber (pis-ton rod path) takes place via the piston valves.
Exact shock absorber function, low-cost manufacturing and long service life are the advantages of the twin-tube telescopic shock absorber. However, it can only be fitted in a vertical or slightly angled positi-on.
In the meantime, the twin-tube telescopic shock absorber is usually gas-pressurised (6–8 bar). As the gas volume does not have to be accommodated in the work chamber, it is shorter than the single-tube telescopic shock absorber with gas pres-sure. For this reason, MAN uses above all twin-tube telescopic shock absorbers in its commercial vehicles.
3.17
3
CH
AS
SIS
3.5W
heel
susp
ensi
ons
3.5.
1Axl
e co
ntro
l and
sta
bilis
atio
n
3.5.
1.1W
heel
con
trol
x
Front axle as torsion crank axle
5 4
1
3 2
LEGEND1 Steering tie rod2 Steering knuckle3 Steering knuckle eye4 Axle body (as torsion spring firmly
connected to trailing arm)5 Trailing arm (firmly connected to axle
body)
BASIC PRINCIPLES
Wheel hubIn order to assess wheel control, the mo-vement of the wheel hub to which the mo-ving gear is attached must be examined.
Each wheel hub has 6 degrees of freedom with regard to its free movement in space. These degrees of freedom refer to the movement according to the three main co-ordinate axes in space and the rotatio-nal movements around these axes.
The wheel suspension must control the gear completely, which means that all the degrees of freedom of the wheel – except for two movement options – are restricted by the wheel suspension if it is assumed that the wheels are blocked.
The remaining directions are the vertical movement (suspension) as well as in the case of steered axles the steering move-ment (rotational movement around the vertical axis).
FUNCTION
Leaf spring wheel controlThe spring packs for leaf-spring suspensi-on handle the longitudinal and transverse wheel control forces. Additional struts (arms) are not required. The leaf springs transfer the wheel control forces as well as the starting and braking torques. Stabili-sers reduce the roll angle of the vehicle on cornering.
Wheel control with air suspensionOn commercial vehicles with air suspensi-on, additional components are required for safe wheel control. These include so-called "struts" (or arms) for axle control (➜ page 3.18) and stabilisers for roll limitation ( page 3.19).
Twist beam rear axleOn a twist beam rear axle, the axle body handles the longitudinal and transverse wheel control forces. As the axle body and the two trailing arms are linked, the axle body acts as a torsion bar or torsion spring (➜ page 3.10). On spring compres-sion of a wheel, the axle body is twisted and stabilises the tendency of the vehicle to roll.
EXAMPLE
A twist beam rear axle, for example, is the MAN front axle with air suspension, desi-gned as a torsion crank axle (➜ Fig.). The axle body is firmly attached to the two trai-ling arms and this has the same effect as a torsion bar spring or torsion spring.
3.18
3
CH
AS
SIS
3.5.
1.2S
trut
s
x
Rear axle with A-arm and stabiliser
5
4
1
3
2
LEGEND1 A-arm2 Shock absorber3 Air bellows4 Trailing arm support5 Stabiliser
BASIC PRINCIPLES
StrutsDue to the high axle loads and economy, rigid axles have become standard for commercial vehicles. Depending on the type of suspension, different components and systems are used in conjunction with rigid axles.
The most important components of the wheel suspension for axles with air sus-pension include so-called struts (or arms). They connect the gear or axle control with the frame, thus absorbing the gear or axle control forces.
FUNCTION
Wishbones and double wishbonesIn the case of independent wheel suspen-sions, these absorb the wheel control forces and in the area of commercial ve-hicles are only used in buses.
Trailing armTrailing arms are used in combination with air suspension. The trailing arm usually only handles longitudinal forces. In the case of one-sided, rigid connection, it also absorbs the drive and braking torques. It is frequently combined with a Panhard (la-teral tie) rod, Watt linkage (not usual in commercial vehicles) or an A-arm.
Panhard rodThe Panhard rod is a simple wishbone that - in combination with trailing arms and air suspension - handles transverse cont-rol of the axle.
Watt linkageThe Watt linkage, in the same way as the Panhard rod, is used for transverse cont-rol of the axle. It is used in conjunction with trailing arms and air suspension.
In contrast to the Panhard rod, the Watt linkage does not lead to a lateral offset of the axle. Due to the great space require-ment, the Watt linkage is rarely used in commercial vehicles.
A-armThese are used to absorb longitudinal and transverse forces; in conjunction with two trailing arms, they assume all the tasks in-volved in axle control. A-arms are fitted above all in the case of rear axles that are subjected to heavy loads, e.g. semitrailer tractors or commercial vehicles with tan-dem axles (➜ Fig.).
3.19
3
CH
AS
SIS
3.5.
1.3S
tabi
liser
x
Rear axle with X strut (four-point strut)
34
1
2
LEGEND1 Shock absorber2 Air bellows3 Trailing arm4 X-strut (four-point strut) from MAN
with stabiliser function
BASIC PRINCIPLES
Roll limitationWithin the framework of driving safety, roll limitation by means of stabilisers plays a significant role, as it counteracts lift of the wheel inside the curve (lower road grip) and the tendency to tilt of the sprung mas-ses.
The unwanted body inclination that oc-curs due to transversal acceleration on cornering (➜ page 18.10) can be avoided in this way.
FUNCTION
StabiliserA stabiliser is usually a torsion bar or torsi-on spring element (➜ page 3.10). When a wheel lifts or if the body tilts to the side from its normal position, the middle sec-tion of the stabiliser (torsion bar) is twisted and thus subjected to torsion. The reac-tion torque that occurs in the torsion bar as a result counteracts the body's ten-dency to roll. A parallel vertical suspension is not influenced by the stabiliser, as it also turns on both sides (➜ Fig. page 3.18).
The moving mounts on the vehicle frame and on the axle body mean that in general stabilisers alone cannot take over wheel control. Some modern commercial vehic-les already have combines components mode of struts and stabilisers. This saves components and weight.
EXAMPLE
X strut (four-point strut)An example of the combination of struts and stabilisers is the X strut (➜ Fig.) deve-loped by MAN. In contrast to the A-arm, it controls the axle not only at one point but at two points. Taking account of the two fixing points on the chassis frame, it is a four-point strut that meets the require-ments for roll stabilisation.
On spring compression of a wheel, the X strut twists, creating a counteracting torsi-on force and thus working simultaneously as a stabiliser. A parallel vertical spring compression of the wheels is not influ-enced by the X strut, as it is raised or lo-wered evenly with the axle.
Axle control remains ensured even in ex-treme driving situations. In comparison with existing systems with stabilisers, ground clearance is increased. This de-sign also reduces the number of compon-ents and the system weight of the rear axle by an average of 20 kg.
3.20
3
CH
AS
SIS
3.6A
xle
geom
etry
3.6.
1Whe
elba
se, t
rack
wid
th a
nd w
heel
cam
ber
x
Wheel camber angle
γ γ
γ > 0˚
1 2
γ < 0˚
LEGEND1 Positive wheel camber (γ > 0°)2 Negative wheel camber (γ < 0°)
BASIC PRINCIPLES
Axle geometryIn order to record the axle geometry of a chassis, physical variables are defined. The most important are:
Wheelbase
Track width
Wheel camber
Other variables that only apply to the area of the front axle are explained on the pa-ges that follow.
FUNCTION
WheelbaseThe distance between the wheel centre points of two axles is referred to as the wheelbase.
Track widthA separate track width can be determined for each axle. It is measured between the centre planes of the two wheels of an axle, in the case of twin tyres between the cen-tre planes.
Wheel camberThe inclination between the wheel centre plane and the vertical to the road surface is referred to as wheel camber (➜ Fig.).
Positive wheel camber
A positive wheel camber (γ > 0°) on the front axle leads to improved straight-ahead tracking and reduces the steering offset (➜ page 3.22). However, the disad-vantages are a reduction in the cornering forces and increased tyre wear of the ou-ter running surface.
Negative wheel camber
A negative wheel camber (γ < 0°) enlarges the steering offset and improves cornering stability. However, the disadvantage is in-creased tyre wear of the inner running sur-face.
EXAMPLE
The wheel camber of wheels on commer-cial vehicle axles is slightly positive or zero. Front axles are usually configured with po-sitive wheel camber angles (γ ≈ 1°).
When a load is placed on the front axle, the wheel camber becomes more neutral (γ = 0°). Tyre wear remains low.
Driven rigid axles have a positive wheel camber angle inherent in their design (γ > 0°).
Non-driven rear axles are fitted without wheel camber (γ = 0°).
3.21
3
CH
AS
SIS
3.6.
2Tra
ck a
nd sp
read
x
Track
2ε
2ε
2
1
l1
l1
l2
l2
Spread
δLEGEND1 Toe-in (l2 – l1 > 0)2 Toe-out (l2 – l1 < 0)
BASIC PRINCIPLES
Front axle geometryA combination of various values of the axle geometry can influence the proper-ties of the chassis and thus the driving characteristics of the vehicle. Slight ten-sioning of the steering linkage balances out the play and the steering reacts more directly.
FUNCTION
TrackTrack refers to the difference in length bet-ween the inner sides of the rear and front wheels (l2 – l1) in the direction of travel (➜ Fig.).
If this difference, measured at the inner si-des of the rim flanges, is positive, this is referred to as the toe-in (the wheel centre planes cross in front of the vehicle and they are swung inwards against the direc-tion of travel by the toe angle ε/2: ε/2 > 0).
If the difference is negative, this is referred to as toe-out. The wheel centre planes cross behind the vehicle.
SpreadThe inclination between the steer axle and the vertical to the road surface is referred to as spread. The spread angle δ is speci-fied in degrees (➜ Fig.).
EXAMPLE
In the case of non-driven front wheels, a toe-in combines with a positive steering offset (➜ page 3.22) ensures good straight-ahead tracking without the wheels wobbling.
The spread with a positive steering offset ensures aligning torque when turning the wheels, as the front section of the vehicle has to be raised against the weight. This aligning torque ensures the straight-ahead tracking of the vehicle and also pre-vents the wheels from wobbling. The spread angle δ is usually between 5° and 10°.
Driven front wheels with a negative stee-ring offset also require a toe-in, as the dri-ve forces wheels towards the outside. For good straight-ahead tracking, this has to be balanced out by a positive toe-in angle.
3.22
3
CH
AS
SIS
3.6.
3Ste
erin
g of
fset
and
cas
ter
x
Examples of various steering offsets
R0 > 0 R0 < 0 R0 = 0
1 2 3
LEGEND1 Positive steering offset2 Negative steering offset3 Zero steering offset
FUNCTION
Steering offsetThe steering offset R0 is defined as the la-teral distance between the wheel contact point and the point of intersection of the steering axis on the road surface (➜ Fig.). It forms a lever arm to the steer axle, on which the peripheral forces of the wheel (friction brake forces) are exerted.
The steering offset influences the aligning torque when longitudinal forces occur in the steering.
Positive steering offset
With a positive steering offset (R0 > 0), the point of intersection of the steering axis on the road surface lies within the track width (➜ page 3.20). On road surfaces with dif-ferent levels of adhesion, the wheel with better adhesion swings outwards on bra-king; the vehicle tends to pull off course.
Small positive values for the steering off-set limit the steering forces and the ten-dency of the wheel to wobble. The alig-ning torque increases on the rolling (non-driven) gear with positive values; on the driven gear, it reduces the alignment ef-fect.
In commercial vehicles, positive steering offsets are usual, in many cases due to the widened drum brakes. The lower hin-ge point of the steer axle is located very far towards the inside.
Negative steering offset
There is a negative steering offset (R0 < 0) when the point of intersection of the stee-ring axis on the road surface lies outside of the track width. The wheel with better ad-hesion on road surfaces with different le-vels of adhesion swings inwards on bra-king; the vehicle is stabilised against the direction of rotation of the possible swer-ving. Also in the event of a tyre burst at the front, the vehicle is kept safely in lane.
Zero steering offset
If the steering offset is zero (R0 = 0), the point of intersection of the steering axis is located exactly on the wheel contact point around which the steered gear swings. The considerable degree of friction means that the steering forces are very high. The braked wheel is swung outwards as in the case of the positive steering offset, but with lower torque.
CasterThe caster or caster angle is specified po-sitively in degrees if the swivelling axis of the steering tends towards the rear in the direction of travel. The point of intersec-tion of the steering axis then lies in front of the wheel contact point and this is refer-red to as positive caster. If it lies behind, this is a negative caster.
The caster offset is the distance between the point of intersection of the steering axis on the road surface and the wheel
contact point. The caster offset is speci-fied in the same way as positive or negati-ve.
A positive caster stabilises the wheel. A large caster leads to good straight-ahead tracking and good steering returnability.
3.23
3
CH
AS
SIS
3.6.
4Toe
diff
eren
ce a
ngle
x
Toe difference angle under Ackermann condition
α
α αβδ
βδ
LEGENDα Steering angle (slewing angle) of the
outer wheelβ Steering angle (slewing angle) of the
inner wheelδ Toe difference angle
BASIC PRINCIPLES
Toe difference angleOn cornering, the inner wheel runs on a smaller radius than the wheel nearest to the curve. The slewing angles have to be of different sizes so that the tyres do not roll obliquely over the road surface and wear out more than necessary.
The toe difference angle δ is the angle by which the wheel inside the curve is turned more compared to the wheel nearest to the curve (➜ Fig.).
FUNCTION
Static steering configurationWith a so-called static steering configura-tion, the toe difference angle is selected in such a way that all the wheels roll without lateral slip. The following applies:
δ = β – αThe consequence of this is that the verti-cals to the wheel centre planes meet at one point (Ackermann condition).
Dynamic steering configurationAs a consequence of the centrifugal forces on cornering, however, the wheels do not run straight: as a general principle, they run under tyre slip angles (angle bet-ween wheel centre plane and direction of movement of the wheel) with lateral slip.
The lateral slip due to the tyre slip angle is to be taken into account when configuring the steering kinematics; this is done with a dynamic steering configuration.
EXAMPLE
The toe difference angle determined at a slewing angle β (steering angle of the inner wheel) of 20°.
In accordance with the dynamic steering configuration, the wheel with the higher load, the one nearest to the curve, is tur-ned slightly more, as the greater tyre slip angle means it builds up higher cornering forces. With greater steer angles, the Ackermann condition is followed to an in-creasing degree.
3.24
3
CH
AS
SIS
3.7C
hass
is e
quip
men
t3.
7.1F
uel t
ank
x
Tank with integrated stages on an tractor
BASIC PRINCIPLES
Fuel tankFuel tanks are mounted on the side of the frame. Long-distance vehicles fully exploit the free space between the axles and fra-me components to accommodate the lar-gest possible fuel volumes of up to 1200 litres.
The high weights when filled (with 1200 l, up to 850 kg), the tank places a very high load on its mounts. With regard to the continuous exposure to vibrations, The securing brackets of the fuel tank are par-ticularly large and strong. Furthermore, the fuel tanks of commercial vehicles are equipped with special components for ventilation and venting.
FUNCTION
Materials for fuel tanksSteel, aluminium and plastic are the mate-rials used to make fuel tanks. Variants of aluminium and plastic containers are used to lower the weight. Their corrosion pro-tection, inherent in the material, is also better that that of steel tanks.
Both the insides and outsides of steel tank have to be given a corrosion protection layer. In the meantime, cathode immersi-on painting on the outside and aluminium-coated inner walls are widespread.
Tank contentTank with contents up to 600 litres are used particularly in long-distance trans-port in order to achieve long ranges and reduce the loss of time due to refuelling stops. For vehicles with powerful hydraulic assemblies, combined tanks are available for separate fuel and oil reserves.
Tank structureThe outer walls of metal tanks consist of rolled sheet metal and usually have deep-drawn or pressed tank bases. In order to increase the rigidity of the fuel tank, the walls are usually curved (convex). In the in-terior of the container, the tank is fitted with baffle partitions. These reinforce the tank walls against excessive deformation and counteract shifts in the position of the fuel.
Tank ventilationFuel tanks are equipped with ventilation and venting connection points: on the one hand, these reduce the overpressure caused by thermal expansion and ensure pressure compensation on refuelling and emptying (consuming) on the other. As diesel fuel expands at increased tempera-tures (up to 2.2 %), tanks should not be filled "to the brim" on very hot days.
Also in the event of extreme inclinations (e.g. accidents), no fuel may escape from the tank. The ventilation and venting con-nections are therefore equipped with gra-vity valves to prevent fuel escaping.
3.25
3
CH
AS
SIS
3.7.
2Equ
ipm
ent c
arrie
r and
side
pro
tect
ion
devi
ces
x
Equipment carrier behind the folded-up side panelling
BASIC PRINCIPLES
Equipment carrierThe equipment carrier is used primarily to accommodate the batteries. Over and above this, the equipment carrier is a plat-form for elements of the compressed air system such as the compressed air drier, tank or external compressed air connec-tions. Combinations with the spare-wheel carrier are also widespread.
Other equipment carriers, e.g. for com-pressed air components, are required if the battery carrier does not provide enough space.
FUNCTION
Battery carrierThe battery carrier is located on the equip-ment carrier in an easily accessible and at the same time protected position. Prefe-rably, it is positioned to the side of the fra-me between the wheels.
Depending on their capacity, batteries weigh up to 100 kg. As it is often the case that two or more batteries are required, the battery carrier and the other compon-ents have to bear heavy loads and the battery carrier is of correspondingly sturdy design.
On some vehicles (e.g. with low frame heights), battery sleds are used to provide better accessibility.
Side protection devicesSide protection devices (➜ page 12.2) are intended to prevent that riders of two-wheel vehicles or pedestrians enter the free spaces between the vehicle axles in the event of an accident and are then run over. In the event of collisions with other vehicles, they also protect the add-on parts on the truck chassis against dama-ge.
Aerodynamic side panelling can already be seen nowadays on many commercial vehicles, in particular on semitrailer trac-tors. Their closed surfaces perform the protective function even better than open profiles.
The fold-up left-hand side panelling on the semitrailer tractor (➜ Fig.) permits free ac-cess to the equipment carrier with battery, spare wheel and compressed air system.
3.26
3
CH
AS
SIS
3.7.
3Cen
tral l
ubric
atio
n sy
stem
x
Reservoir of a central lubrication system
BASIC PRINCIPLES
Lubrication systemOn modern commercial vehicles, a central lubrication system handles most of the usual, regular lubrication work on the ve-hicle. Here, in contrast to manual lubrica-tion, the lubrication points connected to the lubrication system are automatically supplied with lubricant (➜ page17.4 ff.) at regular intervals. Maintenance and wear costs are minimised.
Another advantage is that no lubrication points can be omitted and that only fresh lubricant without contamination gets to the lubrication points. This drastically re-duces wear-related costs. Vehicles with central lubrication systems can get by more or less without wear repairs to the bearings.
However, the development of mainte-nance-free bearings (rubber-bonded me-tal bearings or also plastic bearings) will mean that central lubrication systems will lose significance in future.
FUNCTION
Central lubrication systemAt regular intervals, a lubricant pump (ge-ar-wheel or piston pump) feeds the grea-se from the reservoir of the central lubrica-tion system through the main line to the distributors at the lubrication points. From the distributor, the individual lubrication points are then supplied with a precisely metered amount of grease.
The intervals of the lubrication are cont-rolled on the basis of distance (distance driven), on the basis of time (operating hours) or on the basis of operations (num-ber of braking operations). Manual opera-tion of the pump is no longer usual.
Two principal types of system are distin-guished with regard to the function of their distributors:
Single-line system
Progressive system
Malfunctions in central lubrication sys-tems (both types of system) are indicated by warning lamps in the dashboard. Ho-wever, as the reserves of grease are enough for distances of up to 1000 km, a displayed malfunction does not force the driver to stop.
Single-line lubrication systemWith the single-line lubrication system, each lubrication point has a plunger distri-butor. Against the pressure of a spring-loaded plunger, the lubrication pump fills
the metering chamber with a volume ad-apted to the lubrication point. Once all the metering chambers (distributors) have been filled and the pump switched off, the metered amounts (0.1–0.4 cm3) are pressed by the plunger spring force th-rough a valve to each lubrication point. The single-line lubrication system works with a pressure of up to approx. 60 bar.
Progressive lubrication systemProgressive lubrication distributors each supply a number of lubrication points with grease. Here, the grease is not stored in metering chambers. The grease fed by the pump is distributed by a number of plungers according to requirements. The feed movements of the plungers direct the flow of grease in such a way that the next plunger is energised and then also feeds lubricant to the corresponding lubrication point. In this way, the connected lubricati-on points are supplied with grease (0.1–0.6 cm3) in succession. In a progres-sive lubrication system, pressures of up to approx. 350 bar are used.
