MODULE II WHEELED AND CRAWLER TRACTORS (Wheeled and crawler tractors: Layout of wheeled tractor, power transmission, steering system, brakes and braking system, wheels, rims and tyres and accessories of wheeled tractors, hydraulic control system, power take off, tractor stability and ride characteristics Layout of crawler tractors, power transmission, steering clutch and braking system, crawler details, methods of selection of equipments, selection of machines, basic rules for matching machines, selection of equipments including the nature of operating selection based on the type of soil, selection based on haul distance, selection based on weather conditions.) Introduction: In year 1856 all machines used for traction purposes were known as traction motors and later on, in the year 1906 these were designated as tractor by taking half the word from traction and other half from motor. Now tractor is defined as such a machine which is used to pull or push agricultural implements in the fields. Tractor is one of the basic machines used for accelerating the agricultural production. Types of Tractors Tractors are classified in three different types. These are 1. Type of construction. 3. The purpose for what it is used. 2. Type of drive. Type of Construction There are again two types in this (a) Tractor in which driver can sit and drive. (b) Tractor with which the operator walks along. This type is also known as walking type tractor. Type of Drive There are various types of drives as shown below Track Type Tractors: In this type of tractors, instead of wheels one track side. This track gets drive from the sprocket run by 1
1. MODULE IIWHEELED AND CRAWLER TRACTORS(Wheeled and crawler
tractors: Layout of wheeled tractor, power transmission, steering
system,brakes and braking system, wheels, rims and tyres and
accessories of wheeled tractors, hydrauliccontrol system, power
take off, tractor stability and ride characteristicsLayout of
crawler tractors, power transmission, steering clutch and braking
system, crawlerdetails, methods of selection of equipments,
selection of machines, basic rules for matchingmachines, selection
of equipments including the nature of operating selection based on
the typeof soil, selection based on haul distance, selection based
on weather conditions.)Introduction:In year 1856 all machines used
for traction purposes were known as traction motors and later on,in
the year 1906 these were designated as tractor by taking half the
word from traction and otherhalf from motor. Now tractor is defined
as such a machine which is used to pull or pushagricultural
implements in the fields. Tractor is one of the basic machines used
for acceleratingthe agricultural production.Types of
TractorsTractors are classified in three different types. These
are1. Type of construction.3. The purpose for what it is used.2.
Type of drive.Type of ConstructionThere are again two types in
this(a) Tractor in which driver can sit and drive.(b) Tractor with
which the operator walks along. This type is also known as walking
type tractor.Type of DriveThere are various types of drives as
shown belowTrack Type Tractors: In this type of tractors, instead
of wheels one track side. This track getsdrive from the sprocket
run by rear axle shaft. To steer the tractor there is no steering
gear fitted.The tractor is steered by applying brakes to one side
of the track with the other track moving.Half Track Type: In this
type of tractor a small track chain is fitted at the rear end only
while atfront axle tyres are fitted. Track type of tractors are
mostly used for reclaiming barron lands andare seldom used for
general agricultural purpose. As these are fitted with the tracks
which hasmore contact area to the ground as such the traction power
of the tractor increases considerably.Track type of tractors play a
very important role on dams for earth moving work.Wheel Type of
Tractors: Wheel types of tractors are most commonly used for
agriculturalpurposes. As they can run fast and tyre fitted in it
can absorb a certain amount of field shocksalso.Two wheel types of
tractors are used for small farms, hilly area and for gardening
purposes.Three Wheel or Tricycle Type: Three wheeler tractors were
very popular 25 years back but nowits place has been taken by four
wheelers. These tractors had single or dual wheel fitted at
thefront end in the centre and were considered good for negotiated
shorter turns.According to the purpose for which it is being
used:1
2. i) Utility Tractor : It is general purpose tractor arid is
designed for ploughing and driving anyother equipment through its
P.T.O. drive and it is considered good for such farms where
thefarmer is not able to afford different tractors for specific
jobs.ii) Row Crop Tractor : It is a general purpose tractor
designed in such a way that it can meetpractically all the demands
for agricultural purposes such as ploughing, harrowing,
leveling,pulling, seed drills or for running other machines such as
water pumps, threshers etc.iii) Orchard type tractors: This type of
tractors specially meant for orchard use only. These aremade very
high in height so that in orchards fruits can be easily be plucked
while sitting on thetractors. These are also used for trimming of
trees. Tractor is constructed such that it can easilygo in between
trees safely.iv) Industrial type tractors: These are also known by
the name tuggers. They are used for pullingheavy loads on for
lifting with their craneboon. These kinds of jobs are performed in
industries.v) Garden Tractors: These are mostly used for grass
cutting or making, flower beds in thegarden. These are fitted with
small diameter tyres with thicker in width. Generally Power
istransmitted to driving to driving wheel by the belt. These types
of tractors are smaller inconstruction and power of these tractors
varies from 1 to 10 HP.vi) Rotary Tillers: This is walking type
tractor. These are used in small field or on hills wherefields are
very small and are at different levels, where the ordinary tractors
cant work efficiently.Tines blades are fitted to the tiller for
preparing seed beds by pulverizing the soil.vii) Implement carrier:
This type of tractors having extended classes from between front
and rarewheels. On such extended space all the implements such as
seed drills, sprayers, duster etc. canbe mounted easily.viii) Earth
moving tractors: As the name itself indicates that these tractors
are meant for heavyearth moving marks like in dam construction,
rellowing barren land, quarries and otherconstructional waves.
Hence, naturally these tractors are heavy in weight and strongly
built.These tractors also have strong hydraulic system & may
built in both track & tyre type.LAYOUT OF WHEELED TRACTOR2
3. (i) Engine: A tractor can have petrol engine or diesel
engine fitted in it. But nowadays all thetractors are fitted with
the diesel engine. Engine provides the source of power to drive the
tractor.Generally multicylinder heavy duty diesel engine is used in
recent days.(ii) Clutch: Clutch is fitted between engine and
gearbox and is used to connect or disconnect theengine drive to the
gearbox. Multiplate wet clutches are commonly used for the
tractors. Theseclutches are most commonly hydraulically
operated.(iii) Transmission: Gearbox is fitted in the tractor to
increase the driving torque so as to enablethe tractor to pull more
loads, in the gearbox housing differential and reduction gears are
alsofitted. The differential helps the tractor to take turn without
its inner wheel spinning while thereduction gear fitted further
increases the driving torque to the wheel.(iv) Wheels: Two wheels
are fitted to the front axle and two rear wheels to the reduction
gearshaft, drive is transmitted through the rear wheels. For better
traction rear tyres are provided withhigh lugs.(v) Front Axle:
Front axle is fitted at the front of the tractor and two stub axles
fitted at either endthrough which the steering linkage is connected
and wheels are steered.(vi) Steering System: Steering system
enables the operator to steer the tractor to right or left asthe
case may be. Hydraulic assisted steering is generally employed for
the ease of operation.(vii) Brake System: Brake system is provided
in the tractor so as to enable the operator to stopthe moving
tractor when required. Either hydraulic or air brakes are used for
this purpose.(viii) Electrical System: Electrical system provided
in the tractor enables it to work in the nightthrough the
electricity produced by the dynamo. Moreover engine is also started
through selfstarter which draws its electric current from the
batteries.(ix) Hydraulic Lift: With the hydraulic power the
implements are raised up or lowered down ortheir depth
controlled.3
4. (x) Power Take Off: Provision is made in the tractor to
drive stationary or moving implementssuch as sprayer, duster,
etc.Power Transmission in TractorsThe tractor moves when engine
power flows through the drive train to the driven wheels. Thedrive
train consists of : Clutch Gearbox (sometimes called the
transmission) Differential Power Take Off (PTO) Drive shaftsClutch:
The clutch disengages the engine from the gearbox so the tractor
operator can shift gearsor stop the tractor. The clutch in most
tractors is a dual unit that can pass power to both thegearbox and
the PTO, which provides power to auxiliary implements.Gearbox: A
gearbox is necessary to keep the engine at its most efficient
operating speed whilebeing able to vary ground speeds of the
tractor. Also, it is not possible to accelerate the mass of
atractor and implement using high gears from a dead start, so a low
gear is selected for startingand upward changes made as you get to
the speed limit of that gear.Power is transmitted to the rear
wheels or to all four wheels. Drive to the front wheels
ismechanical or hydrostatic, its purpose being to increase drawbar
pull at the will of the operator.Clutchless hydraulic transmissions
are also used, making it possible to shift gears while
inmotion.Most agricultural operations have an optimum speed to
achieve the best result for the type ofwork it is doing. If the
tractors are going too slowly, it is wasting time and fuel, and if
it is goingtoo fast, the quality of work will decrease and may be
overloading or lugging the engine, whichwill shorten its life.
Generally, from experience, the driver will select a gear and set
his enginespeed for a particular implement before moving off in the
field. He may find during the day that4
5. conditions improve or dry out and he can select the next
higher gear, but in the eveningconditions worsen again and he has
to come down to the original gear or one lower. The engineis
maintained at constant and most efficient speed, usually between
1500 and 2000 rpm.Operations like crop spraying and fertilizer
spreading require set ground speeds to achieve theapplication rate
and precision required, while the engine is running at PTO speed to
produce 540rpm shaft speed, which is driving the chemical pump or
spinning disks. This combination mustnot be varied or application
rates will suffer. The engines governor will take care of changes
inslope in the field.Because of the wide range of jobs carried out
by tractors, a gearbox with many gears is required.Generally, there
are two boxes. The first-main-has 3 forward and 1 reverse gear; the
second-range-has speed ranges, either 2 (low and high), more
commonly 3 (low, medium, and high) oroccasionally 4.Forward first
and reverse gears are usually opposite each other so that we can
shuttle betweenthem for jobs like fore-end loader work. A gearbox
may be described as a 12 x 3-this has 9forward and 3 reverse gears
arranged in three ranges. A 16 x 4 has 12 forward and 4
reversegears in four ranges. Higher gears may have synchromesh for
smooth changing at high speeds onthe road. Lower gears do not need
it as they are nearly always preselected before moving off
inwork.Modern tractors can also have a power shift component where
speed ranges can be changed onthe move, that is, clutchless.
However, they are more expensive and difficult and expensive
torepair. For developing countries, a simple manual transmission is
best.Differential: After the engine power has passed through the
gearbox, the differential sends it tothe driven wheels. The
differential allows the driven wheels to turn at different speeds,
as whenturning a corner. The drive axles are provided with double
reduction to achieve the requiredtraction.In the case of a 4WD or
8WD tractor, power is also sent to the front wheels, which are
equippedwith their own differential. To compensate for rear wheel
slippage, the front wheels of 4WD and8WD tractors turn slightly
faster than the rear ones. These tractors must not be driven on the
roadin four wheel drive, because of excessive tire wear and damage
to the drive train.Power take off: PTO provides power to coupled or
trailed implements such as mowers, sprayers,and cultivators. The
power for the PTO comes from the engine through the dual clutch.
The twotransmissions of power are usually independent. For example,
an operator can stop or move thetractor without affecting the use
of the PTO, or can stop the PTO yet still move the tractor.Tractors
under 100 HP use a PTO speed of 540 rpm, and larger units use a PTO
speed of 1100rpm. Some manufacturers of smaller tractors provide
both speeds regardless of the HP rating.The PTO may also drive
hydraulic pumps for remote work.Drive shafts: Drive shafts connect
the other components of the drive train. For instance, thegearbox
and differential may have a connecting drive shaft.MULTIPLATE
CLUTCHThe power from the engine flywheel is transmitted to clutch
through clutch facing fitted on theclutch plate. The amount of
power which can be transmitted through clutch plate depends uponits
contact area, i.e., the facing area which is in contact with the
flywheel and the pressure plate.To transmit more horsepower, it
require bigger flywheel and clutch plates. But unfortunatelythere
is no much of space available in a vehicle to cope with this
problem, so instead of one bigclutch plate, two or more small
clutch plates are used. In heavy earth-moving
machines/tractors,instead of two plates there are 9 to 11 clutch
plates fitted but in this case in place of asbestoslining,
phosphorus bronze lining is fixed on steel plates and the complete
assembly works in oilso that it remains cool.5
6. Hydraulic Multiplate ClutchIt consists of a number of
friction discs alternatively placed between pressure discs. The
wholeunit runs in hydraulic oil. One such simple unit in engaged
and disengaged position is shown inFigure. The clutch is engaged
when oil under pressure is sent to push the piston against the
discsand plates, clamping them together. The discs are splined to
the drum while the pressure platesare splined to the hub. With the
result the input power from the hub is sent to the drum andoutput
shaft through clutch plates now clamped between pressure discs.The
clutch is disengaged when oil pressure is released and the piston
moves away from theclutch pick. This frees the discs from the
plates, with the result power flow stops. In some of themakes, a
set of springs are placed while in other makes, oil on other side
of piston is used to helpthe release of disc plates.One major
difference between dry and wet clutch is .the disc facing material.
In wet clutch thismaterial must grip when soaked in oil. Cross
hatch marks are usually given on facing of the wetdisc. Cork was
commonly used as lining material and even now used in smaller
clutch plates asfitted to motorcycles, etc. but for bigger vehicles
material made out of bi-metal, is used while thedriving plates are
made out of steel.Hydraulically-Operated Clutch6
7. Nowadays hydraulically operated release bearings are
becoming very common. In this case, amaster cylinder rod is linked
to the clutch pedal and a pipe is taken from the master cylinder
tothe slave cylinder which is fixed on the clutch housing or
gearbox housing.When the clutch pedal is pressed, the hydraulic oil
in the master cylinder gets pushed andoperates the piston in slave
cylinder. This piston is attached to the release fork which in
turnpresses the release bearing. It works similar to that of the
master and wheels cylinders in case ofhydraulic braking system. The
main advantage of providing hydraulic system for operation ofclutch
is that driver has to exert less pressure while disengaging clutch
as compared tomechanical system. Secondly, this system becomes neat
and does not require any linkages.Thirdly, the clutch can be fixed
anywhere and operated easily with the help of a flexible
pipe.TRACTOR GEARBOXIn the previous paragraph you have studied the
necessity of providing gearboxes and type ofgearboxes. With the
help of simple diagrams as used in our Automobile, in tractor too
it servesthe same purpose and works on the same principle.In
tractors the driving wheel require power supply at low r.p.m. and
high torque whereas theengine rum at high r.p.m. and low torque the
transmission of a tractor therefore must providespeed reduction and
torque multiplication, this is done by a suitable design of a
gearbox. Inwheeled tractors the overall gear ratio (engine to
driving wheels) is usually 15:1 to 20:1. UnlikeAutomobile gearboxes
where power in top gear flows from clutch shaft to main shaft of
gear intractor gearboxes to reduce the speed and to multiply the
torque the power from clutch shaft isfirst taken to countershaft
and then to main shaft.In simple tractor gearbox there are three or
four forward speeds and one reverse speed. Butnowadays by providing
another set of gear within the gearbox forward and reverse speeds
aredoubled making 10 forward and two reverse speeds.Gear ShiftingAs
the maximum implement working speed in a field is 10/12 km per hour
as such the tractorwill come to stand still when clutch is
released. This is due to high resistance to rolling as
suchmanufacturers do not recommend shifting gear when tractor is
moving. When working artimplement select the proper gear and then
go instead of working up to it through the lower gearas in road
vehicles.7
8. Ford Tractors are equipped with constant mesh gearbox having
eight forward and two reversespeed selected by two lever mounted on
the top cover. The main lever (longer one) selects fourforward and
one reverse speeds, while the shorter lever provide high and low
range in each gear,thus provide 8 forward and two reverse speeds.
Assembly layout of Ford gearbox is shown infigure given above.All
the gears except the one driving P.T.O. and main countershaft are
helical gears, rest all thegears are Spur gears, and are in
constant mesh. The gears are connected and disconnected withthe
help of sleeve to obtain correct gear engagement.As the system
incorporates transmission type P.T.O. the main countershaft is
splined internallyto accept the external splines of short P.T.O.
countershaft. The power train in different gear isshown in figure
below.8
9. MAIN PARTS OF THE STEERING SYSTEM OF TRACTORSMain parts of
the steering system are1. Steering wheel2. Steering column3.
Steering shaft4. Steering gear box5. Drop arm & Steering
linkagesThese components are similar to that of steering system in
conventional automobiles.9
10. Hydraulic Assisted Mechanical SteeringIn hydraulic assisted
mechanical steering, the force required to turn the tractor is very
negligibleas compared to mechanical steering. In this case the only
force required by the operator is to turnthe steering wheel which
in turn operates the spool valve through which the hydraulic
powergives all the steering force.In almost all the hydraulic
assisted steering, mechanism is so arranged that in the event
ofhydraulic failure, the steering system works as purely mechanical
steering and the force requiredto turn the steering is certainly on
higher side.10
11. Figure shows a simple hydraulic assisted mechanical
steering. The steering shaft is fitted a spoolvalve at the lower
portion it has worm. On this worm is placed a toothed nutteeth of
this are inmesh with sector fixed on sector shaft which is
connected by steering cylinder sector with thehelp of steering link
as shown. On the steering cylinder sector shaft is fitted on other
lever (notshown) which connects the wheels drag link.When the
operator turns the wheel right turn as shown, because of the
resistance in turningsteering wheels the steering shaft is forced
up. This moving of shaft upward moves the inbuiltspool valve up
also, which in turn opens a port and directs hydraulic oil under
pressure to reachsteering cylinder at the front wheel, as such it
pushes the piston to right side as well as move thepinion which is
in mesh with the cylinder rack. This pinion moves the road wheels.
The oil fromother side of cylinder finds its way to reservoir
through spool valve.As long as the steering wheel is turned oil
will continue flowing and move the wheels, but assoon as the
steering wheel motion is stopped, the hydraulic pressure will turn
the wheel slightlyfurther to the right moving the steering linkage
forward and pulling the valve back to neutralposition.Working of
Hydraulic SteeringFigure shows ZF power steering as used in
imported Zetor tractors is shown. In this figure it canbe seen that
piston (4) can go up and down in cylinder (5). Piston (4) is
connected to rocker shaft(7) with the help of connecting rod (6).
On top of the piston a nut (3) is bolted in which steeringspindle
(1) having threads works. A thrust bearing (11) is fixed in the
body on which the steeringspindle moves freely. When the steering
spindle is revolved with the help of steering wheel dueto screwing
action, it pulls or pushes the piston, as the case may be. The
piston is tied up with therocker shaft with the help of connecting
rod as such the rocker shaft moves to and fro. With therocker shaft
is tied up drop arm and drag link as such the wheels get steeredOn
steering spindle is fixed sleeve 8 which moves to and fro through
slight angle, whensteering shaft is moved. This sleeve works as a
hydraulic valve and when it moves in onedirection, it connects the
oil pressure to get into the pressure chamber A. At the same time
theoil trapped in chamber B is allowed to escape through discharge
port. When the hydraulic oilenters the pressure chamber A, pressure
gets built up there and this pressure causes the pistonto move up,
thus helping the driver to steer the vehicle with less effort.
Similarly when steeringshaft is moved in the other direction,
control sleeve connects pressure chamber 4 B with thepressure line
and at the same time allowing the hydraulic oil from chamber A to
ape throughdrain pipe. The hydraulic pressure in chamber-B pushes
the piston down.Such type of steering are classified under
hydraulic assisted mechanical steering and workefficiently and have
long life. These steering can also steer the vehicle in the event
of failure ofhydraulic pump or engine, but in this case steering
will be a little heavier.11
12. BRAKING SYSTEM IN TRACTORSThere are various braking systems
used nowadays which are classified as follows based onoperating
methods:1. Mechanical Brakes 2. Hydraulic Brakes3. Power Assisted
Brakes 4. Power Operated BrakesBased on construction there are two
types1. Drum Brakes 2. Disc Brakes12
13. Hydraulic braking system works on the principle of Pascals
law. Based on this principle,hydraulic brake system of an
automobile is designed and is explained in a simple diagram
givenbelow. In case more pressure is required bigger pistons are
used and for small pressure smallerpistons. In some of the vehicles
in front wheel, front pistons are smaller than rear pistons. It
alldepends upon how much pressure is required to be applied to each
brake shoe.When brake pedal is pressed, the hydraulic oil under
pressure reaches the wheel cylinders (WC)through brake pipe and
pushes the pistons out. Since the pistons are in contact with brake
shoes,these are pushed against drum and vehicle is braked. As soon
as foot from the pedal is removed,brake oil from wheel cylinder
goes back to the master cylinder and brake shoe return springbrings
back the brake shoe to its original position.13