wheeled and crawler tractors

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

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  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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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