FARM TRACTORThe history of tractor development is to lang to relate here .Landmarks along the road include the fires use of an internall combustion engine in a tractor in the United States in 1890;the ‘ Ivel ’ tractor of 1901; the AmericanFord of 1917 , and the International ‘ Farmall ’ of 1923 .During the 1930s , the use of low -pressure pneumatic tyres soon became popular , and the first Ferguson system tracrors incorporating three-point linkage with hydraulic draught control were introduced .Classification of tractor types.Tractors as now manufactured can be classified as follows:A. According to method of securing traction and self-propulsion:1. Wheel tractorsa. Three tractorsb. Four wheels2. Track-type tractorsB. According to utility :1. General-purpose or u tility2. All-purpose or row -crop typeWheel tractors. The wheel -type tractor is the predomingating type,particularly for agriculturalpurposes.Wheel tractors are made either with three wheels or with four wheels .The usual arrangement consists of two rear-drive wheels and one or two front steering members.Track-type tractors. The traction mechanism in thetrack-type tractor consists essentially of twoheavy,endless,metal-linked devices known as tracksthemselves by reducing the movement of one track below the speed of the other.General-purpose or utility tractor.A general -purpose tractor is one ofmore or less conventinal design such as an ordinary four-wheel machine or a track -type machine. This type is also referred to as autility troctor. It is made to perform only the usual tractor jops,including both field and belt work such as plowing ,harrowing,road grading ,combining,feed grinding,and the like.All-purpose tractors.An all -purpose or row -crop type is a tractor designed to handle practically all th e field andbelt jops on the average farm,including the planting and intertillage of row crops.The conventional tractor consists basically of two engine-driwen wheels,usually at the rear, and two support wheels st the front . The type of the engine used t o powerthe heart of the tractor; it is here that the fuel,the chemical potential energy , is converted into the mechanical emergy which causes the wheels to turn.The difference between n internal and external combustion engine is simply that in an intern al combustion engine ,the conversion of potential energy into mechanical energy takes place wholly in the cylinder,whereas with external combustion the fuel is burnt outside the cylinder .A steam engine or a gas turbine is a good example of the latter.How can liquid fuel be changed into rotational mechaanical energy? This is achieved by accurately mixing the fuel with air and then burning it in controlled condition.When this mixture burns ,it expands and pressure builds up ,forcing the piston to move down the cylinder in a straight line. It is said to rotary motion .This linear motion is converted into rotary motion by a connecting rod and crank arrangement joined to the piston.To be able to use this energy effectively, the burning and combustion proce and the force of expansion have to be controlled. To do this , the enginemust have :1. A tube or cylinder closed at one end ,in which the mixture of fuel and air can be compressed and burnt .2. A piston , which slides freely and yet fits closely in the cylinder ,so that the expanding gasforce it down the cylinder ,and do not escape past it .3. Two passages or ports in the cylinder . One for the mixture of fuel and air to enter the cylinder , the inlet prot , and the other to allow the used gases to escape , the exhaust port.4. Two valves , the inlet valve to control the the movement of the mixture to the cylinder , and the exhaust valve to control the escape of the used gases . By carefully timing the ignition of the mixture in relation of the opening and closing ofthe valves and the position of the piston , it is possible to make the piston move up and down the cylinder continuously .The working of an internal combusition engine , once it is started , involves a sequence or cycle of operations ineach cycle and the two-stroke cycle . In the four -stroke cycle , the crankshaft completes two revolutions and thepiston four strokes , a stroke being the movement of thepiston from one end of the cylinder to the other . In the two-stroke cycle , the crankshaft makes one r evolution andthe piston two strokes in each cycle .Your tractor probablyhas acompleteelectricsystemwhich provides electicutyforthe self-starter, forigniting the fuel , andforlights .Insomedesignselectricity is also usedforcertainother ac cessorycircuits.Such complete systems comprise several circuits : (1)generator circuit, (2) starting motor circuit , (3)lighing circuit , (4) ignition circuit . The storage battery may well be considered the basis of the complete system bacause it is co nnected to and works with all four of these principal circuits .We must reduce the relatively fast speed of the engine and transmit its power to the driving menbers to obtain suitable speeds and pulling power for the different farm jops. You may want a fst forward speed for hauling form the field or on the highway but a much slower fortions.Aslo,you msut have some means of reversing the direction of rotation of the driving members.Much of the necessry speed reduction is accomplished by the sliding(speed -changing)gears of the assembly which is ususlly called the “ transmission. ” Further reduction is secured at the differential and by the final drive pinions and bull gears(Fig.9).Fig.9You select the speed you want by shifting one of the sliding gears,which are splined to be transmission shaft.With the gear shift lever,you can move then forward or backward;you can mesh any certain sliding gear with it ’ smating gear on the counter-shaft.In the schematic diagram(Fig.9),gears C,E,G,ang K are sliding gears.Gear A ,at the end of the drive shaft,and gear B,on the ciuntershaft,are in constant mesh.Hence,the countershaft and all it ’ s rigidly connected gears are in motion whenever the drive shaft revolves.When you disengage the clutch, all gears are at rest and no power is transmittedto the drive wheels. However, when you move one of the sliding gears, such as C,into mesh with D,then by engaging the clutch,you complete the connectionbnetween the engine and the drive wheels.Contact between gears C and D(as shown ) would give low,or first, speed; engaging gears E and F would give sceond speed; gear G and H would give third speed. When none of the sliding gers is in mesh with it ’ s countershaft gear, the gears are said to be in “ neutral. ”Reverse speed is obtained by th e use of a small/pinion R between the countershaft gear I and the sliding gear K ,which causes the transmission drive shaft ti be driven in the opposite direction.In today ’ s tractors there are still more gear combingations giving more forward speeds; these are needed for the many operations now performed with farm tractors.In some tractor models,an additional speed -reducing andtoedue-increasing unit is installed between the engine cluth and the transmision driving gear.This unit is controlled with a separate lever near the tractor seat. When the unit incereases the “ torque ” or pulling force obtained from that speed.When you use this unit,you get a ratio of about three to two between the engine crankshaft and the transmission drive gear (when you are not using it you have the usual direct ratio between these two parts).Because this “ torque amplifier ” is ahead of the regular speed-changing gears, it reduces each speed you obtain from the various gear combinations. So it doubles the number of available speeds,Each speed is reduced about one -third andthe torque, or pulling force, from such slower speed is increased by 45 per cent . This is very helpful when plowing through tough spots or diskding through wet spots . You canengage the torque amplifier -reduce speed and increase pull-without stopping.And, when through the heavy pull,you can disengage it and return to normal speed without stopping.农用拖拉机拖拉机的发展历史很长，在这里不再赘述。
拖拉机发展史上 的几个里程碑是：1890年内燃机在美国拖拉机上的首次使用； 1901年Ivel拖拉机的问世；19 17年美国福特拖拉机以及1923 年万国公司Fa rm al l拖拉机的相距出现30年代，低压充气轮 胎的应用迅速普及，并且推广了第一批Forguson系统的拖拉机， 这种拖拉机配有力调节的三点液压悬挂装置拖拉机的分类目前制造的拖拉机可分类如下：A. 按照获得牵引力及自身推进的方法分：1. 轮式拖拉机1) 三轮拖拉机2) 四轮拖拉机2. 履带式拖拉机B. 按照用途分：1.通用型，即多用型拖拉机2 .万能型，即中耕型拖拉机轮式拖拉机：轮式拖拉机是主要的拖拉机类型，特别是在农 用方面现在制造的轮式拖拉机有三轮的或四轮的通常的配置 是在两个后驱动轮及一个或两个前导向轮履带式拖拉机：履带式拖拉机的牵引机构实质上由两条成为 履带的重型封闭金属链条装置构成每以条履带在两个铁轮上运 转，其中一个铁轮具有链齿，用作驱动轮，另一个用作惰轮转 向是通过履带本身来完成的，即通过减慢其中一条履带的运动速。