車輛畢業(yè)設(shè)計-28馬力輪式拖拉機變速箱設(shè)計【含CAD圖紙】
車輛畢業(yè)設(shè)計-28馬力輪式拖拉機變速箱設(shè)計【含CAD圖紙】,含CAD圖紙,車輛,畢業(yè)設(shè)計,28,馬力,輪式拖拉機,變速箱,設(shè)計,cad,圖紙
FARM TRACTOR
The 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 American Ford 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 tractors
a. Three tractors
b. Four wheels
2. Track-type tractors
B. According to utility :
1. General-purpose or utility
2. All-purpose or row-crop type
Wheel tractors. The wheel-type tractor is the predomingating type,particularly for agricultural purposes.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 the track-type tractor consists essentially of two heavy,endless,metal-linked devices known as tracks themselves 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 the field and belt 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 to power the 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 internal 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 gas force 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 of the 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 in each cycle and the two-stroke cycle . In the four-stroke cycle , the crankshaft completes two revolutions and the piston four strokes , a stroke being the movement of the piston from one end of the cylinder to the other . In the two-stroke cycle , the crankshaft makes one revolution and the piston two strokes in each cycle .
Your tractor probably has a complete electric system which provides electicuty for the self-starter , for igniting the fuel , and for lights . Insome designs electricity is also used for certain other accessory circuits.
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 connected 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.9
You 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’s mating 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 connection bnetween 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 the 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 and toedue-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 and the 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 can engage 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.
農(nóng)用拖拉機
拖拉機的發(fā)展歷史很長，在這里不再贅述。拖拉機發(fā)展史上的幾個里程碑是：1890年內(nèi)燃機在美國拖拉機上的首次使用；1901年Ivel拖拉機的問世；1917年美國福特拖拉機以及1923年萬國公司Farmall拖拉機的相距出現(xiàn)。30年代，低壓充氣輪胎的應(yīng)用迅速普及，并且推廣了第一批Forguson系統(tǒng)的拖拉機，這種拖拉機配有力調(diào)節(jié)的三點液壓懸掛裝置。
拖拉機的分類。目前制造的拖拉機可分類如下：
A. 按照獲得牽引力及自身推進(jìn)的方法分：
1.輪式拖拉機
1） 三輪拖拉機。
2） 四輪拖拉機。
2．履帶式拖拉機
B. 按照用途分：
1．通用型，即多用型拖拉機。
2．萬能型，即中耕型拖拉機。
輪式拖拉機：輪式拖拉機是主要的拖拉機類型，特別是在農(nóng)用方面?，F(xiàn)在制造的輪式拖拉機有三輪的或四輪的。通常的配置是在兩個后驅(qū)動輪及一個或兩個前導(dǎo)向輪。
履帶式拖拉機：履帶式拖拉機的牽引機構(gòu)實質(zhì)上由兩條成為履帶的重型封閉金屬鏈條裝置構(gòu)成。每以條履帶在兩個鐵輪上運轉(zhuǎn)，其中一個鐵輪具有鏈齒，用作驅(qū)動輪，另一個用作惰輪。轉(zhuǎn)向是通過履帶本身來完成的，即通過減慢其中一條履帶的運動速度使之低于另一條的速度的辦法來完成的。
通用型即多用型拖拉機：通用型拖拉機基本上是一種傳統(tǒng)設(shè)計的拖拉機，例如普通的四輪拖拉機或履帶式拖拉機。這種類型也被成為多用型拖拉機，它僅用來完成一般的拖拉機作業(yè)，包括田間作業(yè)和皮帶作業(yè)，例如耕地、耙地、平路、聯(lián)合收獲、飼料粉碎等等。
萬能型拖拉機：萬能型拖拉機即中耕型拖拉機，用來完成一般的農(nóng)場的幾乎所有的田間作業(yè)和皮帶作業(yè)，包括中耕作物的播種和中耕在內(nèi)。
傳統(tǒng)的拖拉機通常有兩個用發(fā)動機驅(qū)動的后輪和兩個支重的前輪。用來為拖拉機提供動力的發(fā)動機類型是內(nèi)燃機。發(fā)動機是拖拉機的心臟。正是在這里燃料的化學(xué)年能被轉(zhuǎn)化為使輪子轉(zhuǎn)動的機械能。
內(nèi)燃機和外燃機的區(qū)別僅僅是在：在內(nèi)燃機力，勢能轉(zhuǎn)化為機械能完全與空氣精確的混合，然后在控制的條件下燃燒。這種混合燃燒時就會膨脹，壓力增大，推動活塞沿著氣缸直線地向下運動。我們就說活塞作直線運動。該直線運動通過與活塞連接的連桿和曲柄機構(gòu)轉(zhuǎn)變?yōu)樾D(zhuǎn)運動。
為了能有效地利用這種能量，對燃燒過程和膨脹壓力必須加以控制。為此發(fā)動機必須具有：
1.一端密閉的管子即氣缸，在這里燃油和空氣的混合氣被壓縮和燃燒
2.活塞，它可以在氣缸里只有滑動，但又與氣缸配合嚴(yán)密，這樣，膨脹的氣體就能推動活塞沿著氣缸向下運動而不從周圍漏掉。
3.氣缸的兩個同期道：一個是燃料和空氣的混合氣進(jìn)入氣缸的進(jìn)氣口，另一個是排泄廢氣的排氣口。
4.兩個閥門：控制混合氣進(jìn)入氣缸的進(jìn)氣門和控制廢氣排放的排氣門。根據(jù)閥門的開筆和活塞的位置，使混合氣正時點火，就可以使活塞連續(xù)地在氣缸里上下運動。
內(nèi)燃機一旦運動，其工作過程就是指每個氣缸里所發(fā)生的一系列的動作。一般有兩種不同的循環(huán)：四行程循環(huán)和二行程循環(huán)。在四沖程循環(huán)中，曲柄轉(zhuǎn)兩圈，活塞完成四個行程，一個行程就是指活塞自氣缸的一端運動到另一端。在二行程循環(huán)中，每一個循環(huán)曲柄轉(zhuǎn)一圈，活塞完成兩個行程。
拖拉機大概都有一套完整的電系來為電動機、燃料點火和照明設(shè)備供電。在有些拖拉機的設(shè)計上還要為其它一些輔助線路供電。
一套完整的電系包括下面幾條線路：（1）發(fā)電線路；（2）電起動機線路；（3）照明線路；（4）點火線路。因為蓄電池與所有這四條主要線路連接并為其供電，所以完全可以把它看作是整個電系的基礎(chǔ)。
為了獲得適合各種農(nóng)業(yè)工作的速度和牽引力，我們就必須降低發(fā)動機較快的轉(zhuǎn)速并將其動力傳遞給驅(qū)動裝置。在田間或公路運輸時需要較快的前進(jìn)速度，但在重負(fù)荷田間作業(yè)時需要較慢的前進(jìn)速度和較大的牽引力，此外，還必須有使驅(qū)動裝置反轉(zhuǎn)的某些裝置。
大部分必要的減速通常是通過“傳動箱”總成中的滑動齒輪來完成的。進(jìn)一步的減速則是通過差速器和最終傳動的大、小齒輪來完成的（圖9）。
移動用花鍵連接在傳動軸上的一個滑動齒輪，就可以選擇所需要的某個速度。用變速桿可使各滑動齒輪前后移動，因而能使任何一個化齒輪與中間軸上的和其配對的齒輪嚙合。
在示意圖9中，齒輪C、E、G及K為滑動齒輪。驅(qū)動軸末端的齒輪A與中煎軸上的齒輪B常嚙合。因此每當(dāng)驅(qū)動軸旋轉(zhuǎn)時，中間軸及固定在該軸上的所有齒輪都將轉(zhuǎn)動。
若將離合器分離時，則將所有齒輪都不動，因而也即沒有動力傳到驅(qū)動輪上去。但是如果移動一個滑移齒輪。例如C,與齒輪D嚙合，那么結(jié)合離合器，就可以把發(fā)動機和驅(qū)動輪連接起來。
齒輪C與D嚙合得低擋，即I速，齒輪E與F嚙合得Ⅱ速，齒輪G與H嚙合得Ⅲ速。當(dāng)所有的滑動齒輪都不與中間軸上的齒輪嚙合時，我們就說這些齒輪處于“空擋”位置。
在中間軸齒輪I和K之間使用了一個小齒輪R，使傳動軸被帶著反轉(zhuǎn)，就獲得倒擋。
在現(xiàn)代的拖拉機中，還有更多的齒輪組合可以提供更多的前進(jìn)速度；這些速度是現(xiàn)在農(nóng)用拖拉機完成許多作業(yè)時所需要的。
在有些型號的拖拉機上，在發(fā)動機離合器和變速箱驅(qū)動齒輪之間增裝了一個減速增扭裝置，它由駕駛座旁邊的一個單獨手柄控制；該裝置工作時，能使任一速擋減速，從而增大該速擋所產(chǎn)生的“扭矩”，或者說牽引力。
當(dāng)使用該裝置時，發(fā)動機曲軸和變速箱驅(qū)動齒輪之間約為3：2（不使用該裝置時，兩部件之間為直接傳動，傳動比不見）。
因為“增扭器”一般裝在變速裝置之前，所以它能減低從不同的齒輪組合所獲得的每一種速度，這樣它就把所能得到的速度增加了一倍。每一個速度均減低約1/3，從而這種減慢了的速度所獲得的扭矩，或者說牽引力，則增大了45％。這在犁地通過堅硬地段或用圓盤耙耙地通過潮濕地段時是很有作用的。不必停車，便可結(jié)合增扭器，減低速度，增大牽引力。當(dāng)通過了這個使用大牽引力的地段以后，不必停車，就可以分離增扭器，恢復(fù)常速。
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