580 輕型載貨汽車設(shè)計（底盤設(shè)計）（有cad圖）
580 輕型載貨汽車設(shè)計（底盤設(shè)計）（有cad圖）,580,輕型載貨汽車設(shè)計（底盤設(shè)計）（有cad圖）,輕型,載貨,汽車,設(shè)計,底盤,cad
Classification of Tractors
The tractor is a wheeled or tracked self-propelled vehicle used as a power means for moving agricultural, road building, and other machines equipped with special tools, and also for towing trailers. The tractor engine can be used as a prime mover for active moving tools or starting farm machinery through the intermediary of the power takeoff shaft or belt pulley.
The uses of the tractor in agriculture are many, and so different types of tractors are needed to do different types of farm work.
Farm tractors are classified as follows.
AS TO PURPOSE, modern farm tractors are classed in three groups: general-purpose tractors (land utility), universal-row-crop (row-crop utility) tractors, and special-purpose tractors.
Land utility tractors are used for major farm operations common to the cultivation of most crops, such as tillage, digging, general cultivations, harrowing, sowing, and harvesting. The tractors are characterized by a low ground clearance, increased engine power, and good traction. Thanking to their wide tires or tracks enabling them to develop a high pull.
Universal-row-crop tractors are intended for row-crop work, as well as for many other field tasks. For this purpose, some row-crop utility tractors are provided with replaceable driving wheels of different tread widths-wide for general farm work and narrow for row-drop work, in order not to damage plants, the tractors have a high ground clearance and a wide wheel track that can be adjusted to suit the particular inter-tow distance.
Special-purpose tractors are modifications of standard land or row-crop utility tractor models and are used for definite jobs, and under certain conditions. Thus, special tractors used to mechanize the cultivation of cotton have a single front wheel, swamp tractors are equipped with wide tracks enabling them to operate on marshy soils, and hillside tractors are designed to work on hillsides sloping at up to 16o.
AS TO THE DESIGN OF THE RUNNING GEAR，tractors are divided into crawler (track-laying) and wheeled types.
Crawler tractors are distinguished by a large ground contact area and therefore have a good track adhesion; they crush and compact the soil insignificantly. Such tractors show a high cross-country power and are capable of developing a high pull.
Wheeled tractors are more versatile and can be used for both field and transport work, but their traction is lower than that of crawler tractors.
Main Component Parts of tractor
The tractor is complex self-propelled machine consisting of separate interacting mechanisms and units that can be combined into certain groups.
Irrespective of particular design features, all tractors consist of engine, drive line, running fear, steering mechanism, working attachments, and auxiliary equipments.
THE ENGINE converts thermal energy into mechanical energy.
THE DRIVE LINE comprises a set of mechanisms which transmit the torque developed by the engine to the driving wheels or tracks and change the driving torque both in magnitude and direction. The drive line includes the clutch, flexible coupling, transmission (gearbox) and rear axle.
The clutch serves to disconnect the engine shaft from the transmission for a short period of time while the driver is shifting gears and also to connect smoothly the flow of power from the engine to the driving wheels or tracks when starting the tractor from rest.
The flexible coupling incorporates elastic elements allowing to connect the clutch shaft and the transmission drive shaft with a slight misalignment.
The transmission makes it possible to change the driving torque and the running speed of the tractor by engaging different pairs of gears. With the direction of rotation of the engine shaft remaining the same, the transmission enables the tractor to be put in reveres.
The rear-axle mechanisms increase the driving torque and transmit it to the driving wheels or tracks at right angles to the drive shaft. In most tractors, the rear axle also comprises brakes.
In the wheeled tractor, as distinct from its crawler counterpart the drive line includes the differential which enables the driving wheels to revolve with different speeds when making turns of running over a ragged terrain, at which time the left-and right-hand wheels must travel different distances during one and the same time.
THE RUNNING GEAR is needed for the tractor to move. The rotation of the driving wheels (or the movement of the tracks) in contact with the ground is converted into translatory motion of the tractor.
THE STEERING MECHANISM serves to change the direction of movement of the tractor by turning its front wheels (in wheeled tractors) or by varying the speed of one of the tracks (in crawler tractors).
THE WORKING ATTACHMENTS of the tractor are used to utilize the useful power of the tractor engine for various farm tasks. They include the power takeoff shaft, drawbar (hitch device), implement-attaching (mounting) system, and belt pulley.
THE TRACTOR AUXILIARIES include the driver’s cab with a spring-mounted seat and heating and ventilation equipment, hood, lighting equipment, tell tales (indicators), horns, etc.
The Clutch
The clutch is located in the power train between the engine and the transmission. The clutch allows the driver to couple the engine or to uncouple the engine from the transmission while he is shifting gears or starting the tractor moving from rest.
Modern tractors use friction clutches, ones employing friction forces to transmit power. The friction surfaces in such clutches are provided by discs, whose number depends on the magnitude of torque to single- and double-disc clutches.
Clutch driving disc (pressure plate) is connected to the engine flywheel, while driven disc is mounted on transmission clutch (input) shaft. The driven disc has splines in its hub that match splines on the input shaft. The disc is tightly clamped between the pressure plate and the flywheel by a series of coil springs, called the pressure springs held between the clutch cover and the pressure plate. Owing to the friction forces arising between the friction surfaces of the flywheel, driven disc, and pressure plate , torque transmission input shaft. In this position, the clutch is engaged.
The clutch is operated by the clutch linkage which passes on the movement of clutch pedal to clutch release (throw-out) bearing. When the driver steps on the pedal, the clutch linkage, which includes an operating rod and a release fork, forces the release bearing inward (to the left). As the release bearing moves left, it pushes against the inner ends of three release levers. When the inner ends of three release levers are pushed in by the release bearing, the outer ends of the levers move the pressure plate to the right, compressing pressure springs. With the spring pressure off the driven disc, spaces appear between the disc, the flywheel, and the pressure plate. Now the clutch is disengaged (released), and the flywheel can rotate without sending power through the driven disc. When the clutch pedal is released, the pressure springs force the pressure plate to the left. The driven disc is again clamped tightly between the flywheel and the pressure plate. The driven disc must again rotate with the flywheel. In this position, the clutch is engaged. The initial slipping of the driven disc, which occurs until the disc is fully clamped between the flywheel and the pressure plate, tends to make the engagement smooth. The clutch described above is known as the spring-loaded dry friction type.
Propeller Shaft and Universal Joint
The propeller shaft is a drive shaft to carry the power from the transmission to the rear-wheel axels. It connects the transmission main, or output shaft to the differential at the rear axels. Rotary motion of the transmission main shaft is carried by the propeller shaft to the differential, causing the rear wheels to rotate.
The propeller-shaft design must take two facts into consideration. First, the engine and transmission are more or less rigidly attached to the car frame. Second, the rear-axle housing (with wheels and differential) is attached to the frame by springs. As the rear wheels encounter irregularities in the road, the springs are compressed or expanded. These change the angle of drive and the distance between the transmission and the differential, and the propeller shaft should take care of these two changes. That is to say, as the rear axle housing, with differential and wheels, moves up and down, the angle between the transmission output shaft and propeller shaft changes. The reason why the angle increases is that the rear axle and differential move in a shorter than the propeller shaft. The center pointer of the axle-housing is rear-spring or control-arm attachment to the frame. In order that the propeller shaft may take care of these two changes, it must incorporate two universal joints to permit variations in the angle of drive. There must be a set of slip joint to make the propeller shaft change.
The propeller shaft may be solid or hollow, protected by an outer tube or exposed. Some applications include bearings at or near the center of the propellers which are supported by a center bearing and coupled together by universal joints.
A universal joint is essentially a double-hinged joint consisting of two y-shaped yokes, one on the driving shaft and the other on the driven shaft, and across-shaped member called the spider. The four arms of the spider, known as trunnions, are assembled into bearings in the ends of the two shaft yokes. The driving shaft causes the spider to rotate, and the other two trunnions of the spider cause the driven shaft to rotate. When the two shafts are at an angle to each other, the bearings in the yokes permit the yokes to swing around on the trunnions with each revolution. A variety of universal joints have been used on automobiles, but the types now in most common use are the ball-and-trunnion joints.
A slip joint consists of outside splines on one shaft and matching internal splines in the mating hollow shaft. The splines cause the two shafts to rotate together but permit the two to move endwise with each other. This accommodates any effective change of length of the propeller shaft as the rear axles move toward or away from the car frame.
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