engine
Tractors generally use diesel engines as power, and small walking tractors are also equipped with gasoline engines. Tractor diesel engines are generally high-speed small and medium-sized diesel engines, and the rotating speed is generally 2000 ~ 2800 rpm. Large tractors over 75 kW often use supercharged diesel engines or supercharged intercooled diesel engines. Because the tractor works under heavy load in the field or construction site, the engine must have a large torque reserve coefficient and a good filtering system.
Transmission system
A mechanism for transmitting the power of the engine to the driving wheel and the power output shaft. According to the needs of different operations, the transmission system can make the driving wheel and the power output shaft obtain torques of different sizes and directions and corresponding rotational speeds. At the same time, the power can be cut off to stop the tractor or stop the rotation of the power output shaft. According to the method of changing the transmission ratio, the transmission system can be divided into step type and stepless type.
Step drive system
The stepped drive system of wheeled tractor (Figure 1) generally consists of clutch, gearbox, central drive, differential and final drive. The stepped transmission system of crawler tractor (Figure 2) uses steering mechanism (such as steering clutch) instead of differential, and the rest is basically the same as that of wheeled tractor. In the stepped transmission system of walking tractor (Figure 3), except for a few direct transmission modes between centrifugal clutch and engine, belt transmission is generally adopted in the first stage of deceleration, and claw clutch (embedded clutch) is adopted as the steering mechanism. Tractor tractor
① Clutch. Used to transmit and cut off the power transmitted by the engine to the transmission system. Dry friction clutches are generally used in small and medium-sized tractors, while wet clutches with longer service life and softer operation have been mostly used in modern high-power tractors (above 75 kW). The load shift clutch in gearbox also mostly adopts multi-plate wet clutch, and its lining is generally copper-based powder metallurgy material. Most agricultural wheeled tractors use double-acting clutches, which are characterized by the combination of the main clutch that transmits power to the driving wheel and the auxiliary clutch that transmits power to the power output shaft. Some clutches are operated by pedals. With the downward movement of pedal stroke, the main clutch is disengaged first, and then the auxiliary clutch is disengaged. This structure is called linkage double-acting clutch. Some use pedals and hand levers to control the main clutch and auxiliary clutch respectively, which is called independent control double-acting clutch. Most wheeled tractors use linkage clutches.
② coupling. Used to connect the clutch shaft and the gearbox input shaft, there are two kinds of rigid coupling and elastic coupling. The rigid coupling is a spline sleeve; Elastic coupling is widely used because it uses rubber blocks as elastic elements, which can reduce the impact. However, general wheeled tractors are mostly formed by direct connection of various boxes to form a frameless body. Because of its good rigidity, the clutch shaft and the gearbox input shaft are generally made into a whole without coupling.
③ gearbox. The gearbox of tractor (transmission) traditionally adopts gear transmission structure, which generally consists of the main gearbox of 1 forward gear and 1 reverse gear and the auxiliary gearbox of 1 second gear (or third gear). It can obtain 6 ~ 18 forward gears and 2 ~ 3 reverse gears, with few gears and wide speed range. The number of teeth in modern tractor gearbox tends to increase. Wheeled tractors in Europe and America generally have 12 ~ 16 forward gears, and some even have as many as 24 gears. In order to further improve the mobility and productivity, some large and medium-sized tractors are equipped with load shift torque increasers at the front end of the gearbox. In order to meet the needs of slow operation (such as transplanting rice or ditching), some tractors add a reducer to the gearbox to provide a crawling gear. In the United States, some high-power tractors are equipped with full gear load shifting gearboxes. This kind of gearbox has simple operation and high productivity, but its structure is complex and its cost is high. It is usually provided as an optional component according to the needs of users.
④ Central drive. Used to increase the total transmission ratio of the transmission system and change the transmission direction of torque. In order to improve the stability of transmission, most tractors adopt spiral or cycloidal bevel gears (see bevel gear transmission), except for hand-held belt transmission as the first reduction and the central transmission of small tractors.
⑤ Differential. It can make the left and right driving wheels rotate at different speeds to meet the needs of steering. At the same time, when driving in a straight line, if the actual radii of the left and right driving wheels are not equal due to different tire pressure, load, manufacturing error and wear degree, it is also necessary to use a differential to make the left and right driving wheels rotate at different speeds, otherwise tire wear and power consumption will increase. General wheeled tractors adopt simple bevel gear differential. In order to improve the tractor's working passability in the field and overcome the phenomenon that the driving wheels slip on one side, the general wheeled tractors are equipped with differential locking mechanisms, which can lock the differential function when necessary, so that the additional driving wheels can exert sufficient driving force and the tractor can smoothly pass through bad sections. In crawler tractors, steering clutch, single-stage planetary steering mechanism and double differential are generally used to replace the simple differential of wheeled tractors.
⑥ Final drive. It is used to further reduce the speed from the engine and make the transmission ratio meet the requirements of the total transmission ratio. There are two kinds of final transmission: external meshing cylindrical gear transmission and planetary gear transmission. The former is divided into external type and internal type according to the different arrangement positions. Crawler tractors and some wheeled tractors use external main reducer, which is beneficial to improve the ground clearance of rear axle. The invention has a compact structure and can obtain a larger transmission ratio; But this is not conducive to improving the height of the rear axle from the ground. Walking tractors and high-power crawler tractors often adopt two-stage final drive because of the need of total transmission ratio and steering mechanism characteristics.
Continuously variable transmission system can make the running speed of tractor better meet the requirements of different operations, and make the engine often run under calibrated working conditions, thus improving production efficiency and reducing fuel consumption rate. However, the stepless transmission system has low transmission efficiency, high manufacturing cost and long service life, and is still in the stage of development or small batch production. The stepless transmission systems used by tractors mainly include mechanical transmission (belt or chain), electrical transmission, hydraulic transmission and hydro-mechanical transmission. Among them, the application of hydrostatic CVT in tractors is more concerned. Although its production cost is still higher than that of general gear transmission, it is more convenient to use and operate, and it is still applied to a certain extent. Hydraulic CVT has been produced in small batches in the United States and the Federal Republic of Germany, and has been applied to small four-wheel garden tractors with power of 4.5 ~ 12 kW, and a small number of four-wheel drive tractors with power of 45 ~ 60 kW have adopted this transmission system. In large and medium-sized industrial tractors, some are equipped with hydraulic couplers or torque converters to improve the handling performance, reduce the impact load of the transmission system and improve the service life of the transmission system. Hydromechanical transmission system equipped with hydraulic torque converter can also automatically adapt to the change of external resistance and improve the traction performance of tractor.
Steering system
A mechanical device used to change the direction of a tractor and keep it running in a straight line. The steering system of wheeled tractor can be divided into three types: mechanical, hydraulic booster and cold hydraulic. Steering modes can be divided into front wheel steering, rear wheel steering, four-wheel steering and folding steering. Generally, wheeled tractors mostly use front wheel steering. The steering mechanism of crawler tractor includes steering clutch, single-stage planetary mechanism and double differential. Steering clutch is widely used in general crawler tractors because of its simple structure, small minimum steering radius and good straight-line driving performance. Steering clutches need to transmit a lot of torque, and all of them adopt multi-plates, which are divided into dry and wet types. Most large industrial tractors use wet steering clutch, and the friction lining is made of copper-based powder metallurgy material, which has high cost, long service life and reliable use.
braking system
Used for braking and steering when driving, so that the tractor can stop safely on the slope. There are several types of brakes: block, belt and disc. In addition to small wheeled tractors using block brakes, disc brakes with good sealing performance, small size and convenient operation are also widely used. Because of its convenient arrangement, belt brakes are mostly used on crawler tractors. Disc brakes are divided into dry type and wet type. The wet brake is lined with powder metallurgy material, which is reliable in use and can greatly improve the braking smoothness.
There are several brake operating mechanisms: mechanical, pneumatic and hydraulic. Most small and medium tractors use mechanical control mechanism, while large tractors use hydraulic control mechanism. Considering the need to use brakes to assist steering, the driving wheels on both sides should be able to brake separately. When a wheeled tractor is used for transportation, the left and right brake pedals are interlocked by interlocking plates to ensure that the two driving wheels brake at the same time.
Walking system
Tractor is a device used to support the weight of tractor body, ensure the running performance of tractor, and convert the driving torque transmitted by engine to driving wheels into traction. Common walking mechanisms are pneumatic tires and crawler-type walking devices.
All the driving tires of agricultural tractors adopt large-diameter low-pressure tires with inflation pressure below 0. 15 MPa and pattern height of about 3 cm; The inflation pressure of industrial tractors is 0.15 ~ 0.4 MPa; Off-road pneumatic tires or solid tires with low tread height. The walking device of crawler tractor consists of suspension and walking mechanism. The function of the suspension is to connect the body of the crawler tractor with the walking mechanism and transfer the weight of the tractor to the rollers. The suspension of crawler tractor can be divided into three types: rigid, semi-rigid and elastic, of which semi-rigid suspension is the most widely used. The crawler walking mechanism is used to support the machine body, tension and guide the moving direction of the crawler, and form a closed crawler ring, that is, the tractor's own moving crawler. Crawler walking mechanism is generally composed of driving wheel, guide wheel, roller, sprocket, crawler and tensioning mechanism.
Adding various paddy field iron wheels, high tread tires and narrow carcass high tread tires to wheeled tractors can improve the tractor's adhesion under paddy soil conditions and reduce rolling resistance.
Hydraulic suspension device
There are three connection modes between agricultural tractors and farm tools: traction, suspension and semi-suspension. Modern tractors are mostly suspended and semi-suspended. According to different operation requirements, farm tools can be hung at the rear, front, side or between axles of tractors, thus forming different suspension modes such as rear suspension, front suspension, side suspension and inter-axle suspension, among which the rear suspension device is the most widely used. The hydraulic suspension device consists of hydraulic system and suspension mechanism (Figure 4). Hydraulic suspension device is not only used to lift farm tools, but also used to adjust the tillage depth and transfer the weight of some farm tools to the driving wheels to improve the adhesion and traction performance of tractors.
The hydraulic system is mainly composed of hydraulic pump, distributor, hydraulic cylinder, control mechanism and various accessories (such as oil tank, pipeline and filter, etc.). ). Hydraulic oil pumps are mostly gear pumps or plunger pumps (see reciprocating pumps), and distributors are mostly slide valves. According to whether the oil passes through the control valve when the distributor control valve is in neutral position, the hydraulic system can be divided into open type and closed type. Generally speaking, agricultural tractors mostly adopt open hydraulic system with simple structure. In some modern high-power tractors, closed-center variable flow and constant pressure hydraulic systems are increasingly adopted because of the need for multiple hydraulic manipulations.
The suspension mechanism comprises an upper pull rod, a lower pull rod, a lifting rod, a lifting arm and a connecting piece. Agricultural wheeled tractors generally adopt a three-point suspension mechanism. In order to facilitate the suspension and removal of farm tools, a quick hitch device can be installed behind the three-point suspension mechanism of large and medium-sized wheeled agricultural tractors, which is more and more widely used.
There are two connection modes between industrial tractor and its working device: traction type and suspension type. According to the different belt working devices, there are usually two ways: front suspension and rear suspension, and the working devices can also be hung at the front and rear of the tractor at the same time. When working with traction working device, it is necessary to equip traction mechanism and winch control device. Modern industrial tractors mostly use hydraulic suspension devices. The hydraulic system is mostly open, with simple structure and good heat dissipation, and the oil can be filtered in the oil tank. Because this kind of hydraulic suspension device has more actuators (cylinders) to control at the same time, and the flow rate of its hydraulic system is larger than that of agricultural tractors with the same power level, multi-channel hydraulic control valves are often used.
Power output device A device used by a tractor to output some power to drive a working tool when it is in motion or at rest. Generally, there are three forms: power output shaft, pulley and hydraulic output point.
Power output shaft is mainly used to drive working tools with rotating parts, such as rotary cultivators, winch devices, harvesting machinery, plant protection machinery and trailers with driving shafts. Power output shaft can be divided into standard speed type and synchronous type according to the speed characteristics. ① Standard speed power output shaft. Its speed is proportional to the speed of the engine and does not change with the grade of the gearbox. According to the different tractor power, countries adopt two different standard speeds, namely 540 10 rpm and 1000 25 rpm (when the engine speed is not lower than 80% of the rated speed). ② Synchronous power output shaft. The ratio of its speed to the speed of the driving wheel is a fixed value, which does not change with the grade of the gearbox. It is used in agricultural machinery that needs power output speed to be proportional to tractor speed, such as seeder and trailer with drive axle. The rotating speed of synchronous PTO is about 3.4 ~ 9.8 rpm (the rotating speed of PTO when tractor is running 1 meter).
belt roller
Used to drive fixed agricultural machinery, such as thresher, feed crusher, irrigation and drainage machinery and power generation equipment. The determination of pulley rotation direction should be based on the principle that the tight edge of belt is on the lower side. The rear pulley with gearbox driven by power output shaft is widely used.
Hydraulic output point
With the increase of tractor power and large-scale complex working tools, agricultural tractors should have more hydraulic output points and greater hydraulic output capacity. In industrial tractors, because there are a variety of hydraulically operated working tools, a necessary number of hydraulic output points and corresponding operating mechanisms are usually provided.
Cab and seat
Good cabs and seats and corresponding control and monitoring devices are not only necessary to improve drivers' working conditions and ensure drivers' safety, but also of great significance to improve labor productivity and work quality. In order to prevent personal accidents during rollover, generally medium-sized tractors have optional safety protection frames or safety cabs, and the cabs of large tractors are generally standard. The cab tends to adopt a fully enclosed structure, mostly large glass windows to ensure a good view, and the bottom plate is fixed on the base of the chassis with a damping rubber pad. The cab is covered with sound-absorbing materials, and the floor is covered with rubber plates to reduce the noise, heat and high-frequency vibration of the engine and transmission system. When tractors in Europe and America are not equipped with cab, the noise at the driver's ear is set at 90 ~100 db (a); When installing the cab, the indoor noise is 80 ~ 85dB (a). The air entering the cab is filtered by a paper filter, and the air pressure in the cab is slightly higher than the atmospheric pressure to reduce dust entry. Generally, the cab of a medium-sized tractor has a heating device and no air conditioning equipment, and the front and rear windows of the cab can be opened to facilitate ventilation. The cab of a large tractor is usually equipped with heating and air conditioning equipment.
The tractor driver's seat produced in the early days was generally simple in structure and poor in comfort. Since 1960s, most newly designed tractors have been equipped with comfortable seats with elastic suspension. Elastic suspension includes three parts: bar mechanism, elastic element and hydraulic damping device. For more exquisite seats, the stiffness of the elastic element can be adjusted according to the driver's weight, and the inclination angle of the seat back can also be adjusted. In order to better improve the driver's comfort, the steering wheel height and inclination angle of some modern high-power tractors can also be adjusted.