履帶式行走底盤設(shè)計【履帶式農(nóng)用拖拉機(jī)底盤的設(shè)計】

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That is the reason why the clutch is called a friction mechanism . After engagement, the clutch must continue to transmit all engine torque to transmission depending on the friction without slippage . The clutch is also used to disengage the engine from the drive train whenever the gears in the transmission are being shifted from gear ratio to another . To start the engine or shift the gears , the driver has to depress the clutch pedal with the purpose of disengagement the transmission from the engine . At that time , the driven members connected to the transmission input shaft are either stationary or rotating at a speed that is slower of faster than the driving members connected to engine crankshaft . There is no spring pressure on the clutch assembly parts . So there is no friction between the driving members and driven members . As the driver lets loose the clutch pedal , spring pressure increase on the clutch parts . Friction between the parts also increases . The pressure exerted by the springs on the driven members is controlled by the driver through the clutch pedal and linkage . The positive engagement of the driving and driven members is made possible the friction between the surfaces of the members . When full spring pressure is applied , the speed of the driving and driven members should be the same . At the moment , the clutch must act as a coupling device and transmit all engine power to the transmission , without slipping . However , the transmission should be engaged to the engine gradually in order to operate the car smoothly and minimize torsional shock on the drive train because an engine at idle just develop little power . Otherwise , the driving members are connected with the driven members too quickly and the engine would be stalled . The flywheel is a major part of the clutch . The flywheel mounts to the engines crankshaft and transmits engine torque to the clutch assembly . The flywheel , when coupled with the clutch disc and pressure plate makes and breaks the flow of power the engine to the transmission . The flywheel provides a mounting location for the clutch assembly as well . When the clutch is applied , the flywheel transfers engine torque to the clutch disc . Because of its weight , the flywheel helps to smooth engine operation . The flywheel also has a large ring gear at its outer edge , which engages with a pinion gear on the starter motor during engine cranking . The clutch disc fits between the flywheel and the pressure plate . The clutch disc has a splined hub that fits over splines on the transmission input shaft . A splined hub has grooves that match splines on the shaft . These splines fit in the grooves . Thus , the two parts held together . However , back and forth movement of the disc on the shaft is possible . Attached to the input shaft , the disc turns at the speed of the shaft . The clutch pressure plate is generally made of cast iron . It is round and about the same diameter as the clutch disc . One side of the pressure plate is machined smooth . This side will press the clutch disc facing are against the flywheel . The outer side has shapes to facilitate attachment of spring and release mechanism . The two primary types of pressure plate assemblies are coil spring assembly and diaphragm spring . In a coil spring clutch the pressure plate is backed by a number of coil springs and housed with them in a pressed steed cover bolted to the flywheel . The spring push against the cover . Neither the driven plate nor the pressure plate is connected rigidly to the flywheel and both can move either towards it o away . When the clutch pedal is depressed a thrust pad riding on a carbon or ball thrust bearing is forced towards the flywheel . Levers pivoted so that they engage with the thrust pad at one end and the pressure plate tat the other end pull the pressure plate back against its springs . This releases pressure on the driven plate disconnecting the gearbox from the engine . Diaphragm spring pressure plate assemblies are widely used in most modern cars . The diaphragm spring is a single thin sheet of metal which yields when pressure is applied to it . When pressure is removed the metal spring back to its original shape . The center portion of the diaphragm spring is slit into numerous fingers that act as release levers . When the clutch assembly rotates with the engine these weights are flung outwards by centrifugal plate and cause the levers to press against the pressure plate . During disengagement of the clutch the fingers are moved forward by the release bearing . The spring pivots over the fulcrum ring and its outer rim moves away from the flywheel . The retracting spring pulls the pressure plate away from the clutch plate thus disengaging the clutch .When engaged the release bearing and the fingers of the diaphragm spring move towards the transmission . As the diaphragm pivots over the pivot ring its outer rim forces the pressure plate against the clutch disc so that the clutch plate is engaged to flywheel .The advantages of a diaphragm type pressure plate assembly are its compactness , lower weight , fewer moving parts , less effort to engage , reduces rotational imbalance by providing a balanced force around the pressure plate and less chances of clutch slippage . The clutch pedal is connected to the disengagement mechanism either by a cable or , more commonly , by a hydraulic system . Either way , pushing the pedal down operates the disengagement mechanism which puts pressure on the fingers of the clutch diaphragm via a release bearing and causes the diaphragm to release the clutch plate . With a hydraulic mechanism , the clutch pedal arm operates a piston in the clutch master cylinder . This forces hydraulic fluid through a pipe to the cutch release cylinder where another operates the clutch disengagement mechanism by a cable . The other parts including the clutch fork , release bearing , bell housing , bell housing cover , and pilot bushing are needed to couple and uncouple the transmission . The clutch fork , which connects to the linkage , actually operates the clutch . The release bearing fits between the clutch fork and the pressure plate assembly . The bell housing covers the clutch assembly . The bell housing cover fastens to the bottom of the bell housing . This removable cover allows a mechanic to inspect the clutch without removing the transmission and bell housing . A pilot bushing fits into the back of the crankshaft and holds the transmission input shaft .2.automatic transmission The modern automatic transmission is by far , the most complicated mechanical component in todays automobile . It is a type of transmission that sifts itself . A fluid coupling or torque converter is used instead of a manually operated clutch to connect the transmission to the engine . There are two basic types of automatic transmission based on whether the vehicle is rear wheel drive or front wheel drive . On a rear wheel drive car , the transmission is usually mounted to the back of the engine and is located under the hump in the center of the floorboard alongside the gas pedal position . A drive shaft connects the transmission to the final drive which is located in the rear axle and is used to send power to the rear wheels . Power flow on this system is simple and straight forward going from the engine , through the torque converter , then trough the transmission and drive shaft until it reaches the final drive where it is split and sent to the two rear transmission . On a front wheel drive car , the transmission is usually combined with the final drive to form what is called a transaxle . The engine on a front wheel drive car is usually mounted sideways in the car with the transaxle tucked under it on the side of the engine facing the rear of the car . Front axles are connected directly to the transaxle and provide power to front wheels . In this example , power floes from the engine , through the torque converter to a larger chain that sends the power through a 180 degree turn to the transmission that is along side the engine . From there , the power is routed through the transmission to the final drive where it is split and sent to the two front wheels through the drive axles . There are a number of other arrangements including front drive vehicles where the engine is mounted front to back instead of sideways and there are other systems that drive all four wheels but the two systems described here are by far the most popular . A much less popular rear and is connected by a drive shaft to the torque converter which is still mounted on the engine . This system is found on the new Corvette and is used in order to balance the weight evenly between the front and rear wheels for improved performance and handling . Another rear drive system mounts everything , the engine , transmission and final drive in the rear . This rear engine arrangement is popular on the Porsche. The modern automatic transmission consists of many components and systems that designed to work together in a symphony of planetary gear sets , the hydraulic system, seals and gaskets , the torque converter , the governor and the modulator or throttle cable and computer controls that has evolved over the years into what many mechanical inclined individuals consider to be an art from . Here try to used simple , generic explanation where possible to describe these systems .1）Planetary gear sets Automatic transmission contain many gears in various combinations . In a manual transmission , gears slide along shafts as you move the shift lever from one position to another , engaging various sizes gears as required in order to provide the correct gear ratio . In an automatic transmission , how ever , the gears are never physically moved and are always engaged to the same gears . This is accomplished through the use of planetary gear sets . The basic planetary gear set consists of a sun gear , a ring and two or more planet gears , all remaining in constant mesh . The planet gears are connected to each other through a common carrier which allows the gears to spin on shafts called “pinions” which are attached to the carrier . One example of a way that this system can be used is by connecting the ring gear to the input shaft coming from the engine , connecting the planet carrier to the output shaft , and locking the sun gear so that it cant move . In this scenario , when we turn the ring gear , the planets will “walk” along the sun gear ( which is held stationary ) causing the planet carrier to turn the output shaft in the same direction as the input shaft but at a slower speed causing gear reduction ( similar to a car in first gear ) . If we unlock the sun gear and lock any two elements together , this will cause all three elements to turn at the same speed so that to output shaft will turn at the same rate of speed as the input shaft . This is like a car that is third or high gear . Another way we can use a planetary gear set is by locking the planet carrier from moving , then applying power to the ring gear which will cause the sun gear to turn in opposite direction giving us reverse gear . The illustration in Figure shows how the simple system described above would look in an actual transmission . The input shaft is connected to the ring gear , the output shaft is connected to the planet carrier which is also connected to a “Multi-disk” clutch pack . The sun gear is connected to drum which is also connected to the other half of the clutch pack . Surrounding the outside of the drum is a band that can be tightened around the drum when required to prevent the drum with the attached sun gear from turning . The clutch pack is used , in this instance , to lock the planet carrier with the sun gear forcing both to turn at the same speed . If both the clutch pack and the band were released , the system would be in neutral . Turning the input shaft would turn the planet gears against the sun gear , but since noting is holding the sun gear , it will just spin free and have no effect on the output shaft . To place the unit in first gear , the band is applied to hold the sun gear from moving . To shift from first to high gear , the band is released and the clutch is applied causing the output shaft to turn at the same speed as the input shaft . Many more combinations are possible using two or more planetary sets connected in various way to provide the different forward speeds and reverse that are found in modern automatic transmission . 2）Clutch packA clutch pack consists of alternating disks that fit inside a clutch drum . Half of the disks are steel and have splines that fit into groves on the inside of the drum . The other half have a friction material bonded to their surface and have splines on the inside edge that fit groves on the outer surface of the adjoining hub . There is a piston inside the drum that is activated by oil pressure at the appropriate time to squeeze the clutch pack together so that the two components become locked and turn as one . 3）One-way Clutch A one-way clutch ( also known as a “sprag” clutch ) is a device that will allow a component such as ring gear to turn freely in one direction but not in the other . This effect is just like that bicycle , where the pedals will turn the wheel when pedaling forward , but will spin free when pedaling backward . A common place where a one-way clutch is used is in first gear when the shifter is in the drive position . When you begin to accelerate from a stop , the transmission starts out in first gear . But have you ever noticed what happens if you release the gas while it is still in first gear ? The vehicle continues to coast as if you were in neutral . Now , shift into Low gear instead of Drive . When you let go of the gas in this case , you will feel the engine slow you down just like a standard shift car . The reason for this is that in Drive , one-way clutch is used whereas in Low , a clutch pack or a band is used . 4）Torque Converter On automatic transmission , the torque converter takes the place of the clutch found on standard shift vehicles . It is there to allow the engine to continue running when the vehicle comes to a stop . The principle behind a torque converter is like taking a fan that is plugged into the wall and blowing air into another fan which is unplugged . If you grab the blade on the unplugged fan , you are able to hold it from turning but as soon as you let go , it will begin to speed up until it comes close to speed of the powered fan . The difference with a torque converter is that instead of using air it used oil or transmission fluid , to be more precise . A torque converter is a lager doughnut shaped device that is mounted between the engine and the transmission . It consists of three internal elements that work together to transmit power to the transmission . The three elements of the torque converter are the pump , the Turbine , and the Stator . The pump is mounted directly to the torque housing which in turn is bolted directly to the engines crankshaft and turns at engine speed . The turbine is inside the housing and is connected directly to the input shaft of the transmission providing power to move the vehicle . The stator is mounted to a one-way clutch so that it can spin freely in one direction but not in the other . Each of the three elements has fins mounted in them to precisely direct the flow of oil through the converter . With the engine running , transmission fluid is pulled into the pump section and is pushed outward by centrifugal force until it reaches the turbine section which stars it running . The fluid continues in a circular motion back towards the center of the turbine where it enters the stator . If the turbine is moving considerably slower than the pump , the fluid will make contact with the front of the stator fins which push the stator into the one way clutch and prevent it from turning . With the stator stopped , the fluid is directed by the stator fins to re-enter the pump at a “help” angle providing a torque increase . As the speed of the turbine catches up with the pump , the fluid starts hitting the stator blades on the back-side causing the stator to turn in the same direction as the pump and turbine . As the speed increase , all three elements begin to turn at approximately the same speed . Sine the 80s , in order to improve fuel economy , torque converters have been equipped with a lockup clutch which locks the turbine to the pump as the vehicle reaches approximately 40-50 mph . This lockup is controlled by computer and usually wont engage unless the transmission is in 3rd or 4th gear . 5）Hydraulic System The hydraulic system is a complex maze of passage and tubes that sends that sends transmission fluid and under pressure to all parts of the transmission and torque converter and . Transmission fluid serves a number of purpose including : shift control ,general lubrication and transmission cooling . Unlike the engine ,which uses oil primary for lubrication ,every aspect of a transmission s function is dependant on a constant supply of fluid is send pressure . In order to keep the transmission at normal operating temperature , a portion of the fluid is send to through one of two steel tubes to a special chamber that is submerged in anti-freeze in the radiator . Fluid passing through this chamber is cooled and then returned to the transmission through the other steel tube . A typical transmission has an avenge of ten quarts of fluid between the transmission , torque converter , and cooler tank , In fact , most of the components of a transmission are constantly submerged in fluid including the clutch packs and bands . The friction surfaces on these parts are designed to operate properly only when they are submerged in oil .6）Oil Pump The transmission oil pump ( not to confused with the pump element inside the torque converter ) is responsible for producing all the oil pressure that is required in the transmission . The oil pump is mounted to front of the transmission case and is directly connected to a flange on the engine crankshaft , the pump will produce pressure whenever the engine is running as there is a sufficient amount of transmission fluid available . The oil enters the pump through a filter that is located at bottom of the transmission oil pan and travels up a pickup tube directly to the oil pump . The oil is then sent , under pressure to the pressure regulator , the valve body and the rest of the components , as required . 7）Valve Body The valve body is the control center of the automatic transmission . It contains a maze of channels and passages that direct hydraulic fluid to the numerous valves which