【正文】 int between the engine and driving wheel of the vehicle. It provides a means for changingthe ratio between engine rpm ( revol utions per minute) and driving wheels rpm to best meet each particular driving situation. Given a level road ,an automobile without a transmission could be made to move by accelerating the engine and engaging the clutch. However, a start under these conditions would be slow, noisy and unfortable. In addition, it would place a tremendous strain on the engine and driving parts of the automobile.So in order to get smooth starts and have power to pass and climb hills, a power ratio must be provided to multiply the torque and turning effort of the engine. Also required is a speed ratio to avoid the need for extremely high engine rpm at high road speeds. The transmission is geared to perform these functions.The transmission is designed for changing the torque transmitted from the enginecrankshaft to the propeller shaft, reversing the vehicle movement and disengaging the engine from the drive line for a long time at parking or coasting. A higher to rque should be applied to the wheels to set an automobile in motion or move uphill with a full load than to keep it rolling after it gets under way on level stretches of the road, when inertia is high and tractive resistance is low. To meet these variable torque requirements, special gear boxes are used. Such gear boxes are called fixedratio transmissions.In a gear train consisting of a driving gear and a driven gear, the torque at the driven gear will increase as many times as the number of teeth of the driven gear is larger than that of the driving gear.The figure obtained by dividing the number of driven gear teeth by that of the driving gear is called gear ratio. If a train consists of several pairs of gears, the overall ratio is the product of the gear ratios of all the gear pairs in the train.To provide the different torques required under the varying operating conditionsof a vehicle, the transmission incorporates several pairs of gears with different gear ratios. If an intermediate gear is introduced between the driving gear and the driven gear, the rotation of the driven gear will be reversed.The transmission consists of a housing, an input shaft and gear, an output and gear, an idler shaft, a reverse gear, a cluster of gears and a gear shift mechanism.The cast iron housing has upper and side covers and bores for the installation of shafts. The bottom and side walls are provided with holes for filling and draining oil.The input shaft is made of steel integral with the driving gear and the rim. The front end of the input shaft is installed on the bearing in the bore of the crankshaft, while the rear end rotates in the bore of the front wall of the housing. The input shaft is arranged so that only the driving gear and the rim are acmodated inside the housing while the splined potion of the shaft protruding from the transmission carries the hub of the clutch driven disk. The output shaft is splined and rests with its front end on a roller bearing installed in the bore of the input shaft. The other end of the output shaft rotates in a ball bearing in the transmission housing wall. The output shaft splines carry the sliding gears. The output shaft is coaxial with the input shaft.The idler shaft carries a cluster of gears of different sizes. The cluster is mounted in the roller bearings on an axle or, together with the shaft, on bearings in the housing wall bores. The idler shaft constantly rotates with the input shaft because their gears are in constant mesh. The reverse gear(a single gear or a twogear cluster) rotates on an axle secured in the holes of the housing walls. The gear shift mechanism is designed for engaging gears, setting them neutral and engaging the reverse speed. The speeds are changed by shifting the gears or sleeves along the output shaft. The transmission type depends on the number of the sliding gears or sleeves. The transmission with two sliding gears or synchronizer sleeves is called tworange, with three gears, threerange. Depending on the number of forward speeds there are three,four and fivespeed transmissions. The gear shift mechanism is mounted on the top cover. The shift lever is arranged on the cover or on the bracket of the lever support.