【正文】 conditions and requirements.Plain SpurGear Transmission Figure 187 illustrates the design and operation of the type of transmission originally used in earlier tractors and still used in some small, current models. As shown, the different speeds are obtained by sliding gears of a certain size into me h with others of another size. Reverse requires an additional single gear on a separate short shaft. The entire mechanism operates in an oil bath in a special housing. The bearings may be plain or antifriction type. The different speeds are obtained by a manually operated lever attached to certain gears in such a manner as to slide them into mesh with others that are stationary. These changes can only be made with the main clutch disengaged and the tractor stationary. Figure 188 shows a similar transmission used in a highclearance rowcrop tractor.Deere Horizontal TwoCylinder Transmission Figure 189 shows a twocylinder, horizontal engine equipped with a fourspeed sliding gear transmission and spurgear final drive for the axles and wheels.AllisChalmers DualRange Transmission The AUisChalmers dualrange (Fig. 1810) consists of an eightspeed, constantmesh unit with helical gears and shifting collars. A special doubleunit, handleveroperated disk clutch that runs in oil has three positions: Lowrange, Highrange, and Neutral. The change from one range to the other can be made by means of this clutch and a hydraulic assist mechanism while the tractor is moving.Ford DualPower Transmission Figure 1811 illustrates a manual shift transmission, which normally has eight forward speeds and two reverse speeds. However, if the tractor must operate at slower speeds and under heavier draft conditions, this transmission can be equipped with an optional gear assembly that will provide eight additional forward and four additional reverse speeds. The shift is made by a single lever without stopping the tractor. The travel speed is reduced, but the pulling power is increased to permit the handling of unexpected or difficult situations.Deere Hydraulic HiLo Range Tractor Figure 1812 is a sixcylinder, 80 hp tractor having a tworange transmission with twelve forward and three reverse speeds. Table 181 gives the travel speeds for each range.International Harvester 60 hp Tractor Figure 1813 shows the plete power transmission train for an International Harvester 60 hp tractor having two travel speed ranges with four forward speeds for each one. This is a constantmesh synchronized transmission. A HiLoReverse range lever permits shifting between forward and reverse for shuttletype operation on backandforth operations. The entire power train is pressure lubricated. The wetdisk brakes are hydraulic actuated and selfadjusting.Deere FourWheelDrive Tractor Figure 1814 illustrates the power train and transmission design for a fourwbeeldrive tractor. Power is first transmitted to two sets of primary master gears. These gears, in turn, transmit power to two sets of changespeed gears by which the differentravel speeds are obtained. These gears then transmit the power to the front and rear finaldrive gears and axles. Hydraulically operated front and rear differential locks simultaneously lock both axles. Both locks are operated by a single foot pedal which can be engaged or disengaged on the go.TrackType Tractor Transmissions The transmissions for the tracktype tractors are not unlike those in wheel machines except that the steering mechanism is incorporated in them. That is, the ordinary wheel tractors propelled by the rear wheels and guided by means of the front wheels, whereas the conventional tracktype tractor, having but two traction members, must be both propelled and guided through them.Caterpillar Transmission Figure 1815 shows the changegear set for the Caterpillar tractor. The power is transmitted to a countershaft by means of bevel gears。 then, through two steering clutches located on this countershaft on each side of the bevel gear, to the spurtype finaldrive gears。 and then to the sprocket. Steering is acplished through the multipledrydisk steering clutches。 that is, by means of hand levers, either clutch can be disengaged, which obviously cuts off the power to that particular track and causes the tractor to make a turn. Each clutch is equipped with a footoperated brake that acts on the outside of the clutch drum carrying the driven plates. If a quick, short turn is desired, not only is the clutch released but the brake for that particular clutch is actuated and the track movement virtually stopped on the one side. With all the power going to the other track, the machine obviously will turn very short. By pressing on both brakes at the same time, the machine can be stopped almost instantly. It should be noted that a differential is unnecessary in the Caterpillar transmission.Caterpillar Planetary Transmission Figure 1816 shows a planetary transmission and power train for a tracktype tractor. Power from the engine is first transmitted to the torque converter by the crankshaft, then to the planetary powershift transmission. This unit consists of a certain number of planetary units that vary in size. Large clutch packs surround each gear set and engage with special hydraulic modulation for smooth, cushioned shifting. The clutch plates and gears are cooled by oil. For operations that do not require frequent direction changes this tractor can be equipped with a geartype transmission. Hydrostatic Drives and Power Transmission The basic principles involved in the operation of hydrostatic drives are explained in Chap. 20. A hydrostatic drive mechanism consists essentially of two major units, namely, a positive displacement pump driven by the original power unit and a positive displacement motor, which receives its energy from the pump. In other words, the tractor engine develops and transfers mechanical energy to the pump. The pump unit then conve