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Figure  2-6.Automatic  transmission  cross-sectional  view.
Automatic  Transmission  Operation

Equipment Operator Basic - Beginning construction equipment operators manual
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The principle of fluid drive is shown in figure 2-7. As two fans face each other, the speed of rotation of one fan makes the other fan rotate. When the speed of one fan is changed from medium to low, power is lost at low speeds; but, if the fan speed increases from medium to high, the speed of the driven fan picks up. Torque  Converter The torque converter is a form of and has replaced the fluid coupling. Most automatic transmissions used in automotive and construction equipment have torque converters. The torque converter consists of three parts: the pump (driving member), the turbine (driven member), and the stator (reaction member), all with curved vanes. The stator is located between the load and the power source to act as a fulcrum and is secured to the torque converter housing. Figure 2-8 shows a cutaway view of a  torque  converter  and  the  directional  flow  of  oil.  The pump throws out oil in the same direction in which the pump is turning. As the oil strikes the turbine blade, it forces the turbine to rotate, and the oil is directed toward the center of the turbine. Then the oil leaves the turbine and moves in a direction opposite to that of the pump. As the oil strikes the stator, it is redirected to flow in the same direction as the pump to add its force to that of the pump. Torque is multiplied by the velocity and direction given to the oil by the pump, plus the velocity and direction of the oil entering the pump from the stator. Planetary Gears Automatic  transmissions  use  a  system  of  planetary gears to enable the torque from the torque converter to be used efficiently. Figure 2-7.—Principle of fluid drive. Figure  2-8.—Torque  converter. Planetary units are the heart of the automatic transmission. The four parts that make up the planetary gear system are as follows: the sun gear, the ring (or internal) gear, the planet pinions, and the planet carrier. The sun gear is the center of the system. The term planet fits these pinions and gears, because they rotate around the sun gear, as shown in figure 2-9. The ring gear, or internal gear, is so-called because of its shape and internal teeth. An advantage of the planetary gear system is that it is compact. Additionally, in the planetary system more teeth make contact to carry the load. The reason for this is that each gear of the planetary system usually meshes with at least two other gears. Because the gears are always in mesh, none of the teeth are damaged as a result of teeth clashing or a partial mesh. However, the major advantage of the planetary system is the ease of shifting gears.  Planetary  gears,  set  in  automatic  transmissions, are shifted without any special skill required by the operator. 2-6







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