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.

Figure 2-9.-Planetary gear system.

Power can be transmitted through the planetary gearset in various ways. A shaft from the engine may be connected to drive the sun gear. It may be connected to drive the planet carrier or the shaft may be connected to drive the ring gear. The propeller shaft may also be connected to anyone of these members; however, power can be transmitted in the planetary gear system only when (1) the engine is delivering power to one of the three members, (2) the propeller shaft is connected to one of the other members, and (3) the remaining member is held against rotation. All three conditions must be satisfied for power to be transmitted in the system. Automatic transmissions provide for holding a member through hydraulic servos and spring pressure.

Automatic Transmission Operation

Most automatic transmissions are basically the same. They combine a fluid torque converter with a planetary gearset and control the shifting of the planetary gear with an automatic hydraulic control system. The fluid torque converter is attached to the engine crankshaft and serves as the engine flywheel. This design means that when the engine runs, engine power flows into the converter and drives the converter output (turbine) shaft. There is no neutral in the torque converter. Neutral is provided in the planetary gearset by the release of bands and clutches.

The transmission automatically multiplies and transmits engine torque to the drive shaft as driving conditions demand. The speeds at which the coupling point and the gearshifts occur are controlled partially by the operator. The operator has only a partial control in the D-drive position, because the transmission in the D-drive position shifts the planetary gearset into the higher gears to prevent engine overspeeding regardless of throttle position.

The operation of automatic shift vehicles is quite simple; however, it is imperative that the professional operator learn to operate them smoothly and properly. In vehicles equipped with automatic transmissions, initial gear selection is controlled with a selector lever. When in drive (D or DR), shifting from drive to low (L) and returning to drive is controlled automatically by the engine speed.

Most vehicles have four or five of the following selector positions.

P-PARK POSITION.- On light vehicles, such as sedans and pickups, this position is used for locking the transmission so the vehicle cannot roll while parked. In some heavier vehicles, the park position does not lock the transmission. In vehicles with a park position, the engine should be started from the park position.

N-NEUTRAL POSITION.- Engines of vehicles not equipped with a P-park position are started from the N-neutral position. In this position, the engine is disengaged from the drive shaft of the vehicle.

D-DRIVE POSITION.- With the shift lever at D or DR, the vehicle moves forward as you depress the accelerator. After starting the engine in neutral or park position, step on the brake and change the selector to D or DR for forward movement. To avoid premature forward movement, keep pressure on the brake while in the drive position until you are ready to place the vehicle in motion. Without further operator action, the transmission automatically shifts to higher gears as speed increases.

L-LOW or POWER POSITION.- The transmission will not shift automatically to higher gear ratios when the lever is in the low position. The low position is used when negotiating steep grades and rough terrain or when the braking power of the engine is required. When low range is no longer needed, release the accelerator temporarily and move the shift lever to the drive position for normal gear progression. In the drive position, the low range is engaged automatically when engine speed is reduced. If the accelerator is suddenly fully depressed, the low range becomes engaged. (This procedure may be used to provide a sudden burst of speed for passing.) When a predetermined engine speed has been attained, the transmission automatically returns to driving range.

R-REVERSE POSITION.- Some shift levers must be raised slightly to be moved to the R or reverse position. Others may require the depressing of a button on the end of the lever before moving to R.

Become thoroughly familiar with the operator's manual, vehicle instruments, controls, and selector positions before operating a vehicle or piece of equipment. You may operate equipment that has the R-reverse position on the extreme right on some shift selectors, on the extreme left on others, and the intermediate position on others. From a force of habit, when you are in a different vehicle from the one you have been operating, you could move the selector lever to R, thinking you were moving it to D or L, and cause the vehicle to move in an entirely opposite direction than anticipated.