When the selector control valve lever is moved to the reverse position, oil is admitted from the pump to oil passages. Oil in the piston's passage flows under pressure to the cavity on the face of the rear piston. The oil pressure against the piston moves the piston, causing it to lock the rear clutch plate between the piston and the stationary backplate. This locking action engages the reverse clutch and provides torque to the planetary gear assembly.

The planetary gear assembly (fig. 12-5, view A) is installed in the transmission to provide reverse motion of the output shaft. As shown in figure 12-5, the planetary gear assembly contains the following parts: a sun gear, a ring gear with internal teeth, planet gears, and a planet carrier. In operation, a planetary gear set (fig. 12-5, view B) resembles the solar system. Planet gears orbit, or circle, the sun gear. Planet gears are held in place by the planet carrier and the ring gear. The ring gear surrounds and meshes with the planet gears. In the planetary gear set in view A of figure 12-5, the teeth of a gear are in mesh with the teeth of at least one other gear at all times. Whenever one part of the gear set is turned, all other parts in the gear set are affected. The planetary gear set can be separated into three major parts, each with its own function. To transmit power through a gear set, one gear turns as the drive member (SUN GEAR). Another part, called the reaction member (REVERSE RING GEAR), is held and prevented from moving. A third part, which is free to turn inside the reverse ring gear, is the driven member, or the output member (REVERSE PLANET CARRIER).

To understand how the planetary gear set provides reverse motion, you must keep in mind how the major parts are mounted. The sun gear is splined to the drive plate (forward clutch plate), which is bolted to the flywheel. Consequently, when the engine is running, the sun gear is revolving. The carrier to which the planet gears are mounted is attached by splines to the transmis-sion pinion shaft. The ring gear is attached to the reverse clutch disk.

When the selector valve is in the forward position, the reverse clutch is disengaged, allowing the rear clutch disk to rotate freely. As the clutch disk rotates freely, the ring gear attached to the disk offers no resistance to the motion of the planet gears. When the selector valve is put in the reverse position, hydraulic pressure stops and locks the rear clutch disk and the attached ring gear, resulting in the gear motion shown in view B of figure 12-5. As the sun gear rotates clockwise, planet gear A rotates counterclockwise and planet gear B rotates clockwise. Since the ring gear is locked, gear B must move in a counterclockwise direction. The counterclockwise motion of gear B turns the carrier (on which the gear is mounted) and the pinion shaft (to which the carrier is mounted) in a counterclockwise direction. Thus, the planetary gear set has converted the clockwise motion of the input shaft into the counterclockwise motion of the pinion shaft. This change in direction of the pinion shaft also changes the direction of the drive shaft.