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Planetary Gears
Auxiliary Transmission

Equipment Operator Basic - Beginning construction equipment operators manual
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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.—   T he 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. 2-7







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