The valve body inside the transmission is responsible for directing hydraulic pressure to the appropriate bands and clutches. The pressure of the ATF is regulated by a governor connected to the output shaft, which varies the pressure depending on the vehicle speed. A disadvantage of this arrangement is that there is no oil pressure to operate the transmission when the engine is not running, therefore it is not possible to push start a vehicle equipped with an automatic transmission with no rear pump (aside from several automatics built prior to 1970, which also included a rear pump for towing and push-starting purposes). The input for the main pump is connected to the torque converter housing, which in turn is bolted to the engine's flexplate, so the pump provides pressure whenever the engine is running. The main pump which pressurises the ATF is typically a gear pump mounted between the torque converter and the planetary gear set. Made from petroleum with various refinements and additives, ATF is one of the few parts of the automatic transmission that needs routine service as the vehicle ages.
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The ATF provides lubrication, corrosion prevention, and a hydraulic medium to transmit the power required to operate the transmission. The aforementioned friction bands and clutches are controlled using automatic transmission fluid (ATF), which is pressured by a pump and then directed to the appropriate bands/clutches to obtain the required gear ratio. Bands are not applied when the drive/overdrive range is selected, the torque being transmitted by the sprag clutches instead. The friction bands are often used for manually selected gears (such as low range or reverse) and operate on the planetary drum's circumference. The advantage of a sprag clutch is that it eliminates the sensitivity of timing a simultaneous clutch release/apply on two planetary gearsets, simply "taking up" the drivetrain load when actuated, and releasing automatically when the next gear's sprag clutch assumes the torque transfer. Ī sprag clutch (a ratchet-like device which can freewheel and transmits torque in only one direction) is often used for routine gear shifts. These devices are used to lock certain gears, thus setting which gear ratio is in use at the time. To change gears, the hydraulic automatic uses a combination of internal clutches, friction bands or brake packs.
#AUTOMATIC TRANSMISSION FLUID MANUAL#
Gearsets and shifting mechanism Ī hydraulic automatic transmission uses planetary (epicyclic) gearsets instead of the manual transmission's design of gears lined up along input, output and intermediate shafts. The transmission is connected to the engine via a torque converter (or a fluid coupling prior to the 1960s), instead of the friction clutch used by most manual transmissions. The most common design of automatic transmissions is the hydraulic automatic, which typically uses planetary gearsets that are operated using hydraulics.
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The first mass-produced automatic transmission is the General Motors Hydramatic four-speed hydraulic automatic (using a fluid coupling instead of a torque converter), which was introduced in 1939.Ĭutaway view of a ZF 8HP transmission: torque converter on the left, planetary gearsets in the centre, control mechanisms at the bottom. The 1904 Sturtevant "horseless carriage gearbox" is often considered to be the first true automatic transmission. A hydraulic automatic transmission may also colloquially called a " slushbox" or simply a "torque converter", although the latter term can be incorrect, since torque converters are also used on CVTs. An electronic automatic transmission (EAT) may also be called an electronically controlled transmission (ECT), or electronic automatic transaxle (EATX). Other types of automatic transmissions include continuously variable transmissions (CVT), automated manual transmissions (AMT), and dual-clutch transmissions (DCT).
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The most common type of automatic transmission is the hydraulic automatic, which uses a planetary gearset, hydraulic controls, and a torque converter.
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It typically includes a transmission, axle, and differential in one integrated assembly, thus technically becoming a transaxle.
#AUTOMATIC TRANSMISSION FLUID DRIVER#
Typical gear selector for an automatic transmissionĪn automatic transmission (sometimes abbreviated to auto or AT) is a multi-speed transmission used in internal combustion engine-based motor vehicles that does not require any driver input to change forward gears under normal driving conditions.