Finally understand Automatic Transmissions with this video!

[00:00:01]This is the inside of an automatic transmission.

[00:00:06]At first glance, it looks like an extremely complex mechanism with hundreds of components working simultaneously.

[00:00:14]However, its operation is based on just a few simple mechanical principles.

[00:00:21]In this video, we'll explain how it works step by step until you understand how a real automatic transmission is able to select each of its gears.

[00:00:32]Let's begin by understanding the basic principles behind the transmission.

[00:00:38]It is well known that different torque and speed ratios are required depending on driving conditions.

[00:00:45]>> [music] >> For example, when starting from a standstill, the wheels need greater torque to overcome the vehicle's inertia.

[00:00:56]>> [music] >> At higher speeds, however, a gear ratio that allows the wheels to rotate faster becomes more desirable.

[00:01:08]To achieve these different ratios, automobiles use transmissions.

[00:01:13]>> [music] >> Their job is to receive the engine's rotation and modify the gear ratio before transmitting power to the wheels.

[00:01:24]Traditional automatic transmissions use a remarkably clever mechanism to [music] exchange torque for speed.

[00:01:32]Planetary gear sets.

[00:01:36]>> [music] >> Understanding this mechanism is the key to understanding how most automatic transmissions work. [music] A planetary gear set consists of a central gear called the sun gear, several planet gears, and an outer ring gear with internal teeth.

[00:02:00]A planet carrier allows the planet gears to rotate on their own axes while simultaneously revolving around the sun gear.

[00:02:09]This compact mechanism is capable of producing six different gear ratios as well as reversing the direction of rotation.

[00:02:19]For each operating condition, each of the three main components, the sun gear, the planet carrier, and the ring gear, must assume one of three possible roles: input, output, or reaction.

[00:02:35]The input is the driving member that receives power from the engine.

[00:02:40]The output delivers power to the rest of the drivetrain.

[00:02:45]The reaction member is held stationary by the transmission housing.

[00:02:53]The resulting gear ratio depends entirely on which component is used as the input, which one serves as the output, and which one is held stationary.

[00:03:03]The clever part of an automatic transmission is that it can actively change these roles using hydraulic pressure without any intervention from the driver.

[00:03:14]We'll see how that works shortly.

[00:03:19]Let's begin by holding the sun gear stationary.

[00:03:24]If the ring gear is the input, the planet carrier becomes the output producing a small speed reduction.

[00:03:36]Conversely, if the planet carrier is the input, the ring gear becomes the output producing a small speed increase.

[00:03:47]Now, let's hold the ring gear stationary.

[00:03:50]If the sun gear is the input, the output experiences a large speed reduction.

[00:03:59]Conversely, if the planet carrier is the input, the output rotates much faster, producing a large speed increase.

[00:04:10]Finally, if the planet carrier is held stationary, the output will always rotate in the opposite direction of the input.

[00:04:19]You can study every possible operating condition using [music] the table shown on screen.

[00:04:26]The question now is, how does an automatic transmission decide which component becomes the input, which becomes the output, and which remains stationary?

[00:04:36]>> [music] >> This is the job of multi-disk clutches and brakes.

[00:04:49]These mechanisms connect or disconnect rotating components using hydraulic pressure.

[00:04:58]A series of steel plates are splined to one rotating component.

[00:05:06]Between them are friction discs splined to the component that will be engaged.

[00:05:15]Together, they form what is known as a clutch pack.

[00:05:22]To be operated by a clutch, certain transmission components incorporate splines that engage either the friction discs or the steel plates.

[00:05:35]Under normal conditions, the friction discs rotate independently of the steel plates, meaning no engagement occurs.

[00:05:48]A piston converts hydraulic pressure into linear motion.

[00:05:57]When it compresses the clutch pack, friction eliminates the relative motion between the discs, causing both sets of components to become locked together.

[00:06:11]In an automatic transmission, clutches and brakes operate in exactly the same way.

[00:06:16]The only difference is that brakes lock a component to the transmission housing.

[00:06:28]While clutches connect two rotating components together. For example, a clutch may connect the output of one planetary gear set to the input of another.

[00:06:39]By applying different combinations of clutches and brakes, the transmission [music] can determine which member of each planetary gear set acts as the input, the output, and the reaction member.

[00:06:51]>> [music] >> Finally, we should mention one-way clutches.

[00:07:04]Their operation is very simple.

[00:07:06]>> [music] >> They allow rotation in one direction, but automatically lock when rotated in the opposite direction.

[00:07:15]In this direction, the clutch rotates freely.

[00:07:19]If we reverse the direction of rotation, it locks immediately.

[00:07:27]This behavior is extremely useful inside an automatic transmission.

[00:07:34]During certain gears, a component can be held automatically while the engine is transmitting torque.

[00:07:44]How when the direction of torque reverses, for example, when the driver lifts off the accelerator, the one-way clutch releases that component without requiring any additional hydraulic action.

[00:07:59]In summary, automatic transmissions obtain different gear ratios by applying different combinations of clutches and brakes to one or more planetary gear sets.

[00:08:11]Now, let's return to a real automatic transmission.

[00:08:15]What initially appeared to be an extremely complex mechanism can actually be understood as a combination of planetary gear sets, clutches, and brakes.

[00:08:29]The transmission shown on screen is a four-speed automatic transmission.

[00:08:34]It consists of three planetary gear sets, three clutches, and four multi-disk brakes.

[00:08:42]These planetary gear sets are known as the overdrive, front, and rear planetary gear sets.

[00:08:49]>> [music] >> Let's follow the flow of power from the input shaft to the output shaft in first gear.

[00:08:55]>> [music] >> Engine power enters through the torque converter, whose operation we explained in another video.

[00:09:03]>> [music] >> The input shaft drives the planet carrier of the overdrive planetary gear set.

[00:09:12]At this point, a clutch locks the planet carrier to the sun gear.

[00:09:19]>> [music] >> Because two members of the planetary gear set are locked together, the entire [music] gear set rotates as a single rigid assembly.

[00:09:28]>> [music] >> Power then continues to the ring gear of the front planetary gear set.

[00:09:37]>> [music] >> In this design, the front planet carrier is connected to the rear ring gear through the output shaft.

[00:09:45]While both sun gears are connected together.

[00:09:48]>> [music] >> The amount of torque multiplication depends on the specific transmission design. [music] In this case, first gear multiplies engine torque by approximately 2.8 to 1.

[00:10:01]>> [music] >> In second gear, a brake locks the sun gears of both the front and rear planetary gear sets.

[00:10:07]>> [music] >> Changing the reaction member reduces torque multiplication to approximately 1.5 to 1.

[00:10:18]>> [music] >> In third gear, a clutch transmits power directly to the sun gears.

[00:10:26]>> [music] >> As a result, the planetary gear sets no longer produce a speed reduction.

[00:10:32][music] And the transmission reaches a 1:1 gear ratio. The output shaft [music] now rotates at exactly the same speed as the input shaft. This condition is known as direct drive.

[00:10:45]>> [music] >> Finally, in fourth gear, the overdrive brake locks the sun gear of the overdrive planetary gear set.

[00:10:53]>> [music] >> The gear ratio becomes approximately 0.7 to 1.

[00:10:59]As a result, [music] the output shaft rotates faster than the input shaft, reducing the available torque.

[00:11:06]This is a very specific transmission design, and you don't need to memorize every detail of its construction.

[00:11:14]To understand how automatic transmissions [music] work, it is enough to understand that they change the behavior of planetary gear sets by applying different combinations of clutches and brakes, thereby producing different gear ratios at the output.

[00:11:29]And that concludes our explanation of the automatic transmission.

[00:11:34]Don't forget to leave a comment [music] suggesting the systems or components you'd like us to explain in future videos. Thanks for watching.