Automatic Transmission, How it works ?

Automatic Transmission, How it works ?

When it comes to high-performance automobiles, automatic transmission are the popular option for easy and reliable driving. In this video we will explore the working principles of this complex mechanism in a simple and logical way. We’ll use an automatic transmission developed by Allison Transmission for this purpose. Automatic transmissions work based on the planetary gear set. In order to understand this process, let’s take a look at the basic parts of planetary gear sets. Planetary gear sets have two input and one output. In automatic transmission the output rotation is drawn from the planet carrier. The two inputs are connected to the ring and sun gears. Now let’s see what happens to the output speed when we apply different speeds to the inputs. Here the ring gear is stationary and rotation is given exclusive to the sun gear. This will cause the carrier to spin as you can see here. The ring gear is also rotating. The ring and sun gears rotate at the same speed. In this case the whole mechanism moves as a single unit. The ring gear speed is increased further. Turning the sun gear in the opposite direction will result in a reverse gear. So the operation of automatic transmission is all about transmitting different rotation speeds into the ring and sun gears. The beauty of automatic transmission is that this speed variation can be transmitted simply by engaging few clutch packs. In the coming sessions we well see how this is achieved in Allison. In an automatic transmission there is no direct connection between the input and the output shafts, rather there is an intermediate shaft between the two. In addition to clutch packs are used for transmitting power. Pressing the clutch packs together will lock the hub to the case. This is the simplest form of automatic transmission. With this in mind, let’s see how 1st gear works. If 1st clutch pack is applied the input shaft will connect to the intermediate shaft. On the other hand, if 2nd clutch pack is applied, the ring gear will connect to the case which will make the gear become stationary. In order to achieve 1st gear just apply both the clutch packs together. You can note that the input shaft will turn the sun gear. Since the ring gear is stationary the output shaft will rotate as shown with almost one third of the input speed. To achieve additional gear ratios at another planetary gear set. It is clear that apply C1 will always rotate the both of sun gears at the input shaft speed. If we apply C4 as well the planet carrier of the 2nd set will turn. But here is the tricky part: this planet carrier is permanently linked to the ring gear of the 1st set. So, when C1 and C4 are applied together, the ring gear of the output set will turn. Thus, at the output set the situation is similar to that at 1st gear except that here the ring gear will rotate as well. This means that the speed of the output planet carrier will increase. This is 2nd gear of the transmission. The planet carrier is also fitted with a hollow shaft. The shaft can be directly connected to the input shaft with the help of a rotating clutch module. Take a moment to observe and understand this arrangement – the rest of the process will be easy. With this setup we can achieve 4th gear with a direct drive. We know: in order to get a direct drive the ring and sun gears of the output set should rotate at the speed of input shaft. If we apply C1 and C2, this is what happens: The sun and ring gears will be directly connected to the input shaft. In order to achieve 6th gear just remove C1 and apply C4. Here at 2nd set the input rotation is given to the carrier and the ring gear is stationary. This combination gives a very high rotational speed to the sun gear. Thus, we can achieve a high over drive ratio at the output. 3rd planetary gear set is required to get the remaining gear ratios. The sun gear of this set is always connected to the input shaft as shown here. And here again the output of the set is connected to the input of the adjacent set. This means that applying C3 will always turn the ring gear of 2nd set. For the remaining gear ratios C3 is always applied. For 3rd gear just apply C1 as well. For 5th gear remove C1 and apply C2. In order to go in reverse apply the ring gear clutch C5 as well. Since the planet gear of 2nd set cannot revolve, they will spin as shown here. This will eventually lead to the reverse rotation of the sun gear. Consequently, the final output will be a rotation in the opposite direction. A transmission control unit will decide when to apply which clutch pack. Unlike manual transmissions a torque converter is needed in the automatic transmissions. When the transmission is in gear and we apply the brakes we have to isolate the rotation of the engine from the transmission. With a fluid coupling the torque converter will help accomplish this. A 2D animation will also give you a good idea of the automatic transmission system. Thank you! Subtitles are written by @IEEE_English

About the Author: Michael Flood


  1. Going to have to watch this a few more times. The people who figured out how to make transmissions have a lot to be proud of.

  2. I feel like it didn't explain how the clutches on the ring gear locked them together very well. It could have also showed the speed differences in the different gears to help visualize the the different ratios better.

  3. i am an engineer albeit civil and structural engineer but i could not understand this automatic transmission …….damn i thought i was smart untill now….lol………..the most interesting thing about this is it was invented in 1921 and were used in cars and tanks on 1934……….so many smart people in this world contributed to the easy life we live now.

  4. Some say not to switch from Drive to Neutral while waiting behind a red light. Is this true? Won't transmission be damaged if kept in Drive but not moving? Thanks

  5. I can rebuild engines, manual trannys and differentials but I gave up on auto trannys. My friend does a great job on these and understands them well.

  6. Whenever I become happy thinking about a planetary gear set and how it operates, another planetary gear set is added to make me unhappy about this endless process.

  7. that does not tell how it works. it just tells about gears but nothing about atf and what role it plays in all this. it does not explain fluid drive one bit and that is why i clicked on the video in the first place.

  8. Excellent video information Automatic Dirve Gear box Working education More Really nice, Engineering Student for Best information YouTube video Link, Nice

  9. it's 3am in the morning, and I have exam tomorrow, and not even a quarter of exam materials I have even reviewed… yet I am watching a transmission how-to video, and my Major is not even engineering….

  10. Manual transmission videos are easy to understand, everytime I watch an automatic transmission video I fall asleep for 4 hours.

  11. Yeah I get it, but how does it change gear when the engine reaches a certain rpm? Are the clutches electronically controlled? Or is there some mechanical frictional black magic thats happening in the gearbox that just activates the clutch at an exact rpm?

  12. Awesome video! So well explained! I had a so/so understanding of automatic gearboxes but after watching this, it has really helped fill in the gaps in my knowledge. ๐Ÿ‘๐Ÿ‘

  13. Simplified as much as possible:
    Clutch packs are what engage the planetary sets.
    Different combinations of planetary sets equal different gears.
    So different combinations of engaged clutch packs mean different combinations of different planetary gear sets which mean different drive gears.

    For instance:
    Engage clutch pack 1, that will engage planetary 1 and in turn, gear 1.

    Engage clutch pack 2 and 4, that will engage 2 planetaries that will result in gear 2 or 3 or whatever gear that combination corresponds to.

  14. It's pathetic there are people who can't understand this after such a detailed explanation

  15. Uhhhhhh, yeah. I remember reading that 2 things have been developed by engineers that are real marvels. One is the color television picture tube and the other is the automatic transmission. After viewing this, I can vouch for the transmission.

  16. Can someone help here? I have Mazda mpv 2003, it just slipped down to zero motion from high speed (ATF is at maximum gauge) and since could not move in any of front Drive gears, but moves in Reverse gear. I have changed the ATF inside the gearbox and the fault remains as it is. Any help please!

  17. Whoever made this is clever. I just now began to understand how this works using leverage over the planetary gear systems. Still love my manuals though.

  18. I was pretty good until 6th gear. I'm starting to get it now but it could do with a clearer explanation. From what I can understand, what's usually the output at the 2nd set (the carrier), is now used as the input for the sun gear which carries through to the sun gear for the 1st set, and since the ratio is reversed, the sun gear will be turning around 3 times as fast as the input speed (normally the carrier turns around a 1/3rd of the sun when the ring is stationary).

    Is this about right or have I misunderstood?

  19. I gave up after 30 seconds… Luckily there are people who are (a lot) smarter than me and are able to invent these things…

  20. 1:41 Spin the ring gear in order get higher speeds.
    5:28 Hold the ring gear stationary in order achieve higher speeds.

  21. If it's difficult for me to understand whats happening here, how about the guyz who came up with the initial design?

  22. Ok by watching 4 times I understood 95% of it and by imagining in my mind for the rest of the night I hope I'll get the rest of the 5% too

  23. Does the engine tells an automatic transmission when to shift, or it shifts according to engine RPM, for e.g in a Nissan patrol 3.0 2004

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  25. All that time spent on computer animations and a great clear voice actor.
    Yet no one understands this at all.
    What the fuck do you mean just remove C4 and add C1?

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