How Does Automatic Transmission (AMT) Work?
When it comes to high-performance automobiles, the automatic transmission is the popular option for easy and reliable driving. In this article, 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 inputs and one output. In an 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 exclusively 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 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 a few clutch packs. In the coming sessions, we will 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 the 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 both of the 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 the 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 overdrive 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 the 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 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.
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