In order to select the right speed and torque for the right load some times even the manual transmission could not do the task perfect and it is always near perfect. Which results in the power produced could not be utilized for its optimum and it means always some loss of efficiency in terms of power and fuel consumption. Though the loss seems to be negligible one, but for the fleet operators with larger fleet operation there will be considerable savings in the long run.
In the Automatic Transmission,
1. No clutch pedal is in place and
2. No Gear shift lever available.
The automatic transmission's primary job is to allow the engine to operate in its narrow range of speeds while proving wides range of output speeds. The key difference between a manual and an automatic transmission is that the manual transmission locks and unlocks different sets of gears to the output shaft to achieve the various gear ratios, while in an automatic transmission; the same set of gears produces all of the different gear ratios. This is being achieved by the planetary gear set in the automatic transmission.
The auto transmission unit consists of;
1. Planetary gear set
2. Set of bands to lock parts of a gear set
3. Set of three wet-plate clutches to lock other parts of the gear set
4. An incredibly odd hydraulic system that controls the clutches and bands
5. A Gear pump to move transmission fluid around.
An automatic transmission contains two complete planetary gear sets folded together into one component.
Any planetary gear set has three main components:
1. The sun gear (S)
2. The planet gears and the planet gears' carrier (C)
3. The ring gear (R)
Each of these three components either acting as input or out put or stationary. Choosing which piece plays which role determines the gear ratio for the gear set. Let's take a look at a single planetary gear set.
Assumption: No of teeth on Ring Gear : 72 and No of teeth on Sun Gear: 30
Gear Ratio: 1
Input: Sun gear (S)
Out put: Planet Gear and Planet Carrier (C)
Stationary: Ring Gear (R)
Gear Ratio: 1+ R/S ie. 1+ 72/30 ie 1+2.4= 3.4. Hence the ratio is 3.4:1
The Gear ratio shown above is reduction because the output speed is lower than the input speed.
Input: Planet carrier (C)
Output: Ring gear (R)
Stationary: Sun gear (S)
Gear Ratio: 1/ (1+ S/R), ie 1/ (1+ 30/72)= 0.71. Hence the Gear Ratio is 0.71:1
The Gear Ratio shown above is Overdrive, because the output speed is faster than the input speed.
Input: Sun gear
Output: Ring Gear
Stationary: Planet Carrier (C)
Gear ratio: -R/S ie= – 72/30 = -2.4. Hence the Gear Ratio is -2.4:1
The Gear Ratio shown above is again reduction but in negative. ie: the reverse.
We get several other gear ratios from this planetary gear sets, and these are the ones that are relevant to our Automatic Transmission. This one sets of Gears can produce these different gear ratios without having to engage or disengage any other gears. With two of these gear sets in a row we can get the four forward gears and one reverse gear.
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