Module 2- Apply and Hold Devices (Page 5 of 6)
Lets attempt to put it all together.
In order to obtain Drive 1 gear or maximum reduction, three events must occur (Confused? Refer to Module 1).
- The sun (input) gear must be driven by the engine
- The planet carrier (output) gear must be driven by the sun gear and connected to the wheels.
- The ring gear must be held stationary
Lets see how this is done.
Referring back to module 1, in Drive 1 gear - the sun gear is the input, so it is somehow connected to the engine. The engine turns the sun gear (input) which turns the planet carrier (output). See diagram below.
The planet carrier is attatched to the output shaft of the transmission. The output shaft turns the wheels. The smaller sun gear is turning the larger planet carrier. Therefore we now have maximum reduction in Drive 1. See diagram below.
In the above diagram, notice that the ring / internal gear must be held in order to get Drive 1. The transmission valve body - the brain - applies the band by directing pressurized fluid to the servo which actuates the operating rod. The operating rod tightens the band around the drum that is linked to the internal or ring gear. The ring gear is held stationary. See diagram below.
Clutch Pack Operation in Drive 1
At this point, to obtain Drive 1 gear we understand that
- the sun (input) gear is driven by the engine.
- the planet carrier (output) is driving the engine
- a brake band is applied and holds the ring / internal gear stationary
Let us now turn our attention to clutch pack operation as it relates to #1 and #2
1. The sun (input) gear is driven by the engine
Once again, the power / torque from the engine is transferred to the transmission by means of a torque converter. The torque converter connects to the transmission input shaft.
A clutch pack applies to connect the transmission input shaft to the sun gear of the planetary gear set.
The sun gear is now the input because there is a direct connection between the engine and sun gear through the clutch pack. See diagram below.
Why can't the sun gear be permanently attached to the input shaft?
The sun gear cannot be permanently attached to the engine because there are instances where the sun gear must operate independently of the engine. For instance, overdrive gear requires the sun gear to be held stationary. See module 1.
3. The planet carrier (output) is driving the engine
Next the planet carrier must somehow be connected to the rear wheels. The planet carrier is the output gear.
A second clutch pack applies to connect the planet carrier to the transmission output shaft. Now, the planet carrier is the output - it is connected to the wheels of the vehicle.
In reality, transmissions that do not use overdrive will often have the planet carrier directly splined to the transmission output shaft.
Module 2 (Page 5 of 6)