2. Why Is a Transmission
Necessary?
• Provide torque multiplication at low speeds
• Reduce engine RPM at highway speeds
• Allow the engine to operate within its most
efficient RPM range
• Allows the engine to be disengaged from
the rear wheels while the vehicle is not
moving (torque converter & clutch)
3. What Does a Transmission Do?
• The basic purpose of a transmission breaks
down into 3 parts
– Ability to alter shaft RPM
– Ability to multiply torque
– Ability to reverse the direction of shaft rotation
4. How Does the Transmission
Produce Torque Multiplication
And/or RPM Reduction
• Transmissions use gears
– Spur
– Helical
– Planetary
• Gears are able to change the RPM and the
torque of the power moving through the
transmission as well as the direction of
rotation
5. Types of Gears
• Spur
– Simplest gear design
– Straight cut teeth
– Noisy operation
• Helical
– Spiral cut teeth
– At least two teeth are in mesh at any time
• Distributes the tooth load
• Quieter operation
• Planetary
– Most complex design
– Used in almost all automatic transmissions
– Contains three parts
• Sun gear
• Planet gears
• Internal gear (ring gear)
6. Power Vs. Torque
• Torque – measurement of twisting force
• Power – measurement of how quickly work
can be done
– Power is dependent on torque and RPM
– Horsepower = Torque x RPM
How Stuff Works
5252
7. Gear Ratios
• When two gears are in mesh, a gear ratio exists
• Driven Gear = Ratio
• Example:
– Drive gear has 14 teeth
– Driven gear has 28 teeth
– 28 ÷ 14 = 2:1 ratio (two to one ratio)
– The drive gear must rotate twice to make the driven
gear rotate once
Drive Gear
How Stuff Works
8. Reversal of Direction
• When two gears are in mesh one will spin
the opposite direction of the other
• Idlers are used to reverse direction
9. Speed Change
• The change in RPM from the input gear to
the output gear is directly proportional to
the gear ratio
• Example: 3:1 gear ratio
– Input gear turns at 900 RPM
– Output gear turns at 300 RPM
10. Torque Multiplication
• The change in torque from the input gear to
the output gear is directly proportional to
the gear ratio
• Example: 3:1 gear ratio
– Engine turns input gear at 900 RPM with 50
lb/ft of force
– Output gear turns driveshaft at 300 RPM with
150 lb/ft of force
12. Multiple Gear Ratios
• Individual gear ratios can be multiplied to
calculate a total gear ratio
– Example: Chevy caprice with a TH-350
transmission and a 305 engine
• By removing the differential cover and inspecting
the gearset you are able to count 10 teeth on the
input gear and 41 teeth on the output gear
• 41 ÷ 10 = 4.1:1
• You are able to find the 1st
gear ratio of the TH-350
in a manual which is listed as 2.52:1
13. Multiple Gear Ratios
• Rear end ratio x 1st
gear ratio = total gear ratio
• 4.1 x 2.52 = 10.33:1
– This tells us that the engine turns 10.33 revolutions for
every 1 revolution of the tires (speed reduction)
• Torque multiplication can also be calculated
– The 305 engine produces 245 lb/ft of torque at 3200
RPM
– @ 3200 RPM in 1st
gear the torque acting on the rear
tires = 230 lb/ft x 10.33 = 2375.9 lb/ft torque !!!
15. Automatic Transmission I.D.
• Most automatics are identified by the oil
pan.
– Look at the shift indicator to determine if the
transmission is a 3-speed, 4-speed etc.
• Different transmissions may have been
installed in otherwise identical vehicles.
• Shopkey and other manuals list
transmission application by vehicle.
18. Planetary Gearsets
• Simple planetary gearsets contain three
components
– Internal (ring) gear / (annulus gear)
– Planet gears (and carrier)
– Sun gear
• One component will be the drive member, one the
driven, and one will be held (except direct drive
and neutral)
• Unlike other types of gears, planetary gears are
able to operate on one single axis
20. Planetary Action
• Underdrive
– Planet carrier is the output
• Minimum reduction
– Ring gear is held
– Sun gear is the input
• Maximum reduction
– Ring gear is input
– Sun gear is held
21. Planetary Action
• Overdrive
– Planet carrier is the input
• Minimum overdrive
– Ring gear is the input
– Sun gear is held
• Maximum overdrive
– Ring gear is held
– Sun gear is the input
22. Planetary Action
• Reverse
– Planet carrier is held
• Underdrive
– Ring gear is the output
– Sun gear is the input
• Overdrive
– Ring gear is the input
– Sun gear is output
23. Sun Carrier Internal Speed Torque Direction
Input Output Held Maximum
Reduction
Maximum
Increase
Same as
Input
Held Output Input Minimum
Reduction
Minimum
Increase
Same as
Input
Output Input Held Maximum
Increase
Maximum
Reduction
Same as
Input
Held Input Output Minimum
Increase
Minimum
Reduction
Same as
Input
Input Held Output Reduction Increase Opposite as
Input
Output Held Input Increase Reduction Opposite as
Input
24.
25. Calculating Planetary Gear
Ratios
• Direct Drive = 1:1
• Underdrive
– Carrier is output
# of sun gear teeth + #of ring gear teeth
# of teeth on the driving member
= Ratio
26. Calculating Planetary Gear
Ratios
• Overdrive
– Carrier is input
# of teeth on the driven member .
# of sun gear teeth + #of ring gear teeth
= Ratio
