Gears and Transmissions

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)

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

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

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)

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

Mustang Cobra VS. Caterpillar Diesel

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

Reversal of Direction

• When two gears are in mesh one will spin the opposite direction of the other

• Idlers are used to reverse direction

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

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

Torque Multiplication

1 inch 3 inches

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

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 !!!

Gear Engine Output Torque

Engine Speed

Gear Ratio

Transmission Output Torque

Transmission Output Speed

1 200 ft/lbs 2000 RPM

4:1 800 ft/lbs 500 RPM Underdrive

2 200 ft/lbs 2000 RPM

2:1 400 ft/lbs 1000 RPM Underdrive

3 200 ft/lbs 2000 RPM

1:1 200 ft/lbs 2000 RPM Direct Drive

4 200 ft/lbs 2000 RPM

.5:1 100 ft/lbs 4000 RPM Overdrive

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.

Automatic Transmission I.D.

Automatic Transmission I.D.

1 Aluminum Powerglide 14 bolts

2 TH200 Metric 11 bolts

3 TH350 13 bolts

4 TH400 13 bolts

5 TH200-4R 16 bolts

6TH700-R4, 4L60, 4L60E

16 bolts

7 4L80E 17 bolts

GM I.D.

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

Planetary Action

• Direct Drive– Any two of the components are driven– 1:1 Ratio

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

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

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

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

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

Calculating Planetary Gear Ratios

• Overdrive– Carrier is input

# of teeth on the driven member .# of sun gear teeth + #of ring gear teeth

= Ratio

Calculating Planetary Gear Ratios

• Underdrive– Carrier is held

# of teeth on driven gear= Ratio# of teeth on driving gear