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 Helical Planetary 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) Types of Gears

Power Vs. Torque Torque – measurement of twisting force How Stuff Works 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 5252 Mustang Cobra VS. Caterpillar Diesel

Gear Ratios When two gears are in mesh, a gear ratio exists How Stuff Works 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

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

1 200 ft/lbs 2000 RPM 4:1 800 ft/lbs 500 RPM Underdrive 2 2:1 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 2:1 400 ft/lbs 1000 RPM 3 1:1 Direct Drive 4 .5:1 100 ft/lbs 4000 RPM Overdrive

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