Overview of a Gearbox

• A gearbox is the central component of a transmission system. The gearbox provides gear reduction from a high-speed motor to a slower, but more powerful output system. In this manner, a gearbox converts an engine's speed into torque, or thrust, which propels the motorized object forward. Most gearboxes are used to increase torque, while simultaneously reducing the speed of an engine's driveshaft. As a result, the gearbox's driveshaft (which is connected to the vehicle's drive axle) will rotate at a slower speed than the engine's driveshaft. This reduction in speed results in a conversion of energy from speed to thrust, causing the rotations of the gearbox's driveshaft to be more powerful, leading to an increase in torque.
  
  

Manual Transmission Gearboxes

• Diagram of an Audi Gearbox
  
  Manual transmission gearboxes are the simplest types of gearboxes, and involve the manual movement of a sliding gear along the gearbox's main shaft, using a shifter. The shifter is attached to a gear lever, which moves the sliding gear. When the driver engages the clutch, the sliding gear disengages from its existing position, and can slide up and down the gearbox to re-engage in a higher or lower gear. Modern manual gearboxes contain a diagonal gear that sits side by side with the main gears, and ensures that the sliding gear is synchronized with the main gears. This synchronization allows the sliding gear to seamlessly engage with the main gears, and prevents the gears from clashing with one another, which can damage the transmission. Modern gearboxes contain two sliding gears, one of which engages exclusively with the reverse main gear. This second sliding gear is constantly synchronized with the reverse main gear, and allows the operator to seamlessly shift from forward to reverse.
  
  

Automatic Gearboxes

• Torque Converter
  
  Automatic gearboxes are similar to manual gearboxes, except that the transmission system will automatically select the appropriate gear without any involvement from the operator. Automatic gearboxes use a hydraulic system to detect the pressure of fluids in the engine, and select the appropriate gear based upon this pressure. An automatic gearbox uses a torque converter, rather than a clutch, to engage the sliding gear. The torque converter is placed between the engine and the transmission, with one end connected to the engine's driveshaft, and the other end connected to the gearbox's drive shaft. The torque converter transfers power from the engine to the gearbox, and selects the appropriate gear, based on the fluid pressure within the engine. Faster rotations of the engine's driveshaft lead to higher pressure, which cause the transmission to select a higher gear.