Automobiles need transmissions to efficiently transfer power from the engine to the wheels. Car engines by themselves can only turn out so much power, and the output is a very narrow range. If the wheels of a car were permanently and directly connected to its engine, it would take all the engine’s power to achieve a low speed and it would quickly overheat. Gears of various sizes inside a transmission enable the car to achieve various speeds despite the engine’s inherent limitations. A standard transmission is a transmission system wherein the operator of the vehicle manually controls which gear to engage. Thus another common name for this type of transmission is “‘manual transmission.” Another name often used is “stick shift,” which refers to the lever operated by the drive to select gears.
The clearest indication that an automobile has a standard transmission is the presence of the clutch. The clutch is a coupling device that disconnects or connects the engine to the rest of the transmission system. This is manually controlled by the driver through a foot pedal (also called the clutch or clutch pedal). When the foot pedal is completely depressed the engine is disengaged from the transmission. When it is released the engine is engaged. This is a necessary preparatory action because it would be extremely difficult to shift the transmission’s gears if the engine’s power was continuously running through it. Temporarily isolating the engine also keeps its power output at a constant value, which is necessary when changing gears.
Shafts and Gears
The basic design of a standard transmission is composed of three shafts: the input shaft, the countershaft or layshaft and the output shaft. The input shaft comes from the engine and is controlled by the clutch. It turns in the same speed as the engine. The countershaft connects to the input shaft through meshed gears. There are other fixed gears along the length of the countershaft, and these are in turn meshed with the gears of the output shaft. The output shaft gears are the actual gears referred to when one shifts gears. These output shaft gears are not fixed to the output shaft but rotate independently due to bearings in their hubs. Movable devices known as collars are attached to the output shaft and can slide along it. They have teeth that can attach to the different gears along the output shaft. Collars and their teeth attach the gears to the output shaft so that both gear and shaft rotate as a single unit. The engine’s power is transmitted to the input shaft, countershaft, output shaft and finally the wheels. The final value of the power output depends on which gear is attached and transferring power from the countershaft to the output shaft.
The gears on the output shaft have various sizes. The biggest gear is engaged when the driver selects first gear. This gear is the slowest-turning gear because it has the biggest diameter. As the driver selects higher gears, the actual gears on the output shaft become smaller and consequently turn faster. When the driver slides the stick shift into a gear, this lever moves a particular collar to attach to the designated gear. To facilitate this process of attachment, devices called synchronizers are built into collars. Synchronizers first match the rotational speed of the output shaft and collar with the gear about to be selected before the collar’s teeth actually lock into the gear. This enables a smoother changing of gears even when the vehicle (and thus the output shaft) is moving. When the driver moves the stick shift to the neutral position, no collars are attached and the output shaft and the gears are rotating independently of each other. When the driver moves the stick shift to “reverse,” a collar attaches to a gear that is turning in the opposite direction to all the other forward moving gears. This gear turns in the opposite direction due to an idler gear positioned between it and its corresponding gear on the countershaft.