Part 4 of 6 April 24, 2017
So by now, you should have a basic understanding of a transmission’s purpose: it ensures that your engine spins at an optimal rate (neither too slow nor too fast) while simultaneously providing your wheels with the right amount of power they need to move and stop the car, no matter the situation you find yourself in.
Let’s take a look at the parts of a transmission that allow this to happen:
Input shaft. The input shaft comes from the engine. This spins at the same speed and power of the engine.
Countershaft. The countershaft (aka layshaft) sits just below the output shafts. The countershaft connects directly to the input shaft via a fixed speed gear. Whenever the input shaft spins, so does the countershaft, and at the same speed as the input shaft.
In addition to the gear that takes power from the input shaft, the countershaft also has several gears on it, one for each of the car’s “gears” (1st-5th), including reverse.
Output shaft. The output shaft runs parallel above the countershaft. This is the shaft that delivers power to the rest of the drivetrain. The amount of power the output shaft delivers all depends on which gears are engaged on it. The output shaft has freely rotating gears that are mounted on it by ball bearings. The speed of the output shaft is determined by which of the five gears are in “gear,” or engaged.
1st-5th gears. These are the gears that are mounted on the output shaft by bearings and determine which “gear” your car is in. Each of these gears is constantly enmeshed with one of the gears on the countershaft and are constantly spinning. This constantly enmeshed arrangement is what you see in synchronized transmissions or constant mesh transmissions, which most modern vehicles use. (We’ll go into how all the gears can always be spinning while only one of them is actually delivering power to the drivetrain here in a bit.)
First gear is the largest gear, and the gears get progressively smaller as you get to fifth gear. Remember, gear ratios. Because first gear is bigger than the countershaft gear it’s connected to, it can spin slower than the input shaft (remember, the countershaft moves at the same speed as the input shaft), but deliver more power to the output shaft. As you move up in gears, the gear ratio decreases until you reach the point that the input and output shafts are moving at the same speed and delivering the same amount of power.