Gears are round wheels with teeth machined on their outer diameter. They are commonly used to transmit turning effort from one shaft to another. Basically, one size gear is used to turn another size gear to change output speed and torque (turning power). This is illustrated in Fig. 02.
A gear ratio is the number of turns a driving gear must turn before the driven gear turns one complete revolution. Gear ratio is calculated by dividing the number of teeth on the driven gear by the number of teeth on the driving gear. For example, look at Fig. 03. If the drive gear has 12 teeth and the driven gear 24 teeth (24 divided by 12), the gear ratio would be TWO TO ONE, written 2: 1. In this example, the drive gear would have to revolve two times to turn the other gear once. As a result, the speed of the larger, driven gear would be half as fast as the drive gear. However, the torque on the shaft of the larger gear would be twice that of the input shaft. Various sizes of drive and driven gears can be used to produce any number of gear ratios. As the number of teeth on the driven gear increase in relation to the number of teeth on the drive gear, the gear ratio increases. A gear ratio of 10: 1 would be larger than a ratio of 5:1, for example.
Transmission gear ratios vary with the manufacturer. However, approximate gear ratios average ~: 1 for first gear, 2:1 for second gear, 1:1 for third or high gear, and ~: 1 for reverse gear
In first or low gear, there would be a high gear ratio. A small gear would drive a large gear. This would reduce output speed but increase output torque. The car would accelerate easily, even with low engine rpm and low power conditions.
In high gear, the transmission frequently has a 1: 1 ratio. The transmission output shaft would spin at the same speed as the engine crankshaft. There would be NO torque multiplication (increase), but the car would travel faster. Very little torque is needed to propel a car at a constant speed on level ground
Gear reduction occurs when a small gear drives a large gear to increase turning force. Gear reduction is used in the lower transmission gears. Fig. 02.
An overdrive ratio results when a larger gear drives small gear. As shown in Fig. 02, the speed of the input gear increases, but torque drops.
Manual transmissions commonly use two types of gears: spur gears and helical gears.
Spur gears have their teeth cut parallel to the centreline of the gear shaft. As shown in Fig. 04A, they are sometimes called straight-cut gears.
Spur gears are somewhat noisy and are no longer used as the main drive gears in a transmission. They ay are used for sliding reverse gear, however.
Helical gears have their teeth machined at an angle I the centreline of gear rotation. Modern transmissions commonly use helical gears as the main “five gears. See Fig. 04B. Helical gears are quieter and stronger than spur gears.