Gear pitch is a very important factor in gear design and operation. Gear pitch refers to the number of teeth per given unit of pitch diameter. A simple way of determining gear pitch is to divide the number of teeth by the pitch diameter of the gear. The important fact to remember is that gears must have the same pitch to operate together. A 5-pitch gear meshes only with another 5-pitch gear, a 6-pitch only with a 6-pitch, and so on.

Spur Gears

The spur gear is the simplest gear design used in manual transmissions and transaxles. Spur gear teeth are cut straight across the edge parallel to the gear¡¯s shaft. During operation, meshed spur gears have only one tooth in full contact at a time.

Its straight tooth design is the spur gear¡¯s main advantage. It minimizes the chances of popping out of gear, an important consideration during acceleration/deceleration and reverse operation. For this reason, spur gears are often used for the reverse gear.

The spur gear¡¯s major drawback is the clicking noise that occurs as teeth contact one another. At higher speeds, this clicking becomes a constant whine. Quieter gears, such as the helical design, are often used to eliminate this gear whine problem.

Helical Gears

A helical gear has teeth that are cut at an angle or are spiral to the gear¡¯s axis of rotation. This allows two or more teeth to mesh at the same time. This distributes tooth load and produces a very strong gear. Helical gears also run more quietly than spur gears because they create a wiping action as they engage and disengage the teeth on another gear. One disadvantage is that helical teeth on a gear cause the gear to move fore or aft (axial thrust) on a shaft, depending on the direction of the angle of the gear teeth. This axial thrust must be absorbed by thrust washers and other transmission gears, shafts, or the transmission case.

Helical gears can be either righthanded or lefthanded, depending on the direction the spiral appears to go when the gear is viewed face-on. When mounted on parallel shafts, one helical gear must be righthanded and the other lefthanded. Two gears with the same direction spiral do not mesh in a parallel mounted arrangement.

External and Internal Gear Teeth

Spur and helical gears having teeth cut around their outer edge are called external gears. When the teeth of two external gears are meshed, the direction of rotation is reversed as the gears turn. In other words, the driven or output gear rotates in the opposite direction of the drive or input gear. As internal gear has teeth around its inside diameter. When an external gear and an internal gear are meshed and rotate, they do so in the same direction.

Idler gears

An idler gear is a gear that is placed between a drive gear and driven gear. Its purpose is to transfer motion from the drive gear to the driven gear without changing the direction of rotation. It can do this because all three gears have external teeth.

Idler gears are used in reverse gear trains to reverse the directional rotation of the output shaft. In all forward gears, the output shaft rotates in the opposite direction as the input shaft. With the placement of an idler gear, the input and output shafts now rotate in the same direction. This allows the vehicle drive wheels to turn backward.

Gear Ratios

As mentioned earlier, only gears with matching pitches can be meshed together. The speeds at which two meshed gears turn depend on the number of teeth on each gear. If both gears are the same size (same number of teeth), then the speed of both gears is equal. If the driving gear is smaller (fewer teeth) than the driven gear, the speed of the driven (output) gear decreases. However, when the driving gear is larger (more teeth) than the driven (output) gear, the speed of the driven gear increases.

To calculate the speed of either gear, multiply the number of teeth on one gear by the speed of that gear. Divide that number by the number of teeth on the other gear.

For example, calculate the speed of the driven gear when the driving gear has 45 teeth and is rotating at 200 rpm, while the driven gear has 75 teeth. The formula follows.

(Teeth on driven gear)/(Teeth on driving gear) = (rpm of driving gear)/(rpm of driven gear)

Putting in the known data leads to

75/45 = 200/x Therefore,

x = (45*200)/75 = 9000/75

x = 120 rpm of driven gear

The input speed of 200 rpm is reduced to an output speed of 120 rpm. The relationship of input and output speeds is stated in a gear ratio. This gear ratio can be calculated in a number of ways depending on the data that is known. One method is to divide the speed of the driving (input) gear or shaft by the speed of the driven (output) gear or shaft. In the given example, this results in

200 rpm / 120 rpm = 1.66 : gear ratio

This gear ratio can also be found by dividing the number of teeth on the driven gear (d) by the number of teeth on the driving gear (D). Again, in the sample problem, this leads to

d/D = 75 driven gear teeth / 45 driving gear teeth = 1.66 : gear ratio

This means the driven (output) gear is rotating 1.66 times slower than the driving (input) gear.

Speed Versus Torque A rotating gear also produces a rotational force known as torque. Torque is calculated by multiplying the applied force by its distance from the centerline of rotation. A simple example of this law of physics is demonstrated whenever a torque wrench is used. When a force of 10 pounds is applied perpendicular to the centerline of the bolt's rotation at a distance 1 foot from this centerline, 10 foot-pounds of torque are generated at the centerline of rotation. The torque wrench acts as a lever to apply this force.

Meshed gears use this same leverage principle to transfer torque. If two gears in mesh have the same number of teeth, they rotate at the same speed, and the input gear transfers an equal amount of torque to the output gear.

It has already been explained that when a driven gear is larger than its driving gear, output speed decreases. But how is torque affected?

A small gear with 12 teeth driving a large gear with 24 teeth. This results in a gear ratio of 2:1 with the output speed being half of the input speed.

In this example, torque at the input shaft of the driving gear is 10 foot-pounds. The distance from the centerline of this input shaft to the gear teeth is 1 foot. This means the driving gear transfers 10 pounds of force to the teeth of the larger driven gear. The distance between the teeth of the driven gear and the centerline of its output shaft is 2 feet. This means the torque at the output shaft is 10 pounds x 2 feet, or 20 foot-pounds. Torque has doubled.

The amount of torque increase from a driving gear to a driven gear is directly proportional to the speed decrease. When speed is halved, torque doubles.

Review the previous example. With a gear ratio of 1.66:1, the driven gear is rotating 1.66 times slower than the driving gear, but it is producing 1.66 times the torque of the driven gear. If torque at the driving gear input is 100 foot-pounds, torque at the driven gear output is 1.66x100 foot-pounds, or 166 foot-pounds.

Most manual transmission gearing is speed reducing/torque increasing. In some transmissions, the top gear is a 1:1 gear ratio in which speed and torque are transferred directly from the input to the output shaft. In most cases, the top gear is an overdrive gear combination. This means it is a speed-increasing/torque-reducing gearing.