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.