gears use to transmit power positively from one shaft to another by means of successively engaging teeth (in two gears). they are used in place of belt drives and other forms of friction drive when exact speed ratios and power transmission must be maintained. gears may also be used to increase or decrease the speed of the driven shaft, thus decreasing or increasing the torque of the driven number.

TypeS  of gears
   Spur gear
 Spur gears, are generally used to transmit power between two parallel shafts.  The teeth on these gears are straight and parallel to the shafts to which they are attached.  When two gears of different sizes are in mesh, the larger is called the gear while the smaller is called the pinion. Spur gears are used where slow to moderate- speed drive are required. 
Internal gears

            Internal gears, are used where the shafts are parallel and the centers must be closer together and that could be achieved with spur or helical gearing.  This arrangement,  provides  a stronger drive since there is the greater area of contact than with the conventional gear drive.  It also provides speed reductions with a minimum space requirement.  Internal gears are used on heavy duty tractors where much torque is required.

Helical gears

Helical gears,  may be used to connect parallel shafts or shafts which are at an angle.  Because of the progressive rather than intermittent action of the teeth, helical gears run more smoothly and quietly than spur gears.  Since there is more than one tooth in engagement at any one time, helical gears are stronger than spur gears of the same size and pitch.  However, special bearing (thrust bearings) are often required on shafts to overcome the end thrust produced by these gears as they turn.

Herringbone gears

Herringbone gears, Fig. 2.4., are resembles of two helical
gears  placed  side by side, with one half having a left-hand helix and the other half a right-hand helix.  These gears have a smooth continuous action and eliminate the need for thrust bearings. 

Bevel gears
When two shafts are located at an angle with their axial lines intersecting at 90o, power is generally transmitted by means of bevel gears.

Angular bevel gears
However, it is not necessary that the shafts be only at right angles in order to transmit power.  If the axes of the shafts intersect at any angle other 90o,  the gears are known as angular bevel gears.

Miter gears
When the shafts are at right angles and the gears are of the same size, they are called miter gears.

Hypoid gears
Bevel gears have straight teeth very similar to spur gears.  Modified bevel gears having helical teeth are known as hypoid gears.  The shafts of these gears, although at right angles, are not in the same plane and, therefore, do not intersect.  Hypoid gears are used in automobile drives.

Worm and worm gear
When shafts are at right angles and considerable reduction in speed is required, a worm and worm gear may be used, Fig. 2.9.  The worm, which meshes with the worm gear, may be single or multiple start thread.  A worm with a double-start thread will revolve the worm gear twice as fast as a worm with a single-start thread and the same pitch.

Rack and pinion
When it is necessary to convert rotary motion to linear motion, a rack and pinion may be used, Fig. 2.10.  The rack, which is actually a straight or flat gear, may have straight teeth to mesh with a spur gear, or angular teeth to mesh with a helical gear.


Addendum is the radial distance between the pitch circle and the outside diameter or the height of the tooth above the pitch.
Dedendum is the radial distance from the pitch circle to the bottom of the tooth space.
Pitch diameter
Pitch diameter is the diameter of the pitch circle which is equal to the outside diameter minus two addendums.
Base diameter
The diameter of  the circle from which the  involute  is generated; which is equals to pitch diameter times the cosine of the pressure angle. 
 Pitch circle
Pitch circle is the circle through the pitch point having its centre at the axis of the gear.
Pitch line
The line formed by the intersection of the pitch surface and the tooth surface.
Face width - The width of the pitch surface.
Tooth thickness
The thickness of the tooth measured on the pitch circle.
Top land - The surface of the pitch cylinder.
Base diameter - The diameter of the root circle.
Root - The bottoms of the tooth surface.