A gear or cogwheel is a rotating machine part having cut teeth, or cogs , which mesh with another toothed part to transmit torque , in most cases with teeth on the one gear being of identical shape, and often also with that shape on the other gear.
Nearly all mechanical devices rely on some form of gearing, and the key to a reliable gear is a good and appropriate design taking into account such factors as strength, wear resistance, noise, and fatigue resistance.


               Meshing Gears

Two or more gears working in a sequence (train) are called a gear train or, in many cases, a transmission ; such gear arrangements can produce a
mechanical advantage through a gear ratio and thus may be considered a
simple machine. Geared devices can change the speed, torque, and direction of a power source. The most common situation is for a gear to mesh with another gear; however, a gear can also mesh with a non-rotating toothed part, called a rack, thereby producing
translation instead of rotation.

1-Bevel gear:
These gears have teeth cut on a cone instead of a cylinder blank. They are used in pairs to transmit rotary motion and torque where the bevel gear shafts are at right angles (90 degree) to each other.
2-Crossed helical gear
These gears also transmit rotary motion and torque through a right angle. The teeth of a helical gear are inclined at an angle to the axis of rotation of the gear.
3-Worm and worm wheel
A gear which has one tooth is called worm. The tooth is in the form of a screw thread. A worm wheel meshes with the worm. The worm wheel is a helical gear with teeth inclined to that they can engage with the tread like worm. Like the cross helical gears, the worm and the worm wheel transmit torque and rotary motion through a right angle. The worm always drives the worm wheel and never the other way round. The mechanism locks if the worm wheel tries to drive the worm. Worm mechanism is very quiet running.
4-Helical gear
The gear is used for application that requires very quite and smooth running, at high rotational velocities. Parallel helical gears have their teeth inclined at a small angle to their axis of rotation. Each tooth is part of a spiral or helix. The helical gears have splines cut in their center holes. The gear can move along a splined shaft, although they rotate with the shaft. Double helical gears give an efficient transfer of torque and smooth motion at very high rotational velocities.
5-Spiral bevel gears
When it is necessary to transmit quietly and smoothly a large torque through a right angle at high velocities, spiral bevel gears can be used. Spiral bevel gears have teeth cut in helix spiral form on the surface of a cone. They are quieter running than straight bevel gears and have a longer life. Spiral bevel gears are used in motorcar axle gearboxes.
6-Internal gears
It is possible to cut gear teeth on the face of a gear wheel. Also, gear teeth can be cut the inclined of a gear ring. Internal gears have better load carrying capacity than external spur gear. They are safer in use because the teeth are guarded.
         Reference: Wikipedia , http://www.enggpedia.com.

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