Gears are used to transfer transfer kinetic energy in a rotary motion using levers. Each tooth on a gear can be simplified as a lever which can be used to produce a mechanical advantage in a system. In a geared system there is a 'Driver', the gear which is powered, and a 'Driven', the gear that is moved. When the two engage the rotary motion is reversed. The rate of rotation is given in RPM (revolutions per minute) and calculated; input speed ÷ gear ratio 
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There are 3  reasons for implementing gears into a system; 

- Change Speed; This is achieved by making the number of teeth on each gear different. The larger 'Driver' gear has a greater number of teeth than the smaller 'Driven' gear, increasing the number of rotations it makes. The difference between each gear is given as a ratio. Driven gear teeth ÷ Driver gear teeth =  Ratio

- Change Force; If the 'Driver' gear is smaller than  the 'Driven' gear the speed of rotation is decreased, however, the force applied is increased. The increase in torque occurs much in the same way a lever is given a mechanical advantage by using differing lengths. A gear is simply multiple levers arranged in a circle

- Change Direction of Force; When two gears engage the direction of force is reversed as one pushes the other away. Motion can be changed in a linear direction as shown or at an angle by altering the shape of the tooth profiles
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Gears can be cut into different profiles for different applications; 
- Spur gears are cut with straight toothed profiles that run parallel to the axis of the gear. They are simple to manufacture, low cost and easy to maintain. Often they are used for low speed applications.
- Helical gears follow the gear axis in a helix shape and are suitable for higher speed/load due to smoother engagement and reduced noise

Bevel Gears are angled in such a way that the rotary motion is translated typically at a 90 degree angle. Each tooth is conically shaped and cut to engage fully with each other to transfer force between the two. These are commonly found in a hand drill as a way of transferring the users downward rotary motion to lateral motion at the chuck

The shape of the gear profiles can be adjusted from linear to spiralled to allow for a more gradual engaging of gears. This prevents an abrupt engaging at high speed or under high load which can cause noise and damage to the system
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Worm Gears are used when large gear reductions are required of around 20:1 to in excess of 200:1. The 'driver' worm gear turns the driven circular gear as it rotates on the axel. Uniquely the circular gear cannot drive the worm gear due to a excessive friction and a shallow tooth angle. This is useful as a method for braking in a mechanical system when the motor is stopped, as found in a conveyor belt

Rack and Pinion gears consist of a toothed rod (rack) and a single slot gear (pinion). The rotary torque of the driven slot gear transfers to linear motion as the toothed rack is moved along the gear profile. Such a mechanism is responsible for transferring the rotary motion of a steering wheel to move the wheels of a car

Planetary Gears consist of a number of smaller slot gears inside a larger slot gear. They consist of a 'sun' gear at the centre, which is typically driven, and planet gears in mesh around the outside, held within a larger 'ring' gear. Planetary gears allow for gear reduction to be achieved in a small space, and a number of ratios to be achieved in one system by alternating the powered 'driven' gear 
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The 'Differential' within a cars drive system is a vital mechanical system which relies on gears. When cornering a cars inside wheel rotates faster than the outside wheel due to the shorter distance covered. This would be problematic if the wheels powered by the drive shaft were on a fixed axel. To overcome this the axel is separated by a geared differential system

As power is provided by the 'propeller shaft' via a bevel gear to the 'pinion', the fixed 'cage' around the smaller gears rotates with it. Driving in a straight line the 'small gears' do not rotate and torque is transferred equally. When cornering the 'small gears' rotate to allow for the differing speed of each axel to ensure constant rotation





Mechanical Devices - Gears 

The old ones are the best!

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