The spur gear is is simplest type of gear manufactured and is generally used for transmission of rotary motion between parallel shafts.  The spur gear is the first choice option for gears except when high speeds, loads, and ratios direct towards other options.  Other gear types may also be preferred to provide more silent low-vibration operation.  A single spur gear is generally selected to have a ratio range of between 1:1 and 1:6 with a pitch line velocity up to 25 m/s. The spur gear has an operating efficiency of 98-99%.  The pinion is made from a harder material than the wheel.  A gear pair should be selected to have the highest number of teeth consistent with a suitable safety margin in strength and wear.The minimum number of teeth on a gear with a normal pressure angle of 20 desgrees is 18.

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Helical gear is a cylindrical shaped gear with helicoid teeth. Helical gears operate with less noise and vibration than spur gears. At any time, the load on helical gears is distributed over several teeth, resulting in reduced wear. Due to their angular cut, teeth meshing results in thrust loads along the gear shaft. This action requires thrust bearings to absorb the thrust load and maintain gear alignment. They are widely used in industry. A negative is the axial thrust force the helix form causes.

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Sprocket are very popular and common gear or wheel that comes with a metal teeth which meshes perfectly with a chain or track. The term pitch is used to refer to the distance between two sprockets.
Industrial sprockets today are made in a variety of sizes. Sprockets are available in metric and also in English measurements. Sprockets do not allow gears to slip away and are specifically applied in cases where chains get engaged by gears or wheels and also in cases of motors producing rotational motions. Industrial sprockets are essentially part of the chain drives. It is important in a chain drive that the sprocket should match with the type of chain used. Interestingly, though Sprockets look like gears but it differs in few ways from Gears

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Worm gears teeth resembles ACME screw thread which mates with a helical gear, except that it is made to envelope the worm as seen along the worm's axis. Operation of worm gears is analogous to a screw. The relative motion between these gears is sliding rather than rolling. The uniform distribution of tooth pressures on these gears enables use of metals with inherently low coefficients of friction such as bronze wheel gears with hardened steel worm gears. These gears rely on full fluid film lubrication and require heavy oil compounded to enhance lubricity and film strength to prevent metal contact.

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The rack and pinion is used to convert between rotary and linear motion. The rack is the flat, toothed part, the pinion is the gear. Rack and pinion can convert from rotary to linear of from linear to rotary.The diameter of the gear determines the speed that the rack moves as the pinion turns.Rack and pinions are commonly used in the steering system of cars to convert the rotary motion of the steering wheel to the side to side motion in the wheels.
Rack and pinion gears give a positive motion especially compared to the friction drive of a wheel in tarmac. In the rack and pinion railway a central rack between the two rails engages with a pinion on the engine allowing the train to be pulled up very steep slopes.

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Where two axles cross at point and engage by means of a pair of conical gears, the gears themselves are referred to as bevel gears. These gears enable a change in the axes of rotation of the respective shafts, commonly 90°. A set of four bevel gears in a square make a differential gear, which can transmit power to two axles spinning at different speeds, such as those on a cornering automobile.

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Timing belt pulley

A timing belt pulley is part of the timing belt system for an automobile. A timing belt is one way to control the opening and closing of engine valves by connecting the crankshaft to the camshaft of the vehicle. Generally, the timing belt does this by having teeth that lock into the teeth of the two gears for the crankshaft and camshaft.
A timing belt is one of several belts used in conventional cars and other vehicles. A serpentine belt is a belt that drives various other engine accessories off of the crankshaft. The serpentine belt can run an alternator, power steering pump, and air conditioning compressor. In some vehicles, these jobs are split between multiple belts,   where a vehicle usually has only one timing belt pulley.

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