Gear

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For other uses of this term, see Gear (disambiguation).
Spur gears found on a piece of farm equipment
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Spur gears found on a piece of farm equipment

Gears, toothed wheels or cogs are positive type drives which are used to transmit motion between two shafts or a shaft and a component having linear motion, by the meshing of two or more gears. Their definite velocity ratio is an advantage over other drives (such as traction drives and V-belts) in precision machines such as watches that depend upon an exact velocity ratio. In cases where driver and follower are in close proximity gears also have an advantage over other drives in the reduced number of parts required; the downside is that gears are more expensive to manufacture and their lubrication requirements may impose a higher operating cost.

Gears of differing size are often used in pairs for a mechanical advantage, allowing the torque of the driving gear to produce a larger torque in the driven gear at lower speed, or a smaller torque at higher speed. The larger gear is known as a wheel and the smaller as a pinion. This is the principle of the automobile transmission, allowing selection between various mechanical advantages.

A vital aspect of two meshed gears is the gear ratio. It is the ratio of their rotational speeds.

A gearbox is not an amplifier or a servomechanism. Conservation of energy requires that the amount of power delivered by the output gear or shaft will never exceed the power applied to the input gear, regardless of the gear ratio. Work equals the product of force and distance, therefore the small gear is required to run a longer distance and in the process is able to exert a larger twisting force or torque, than would have been the case if the gears were the same size. There is actually some loss of output power due to friction.

Intermeshing gears in motion
Intermeshing gears in motion

Contents

[edit] Spur gears

The most common type of gear wheel, spur gears, are flat and have teeth projecting radially and in the plane of the wheel. The teeth of these "straight-cut gears" are cut so that the leading edges are parallel to the line of the axis of rotation. These gears can only mesh correctly if they are fitted to parallel axles.

[edit] Helical gears

Helical gears from a Meccano construction set.
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Helical gears from a Meccano construction set.

Helical gears offer a refinement over spur gears. The teeth are cut at an angle, allowing for more gradual, hence smoother meshing between gear wheels, eliminating the whine characteristic of straight-cut gears. A disadvantage of helical gears is a resultant thrust along the axis of the gear, which needs to be accommodated by appropriate thrust bearings, and a greater degree of sliding friction between the meshing teeth, often addressed with specific additives in the lubricant. Whereas spur gears are used for low speed applications and those situations where noise control is not a problem, the use of helical gears is indicated when the application involves high speeds, large power transmission, or where noise abatement is important. The speed is considered to be high when the pitch line velocity (that is, the circumferential velocity) exceeds 5000 ft/min or the rotational speed of the pinion (ie. smaller gear) exceeds 3600 rpm.

[edit] Double helical gears

Double helical gears, invented by André Citroën and also known as herringbone gears, overcome the problem of axial thrust presented by Single helical gears by having teeth that are 'V' shaped. Each gear in a double helical gear can be thought of as two standard, but mirror image, helical gears stacked. This cancels out the thrust since each half of the gear thrusts in the opposite direction. They can be directly interchanged with spur gears without any need for different bearings.

[edit] Bevel gears

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 [1]. 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.

Helical gears can also be designed to allow a ninety degree rotation of the axis of rotation.

[edit] Worm gear

Main article: Worm gear
A Worm Gear and Pinion from a Meccano construction set
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A Worm Gear and Pinion from a Meccano construction set

If the axles are skewed, that is, not co-planar, then a worm gear can be used. This is a gear that resembles a screw, with parallel helical teeth, and mates with a normal spur gear. The worm is in most cases the driving gear, there are however a few exceptions where the spur gear drives the worm [2]. The worm gear can achieve a higher gear ratio than spur gears of a comparable size. Designed properly, a built in safety feature can be obtained: This gear style will self-lock if power is lost to the drive (worm). It doesn't work if the pinion is powered.

[edit] Sun gear

The central gear of a Planetary gear

[edit] Sector gear

A sector gear is merely a segment of a spur gear, such as one half or one quarter of the circumference, but still attached to the axle in the normal fashion. Such a gear will operate normally as long as the gear with which it meshes does not drive off the edge of the sector, for instance in a worm and sector automotive steering gear or its descendant the recirculating ball. It is useful for saving space and weight when only limited movement is necessary rather than the full 360 degrees of rotation.

[edit] Rack and pinion

Rack and pinion animation
Rack and pinion animation

Torque can be converted to linear force by a rack and pinion. The pinion is a spur gear, and meshes with a toothed bar or rod that can be thought of as a sector gear with an infinitely large radius of curvature. Such a mechanism is used in automobiles to convert the rotation of the steering wheel into the left-to-right motion of the tie rod(s).

Image:Crown gear.png
A crown gear

[edit] See also

[edit] Crown gear

A crown gear or contrate gear is a special form of bevel gear which has teeth at right angles to the plane of the wheel; it meshes with a straight cut spur gear or pinion on a right-angled axis to its own, or with an escapement such as found in mechanical clocks.

Simple gears suffer from backlash, which is the error in motion that occurs when gears change direction, resulting from hard to eliminate manufacturing errors. When moving forwards, the front face of the drive gear tooth pushes on the rear face of the driven gear. When the drive gear changes direction, its rear face is now pushing on the front face of the driven gear. Unless deliberately designed to eliminate it, there is slight 'slop' in any gearing where briefly neither face of the driving gear is pushing the driven gear. This means that input motion briefly causes no output motion. Assorted schemes exist to minimize or avoid problems this creates.

[edit] Epicyclic gearing

See epicyclic gearing, also known as a planetary gear.

[edit] Shifting of gears

In some machines (e.g. automobiles) it is necessary to change the gear ratio to suit the task. There are several ways of doing this. For example:

Friction and wear between two gears is highly dependent on the profile of the teeth. The tooth form used for most applications is involute but there are other tooth forms such as cycloidal (used in mechanical clocks) or rack (used in automobile steering).

[edit] Gear materials

Numerous nonferrous alloys, cast irons, powder-metallurgy and even plastics are used in the manufacture of gears. However steels are most commonly used because of their high strength to weight ratio and low cost.

[edit] See also

Molecular gears are a promise of nanotechnology.
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Molecular gears are a promise of nanotechnology.

[edit] External links

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