Gear
For other uses of this term, see Gear (disambiguation). |
Spur gears found on a piece of farm equipment. |
Gears or toothed wheels are positive type drives which are used to
transmit motion between two shafts or a shaft and a component having linear motion, by meshing of two or more gears. They have advantage over other drives like chains, belts etc. in case of precision machines where a definite velocity ratio is of importance and also in case where the driver and the follower are in close proximity; the downside is that gears are more expensive to manufacture and their operating cost is also relatively high.
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.
The ratio of the rotational speeds of two messed gears is called the
Gear ratio.
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.
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Intermeshing gears in motion |
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.
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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 (ie. circumferential velocity) exceeds 5000 ft/min or the rotational speed of the pinion (ie. smaller gear) exceeds 3600 rpm.
Double helical gears, invented by
Citroen 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.
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.
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[A Worm Gear and Pinion from a Meccano construction set |
If the axles are skewed, that is, non-intersecting, 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 gear can achieve a higher gear ratio than spur gears of a comparable size.
The central gear of a
Planetary gearA
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.
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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).
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Crown_gear.png |
A crown gearA
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.
See
Epicyclic gearingAlso known as a
Planetary gearIn some machines it is necessary to change the gear ratio to suit the task. There are several ways of doing this. For example:
*
Manual transmission*
automatic gearbox*
derailleur gears which are actually
sprockets in combination with a
roller chain*
hub gears (also called
epicyclic gearing or sun-and-planet gears)
*
continuously variable transmission*
transmission (mechanics)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).
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Antikythera mechanism*
Ball screw*
Cycloid gear*
Differential gear*
Planetary gear*
Epicyclic gearing*
Gear coupling*
Hobbing machine*
Hypoid*
Jackscrew*
Non-circular gear*
Rack and pinion*
South Pointing Chariot *
Sprocket*
Mathematical Tutorial for Gearing (Relating to Robotics)*
Flash Animation of an Involute Gear*
Animation of an Involute Rack and Pinion
