A gear is a rotating machine part having cut teeth, or cogs, which mesh with another toothed part in order to transmit torque.
Two
or more gears working in tandem are called a transmission and can
produce a mechanical advantage through a gear ratio and thus may be
considered a simple machine .
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.
When two gears of unequal number of teeth are combined, a mechanical advantage is produced, with both the rotational speeds and the torques of the two gears differing in a simple relationship.
In transmissions which offer multiple gear ratios, such as bicycles and cars, the term gear, as in first gear, refers to a gear ratio rather than an actual physical gear.
The term is used to describe similar devices even when the gear ratio is continuous rather than discrete , or when the device does not actually contain any gears, as in a continuously variable transmission .
With parallel helical gears,
each pair of teeth first make contact at a single point at one side of
the gear wheel; a moving curve of contact then grows gradually across
the tooth face to a maximum then recedes until the teeth break contact
at a single point on the opposite side.
Spur gears make a characteristic whine at high speeds. 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.
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 additives in the lubricant.
The relationship between the two shafts is actually defined by the helix angle of the two shafts and the handedness, as defined: for gears of the same handedness for gears of opposite handedness
For shafts crossed at right angles, the helix angles are of the same hand because they must add to 90 degrees.Double helical gears, or herringbone gears , overcome the problem of axial thrust presented by "single" helical gears, by having two sets of teeth that are set in a V shape.
A double helical gear can be thought of as two mirrored helical gears joined together. This arrangement cancels out the net axial thrust, since each half of the gear thrusts in the opposite direction. However, double helical gears are more difficult to manufacture due to their more complicated shape.
For both possible rotational directions, there exist two possible arrangements for the oppositely-oriented helical gears or gear faces.
In a stable orientation, the helical gear faces are oriented so that each axial force is directed toward the center of the gear.
If the gears become misaligned in the axial direction, the unstable arrangement will generate a net force that may lead to disassembly of the gear train, while the stable arrangement generates a net corrective force.
The teeth of a bevel gear may be straight-cut as with spur gears, or they may be cut in a variety of other shapes.
Spiral bevel gear teeth are curved along the tooth's length and set at an angle, analogously to the way helical gear teeth are set at an angle compared to spur gear teeth.
Spiral bevel gears have the same advantages and disadvantages relative to their straight-cut cousins as helical gears do to spur gears.
Hypoid
Depending on which side the shaft is offset to, relative to the angling of the teeth, contact between hypoid gear teeth may be even smoother and more gradual than with spiral bevel gear teeth. Also, the pinion can be designed with fewer teeth than a spiral bevel pinion, with the result that gear ratios of 60:1 and higher are feasible using a single set of hypoid gears. This style of gear is most commonly found driving mechanical differentials; which are normally straight cut bevel gears; in motor vehicle axles.
Worm
A worm gear is usually meshed with a spur gear or a helical gear , which is called the gear, wheel, or worm wheel.
Worm-and-gear sets are a simple and compact way to achieve a high torque, low speed gear ratio.
For example, helical gears are normally limited to gear ratios of less than 10:1 while worm-and-gear sets vary from 10:1 to 500:1.
Worm gears can be considered a species of helical gear, but its helix angle is usually somewhat large and its body is usually fairly long in the axial direction; and it is these attributes which give it screw like qualities.
In a worm-and-gear set, the worm can always drive the gear. However, if the gear attempts to drive the worm, it may or may not succeed.
If the gear in a worm-and-gear set is an ordinary helical gear only a single point of contact will be achieved.
If medium to high power transmission is desired, the tooth shape of the gear is modified to achieve more intimate contact by making both gears partially envelop each other. This is done by making both concave and joining them at a saddle point ; this is called a cone-drive. or "Double enveloping"
Worm gears can be right or left-handed, following the long-established practice for screw threads.A rack is a toothed bar or rod that can be thought of as a sector gear with an infinitely large radius of curvature.
Racks and pinion
Racks also feature in the theory of gear geometry, where, for instance, the tooth shape of an interchangeable set of gears may be specified for the rack , and the tooth shapes for gears of particular actual radii are then derived from that.
Cage gears
A cage gear, also called a lantern gear or lantern pinion has cylindrical rods for teeth, parallel to the axle and arranged in a circle around it, much as the bars on a round bird cage or lantern.
In the case of a
large gear and a small pinion, however, the backlash is usually taken
entirely off the gear and the pinion is given full sized teeth.
For instance, the gear can be split along a plane perpendicular to the axis, one half fixed to the shaft in the usual manner, the other half placed alongside it, free to rotate about the shaft, but with springs between the two halves providing relative torque between them, so that one achieves, in effect, a single gear with expanding teeth.
In some machines it is necessary to alter the gear ratio to suit the task, a process known as gear shifting or changing gear.
The design life of the lower ratio gears is shorter, so cheaper gears may be used which tends to generate more noise due to smaller overlap ratio and a lower mesh stiffness etc. than the helical gears used for the high ratios. This fact has been utilized in analyzing vehicle generated sound since the late 1960s, and has been incorporated into the simulation of urban roadway noise and corresponding design of urban noise barriers along roadways. Tooth profile
There are a great many tooth profiles that will give a constant velocity ratio, and in many cases, given an arbitrary tooth shape, it is possible to develop a tooth profile for the mating gear that will give a constant velocity ratio. However, two constant velocity tooth profiles have been by far the most commonly used in modern times.
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.
When two gears of unequal number of teeth are combined, a mechanical advantage is produced, with both the rotational speeds and the torques of the two gears differing in a simple relationship.
In transmissions which offer multiple gear ratios, such as bicycles and cars, the term gear, as in first gear, refers to a gear ratio rather than an actual physical gear.
The term is used to describe similar devices even when the gear ratio is continuous rather than discrete , or when the device does not actually contain any gears, as in a continuously variable transmission .
Types
External vs internal gears
An external gear is one with the teeth formed on the outer surface of a cylinder or cone. Conversely, an internal gear is one with the teeth formed on the inner surface of a cylinder or cone.
Helical
Spur gears make a characteristic whine at high speeds. 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.
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 additives in the lubricant.
The relationship between the two shafts is actually defined by the helix angle of the two shafts and the handedness, as defined: for gears of the same handedness for gears of opposite handedness
For shafts crossed at right angles, the helix angles are of the same hand because they must add to 90 degrees.Double helical gears, or herringbone gears , overcome the problem of axial thrust presented by "single" helical gears, by having two sets of teeth that are set in a V shape.
A double helical gear can be thought of as two mirrored helical gears joined together. This arrangement cancels out the net axial thrust, since each half of the gear thrusts in the opposite direction. However, double helical gears are more difficult to manufacture due to their more complicated shape.
For both possible rotational directions, there exist two possible arrangements for the oppositely-oriented helical gears or gear faces.
In a stable orientation, the helical gear faces are oriented so that each axial force is directed toward the center of the gear.
If the gears become misaligned in the axial direction, the unstable arrangement will generate a net force that may lead to disassembly of the gear train, while the stable arrangement generates a net corrective force.
Bevel
Spiral bevel gear teeth are curved along the tooth's length and set at an angle, analogously to the way helical gear teeth are set at an angle compared to spur gear teeth.
Spiral bevel gears have the same advantages and disadvantages relative to their straight-cut cousins as helical gears do to spur gears.
Hypoid
Depending on which side the shaft is offset to, relative to the angling of the teeth, contact between hypoid gear teeth may be even smoother and more gradual than with spiral bevel gear teeth. Also, the pinion can be designed with fewer teeth than a spiral bevel pinion, with the result that gear ratios of 60:1 and higher are feasible using a single set of hypoid gears. This style of gear is most commonly found driving mechanical differentials; which are normally straight cut bevel gears; in motor vehicle axles.
A worm gear is usually meshed with a spur gear or a helical gear , which is called the gear, wheel, or worm wheel.
Worm-and-gear sets are a simple and compact way to achieve a high torque, low speed gear ratio.
For example, helical gears are normally limited to gear ratios of less than 10:1 while worm-and-gear sets vary from 10:1 to 500:1.
Worm gears can be considered a species of helical gear, but its helix angle is usually somewhat large and its body is usually fairly long in the axial direction; and it is these attributes which give it screw like qualities.
In a worm-and-gear set, the worm can always drive the gear. However, if the gear attempts to drive the worm, it may or may not succeed.
If the gear in a worm-and-gear set is an ordinary helical gear only a single point of contact will be achieved.
If medium to high power transmission is desired, the tooth shape of the gear is modified to achieve more intimate contact by making both gears partially envelop each other. This is done by making both concave and joining them at a saddle point ; this is called a cone-drive. or "Double enveloping"
Worm gears can be right or left-handed, following the long-established practice for screw threads.A rack is a toothed bar or rod that can be thought of as a sector gear with an infinitely large radius of curvature.
Racks and pinion
Racks also feature in the theory of gear geometry, where, for instance, the tooth shape of an interchangeable set of gears may be specified for the rack , and the tooth shapes for gears of particular actual radii are then derived from that.
A cage gear, also called a lantern gear or lantern pinion has cylindrical rods for teeth, parallel to the axle and arranged in a circle around it, much as the bars on a round bird cage or lantern.
Backlash
For instance, the gear can be split along a plane perpendicular to the axis, one half fixed to the shaft in the usual manner, the other half placed alongside it, free to rotate about the shaft, but with springs between the two halves providing relative torque between them, so that one achieves, in effect, a single gear with expanding teeth.
Shifting of gears
In some machines it is necessary to alter the gear ratio to suit the task, a process known as gear shifting or changing gear.
The design life of the lower ratio gears is shorter, so cheaper gears may be used which tends to generate more noise due to smaller overlap ratio and a lower mesh stiffness etc. than the helical gears used for the high ratios. This fact has been utilized in analyzing vehicle generated sound since the late 1960s, and has been incorporated into the simulation of urban roadway noise and corresponding design of urban noise barriers along roadways. Tooth profile
There are a great many tooth profiles that will give a constant velocity ratio, and in many cases, given an arbitrary tooth shape, it is possible to develop a tooth profile for the mating gear that will give a constant velocity ratio. However, two constant velocity tooth profiles have been by far the most commonly used in modern times.
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