Gear

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The BEAM Glossary says:


 Two meshing gears transmitting rotational motion. Note that the smaller gear is rotating faster. Although the larger gear is rotating less quickly, its torque is proportionally greater.
Two meshing gears transmitting rotational motion. Note that the smaller gear is rotating faster. Although the larger gear is rotating less quickly, its torque is proportionally greater.


There are a wide variety of gear types, each of which serves its particular function by its own characteristic transfer of motion to another gear or mechanism that is mated to it. The following is a list and short description of a few of the kinds of gears that a BEAMer might run across while attempting to salvage useful parts from toys, mechanical timers and various scrap mechanisms.


Contents

[edit] Spur gears

This is the simplest, most common type of gear used. They consist of a cylinder or disk with the teeth projecting around the entire circumference, and although these teeth are not straight-sided in form, the edge of each tooth is straight and aligned parallel to the axis of rotation. These gears can be meshed together correctly only if they are fitted to parallel shafts.


Spur gears teeth meet suddenly, at a line of contact across their entire width. This causes stress and noise. As a result, spur gears make a characteristic whine at high speeds and can not take as much torque as helical gears.



[edit] Compound Gears

Put simply, a compound gear is two or more gears of different diameters and/or type that are either cut or molded as a single unit, or otherwise keyed to rotate together as a single unit.

Compound gears are commonly used to make gear trains having a greater gear reduction, within a smaller space than would be possible with individual spur gears. This allows the creation of more compact gearboxes.



[edit] Helical gears

Upper Image: Parallel Helical gearLower Image: Crossed Helical gear
Upper Image: Parallel Helical gear
Lower Image: Crossed Helical gear


These offer a refinement over spur gears. The leading edges of the teeth are not parallel to the axis of rotation, but are set at an angle. Since the gear is curved, this angling causes the tooth shape to be a segment of a helix. The angled teeth engage more gradually than do spur gear teeth, causing them to run more smoothly and with vibration, making then quieter than spur gears. Add to this, the fact that at any time, the load on helical gears is distributed over several teeth, resulting in reduced wear.


However, 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.


Helical gears can be meshed in a parallel or crossed orientations. The former refers to when the shafts are parallel to each other; this is the most common orientation. With parallel helical gears, the teeth of the meshing gears are cut at opposite angles.


In operation, 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.


In the crossed orientation, the shafts are non-parallel, and teeth of the meshing gears are cut at the same angle. In this configuration they are sometimes known as "skew gears".



[edit] Herringbone gears

Herringbone gear AKA Double helical gearIllustration from Bentley, Sketches of Engine and Machine Details
Herringbone gear AKA Double helical gear
Illustration from Bentley, Sketches of Engine and Machine Details

These 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 herringbone gear (AKA double helical gears) can be thought of as two standard mirror image helical gears bonded together side-by-side to form a single gear. This cancels out the thrust load along the gear shaft, since each half of the gear thrusts in the opposite direction. As a result of their more complicated shape, double helical gears are more difficult to manufacture, and therefore tend to be more expensive.


For each possible direction of rotation, there are two possible, opposite orientations for the gear faces. In one, the helical gear faces are oriented so that the axial force generated by each is in the axial direction away from the center of the gear. This arrangement is unstable. In the other, which is stable, the helical gear faces are oriented so that each axial force is toward the mid-line of the gear. In both arrangements, alignment is critical to ensure correct teeth engagement. When the gears are aligned correctly, the total (or net) axial force on each gear is zero.


If the gears become misaligned in the axial direction, the unstable arrangement will generate a net force for disassembly of the gear train, while the stable arrangement will generate a net corrective force. If the direction of rotation is reversed, the direction of the axial thrusts is also reversed. Thus a stable configuration becomes unstable, and vice versa.


[edit] Bevel gear

These are gears having tooth-bearing faces that are conical in shape, and that are used to transmit motion between shafts that have intersecting axes.


The intersecting angle is normally 90 deg but can also be designed to work at other angles. When the mating gears are equal in size and the shafts are positioned at 90 degrees to each other, they are referred to as miter gears.


The teeth of bevel gears can also be cut in a curved manner to produce spiral bevel gears, which produce smoother and quieter operation than straight cut bevels.



[edit] Crown gears


Also called contrate gears, these are a particular form of bevel gear whose teeth project at right angles to the plane of the wheel and thus they resemble the points on a crown. A crown gear can only mesh accurately with another bevel gear, but they are sometimes seen meshing with spur gears.



[edit] Worm gears

Analogous to screws, the relative motion between these gears is sliding rather than rolling. They can be right or left-handed (following the long established practice for screw threads) and are usually meshed with an ordinary looking, disk-shaped gear wheel, which is called the worm wheel.


The uniform distribution of tooth pressures on a worm wheel enables it to be made with of metals that have inherently low coefficients of friction (such as bronze) and still be used with hardened steel worm gears. Worm-and-gear sets are a simple and compact way to achieve a high torque, low speed gear ratio. For example, worm-and-gear sets range from 10:1' to 500:1. By comparison helical gears are normally limited to gear ratios of less than 10:1.


A disadvantage is the potential for considerable sliding action, leading to low efficiency.


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. Particularly if the lead angle is small, the gear's teeth may simply lock against the worm's teeth, because the force component circumferential to the worm is not sufficient to overcome friction. Worm-and-gear sets that do lock are called self locking, which can be used to advantage, as for instance when it is desired to set the position of a mechanism by turning the worm and then have the mechanism hold that position. An example is the machine head found on some types of stringed instruments.



[edit] Face gears


A close cousin of the crown gear, this is a disk-shaped gear, grooved on at least one face, which works in combination with a spur, helical, or conical pinion.


A face gear has a planar pitch surface] and a planar root surface], both of which are perpendicular to the axis of rotation. gears are at various times also referred to as a face wheels, crown gears, crown wheels, contrate gears or contrate wheels.


[edit] Hypoid gears

These resemble spiral bevel and face gears except that the shaft axes do not intersect. They combine the rolling action and high tooth pressure of spiral bevels with the sliding action of worm gears.


The pitch surfaces appear conical but, to compensate for the offset shaft, are in fact hyperboloids of revolution. Hypoid gears are almost always designed to operate with shafts at 90 degrees. 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 being 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.


[edit] Rack and pinion

Rack and pinion gears are essentially a variation of spur gears. The rack is a linear "gear", that is, it is a toothed bar or rod that can be thought of as a sector gear with an infinitely large radius of curvature. The circular pinion engages teeth on the the rack. Rotational motion applied to the pinion causes the rack to move to one side or the other (depending on the direction of rotation of the pinion), up to the limit of its travel. Thus torque is converted to linear force.


The rack and pinion arrangement is commonly found in the steering mechanism of cars or other wheeled, steered vehicles. This arrangement provides a lesser mechanical advantage than other mechanisms such as recirculating ball, but much less backlash and greater feedback, or steering "feel".



[edit] Annular gears

These have a similar tooth design to that of a spur and helical gears, except that the teeth are formed on the inner surface of a cylinder or cone, and they are more concave in shaped, whereas the teeth of a spur gear are more convex in shape. In fact, the teeth on a annular gear are basically the same shape as the space between the teeth of a spur gear.


Unlike spur or helical gears, annular (AKA Internal gears) do not cause direction reversal. The pitch angle of a beveled internal gear will exceeding 90 degrees.




[edit] Related Pages


[edit] External References


[edit] Related Videos & Animations

[edit] Unique Gearing




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