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External versus internal gears

External gears are gears with teeth formed on the outer surface of a cylinder or cone.In contrast, an internal gear is a gear with teeth formed on the inner surface of a cylinder or cone.As with bevel gears, internal gears are gears with pitch angles greater than 90 degrees. The internal gear does not cause the direction of the output shaft to reverse.

Spur

Spur or spur gears are the simplest type of gears.They consist of cylinders or discs with radially protruding teeth.While the teeth are not straight-sided (but usually in a special form to achieve a constant ratio, mostly involute but rarely cycloidal), the edge of each tooth is straight and aligned parallel to the axis of rotation.These gears mesh properly only when mounted on parallel shafts.Tooth loads do not generate axial thrust.Spur gears are great at medium speeds but tend to be noisy at high speeds.

Helical 

Helical or "dry fixed" gears provide an improvement 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 inclination causes the teeth to form part of the helix.Helical gears can mesh in parallel or cross directions.The former refers to when the axes are parallel to each other; this is the most common orientation. In the latter, the shafts are not parallel,and in this configuration the gears are sometimes called "helical gears".Slanted teeth mesh more slowly than spur gear teeth, making them run smoother and quieter.For parallel helical gears, each pair of teeth first contacts at a point on one side of the gear; then, a moving contact curve gradually grows across the tooth faces to a maximum value and then recedes until the teeth are at a point on the other side break contact.In a spur gear, the teeth suddenly meet in line contact across the width, causing stress and noise.The spur gears make a characteristic whine at high speeds.Therefore,spur gears are used in low speed applications and where noise control is not an issue,while helical gears are used in high speed applications,high power transmission or where noise suppression is important.Velocity is considered high when the pitch line velocity exceeds 25 m/s.A

disadvantage of helical gears is that the thrust generated along the gear axis must be accommodated by suitable thrust bearings. However,this problem can be circumvented by using herringbone gears or double helical gears with no axial thrust and also providing self-alignment of the gears.This results in less axial thrust than a similar spur gear.A second disadvantage of helical gears is also a greater degree of sliding friction between the meshing teeth, usually addressed by additives in the lubricant.

Skew gears

For a "crossed" or "slanted" configuration,the gears must have the same pressure angle and normal pitch; however, the helix angle and handedness can differ.The relationship between the two axes is actually defined by the helix angle and handedness of the two axes, defined as follows:

 For concentric gears,

For counter-rotating gears,

Where is the helix angle of the gear.The cross configuration is mechanically less reliable as there is only point contact between the gears as opposed to wired contact in a parallel configuration.It is very common that helical gears are used with negative values of one helix angle and the other; such a pair may also be called a right-handed and left-handed helix with equal angles.Two equal but opposite angles add to zero: the angle between the axes is zero, that is, the axes are parallel. If the sum or difference (as in the formula above) is non-zero, then the axes cross.For axes intersecting at right angles, the helix angles are in the same direction because they must add 90 degrees.(This is the case with the gears pictured above: they mesh correctly in a cross configuration: for a parallel configuration, one of the helix angles should be reversed.The gears pictured cannot mesh parallel to the shaft.)

Double helical

Double helical gears overcome the axial thrust problem of single helical gears by using double sets of teeth that are inclined in opposite directions.A double helical gear can be thought of as two mirrored helical gears mounted closely on the same shaft.This arrangement cancels out the net axial thrust because each half of the gear pushes in opposite directions, resulting in a net axial force of zero.This arrangement can also eliminate the need for thrust bearings. However,double helical gears are more difficult to manufacture due to their more complex shape.A herringbone gear is a special type of helical gear.They don't have a groove in the middle like other double helical gears; two mirrored helical gears are joined so that their teeth form a V shape.This also applies to bevel gears,such as final drives of Citroën Type A.For the two possible directions of rotation, there are two possible arrangements for oppositely oriented helical gears or gear faces.One arrangement is called stable and the other is called unstable.In a stable arrangement,the helical gear faces are oriented such that every axial force is directed toward the center of the gear.In an unstable arrangement,both axial forces are away from the center of the gear.In either arrangement, when the gears are properly aligned, the total (or net) axial force on each gear is zero.An unstable arrangement creates a net force that can cause the gear train to disassemble if the gears become misaligned in the axial direction,while a stable arrangement creates a net correcting force.If the direction of rotation is reversed, the direction of axial thrust is also reversed, so a stable configuration becomes unstable,and vice versa.Stable double helical gears are directly interchangeable with spur gears without the need for different bearings.