Editing Gear (section)

===Helical===
[[File:Helical Gears.jpg|left|thumb|Helical gears<br/>Top: parallel configuration<br/>Bottom: crossed configuration]]

In a ''[[helical gear|helical]]'' or ''dry fixed'' gear the tooth walls are not parallel to the axis of rotation, but are set at an angle.  An imaginary pitch surface (cylinder, cone, or hyperboloid, depending on the relative axis positions) intersects  each tooth face along an arc of a [[helix]]. Helical gears can be meshed in either ''parallel'' or  ''crossed'' orientations. The former refers to when the shafts are parallel to each other; this is the most common orientation. In the latter, the shafts are non-parallel, and in this configuration the gears are sometimes known as "skew gears".

[[File:Anim engrenages helicoidaux.gif|thumb|An external contact helical gear in action]]
The angled teeth engage more gradually than do spur gear teeth, causing them to run more smoothly and quietly.<ref>{{Citation|last=Khurmi|first=R. S.|title=Theory of Machines|publisher=S.CHAND}}</ref> 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. In spur gears, teeth suddenly meet at a line contact across their entire width, causing stress and noise. Spur gears make a characteristic whine at high speeds. For this reason spur gears are used in low-speed applications and in situations where noise control is not a problem, and helical gears are used in high-speed applications, large power transmission, or where [[noise abatement]] is important.<ref>{{citation|chapter-url=https://kyotoo.org/|chapter=Minimizing gearbox noise inside and outside the box|first=Richard|last=Schunck|title=Motion System Design|postscript=.}}{{Dead link|date=May 2024 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> The speed is considered high when the pitch line velocity exceeds 25&nbsp;m/s.<ref name="pitchlinespeed">{{harvnb|Vallance|Doughtie|1964|p=281|quote=Pitch-line speeds of 4,000 to 7,000 fpm [20 to 36 m/s] are common with automobile and turbine gears, and speeds of 12,000 fpm [61 m/s] have been successfully used.}}</ref>

A disadvantage of helical gears is a resultant [[thrust]] along the axis of the gear, which must be accommodated by appropriate [[thrust bearing]]s. However, this issue can be circumvented by using a [[herringbone gear]] or ''double helical gear'', which has no axial thrust - and also provides self-aligning of the gears. This results in less axial thrust than a comparable spur gear.

A second disadvantage of helical gears is a greater degree of [[sliding friction]] between the meshing teeth, often addressed with additives in the lubricant.

For a "crossed" or "skew" configuration, the gears must have the same pressure angle and normal pitch; however, the helix angle and handedness can be different. The relationship between the two shafts is actually defined by the helix angle(s) of the two shafts and the handedness, as defined:<ref name="roymech">{{Citation |title=Helical gears |url=http://www.roymech.co.uk/Useful_Tables/Drive/Hellical_Gears.html |access-date=15 June 2009 |postscript=. |url-status=dead |archive-url=https://web.archive.org/web/20090626030945/http://www.roymech.co.uk/Useful_Tables/Drive/Hellical_Gears.html |archive-date=26 June 2009 }}</ref>
:<math>E = \beta_1 + \beta_2</math> for gears of the same handedness,
:<math>E = \beta_1 - \beta_2</math> for gears of opposite handedness,
where <math>\beta</math> is the helix angle for the gear. The crossed configuration is less mechanically sound because there is only a point contact between the gears, whereas in the parallel configuration there is a line contact.<ref name="roymech"/>

Quite commonly, helical gears are used with the helix angle of one having the negative of the helix angle of the other; such a pair might also be referred to as having a right-handed helix and a left-handed helix of equal angles. The two equal but opposite angles add to zero: the angle between shafts is zero—that is, the shafts are ''parallel''. Where the sum or the difference (as described in the equations above) is not zero, the shafts are ''crossed''. For shafts ''crossed'' at right angles, the helix angles are of the same hand because they must add to 90 degrees. (This is the case with the gears in the illustration above: they mesh correctly in the crossed configuration: for the parallel configuration, one of the helix angles should be reversed. The gears illustrated cannot mesh with the shafts parallel.)

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* [https://www.youtube.com/watch?v=Qcgjsor1Q-Y 3D animation of helical gears (parallel axis)]
* [https://www.youtube.com/watch?v=ZpJuyK842RQ 3D animation of helical gears (crossed axis)]