Helical Gear

an external contact helical gear in action



helical gears

top: parallel configuration

bottom: crossed configuration

helical or dry fixed gears offer refinement on spur gears. leading edges of teeth not parallel axis of rotation, set @ angle. since gear curved, angling makes tooth shape segment of helix. helical gears can meshed in parallel or crossed orientations. former refers when shafts parallel each other; common orientation. in latter, shafts non-parallel, , in configuration gears known skew gears .

the angled teeth engage more gradually spur gear teeth, causing them run more smoothly , quietly. parallel helical gears, each pair of teeth first make contact @ single point @ 1 side of gear wheel; moving curve of contact grows gradually across tooth face maximum, recedes until teeth break contact @ single point on opposite side. in spur gears, teeth meet @ line contact across entire width, causing stress , noise. spur gears make characteristic whine @ high speeds. reason spur gears used in low-speed applications , in situations noise control not problem, , helical gears used in high-speed applications, large power transmission, or noise abatement important. speed considered high when pitch line velocity exceeds 25 m/s.

a disadvantage of helical gears resultant thrust along axis of gear, must accommodated appropriate thrust bearings, , greater degree of sliding friction between meshing teeth, addressed additives in lubricant.

skew gears

for crossed or skew configuration, gears must have same pressure angle , normal pitch; however, helix angle , handedness can different. relationship between 2 shafts defined helix angle(s) of 2 shafts , handedness, defined:

e
=

β

1


+

β

2




{\displaystyle e=\beta _{1}+\beta _{2}}

gears of same handedness,




e
=

β

1


−

β

2




{\displaystyle e=\beta _{1}-\beta _{2}}

gears of opposite handedness,

where



β


{\displaystyle \beta }

helix angle gear. crossed configuration less mechanically sound because there point contact between gears, whereas in parallel configuration there line contact.

quite commonly, helical gears used helix angle of 1 having negative of helix angle of other; such pair might referred having right-handed helix , left-handed helix of equal angles. 2 equal opposite angles add zero: angle between shafts zero—that is, shafts parallel. sum or difference (as described in equations above) not zero, shafts crossed. shafts crossed @ right angles, helix angles of same hand because must add 90 degrees. (this case gears in illustration above: mesh correctly in crossed configuration: parallel configuration, 1 of helix angles should reversed. gears illustrated cannot mesh shafts parallel.)

3d animation of helical gears (parallel axis)
3d animation of helical gears (crossed axis)




^ khurmi, r. s., theory of machines, s.chand 
^ schunck, richard, minimizing gearbox noise inside , outside box , motion system design. 
^ vallance & doughtie 1964, p. 281
^ helical gears, archived original on 26 june 2009, retrieved 15 june 2009.