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Helical gears
Intermeshing gears in motion
Unlike most gears, an internal gear (shown here) does not cause direction reversal.
Helical gears from a Meccano construction set.
Helical gears
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. This
causes helical gears to run more smoothly and quietly than spur gears.
Helical gears also offer the possibility of using non-parallel shafts.
A pair of helical gears can be meshed in two ways: with shafts oriented
at either the sum or the difference of the helix angles of the gears.
These configurations are referred to as parallel or crossed,
respectively. The parallel configuration is the more mechanically
sound. In it, the helices of a pair of meshing teeth meet at a common
tangent, and the contact between the tooth surfaces will, generally, be
a curve extending some distance across their face widths. In the
crossed configuration, the helices do not meet tangentially, and only
point contact is achieved between tooth surfaces. Because of the small
area of contact, crossed helical gears can only be used with light
loads.
Quite commonly, helical gears come in pairs where the helix angle of
one is 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. If such a pair is meshed in the 'parallel' mode,
the two equal but opposite angles add to zero: the angle between shafts
is zero -- that is, the shafts are parallel. If the pair is meshed in
the 'crossed' mode, the angle between shafts will be twice the absolute
value of either helix angle.
Note that 'parallel' helical gears need not have parallel shafts --
this only occurs if their helix angles are equal but opposite. The
'parallel' in 'parallel helical gears' must refer, if anything, to the
(quasi) parallelism of the teeth, not to the shaft orientation.
As mentioned at the start of this section, helical gears operate
more smoothly than do spur gears. 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. It may span the entire width of the tooth for a time.
Finally, it recedes until the teeth break contact at a single point on
the opposite side of the wheel. Thus force is taken up and released
gradually. With spur gears, the situation is quite different. When a
pair of teeth meet, they immediately make line contact across their
entire width. This causes impact stress and
noise. Spur gears make a
characteristic whine at high speeds and can not take as much torque as
helical gears because their teeth are receiving impact blows. 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. The speed is
considered to be high when the pitch line velocity (that is, the
circumferential velocity) exceeds 5000 ft/min.8
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 specific additives in the lubricant.
