Reasons for choosing them
cylindrical spur gear, the contact (and seperation) is simultaneous across
the full width of the tooth.
On the other
hand, when the gear is helical, contact is made first at one end of the
tooth and then progressively across it's width. The final seperation point
is at the other end of the tooth. This makes for a much longer contact
period for each tooth and thus makes for smoother operation, with less
noise and vibration and a much smoother transmission.
The Helical Angle (HA)
with a spur gear, the helical gear introduces the notion of a helical lead
angle. The sum of the helical lead angles of the meshed gears will
determine the angle between the two axes.
totally arbitrary manner, we'll call Right hand inclinations positive (+)
and Left hand negative
(-) . If a gear with teeth inclined to the right
by A?meshes with a gear with teeth inclined to the left of A?the
resulting angular difference between the two gears will be :
angles cancel each other out. Therefore the system is said to be made up
of parallel axed
helical gears .On the other hand, if a RH gear at 45?meshes with
another RH gear of 45?The resultant will be:
+ (+) 45?= (+) 90?BR>
angles are complementary. Producing crossed
axle helical gears
The inclination of the teeth in such a
manner causes axial forces. This force becomes rapidly stronger the more
the lead angle is increased. It is therefore necessary to oppose this
force by adding a system of thrust bearings.
because of this lead angle, the basic formulas used for stating the
dimensions of the gears must be subjected to a small
- Helical pitch: pz = p d / tg ?/FONT>
- Apparent module: mt = mn / cos?/FONT>
- Apparent pitch :pt = mt
- Real pitch: pn = mn = pt
- Total depth: h = ha + hf = 2.25 mn
Helical gears with parallel axes.
inclination of the teeth generates an axial force. This axial force
increases rapidly with the increase in the angle of inclination. It is
therefore necessary to counter these forces by using thrust
of this, one hardly ever uses values for the lead angle of over 25? This
angle is not standardised between the different manufacturers. HPC has
decided to use an angle of 17?5' for its gears. Therefor it is necessary,
for reasons of compatibility, that the client uses a pair of wheels from
the same manufacturer.
summary, the same Helical angle must be used on the two gears so that they
are compatible : also, the helical angle has an effect on the Pitch
diameter, therefore on the centre to centre distance.
LH helical gears
mesh with RH helical gears.
Advantages, disadvantages and advice.
gearing is much quieter, there is less vibration.
- Slightly less efficient than the equivalent spur gear.
- Thrust bearings are needed to support axial loading
- Helical angles must be compatible.
Forces generated by the system
with crossed axes.
The inclination of the teeth generates an axial
force. This axial force increases rapidly with the increase in the angle
of inclination. It is therefore necessary to counter these forces by using
thrust bearings. The forces generated by an HA of 45?are high. Thus
resulting in an important loss in system efficiency, which, with this type
of transmission can only be described as average.
between the two gears is made at one single point. Which means that the
mechanism is capable of transmitting only quite weak forces. The friction
levels are relatively high, which can sometimes lead to the choice of
bronze as the material for one of the gears, as in, for example, worms
and wormwheels , which are no more and no less than helical gears with
crossed axes but with a very large reduction ratio.
Crossed axe helical gears will only mesh with gears which have teeth
inclined in the same direction
Advantages, disadvantages and
system is quiet, with little vibration
is a large choice of different reduction ratios available.
has a relatively low level of efficiency (from 40% to 70%), which
depends on the type of material and lubrication.
thrust bearings must be capable of supporting the high levels of axial
generated by the system
|Left hand helices work together
||Right hand helices work together