With a 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.
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The Helical Angle (HA)
In comparison
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.
In a
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 :
(+) A?+ (-)A?= 0?BR>
The two 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?
+ (+) 45?= (+) 90?BR>
The two
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.
Also
because of this lead angle, the basic formulas used for stating the
dimensions of the gears must be subjected to a small
adjustment..
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STANDARD
TOOTH |
Symbols | Formula | Unit |
| Module | m | p   |
mm |
| Pitch | p | m x |
mm |
| Pitch diameter | d | Zm x cos?/FONT> | mm |
| External diameter | D | m + (2 x m) | mm |
| Number of teeth | Z | d + cos?/U> m |
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| Adendum | ha | m | mm |
| Helical pitch | Pz | d x
tan?/FONT> |
mm |
Helical gears with parallel 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.
Because
of this, one hardly 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.
In
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.
- The 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.
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SH
and PSH
Forces generated by the system
Helical gears
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.
The contact
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.
Attention :
Crossed axe helical gears will only mesh with gears which have teeth
inclined in the same direction
- The
system is quiet, with little vibration
- There
is a large choice of different reduction ratios available.
- It
has a relatively low level of efficiency (from 40% to 70%), which
depends on the type of material and lubrication.
- The
thrust bearings must be capable of supporting the high levels of axial
forces.
| LH Teeth |  drive wheel |
 drive wheel |
RH Teeth |
| Left hand helices work together | Right hand helices work together | ||
| LH Teeth |  drive wheel |
 drive wheel |
RH Teeth |