The Art of the Helicopter John Watkinson - Karatunov.net

240 The Art of the Helicopter
two reduction stages as was shown in Figure 6.1. The first will reduce engine RPM
to approximately tail rotor RPM, and the second will reduce the tail rotor RPM to
main rotor RPM. The tail rotor gearbox contains a pair of bevel gears to turn the drive
through a right angle. This has usually a near 1:1 ratio, but some variation is possible.
In development the tail rotor RPM may need to be changed to obtain a different
compromise between noise and available thrust, or if the solidity is changed. This can
be achieved by changing the tail gearbox ratio. In machines with cranked tails a further
gearbox may be needed to turn the drive at the base of the fin.
Although simple at one level, gearboxes incorporate some subtle detailing. Consider
a gearbox with a ratio of 2 : 1. The gears have, for example, 40 and 20 teeth. As these
numbers factorize, each tooth on the small gear only touches two teeth on the large
gear and any irregularities would form a wear pattern. However, if the number of teeth
were to be relatively prime, every tooth would touch every other tooth and this would
result in the irregularities averaging out. This could be achieved by using 20 and 41
teeth. For this reason gearboxes seldom have simple ratios.
If the gear teeth were parallel to the shafts, the gearbox would make a lot of noise and
vibration as the drive jumps from one pair of teeth to the next. In practice the teeth are
twisted so that the point of contact slides along the shaft as the gears rotate. Contact
between the next pair of teeth begins just before the current pair part company and the
result is a more even power transmission and less noise and wear. Helical gears produce
end thrust on the shafts because of the angular contact. The angled contact faces of
the gear teeth also cause the shafts to be forced apart. The gear case, the shafts and
the bearings must be stiff enough to contain the forces generated without distorting.
Distortion will reduce the accuracy of tooth mesh and destroy the uniformity of the
tooth loading. Tapered roller bearings resist combinations of radial and axial forces.
When used in opposed pairs, the outer race of one of the bearings can be adjusted
axially to remove all play in the shaft and apply a slight pre-load to the rollers to stop
them skidding.
Figure 6.32 shows that in most gear systems both gearwheels have convex teeth
making the contact patch between the teeth very small. In conformal gearing, one of
the gearwheels has concave teeth designed to rotate around the convex teeth of the
other gear. This allows a much larger contact area so that a given size of gear can
transmit more torque. The Westland Lynx has conformal gearing.
The epicyclic or planetary reduction gearbox is popular in helicopters because it can
be built with low weight for the torque handled. Figure 6.2(a) showed that there is
a fixed internally toothed gear which is coaxial with the input or sun gear. Between
these gears are three planet gears that are fitted to pins on the output shaft. As the sun
gear rotates, the planets orbit the ring gear and revolve at the same time. The part of
the planet gear in contact with the sun gear is travelling twice as fast as the axis of
the planet gear. As a result the planetary mechanism itself gives a 2:1 speed reduction
which may further be increased by reducing the size of the sun gear. The torque is
shared between the three planets and applied at three different places on the sun gear
and the ring gear. This makes the epicyclic gearbox ideal for the final stage of rotor
drive where the torque is greatest. Figure 6.33 shows a Chinook gearbox, in which the
final stage epicyclic gear ring can be seen.
The gear teeth slide over each other, and metal-to-metal contact is prevented by
special gear oil known as extreme pressure (EP) oil. The viscosity of the oil means that
it is not squeezed out from between the teeth by the contact pressure. The sliding
then takes place in the shearing of the oil film. Shearing viscous oil produces heat and
the gearbox casing may be made with fins to dissipate the heat. In large machines an
oil cooler may be fitted, perhaps with a fan and oil pump. In some machines the oil