The principle of fluid drive is shown in figure 2-7.
As two fans face each other, the speed of rotation of one
fan makes the other fan rotate. When the speed of one
fan is changed from medium to low, power is lost at low
speeds; but, if the fan speed increases from medium to
high, the speed of the driven fan picks up.
Torque Converter
The torque converter is a form of and has replaced
the fluid coupling. Most automatic transmissions used
in automotive and construction equipment have torque
converters.
The torque converter consists of three parts: the
pump (driving member), the turbine (driven member),
and the stator (reaction member), all with curved vanes.
The stator is located between the load and the power
source to act as a fulcrum and is secured to the torque
converter housing. Figure 2-8 shows a cutaway view of
a torque converter and the directional flow of oil. The
pump throws out oil in the same direction in which the
pump is turning. As the oil strikes the turbine blade, it
forces the turbine to rotate, and the oil is directed toward
the center of the turbine. Then the oil leaves the turbine
and moves in a direction opposite to that of the pump.
As the oil strikes the stator, it is redirected to flow in the
same direction as the pump to add its force to that of the
pump. Torque is multiplied by the velocity and direction
given to the oil by the pump, plus the velocity and
direction of the oil entering the pump from the stator.
Planetary Gears
Automatic transmissions use a system of planetary
gears to enable the torque from the torque converter to
be used efficiently.
Figure 2-7.—Principle of fluid drive.
Figure 2-8.—Torque converter.
Planetary units are the heart of the automatic
transmission. The four parts that make up the planetary
gear system are as follows: the sun gear, the ring (or
internal) gear, the planet pinions, and the planet carrier.
The sun gear is the center of the system. The term
planet fits these pinions and gears, because they rotate
around the sun gear, as shown in figure 2-9. The ring
gear, or internal gear, is so-called because of its shape
and internal teeth.
An advantage of the planetary gear system is that it
is compact. Additionally, in the planetary system more
teeth make contact to carry the load. The reason for this
is that each gear of the planetary system usually meshes
with at least two other gears. Because the gears are
always in mesh, none of the teeth are damaged as a result
of teeth clashing or a partial mesh. However, the major
advantage of the planetary system is the ease of shifting
gears. Planetary gears, set in automatic transmissions,
are shifted without any special skill required by the
operator.
2-6
