Page 200 - Mechatronics with Experiments

186 MECHATRONICS
axis, w . As indicated above,
32
w 32 = w − w 2 (3.305)
3
w 32 =−20 − 20 rpm (3.306)
=−40 rpm (3.307)

which is the angular velocity of the planetary gear about its own rotation axis.
Then the net gear ratios between sun gear and carrier (N ), and sun gear and planetary
21
gear (N )
31
w 2 20 1
N 21 = = = (3.308)
w 1 100 5
w 3 −20 1
N 31 = = =− (3.309)
w 1 100 5
The absolute gear ratios could also be calculated for this case using the equations
N 1 25 1
N 21 = = = (3.310)
N + N 4 25 + 100 5
1
( ) ( )
N 1 N + N 3 25 25 + 50 1
1
N 31 = − 1 = − 1 =− (3.311)
N 3 N + N 4 50 25 + 100 5
1
In transmission applications, we could obtain two different gear ratios from this single
planetary gear stage: use a brake (B1) to lock the ring gear, and clutch (C1) to engage the
1
carrier to the output shaft, hence obtain a gear ratio of between the input and output
5
shafts. Alternatively, use a brake (B1) to lock the ring gear, and a second clutch (C2) to
1
engage the planetary gear to the output shaft to obtain a gear ratio of − between input and
5
output shafts.
3.8.4 Torque Converter
Figure 3.27 shows an automatic transmission based on a planetary gear mechanism. The
input section of an automatic transmission has a torque converter which functions as a
hydrodynamic flexible coupling between the engine and the transmission (Figure 3.29a and
1
b) . It can also be viewed as an automatic clutch with slip capability. As a result, it provides
a damping to the power transmission line. Torque converter provides a variable gear ratio
between its input–output shafts. The main disadvantage of a torque converter is its lower
power conversion efficiency compared to direct drive. Because the power is transmitted by
the moving fluid, it creates heat, hence the losses. In addition, a cooling system may be
needed to remove the heat.
The torque converter has three main components:

1. The impeller, which is connected to the input shaft of the torque converter, which is
also the engine output shaft. The impeller converts the mechanical energy of its input
shaft to the hydrodynamic energy of the coupling fluid.
2. The turbine, which is connected to the output shaft of the torque converter, which is
connected to the input shaft of the transmission’s planetary gear set, and converts the
hydrodynamic power of the fluid back into mechanical rotational motion of its shaft.
3. The stator is a component that redirects the coupling fluid between the turbine and
the impeller, typically stationary, however in some cases it can be free-wheeling or

1 This sub-section is based on Lectures by Dr. Richard Ingram.