The following sections
describe the power flow paths in a typical four-speed manual
The input shaft rotates at
engine speed whenever the clutch is engaged. The clutch gear is mounted
on the input shaft and rotates with it. The clutch gear meshes with the
counter gear, which rotates around the contershaft.
The counter gear transfers
power to the speed gears on the mainshaft. However, since speed gears
one, two, three, and four are not locked to the mainshaft when the
transmission is in neutral, they cannot transfer power to the mainshaft.
The mainshaft does not turn, and there is no power output to the
All gear changes pass
through the neutral gear position. When changing gears, one speed gear
is disengaged, resulting in neutral, before the chose gear is engaged.
This is important to remember when diagnosing hard-to-shift problems.
Power or torque flows
through the input shaft and clutch gear to the counter gear. The counter
gear rotates. The first gear on the cluster drives the first speed gear
on the mainshaft. When the driver selects first gear, the first/second
synchronizer moves to the rear to engage the first speed gear and lock
it to the mainshaft, the first speed gear drives the main (output)
shaft, which transfers power to the driveline. A typical first speed
gear ratio is 3:1 (three full turns of the input shaft to one full turn
of the output shaft). So, if the engine torque entering the transmission
is 220 foot-pounds it is multiplied three times to 660 foot-pounds by
the time it is transferred to the driveline.
When the shift from first
to second gear is made, the shift fork disengages the first/second
synchronizer from the first speed gear and moves it until it locks the
second speed gear to the mainshaft. Power flow is still through the
input shaft and clutch gear to the counter gear. However, now the second
counter gear on the cluster transfers power to the second speed gear
locked on the mainshaft. Power flows from the second speed gear through
the synchronizer to the mainshaft (output shaft) and driveline.
In second gear, the need
for vehicle speed and acceleration is greater than the need for maximum
torque multiplication. To meet these needs, the second speed gear on the
mainshaft is designed slightly smaller than the first speed gear. This
results in a typical gear ratio of 2.2:1, which reflects a drop in
torque and an increase in speed.
When the shift from second
to third gear is made, the shift fork returns the first/second
synchronizer to its neutral position. A second shift fork slides the
third/fourth synchronizer until it locks the third speed gear to the
mainshaft. Power flow now goes through the third gear of the counter
gear to the third speed gear, through the synchronizer to the mainshaft,
Third gear permits a
further decrease in torque and increase in speed. As you can see, the
third speed gear is smaller than the second speed gear. This results in
a typical gear ration of 1.7:1.
In fourth gear, the
third/fourth synchronizer is moved to lock the clutch gear on the input
shaft to the mainshaft. This means power flow is directly from the input
shaft to the mainshaft (output shaft) at a gear ratio of 1:1. This ratio
results in maximum speed output and no torque multiplication. Fourth
gear has no torque multiplication because it is used at cruising speed
to promote maximum fuel economy. The vehicle is normally downshifted to
lower gears to take advantage of torque multiplication and acceleration
when passing slower vehicles or climbing grades.
In reverse gear, it is
necessary to reverse the direction of the mainshaft (output shaft). This
is done by introducing a reverse idler gear into the power flow path.
The idler gear is located between the countershaft reverse gear and the
reverse speed gear on the mainshaft. The idler assembly is made of a
short drive shaft independently mounted in the transmission case
parallel to the countershaft. The idler gear may be mounted near the
mid-point of the shaft.
In other transmissions,
there are two separate idler gears, one near each end of the shaft. The
reverse speed gear may be an independent gear located at the rear of the
mainshaft. The reverse speed gear is actually the external toothe sleeve
of the first-second synchronizer.
When reverse gear is
selected, both synchronizers are disengaged. In the transmission, the
shifting linkage moves the reverse idler gear into mesh with the
first/second synchronizer sleeve. Power flows through the input shaft
and clutch gear to the countershaft. From the countershaft, it passes to
the reverse idler gear, where it changes rotational direction. It then
passes to the mainshaft and driveline. In the transmission, the reverse
slides the reverse speed gear forward until it meshes the gear idler
gear. Power flows from the input shaft and clutch gear to the
countershaft. It then passes through the front idler gear (direction
change), rear idler gear, rear speed gear (direction change), and out
through the mainshaft to the driveline.
As discussed earlier, when
a large gear drives a smaller gear, an overdrive condition occurs. The
large driving gear may rotate three-quarters of a revolution while the
smaller driven gear rotates one full turn. Overdrive permits an engine
speed reduction at higher cruising speeds. Because the engine (rpm) is
running slower, fuel economy is greater. However, engine torque also
drops, so power is sacrificed for better mileage.
Overdrive gears are usually
located in the transmission housing. The gear ratio of this fifth gear
is 0.87:1. The reverse gear train is designed with spur-type gearing.
Unlike the four-speed transmission covered earlier, reverse shifting in
this transmission is controlled by a synchronizer. As you can see, this
synchronizer is also used to control engagement of fifth gear overdrive.
Power flows for first,
second, third, fourth, and fifth gears are similar to those in the
four-speed transmission described earlier. In each case, a shift fork
moves the appropriate synchronizer to lock the required speed gear to
the mainshaft. Power flows through the input shaft to the counter gear,
and back through the mainshaft to the driveline.
When reverse gear is
selected, the fifth/reverse synchronizer is moved by the fifth/reverse
shift rail and shift fork. The synchronizer locks the reverse gear to
the mainshaft. The clockwise rotation of the input shaft and clutch gear
drive the counter gear in a counterclockwise direction. The reverse
idler gearing is driven clockwise by the counter gear. The clockwise
turning reverse idler gear drives the reverse speed gear, synchronizer,
mainshaft, and driveline counterclockwise. The vehicle can then back up.