Figure 13-7.-Constant-mesh transmission assembly—sectional view.
All transmissions do not have four speeds forward,
and all do not have the same gear reductions at the
various speeds. Passenger cars, for example, usually
have only three forward speeds and one reverse speed.
Their gear ratios are about 3 to 1 in both low and reverse
gear combinations. You must remember, the gear
reduction in the transmission is only between the engine
and the propeller shaft. Another reduction gear ratio is
provided in the rear axle assembly. If you have a
common rear axle ratio of about 4 to 1, the gear
reduction from the engine of a passenger car to the rear
wheels in low gear would be approximately 12 to 1. In
high gear the ratio would be 4 to 1 since the transmission
would have no reduction of speed.
CONSTANT MESH TRANSMISSION
To eliminate the noise developed in the old spur-
tooth type of gears used in the sliding gear transmission,
the automotive manufacturers developed the constant-
mesh transmission that contains helical gears.
In this type of transmission, certain countershaft
gears are constantly in mesh with the main shaft gears.
The main shaft meshing gears are arranged so that they
cannot move endwise. They are supported by roller
bearings that allow them to rotate independently of the
main shaft (figs. 13-7 and 13-8).
In operation, when you move the shift lever to third,
the third and fourth shifter fork moves the clutch gear
13-7
