Chapter 30 Engine Starting Systems 507
Pinion Gear
A pinion gear is a small gear mounted on the
armature shaft that engages a large ring gear on the
engine fl ywheel. It moves into and meshes with the
fl ywheel ring gear any time the starter is energized.
Most pinion gears are made as part of a pinion drive
assembly (pinion gear, overrunning clutch, and
housing). The pinion drive assembly slides over one
end of the armature shaft, Figures 30-8 and 30-9.
A starting motor must produce high torque to
start an engine. The pinion gear is much smaller
than the fl ywheel ring gear. Therefore, the starting
motor armature turns at a relatively high speed.
This helps prevent stalling of the motor. The differ-
ence in gear size also increases turning force applied
to the crankshaft.
The overrunning clutch locks the pinion gear in
one direction and releases it in the other. This allows
the pinion gear to turn the fl ywheel ring gear for
starting. It also lets the pinion gear freewheel when
the engine begins to run. Refer to Figure 30-10.
Small spring-loaded rollers are located between the
pinion gear collar and the clutch shell, Figure 30-11.
These rollers wedge into the notches in the shell in
Flywheel ring
gear
Engagement
mechanism
Starter
terminal
Starter mounting
flange
Overrunning
clutch
Pinion
gear
Clutch
housing
Shell
Roller
retainer
Roller
spring
Bushing
Pinion gear
Roller
Engagement
flange
Meshing
spring
Mechanical
connection in
this direction
Tapered notch
Pinion
gear
Roller
Pinion collar Clutch shell
Roller spring
Overrunning
clutch
Armature
Armature shaft
Brush Field
Pinion
gear
Figure 30-8. The starter pinion gear meshes with the large
ring gear on the engine fl ywheel.
Figure 30-10. Study the construction of the starter overrunning
clutch. It is simply a one-way clutch. It locks the fl ange to the pinion
gear in one direction and releases in the other direction. (Ford)
Figure 30-11. This cutaway shows how rollers fi t in the clutch.
Rollers jam and lock units together one way. Going the other way,
they release, allowing the pinion to freewheel. (Robert Bosch)
Figure 30-9. The pinion gear clutch assembly slides onto one
end of the armature shaft. Note the other parts. (Robert Bosch)