The starting motor is equipped with an overrunning
The overrunning clutch drive starting motor
clutch drive mechanism (fig. 5-5) that transmits the
provides positive engaging and disengaging of the
power from the motor to the engine. The drive
starting motor drive pinion and the flywheel ring gear.
mechanism performs the following functions:
This drive mechanism uses a shift lever that slides the
clutch and drive pinion assembly along the armature
1. Engages the drive pinion with the flywheel for
shaft so that it can be engaged and disengaged with the
cranking the engine. When the starting motor is
flywheel ring gear. The clutch transmits cranking
operated, the drive mechanism causes the drive
torque from the starting motor to the engine flywheel
pinion to mesh with the teeth of the flywheel ring
but permits the pinion to overrun the armature after the
gear, thereby cranking the engine.
engine starts. Thus power can be transmitted through
2. Provides a gear reduction between the drive
the overrunning clutch in only one direction. This
pinion and the flywheel. The gear reduction is
action protects the starting motor from excessive speed
necessary because the starting motor must rotate
during the brief interval that the drive pinion remains
at a relatively high speed with respect to the
with the flywheel ring gear after the engine has started.
engine cranking speed to produce sufficient
When the shift lever is operated, the clutch
output power to crank the engine. Thus a gear
assembly is moved along the armature shaft until the
reduction ratio of 15 to 1 will permit the starting
pinion engages with the flywheel ring gear. The
motor to rotate at 1,500 rpm while cranking the
starting-motor contacts are closed when the movement
engine at 100 rpm.
of the shift lever is completed, causing the armature to
3. Disengages the drive pinion and the flywheel
rotate, and thereby cranking the engine.
after the engine is started As soon as the engine
Once the engine has started the speed of rotation of
is started, the drive mechanism causes the drive
the engine flywheel causes the pinion to spin faster than
pinion to disengage from the flywheel. The
the armature of the starting motor. This action causes
engine speed increases immediately and may
the pinion to spin independently or overrun. When the
soon attain speeds up to 1,000 rpm. If the drive
starting-motor switch is opened, the shift lever releases,
pinion is allowed to remain in mesh with the
causing the drive spring to pull the overrunning clutch
flywheel, the engine would drive the starting
motor at speeds up to 15,000 rpm, resulting in
drive pinion out of engagement with the engine flywheel
serious damage to the motor.
Figure 5-5.--Starting motor with an overrunning clutch drive and a solenoid-operated switch.