100
Two-Stroke Engines
Copyright Goodheart-Willcox Co., Inc.
A direct-drive PTO is a type of PTO in which
rotational output from the engine has a constant
mechanical connection to the work-performing
end of the equipment. For example, this type of
PTO is used in a string trimmer. A drive shaft
connects the engine to the string trimming
head (the part of the working end that holds the
string). The drive shaft is a solid metal or flex-
ible shaft that transmits the engine rotation to
the working end of the equipment. One end of
the shaft is connected to a crankshaft adapter,
Figure 5-28, and the other end is connected
to the string trimming head. This direct-drive
process can be identified by disconnecting the
spark plug and pulling the starter rope while
observing the string trimming head for rota-
tion. Because of the direct-drive PTO, the string
trimming head rotates whenever the engine’s
crankshaft is rotating, even if the engine is
being turned over by hand.
A clutch-drive PTO is a type of PTO with a
clutch between the engine output and the work-
ing end of the equipment. The clutch is used to
connect and disconnect power. The clutch typ-
ically used in two-stroke equipment is a cen-
trifugal clutch. The centrifugal clutch uses the
rotation of the crankshaft to apply a centrifu-
gal force to the clutch components, causing the
clutch to engage. When the clutch is engaged,
a drive component transmits power to a driven
component.
A centrifugal clutch is also called an expand-
ing shoe clutch. The “expanding shoe” part of
the clutch is connected to the crankshaft and
consists of the shoes and springs that rotate with
crankshaft rotation. The clutch shoes are held
A B
Starter pulley Pawls Notched vane
Lugs Notched vane
Goodheart-Willcox Publisher
Figure 5-27. In this rewind starter assembly, the pawls are mounted on the flywheel. When the starter
rope is pulled, lugs on the starter pulley engage the pawls on the flywheel. As the flywheel speeds up,
centrifugal force slings the pawls outward into the notches in the flywheel vanes. A—Starter pulley.
B—Flywheel and pawls.
Square hole
for drive shaft
PTO adapter
Goodheart-Willcox Publisher
Figure 5-28. The square-hole adapter is attached
to the engine. The drive shaft end is square and is
driven by the adapter.