Primary Sheave Assembly - Yamaha MM700A Manual

(Primary)
Figure 2.
The shift sequence is determined by a combination of
engine torque and engine rpm. When track resistance (or
load) increases, such as when going up hill, the sheaves
change the reduction ratio. Engine q m will remain nearly
the same but the vehicle's speed drops. When track resis-
tance decreases, the sheaves automatically shift toward a
higher ratio. The engine rpm remains the same but the ve-

PRIMARY SHEAVE ASSEMBLY

Major components of the primary sheave assembly are
the sliding sheave half, fixed sheave half, weight levers,
weight ramps, primary spring and V-belt. The V-belt con-
nects the drive and secondary sheaves.
Fixed and Sliding Sheave Halves
The smooth tapered surfaces of the sheaves are preei-
sion machined to match the V-belt gripping surface (Fig-
ure 3) and the sheaves are carefully balanced to prevent
vibration.
The primary sheave assembly (A, Figure 4) is mounted
on the left end of the engine crankshaft. When the engine
is at idle or stopped, the fixed and sliding halves of the
sheave are held apart by the primary spring. At slow idle
speed or when the engine is stopped, the groove should be
High-speed
Low-speed
Driven sheave
(Secondary)
wide enough for the V-belt to drop down between the
sides of the sheave. There is no engagement because the
width of the belt is less than the space between the sides of
the primary sheave.
At low speed, the belt is located as shown at the
low-speed position in Figure 2. When engine speed in-
creases from idle, centrifugal force causes the weight le-
vers mounted on the sliding half of the primary sheave to
swing out. When centrifugal force of the weights (1 0, Fig-
ure 5) is sufficient to overcome the pressure of the pri-
mary spring (24, Figure 5), the sliding half of the sheave
( 5 , Figure 5) is moved closer to the fixed half (3, Figure
5). This movement narrows the groove between the
sheave halves until the sides of the sheave grip the belt.
The point at which the sheave grips the belt is called the
engagement rpm. Refer to Table 1 for the recommended
engagement rpm for specific models.
As engine rpm increases, centrifugal force causes the
weights (1 0, Figure 5) of the primary sheave to swing fur-
ther out against the rollers (21, Figure 5) and force the
sliding half of the sheave closer to the fixed half. As the
groove of the primary sheave becomes narrower, the
V-belt is forced upward in the groove toward the outer
edge of the sheave. Since the V-belt is a fixed length and
width, the belt is forced to move deeper into the groove of
the secondary sheave as indicated by the high-speed posi-
tion of the belt in Figure 2.
Though not part of the primary sheave, it should be
noted that the secondary spring of the secondary sheave
forces the sides of that sheave together. As the belt wedges
/