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The illustration below shows how the engage-
ment rpm can be changed by varying the ramp
profile. For example, ramp
"X"
allows the clutch
to engage at 3500 rpm. Notice the point at which
the roller begins its travel up the angled incline of
the ramp. On ramp "Y", the angle of incline at this
point of engagement is steeper, making it
necessary for the engine to develop higher rpm
before the roller begins its climb up the ramp.
Thus, the engagement speed with ramp "Y"
would be more than 3500 rpm. On ramp "Z", the
angle of incline at the point of engagement is less
than it is on ramp
"X".
Thus, it will be easier for
centrifugal force to cause the roller to move up
ramp "Z" and therefore, the clutch will engage at
less than 3500 rpm.
Fig. 6-3
iJ
-
A M PY
ENGAGEMENT
POINT
ROLLER
0725·233
Shown below are three different ramp profiles
and the position of the roller when the clutch has
completed its shift pattern. Note the three
different angles of incline. For example, the
engine is running at 6500 rpm when the drive
clutch, with ramp
"X",
has full shifted. On ramp
"Y", the profile is cut back (less angle of incline)
toward the top. It is easier for centrifugal force to
move the roller up ramp "Y" and the engine speed
required is less than 6500 rpm. On the other hand,
ramp "Z" is not cut back as far as ramp
"X".
The
engine will have to run at a speed greater than
6500 rpm to move the roller to the top of the ramp
against spring pressure and complete the shift
pattern.
Fig. 6-4
0725-232
II
NOTE: The only change made in these
examples was to the ramp profile. The
spring rate and the weights were left constant.
Ramps are designed to match the shifting
characteristics of the drive clutch to the torque
and horsepower curves of the engine. Thus, with
several different types of engines used in
snowmobiles, ramp profiles vary accordingly.
83
The illustration below shows three different
ramps. Ramp "A" was made for an engine that
develops maximum horsepower at low rpm
(approximately 6000). With its relatively shallow
angle of incline, this ramp allows the clutch to
shift through its pattern faster, pulling the rpm of
the engine down. Such a ramp would not work on
a high rpm race engine.
Ramp "8" was made specifically for a high rpm
race engine. The relatively steep angle of incline
causes high rpm engagement and high rpm
throughout the pattern. This is exactly what a
race engine requires as it has very little torque
and needs a high engagement speed.
Ramp "C", on the other hand, was designed for a
high rpm engine that requires a low rpm engage-
ment. After the low rpm engagement, the ramp
incline increases, causing the engine to run
through the balance of its shift pattern at high
rpm.
Fig. 6-5
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Driven Clutch
Theory
Operation
The driven clutch is connected to the drive shaft
and track of the machine through the chain case
and drive chain. Its purpose is to sense the load
on the machine and keep enough tension on the
drive belt to keep from slipping. The driven clutch
is a very important element in the drive system,
and, unless it is doing its proper job, the machine
will not perform up to its capabilities.
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