Polaris 2007 Sportsman 450 EFI, Service Manual

Page: 196 / 406

6.6
CLUTCH SYSTEM
PVT SYSTEM
PVT Operation Overview
The Polaris Variable Transmission (PVT) consists of three
major assemblies: 1) The Drive Clutch; 2) The Driven Clutch;
and 3) The Drive Belt. The internal components of the drive
clutch and driven clutch control engagement (initial vehicle
movement), clutch upshift and backshift. During the
development of a Polaris ATV, the PVT system is matched first
to the engine power curve; then to average riding conditions and
the vehicle's intended usage. Therefore, modifications or
variations of components at random are never recommended.
Proper clutch setup and careful inspection of existing
components must be the primary objective when
troubleshooting and tuning.
Drive Clutch Operation
Drive clutches primarily sense engine RPM. The two major
components which control its shifting function are the shift
weights and the coil spring. Whenever engine RPM is
increased, centrifugal force is created, causing the shift weights
to push against rollers on the moveable sheave, which is held
open by coil spring preload. When this force becomes higher
than the preload in the spring, the outer sheave moves inward
and contacts the drive belt. This motion pinches the drive belt
between the spinning sheaves and causes it to rotate, which in
turn rotates the driven clutch.
At lower RPM, the drive belt rotates low in the drive clutch
sheaves. As engine RPM increases, centrifugal force causes the
drive belt to be forced upward on drive clutch sheaves.
Driven Clutch Operation
Driven clutches primarily sense torque, opening and closing
according to the forces applied to it from the drive belt and the
transmission input shaft. If the torque resistance at the
transmission input shaft is greater than the load from the drive
belt, the drive belt is kept at the outer diameter of the driven
clutch sheaves.
As engine RPM and horsepower increase, the load from the
drive belt increases, resulting in the belt rotating up toward the
outer diameter of the drive clutch sheaves and downward into
the sheaves of the driven clutch. This action, which increases
the driven clutch speed, is called upshifting.
Should the throttle setting remain the same and the vehicle is
subjected to a heavier load, the drive belt rotates back up toward
the outer diameter of the driven clutch and downward into the
sheaves of the drive clutch. This action, which decreases the
driven clutch speed, is called backshifting.
In situations where loads vary (such as uphill and downhill) and
throttle settings are constant, the drive and driven clutches are
continually shifting to maintain optimum engine RPM. At full
throttle a perfectly matched PVT system should hold engine
RPM at the peak of the power curve. This RPM should be
maintained during clutch upshift and backshift. In this respect,
the PVT system is similar to a power governor. Rather than vary
throttle position, as a conventional governor does, the PVT
system changes engine load requirements by either upshifting or
backshifting.
WARNING
All PVT maintenance or repairs should be
performed only by a certified Polaris Master Service
Dealer (MSD) technician who has received the
proper training and understands the procedures
outlined in this manual. Because of the critical
nature and precision balance incorporated into the
PVT components, it is absolutely essential that no
disassembly or repair be made without factory
authorized special tools and service procedures.