Centrifugal Clutches: Design and Function

Research Project
Centrifugal clutches
Project Proposal:
A centrifugal clutch is a clutch that uses centrifugal force to connect two concentric shafts, with
the driving shaft nested inside the driven shaft.
The input of the clutch is connected to the engine crankshaft while the output may drive a shaft,
chain, or belt. As engine revolutions per minute increase, weighted arms in the clutch swing
outward and force the clutch to engage. The most common types have friction pads or shoes
radially mounted that engage the inside of the rim of a housing. On the center shaft there are an
assorted number of extension springs, which connect to a clutch shoe. When the center shaft
spins fast enough, the springs extend causing the clutch shoes to engage the friction face. It can
be compared to a drum brake in reverse. This type can be found on most home built karts, lawn
and garden equipment, fuel-powered model cars and low power chainsaws. Another type used in
racing karts has friction and clutch disks stacked together like a motorcycle clutch. The weighted
arms force these disks together and engage the clutch.
When the engine reaches a certain speed, the clutch activates, working somewhat like a
continuously variable transmission. As the load increases, the speed drops, disengaging the
clutch, letting the speed rise again and reengaging the clutch. If tuned properly, the clutch will
tend to keep the speed at or near the torque peak of the engine. This results in a fair bit of waste
heat, but over a broad range of speeds it is much more useful than a direct drive in many
applications.
The centrifugal clutch offers many advantages in motor and engine drive applications. Utilizing
the centrifugal clutch enables the selection of normal torque motors for running loads rather than
the selection of high torque motors for starting loads. The aim of this project is to be describing
the torque carrying capacity at different speed which using in transmitting power. The entire
work based on conventional design with Frodo lining, driving shaft, spring , shoe , spider, cover
plate, driven shaft. The detail design for various speed is made in the centrifugal clutch to select
an operating range of speed under which the application are designed for torque transmission. In
this project the design calculation are worked out analytically. Along with this we have
introduced a compliant clutch.
Compliant clutch is made from polypropylene material and there is no anyone connecting parts.
it has rigid body design since revolute joints are replaced by flexible segment. The potential
energy store in flexible segment can replace springs and reduction in revolute joints reduces
problem with backlash and wear. The compliant clutch includes rigid body mechanism through
dimensional synthesis by graphical method. A mechanism is evaluated and synthesized specific
force-deflection relationship. In many applications, compliant mechanism can maintain or even
improve performance relative to conventional rigid body designs. It is cost benefitted. Also
conventional clutch is expensive rather than compliant clutch.

BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to an engine controller for a vehicle equipped with an automatic
centrifugal clutch, such as a snowmobile, ATV (all-terrain vehicle), motorcycle, etc., including
an idle speed control (abbreviated hereinbelow as ISC) device disposed in the engine intake
system for adjusting the amount of intake air by a throttle valve.
Description of the Prior Art
Concerning engine intake systems for regulating the amount of intake air based on control of the
opening degree at the throttle valve of an engine mounted on vehicles, such as snowmobiles,
having an automatic centrifugal clutch, there are those that adopt ISC devices for controlling the
idle speed by adjusting the amount of bypass air.
In a vehicle including such an ISC device, an ECM (electronic control module) is provided for
the ISC device, so as to control the amount of intake air at the time of the throttle valve being
fully closed, for each of the following modes (as to the prior art relating to the ISC device, refer
to Japanese Utility Model Application Laid-open Sho 64 No. 34441 (FIG. 1 and the full text of
the specification)).
In ISC, in startup mode, control at a start of the engine is performed by adjusting the amount of
bypass air to the controlled amount for start in accordance with the cooling water temperature
plus corrections for altitude and other factors.
In the complete explosion mode other than the startup mode, the basic control amount is
corrected in accordance with the cooling water temperature and at the same time the idle speed is
controlled by regulating the amount of bypass air.
SUMMARY OF THE INVENTION
The present invention has been devised in view of the above conventional problems, it is
therefore an object of the present invention to provide an engine controller of an engine unit with
an idle speed control (ISC) device for use in a vehicle including an automatic centrifugal clutch,
the controller capable of preventing unintentional increase in engine speed even if the ISC device
breaks down and capable of preventing excessive effect of engine brake resulting from fuel
cutoff or ignition cutoff hence preventing degradation of driver's sensation during traveling.
In order to achieve the above object, the present invention is configured as follows:
In accordance with the present invention, an engine controller for a vehicle using an automatic
centrifugal clutch device for power transmission, is characterized in that an idle speed control
means is provided for an engine intake system that adjusts the amount of intake air by
implementing control of the opening degree of a throttle valve, and the engine controller
comprises: a full-close state detecting means for detecting the full-close state of the throttle
valve;
an engine speed detecting means for detecting the engine speed; and an engine halt controlling
means for implementing engine halt control based on the halt control starting conditions that the
detected engine speed is equal to or greater than a first set rotational frequency, which is set to be

equal to or lower than the clutch engagement rotational frequency of the automatic centrifugal
clutch device, and that the full-closed state of the throttle valve is detected.
INTRODUCTION:
A CENTRIFUGAL CLUTCH:
A clutch is a machine member used to connect the driving shaft to a driven shaft, so that the
driven shaft may be started or stopped at will, without stopping the driving shaft. A clutch
provides an interruptible connection between two shafts. The centrifugal clutch is usually used
into motor pulley. It consists of number of shoe on the inside of a rim of pulley. The outer
surface of pulley is covered with friction material. These shoes which can move radially in
guides are held against the boss on the driving shaft by means of springs.The spring exert a
radialy inward force which assumed to be constant.
The weight shoes when revolving cause it to exert a radialy outward force (centrifugal force).The
magnitude of centrifugal force depeded on speed at which shoes is revolving. A little
consideration show that when centrifugal force is less than the spring force, the shoes remain
same position as when driving shaft was stationary, but when centrifugal force is equal to spring
force, spring is floating.
Figure: 1 Centrifugal clutch
When centrifugal force exceed the spring force, the shoe moves outward and comes in contact
with driven member and press against it. The force with which the shoe press against the driven
member is the difference of the centrifugal and spring force. The increase of speed causes the
shoe to press harder and enables to be transmitted.
COMPLIANT CLUTCH:
Compliant mechanisms are mechanisms that obtain some or all of their motion through the
deflection of their members. In a compliant mechanism, a single flexible link often replaces two
or more rigid links of an equivalent rigid-body mechanism. This decreases the mechanism’s part
count, wear points, and backlash. In the mechanism the centrifugal force is utilised to deflect the
members and get friction on rotating drum to synchronise motion.