FIELD OF INVENTION

This invention relates to design of Clutch for motor vehicles, more particularly, two wheeler scooters and motorcycles.

BACKGROUND OF THE INVENTION

Two wheeler scooters and motorcycles are conventionally powered by gasoline engine equipped with a manual, sequential speed change gear box transmission to extend the driving speed range of the vehicle. A clutch is an essential part of the power transmission without which it will be almost impossible to drive the vehicle. Placed upstream of the gear box, the clutch allows full or partial disconnection of the engine with the gearbox and the vehicle's driving wheel which is necessary at very low vehicle speeds to prevent engine shutdown. During the course of driving, depending on the vehicle speed, the rider needs to shift to an appropriate gear to maintain the engine RPM within its operating range and to derive good performance in terms of fuel economy and drivability. Declutching, i.e. disengaging the drive between the engine and gearbox by actuating the clutch, is necessary during gear shifting also, to avoid jerk and possible internal damage.

Geared two wheelers commonly use conventional axial engagement friction disc clutches. These clutches require certain force for de clutching causing rider fatigue.

With no requirement of declutching and gear shifting, scooters with Continuously Variable Transmission, or -'CVT", as they are commonly called, offer an alternate solution to the problem of driving fatigue due to frequent actuation of clutch & gear shifting. However. CVTs employ an arrangement including Belt Drive, which is prone to slippage and also has significantly lower power transmission efficiency as compared to gear drives. The fuel economy of CVT two wheeler is inferior to the conventional geared two wheeler. With rapid developments in the recent years - expansion of cities, improved road infrastructure & connectivity of rural areas- the overall commuting levels have increased exponentially. Two wheelers, with their unique & distinct advantages, continue to be the most preferred and popular means of personal transport. In the emerging scenario, it is imperative that the issues such as of driving fatigue deserve better solution without sacrificing the excellent drivability and fuel economy offered by geared transmission. The present innovation addresses this requirement.

The technology of Centrifugal Clutch is a well established art. A typical Centrifugal Clutch is normally in a state of disengagement and automatically engages beyond a threshold rotational speed, by virtue of centrifugal action. As a general rule, the torque transmission capacity of Centrifugal Clutches increases with the rotational speed. These characteristics make them a suitable choice for variety of applications. However, for their adoption in two wheelers with geared transmission, the following specific issues need to be addressed - - With geared transmission declutching is necessary during gear shifting.

- Kick for manual cranking of the engine is a standard essential feature of two wheelers. In almost all kick mechanisms, clutch is an intermediate connecting link between the kick shaft and the crank. With conventional Centrifugal Clutch, this drive from kick to crank will need to be re- established.

US Patent 4106605 of August 15, 1978 describes a Centrifugal Clutch arrangement for vehicular engine. However, the present invention offers completely different solutions for addressing the key issues as explained above. Those conversant with the art, would readily appreciate the improvisation over the sighted patent in terms of simplicity, cost effectiveness and compactness of the design as per the present invention.

OBJECT OF THE INVENTION The object of the invention is to significantly reduce the driver's fatigue caused by effort in frequent de-clutching while retaining the performance and fuel economy of a two wheeler equipped with gearbox transmission.

An important target of the invention is to design a compact system which can be accommodated in the two wheeler engine and preferably with no or minimal changes in the interfacing parts and mechanisms such as the drive train and clutch actuation linkages.

SUMMARY OF THE INVENTION

A Multi Mode Centrifugal Clutch has been designed which can be used in two wheeler engines with gearbox transmission.

In addition to automatic engagement and disengagement depending on the engine speed, the Multi Mode Centrifugal Clutch, according to this invention, can be manually disengaged during gear shifting and, in the disengaged state when the engine is not running, allows reverse power flow to facilitate kick starting of the engine.

The accompanying diagrams and the description which follows explain a sample design as per the invention. Readers will appreciate that the sample design is only one example for the purpose of illustration and does not limit the scope of the invention. BRIEF DESCRIPTION OF THE DRAWINGS

Figures 1 is perspective view showing the Multi Mode Centrifugal clutch assembly mounted on the engine crank shaft.

Figure 2 is a sectioned view of the Multi Mode Centrifugal clutch assembly showing its internal construction.

Illustrated in Figure 3 & 4 is the coupling arrangement of the drive and driven plates of the clutch with their respective mating parts.

Figure 5 shows the guided path of Roller and defines its extreme positions.

Figure 6 shows the Multi Mode Centrifugal clutch in free and engaged conditions.

Figure 7 explains the forces which develop interfacial pressure between the friction plates. It includes free body diagram of the Roller, defining the relationship between the radial centrifugal force acting on the roller, the axial pressure between the drive and driven friction plates, and the force of the Compression Springs.

Figure 8 shows the Multi Mode Centrifugal Clutch in manual de-clutching mode.

Figure 9 is a perspective view of the square tread coupling explaining the axial trust exerted by the mating parts. Figure 10 shows the Multi Mode Centrifugal Clutch in automatic engagement during kick starting of the engine. DETAILED DESCRIPTION

The construction of the Multi Mode Centrifugal Clutch as per the present invention is shown in detail in Figure 2. Rotor (1 ) is rotationally and axially coupled with the crank shaft of the engine as shown. On the other side, the Rotor (1) is rotationally coupled to the Clutch Casing (2) by virtue of engagement of its radial lugs with the axial slots of the Clutch Casing as shown more clearly in Figure 3. Stepped pins (3) and Compression Springs (4) define a normal axial position and allow a restricted movement of the Clutch Casing (2) with respect to the Rotor ( I ). A set of Friction Drive Plates (5) are placed in the Clutch Casing (2) as shown. These Plates also have radial projecting lugs, similar to Rotor (1 ), which also are engaged in the axial slots of the Casing and are therefore, rotationally coupled with it. Internal Circlip (6) prevents the Friction Drive Plates from coming out of the Clutch Casing. Thus, the Rotor ( 1 ), Clutch Casing (2) and Friction Drive Plates (5) rotate together with the crank shaft and constitute the driving half of the clutch.

Referring again to Figure 2, Primary Drive Pinion (8) is assembled on the Rotor ( 1 ) such that it is axially restrained but is free to rotate on its axis, which coincides with the crank axis. The Pinion meshes with the mating gear on the gearbox drive shaft which transmits the crank power to the gearbox. External square helical threads on the other side of the part (8), shown more clearly in Figure 8, engage with mating internal threads of Slider (9). The axial float of the Slider (9) on Primary Drive Pinion (8) is restricted by a step on one side and a circlip on the other. A set of Friction Driven plates ( 10) are assembled on the Slider (9). As can be seen in figure 2, each plate is sandwiched between adjoining Friction Drive Plates and is concentric with the crank axis. Teeth are provided in the bore of these Plates which mesh with external teeth (splines) of the slider as shown in Figure 4. The Primary Derive Pinion (8), Slider (9) and the Friction Driven Plates (10) thus rotate together and constitute the driven half of the clutch.

Rollers ( 1 1), shown in Figure 2 and more clearly in Figure 5, are positioned in specially profiled cavities made in Rotor ( 1 ). Supported on one side by the Friction Drive Plate (5), the Rollers have a limited radial movement between extreme lower position "A" and extreme upper position "B". Multiple Rollers are deployed and spaced uniformly in the circumferential direction. As the crank shaft and the Rotor ( 1 ) start rotating, the centrifugal force on these Rollers causes engagement of the drive and driven half of the clutch as explained below. FREE DISENGAGED MODE

The dimensions of various parts are so controlled that in the assembled condition with the Rollers at position "A ^'* , the available space ^U X" (refer figure 6) is more than the width of the stack of the Friction Drive and Driven Plates. As a result, there is no interfacial pressure between the plates and the clutch is in no position to transmit any power.

NORMAL ENGAGEMENT MODE

As the crank shaft starts rotating, the rollers experience an outward acting centrifugal force and move in that direction. However, the profile of the guiding surface of the cavity in Rotor ( 1 ) makes it move axially as well, thereby, causing a reduction in dimension "X" till the gaps between Friction Drive and Driven Plates are eliminated. Herein after, further outward, and the corresponding axial, movement of the Rollers requires axial displacement of the Clutch Casing (2). In this condition the force of Compression Springs (4) gets transferred on, and leads to interfacial pressure between, the Friction Plates. The clutch thus gets engaged and starts transmitting power. The equilibrium position depends on the rotational speed of the crank shaft and the force exerted by the Compression Springs (4) as explained by the free body diagram of the Roller in Figure 7. MANUAL DECLUTCHING MODE

As the crank shaft revs up, the Rollers move further outward till they reach position "B" shown in Figure 5. In this position, the pressure between the Friction Plates is at its maximum and so is the torque transmission capacity of the clutch. It may be noted that in this condition, any further axial displacement of the Clutch Casing (2) will relieve the interfacial pressure between the Drive and Driven Plates resulting in disengagement of the clutch. This movement is achieved by manual actuation of the clutch lever at the handle bar by the rider, which, through a linkage moves Presser Plate (7) and the Clutch Casing (2). The conditions of Maximum torque capacity and manual de-clutching are shown in Figure 8.

AUTOMATIC REVERSE POWER ENGAGEMENT MODE (FOR KICK START)

As described earlier, the Slider (9) and Primary Drive Pinion (8) are coupled through a helical square thread. It is well known that, two mating parts exert equal and opposite force on each other and that this force acts along the common normal to their contacting surfaces. Since the contacting faces of helical thread are at an angle to the axis of rotation, both parts experience an axial thrust as well while transmitting torque. This is diagrammatical ly clarified in Figure 9. Accordingly, as shown in Figure 10, during reverse flow of power, as during kick or push start, the Slider (9) experiences an axial force in direction "Y" and since it is free to move, starts shifting till the clearance between its collar and the pack of Friction Drive and Driven plates is eliminated. The axial force then gets transmitted to and develops interfacial pressure between the Friction Drive and Driven Plates enabling the clutch to transmit the torque to the crank shaft.

The figures and the description above provide details of one particular design which is also specific to the design and configuration of the base engine being considered. It would be obvious to those skilled in the art that, based on the concepts, ideas and issues described herein, several variations of the proposed design for the design and configuration of the base engine considered, as well as, for engines with distinctly different design and configuration, will be possible without deviating from the scope of this invention.