BACKGROUND TO THE INVENTION In this specification the term one way band clutch refers to a clutch comprising a gripping band adapted to engage an output shaft, on which the gripping band is located, when the band is rotated in one direction thereby rotating the shaft and to release the shaft when the band is rotated in the opposite direction.

Through the years of development in industry, agriculture and commerce it has been important to be able to transmit the rotational output from a prime mover, such as an internal combustion engine, to a suitable output shaft and furthermore to be able to control the output torque and speed. This has been achieved by means of gearboxes.

One of the most common examples of this is the modern motor vehicle that has an internal combustion engine that is connected via a gearbox to an output shaft, which is the drive shaft. The internal combustion engine and the gearbox are connected by means of a clutch.

The traditional gearbox or transmission typically has only a few ratios because of size considerations. The limited number of ratios must be applied over a broad range of torque and speed requirements. In effect, each ratio only has a very small portion of the range in which it can be considered to be the most effective ratio for specific torque and speed requirements. For the rest of the range of required torque and speed it is a compromise, and the specific ratio is only used because none of the other available ratios can offer a better combination of torque and speed.

Some of the developments in the field of transmissions have been attempts to develop continuously variable transmissions (CVT). This type of transmission has an infinite number of ratios within a specific range, which is also where its advantage over the traditional transmission lies.

USA patent 5 392 664 deals with a CVT. It uses variable throw eccentric discs in conjunction with several types of one way clutches to produce continuously variable transmission of rotational input from a prime mover.

In theory CVT's have the capability to have a perfect ratio available for every torque and speed requirement. In practice problems has been experienced with building a CVT that is small enough to be practical, yet strong enough to be able to transmit the required high torque levels.

One of the most important aspects of a CVT is the clutch mechanism used to transmit the output of the engine or other prime mover to the output shaft and specifically one way clutches. Various types of one way clutches have been developed over the years. In US patent 5 392 664 hydraulic and mechanical one way clutches are described.

One type uses a set of hinged vanes attached to a rotor inside a chamber, with the chamber filled with a suitable liquid. The rotor is attached to an output shaft.

The hinged vanes are attached to the rotor in such a way as to be able to hinge to one side, where they would be flat against the rotor, but to stand up away from the rotor if rotated in the opposite direction. Thus, rotation in one direction is allowed, whilst it is resisted in the other direction. Other types of hydraulic one way clutches are also described, with one of the common factors being the use of a liquid in the operation of the clutch.

Mechanical one way clutches that are described include mechanical types such as roller clutches and sprag clutches.

Some of the problems associated with the known one way clutches are their inability to transmit large loads when the device is relatively small and also their inability to operate at a high frequency.

A development of importance to this invention and specifically to one way clutches has been in the field of gripping bands or band brakes. Band brakes are used to provide holdback on a rotating shaft, that is to prevent a shaft from rotating in one direction whilst allowing it to freely rotate in the other direction.

This has particular application in the hold back of for example a conveyor belt carrying materials up a slope. If the system failed and there was nothing to hold the belt back, the belt and material would run down to the base of the slope, with possible serious results. By using a band brake system on the drive shaft, the belt will be prevented from running back in case of a failure as described. This type of application is described in US Patent 3 837 440.

The inventors also know that the concept of a hold back band brake has been applied in the field of one way clutches. The specific application was in a fixed ratio transmission situation and utilise two gripping bands to act as one way clutches to drive an output shaft. Problems associated with this device were the inability to operate at high frequencies and the fact that it was never adapted to operate with more than two gripping bands.

OBJECT OF THE INVENTION It is an object of this invention to provide a continuously variable transmission.

SUMMARY OF THE INVENTION In accordance with this invention a continuously variable transmission comprises at least one variable throw reciprocating device adapted to be moved by a prime mover; an output shaft; at least one, one way band clutch located on the shaft and adapted to engage the shaft when rotated in one direction and to release the shaft when rotated in the opposite direction, the clutch being connected to the variable throw reciprocating device to be rotated thereby.

There is further provided for the gripping band one way clutch to comprise a part circular flexible gripping band having an inner gripping surface, a leading end and a trailing end, extending in length for more than 180 degrees around its circular axis and adapted to be in close fitting relationship around the circumference of a shaft it is intended to rotate; an activation lever for rotating the band about its axis and that of the shaft, the lever being mounted by way of a pivotal mounting to the leading end of the band and having a first part of the lever remote from the trailing end of the band being adapted to be moved in a reciprocating manner by a prime mover, and a second part of the lever on the trailing end side of the pivotal mounting; and releasably engageable formations on the second part of the lever and the trailing end of the band the formations being engaged when the band is rotated by the lever in a leading end first direction and disengaged when the band is rotated by the lever in a trailing end first direction.

Further according to the invention the band is a metal band or preferably a steel band or other materials such as a plastics materai.

Still further according to the invention the band tapers in thickness from the leading end to the trailing end.

Still further according to the invention the band extends for at least 270 degrees.

Still further according to the invention the activation lever is curved around the band with the first part extending for about 180 degrees around the band.

Still further according to the invention the engageable formations comprise an outwardly extending lip at the trailing end of the band and a co-operating gripping projection on the second part of the lever.

There is further provided for the variable reciprocating device to comprise a connecting arm a first end of which is adapted to be rotated to produce a reciprocating motion of the second end; an operating arm having a first end pivotably connected to the second end of the connecting arm and a second end adapted to produce usable reciprocating motion, the second end being constrained to move generally transversely to the direction of movement of the connecting arm; and variable control means for constraining the second end of the connecting arm to produce a reciprocating motion with a variable component in the direction of reciprocation of the second end of the operating arm.

Further according to the invention the first end of the connecting arm includes a big end for rotational mounting on a crankshaft.

Still further according to the invention the second end of the operating arm may be constrained by a pivotably mounted linkage or by a guide system.

Still further according to one aspect of the invention the control means may be a control arm pivotably mounted to the first end of the operating arm and the axis of the connection of the second end of the connecting arm and the first end of the operating arm.

Still further according to another aspect of the invention the control means may be a control arm being an extension of the operating arm beyond the pivotal connection, which extension runs in a suitable guide slot.

The invention also provides a device comprising a rotable shaft rotatable by means of at least one reciprocating one way clutch device mounted thereon and connected to a variable reciprocating device as defined.

Further according to this aspect of the invention the. device includes two or more of the clutches as defined preferably with at least some of the clutches being operated out of phase with each other.

BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the invention will be described below by way of example only and with reference to the accompanying drawings in which: FIG. 1 is a general schematic arrangement of a continuously variable transmission incorporating a one way band clutch and a variable throw reciprocating device; FIG. 2 is a general schematic arrangement of a continuously variable transmission incorporating a one way band clutch and a variable throw reciprocating device adjusted to produce zero output; FIG. 3 is a general schematic arrangement of a continuously variable transmission incorporating a one way band clutch and a variable throw reciprocating device with the control arm, operating arm and connecting rod sharing a point of rotation; FIG. 4 is a general schematic arrangement of a continuously variable transmission incorporating a one way band clutch and a variable throw reciprocating device with a guide slot as the control means; FIG. 5 is a schematic representation showing the travel of the various points for some given position of point D; FIG. 6 is a schematic representation showing the travel of the various points with points D and E coincident, resulting in zero output motion.

DETAILED DESCRIPTION WITH REFERENCE TO THE DRAWINGS An embodiment of the invention is shown in figure 1 where a continuously variable transmission (1) incorporating a one way clutch (2) for use in rotating a shaft (3) of circular cross section and a variable throw reciprocating device (20) is provided.

The one way clutch comprises a part circular flexible gripping band (4). The band (4) has a leading end (12) and a trailing end (13).

The gripping band (4) terminates in a trailing end (13). At the trailing end (13) of the gripping band (4) is an outwardly projecting formation (14) having a part circular surface (not shown) directed away from the band (4) and away from the leading end (12). There is a gap (15) between the leading end (12) and the trailing end (13) which gap allows for flexing of band (4).

An activation lever (5) is mounted to the leading end (12) of the gripping band (4) by way of the pivot pin (6) passing through a circular aperture (not shown). The activation lever (5) includes first (7) and second parts (8) on opposite sides of the pin (6), which acts as a fulcrum.

The first part (7) of the lever is of a generally curved configuration and extends around the gripping band (4) for about 180 degrees (i. e. about halfway around the band (4)). This first part terminates in an end (9) in which there is a circular aperture (10) by which the first part of the lever is adapted to be attached to a reciprocating member of a variable throw reciprocating device. The second part (8) of the activation lever (5) is considerably shorter than the first part (7) and is arranged to be in close proximity with the trailing end (13) of the band (4). The second part (8) of the activation lever (5) includes a part circular projection (16) which is adapted to releasably engage with the projection (14) on the trailing end of the gripping bond (4).

Referring to figure 1 it will be seen than when the activation lever is rotated in a clockwise direction about the pivot pin (6) the formations (16) and (14) on the second part of the lever (8) and the trailing end (13) of the band (4) respectively engage with each other and tend to flex the band (4) to a condition of smaller diameter and consequent partial closure of the gap (15). When the activation lever (5) is rotated in an anti-clockwise direction the formations (16) and (14) disengage allowing the band (4) to return to its normal condition.

It will thus be appreciated that when the activation lever (5) is pivoted in a clockwise direction the gripping band is caused to grip the shaft (3) and when the activation lever is rotated in an anti clockwise direction the gripping band releases the shaft (3). In this way the clutch may be used to rotate the shaft in a clockwise direction through a rotational arc indicated by numeral (11) and then returned to its original position so as to rotate the shaft again and so on depending on a reciprocating motion applied to the activation lever.

The input motion from a reciprocating input source (FE) is transmitted to the activation lever (5) by means of a connecting rod connected to the activation lever (5) at an aperture (10) which is, as previously stated, about 180 degrees from the pin (6).

The continuously variable transmission also comprises a variable throw reciprocating device (20). It is connected to a crankshaft (21). The variable throw reciprocating device comprises a connecting arm or rod (22), an operating arm (23)) and a control arm (24).

Each big end journal of the crankshaft (21) is rotatably connected in known manner to a connecting rod (22) at its first end (22a) with the axis thereof shown by point A. The connecting rod (22) is also rotably connected at its second end (22b) to the first end (23a) of the operating arm (23) around point B. The second end (23b) of the operating arm (23) is rotatably connected to the end (9) of the activation lever (5) around point E.

The control arm (24) is rotatably fixed at one end (24a), around point D. At the other end (24b), around point C, it is rotably connected to the operating arm (23).

The fixed throw crankshaft imparts an approximately sinusoidal motion to one end of the operating arm through force FA, causing point B to rotate in a complex arc about points D and C, and resulting in a force FB acting on point B. The travel of the various points are illustrated in figures 5 and 6.

When the arc thus described is not centered at point E, it forces a force FE to act on point E. This results in a reciprocating motion of point E (from force FE) and in a direction generally transverse to the motion of point B. However, if point D is moved to coincide with point E, the centre of the arc described by point B is at point E, then there is no resultant force FE acting on point E and thus no reciprocating motion on point E.

The effect is that by varying the position of point D relative to point E, varying degrees of reciprocating output motion can be induced in point E. As shown in figure 5, point E describes an arc, which is the reciprocating output that is used to operate the band clutch (2). Figure 6 shows the situation when the control arm has been adjusted so as to produce zero reciprocating output. In this case there is no travel at point E and therefore there will be no output from the band clutch.

Referring to figure 3 it is possible to change the position of point C relative to the operating arm (23), to be anywhere along the length of the operating arm (23) and for instance to be coincidental with point B. It can also be positioned at an extended point on the operating arm (23) outside of the connection points B and E. The changing of the position of point C adds to the level of control over the reciprocating output of the variable throw reciprocating device.

Referring to figure 4, it may be required in certain circumstances to specifically predetermine the output velocity and acceleration characteristics of the output reciprocating motion. To achieve this, it is possible to replace the control arm (24) with a guide slot (25). By changing the position of the guide slot (25), and also specifically the shape of the slot it is possible to achieve an output motion of any desired form and allows for the capability to reduce the output motion at point E to zero.

The variable throw mechanism and gripping band one way clutch described above is used in a continuously variable transmission. The transmission may comprise an output shaft having a plurality of gripping band clutches operated out of phase with each other.

The gripping bands will be used to rotate the shaft in individually incremental angular distances in accordance with the reciprocation of a prime mover. The reciprocating motion is used to activate the clutches that engage with the shaft in one direction only. It will be appreciated that for a given prime mover or crankshaft rotational speed the reciprocating device may be varied from an output throw or amplitude between zero and a designed maximum. Starting at zero the throw can be increased to start driving the clutch mechanisms. As the throw is increased so will the angular displacement of the clutches be increased with an increase in the rotational speed of the output shaft.

This system has obvious applications in many areas of industry including automotive and agricultural devices and vehicles.

Other embodiments are envisaged within the scope of the invention and include other configurations of the one way band clutch and variable reciprocating device and other uses thereof.