GB2215000A

GB2215000A - Continuously variable transmission

Info

Publication number: GB2215000A
Application number: GB8802740A
Authority: GB
Inventors: Thomas Tei-Ho Ma
Original assignee: Ford Motor Co
Current assignee: Ford Motor Co
Priority date: 1988-02-06
Filing date: 1988-02-06
Publication date: 1989-09-13
Also published as: GB8802740D0

Abstract

A continuously variable transmission comprises two conical members 10 arranged adjacent one another for rotation about parallel axes and tapering in opposite directions with a chain or belt 18 passing over both conical members to transmit torque. The chain or belt is slidable axially along the two conical members so as to vary the transmission ratio continuously. Each conical member is formed on its surface with corrugations 12 which, in the development of the conical surface onto a flat plane, are parallel to one another and equally spaced. This produces a line of discontinuity 16 along one slope line of each cone. The belt comprises cups 20 which have a mutual spacing equal to that between the corrugations. Balls 22 having a radius of curvature smaller than that of the cups. A chain would comprise pivoted links 34, each carrying a ball 32. The balls of the chain or belt transmit the torque. <IMAGE>

Description

CONTINUOUSLY VARIABLE TRANSXISSION The present invention relates to a continuously variable transmission.

Continuously variable transmissions are known for use in motor vehicle and all known forms of such transmissions rely on friction to transmit torque. For example, there are known transmissions employing a V-belt which passes over two pulleys each consisting of two cone sheaves the distance between which can be adjusted to vary the radius at which the V-belt engages the pulley. Here, the frictional engagement is between the V-belt and the two pulleys.

A first disadvantage with transmissions relying upon friction is that the amount of torque that can be transmitted is limited and consequently in motor vehicles such transmissions are only commonly used with smaller engine sizes. To maintain the grip between the friction surfaces, a hydraulic control system is required which is expensive to manufacture and consumes power while the engine is running. Lastly, in the event of slipping, the usual problems caused by friction, namely noise and wear, are noticeable.

The invention seeks to provide a transmission with continuously variable transmission ratio which mitigates at least some of the foregoing disadvantages.

According to one aspect of the present invention, a continuously variable transmission comprises two conical members arranged adjacent one another for rotation about parallel axes and tapering in opposite directions, and a belt passing over both conical members to transmit torque from one conical member to the other, the belt being slidable axially along the two conical members so as to vary the transmission ratio continuously, wherein each of the two conical members is formed on its surface with corrugations which, in the development of the conical surface onto a flat plane, are parallel to one another and equally spaced, and wherein the belt comprises, on its side facing the conical members, cups which have a mutual spacing equal to that between the corrugations on the surface of the conical members, balls having a radius of curvature smaller than that of the cups of the belt and the corrugations on the surfaces of the conical members being trapped between the cups and the corrugations to transmit torque between the conical members and the belt.

According to a second aspect of the present invention, a continuously variable transmission comprises two conical members arranged adjacent one another for rotation about parallel axes and tapering in opposite directions, and a chain passing over both conical members to transmit torque from one conical member to the other, the chain being slidable axially along the two conical members so as to vary the transmission ratio continuously, wherein each of the two conical members is formed on its surface with corrugations which, in the development of the conical surface onto a flat plane, are equally spaced and parallel to one another and wherein the chain comprises a plurality of mutually pivotable links of the same mutual spacing as the corrugations on the surface of the conical members and carrying balls which engage in the grooves of corrugations of the conical members and which have a smaller radius of curvature than that of the latter grooves, the centres of the balls being offset from the plane of the pivotal axes between adjoining links whereby as the links pivot relative to one another the distance between adjacent balls measured along the chain is varied.

One way to visualise the construction of the conical member is to consider it as being constructed by wrapping a corrugated sheet about a solid cone. If one starts with a central corrugation lying on the line of greatest slope on the surface of the cone then as the corrugated sheet is wrapped around the cone in both directions, the sheet will envelope the smaller diameter end more quickly than the larger diameter end. If the sheet is now trimmed along the line of greatest slope diametrically opposite the first line, then the continued wrapping of the corrugated sheet will produce a chevron like line of discontinuity. It must be stressed that the above method of construction is only described to assist in visualising the conical member.

In practice, the conical members must be manufactured from a solid as a corrugated sheet cannot be wrapped around a cone in this way.

Because the corrugations are parallel when the surface of each conical members developed onto a flat plane, the grooves are of the same pitch at all radii of the conical members. The conical member can be regarded as as sprockets of continuously increasing diameters but equal tooth spacing arranged side by side. Of course, because the number of teeth can only increase in discreet steps while the diameter is increasing continuously, the circumference will not always be equal to an exact multiple of the pitch and this discrepancy is taken up by having one groove wider than all the others, all the wider grooves being aligned with one another.

A conventional chain with a fixed link size cannot pass easily over a sprocket with one tooth wider that all the others and the invention in its difference aspects seeks to permit the "teeth" on the belt or chain transmitting torque between the two conical members to move closer to each other so that two teeth may be accommodated in the same groove if necessary when the discontinuity is encountered.

In the embodiment with cups in the belt and balls trapped between the cups and the grooves in the corrugated conical surfaces, the balls can move relative to the two curved surfaces and effect jamming in a manner analogous to the operation of the balls in a one way running clutch. The sloping surfaces of the curved cups and grooves act as ramps between which the balls become wedged and should the cup be misaligned with the groove because of the discontinuity, then the ball can itself move to a new locking position. Even if there is no locking position for the ball on the discontinuity, there is no risk of slipping since the torque, at any one time, is transmitted by several balls.

In the embodiment using triangular chain links, the links can move out of a regular curve on encountering the discontinuity thereby permitting engagement with irregular sprocket teeth or even permitting the balls of two adjacent links to engage in the same wide groove.

The balls engaging the grooves in the case of both the embodiments considered above must be capable of movement in all directions and it is not believed possible to substitute barrels or other forms of rolling elements for the balls.

The peaks of the corrugated surface must be narrow in order that a ball falling directly onto the peak should be naturally deflected into one or other of the adjacent grooves. Furthermore, adjacent the discontinuity, the corrugations should be rounded to avoid any sharp edge at which ball movement may be obstructed.

In the case of the embodiment using chain links, the balls can be captive in the links but this is not possible in the embodiment employing curved cups of greater radius of curvature than the balls. In this case, two guides are preferably provided to maintain the balls captive in the cups of the two runs of the belt between the two conical surfaces.

The action of the balls relies on the belt or chain being substantially inextensible and incapable of movement away from the axis of rotation of the two conical members. If the belt or chain should not have sufficient stiffness for this purpose, then it is possible to position a further guide to restrict such movement of the belt of chain.

In a further aspect of the invention, there is provided a flexible inextensible drive element having coupling elements for engagement with a sprocket of uneven tooth spacing, each coupling element being capable of a limited degree of lost motion relative to the drive element.

Because the balls acting as coupling elements can move relative to one another, it is not vital that their pitch should initially be uniform nor indeed precisely the same as the pitch of the corrugations on the conical members. It may even be advantageous to introduce intentionally a random variation in the pitch of the balls, the size of the balls or the pitch of the corrugations to avoid the possibility of all the balls engaging or disengaging simultaneously, which would result in jerks.

The invention will now be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is an end view of a corrugated conical member, Figures 2 and 3 are developments of the surface of the conical member of Figure 1 onto a flat plane, Figure 4 is a plan view of two conical members coupled to one another by means of a belt to form a variable transmission, Figure 5 is an end view of the two conical members in Figure 4, with the belt omitted, Figure 6 is a detail showing the engagement of a ball between the corrugations on the conical members and the belt of Figure 4, and Figure 7 is a view similar to Figure 6 of an alternative embodiment employing a chain in place of the belt.

In Figure 1, there is shown a conical member 10 having corrugations 12 on its surface. The member is to be mounted on a shaft and 9 key 14 is provided on the inner surface of the conical member 10 for this purpose. The corrugations are of constant pitch at all diameter of the conical member and this inevitably results in a discontinuity 16 at one point on the circumference of the conical member 10.

Two developments of the conical member 10 on a flat surface are shown in Figures 2 and 3. Figure 2 would be the impression formed on a flat plane by rolling the conical member while starting and terminating on the discontinuity 16 while Figure 3 would be the impression made when starting and terminating on the opposite side of the conical member 10 from the discontinuity 16.

To form a variable transmission, two such conical member 10 and 10'are arranged as shown in Figures 4 and 5 adjacent one another with their axes parallel and tapering in opposite senses. A belt 18 carrying balls passes around the two conical members 10 and 10' and its position along the axes of the conical members 10 and 10' is adjustable to vary the transmission ratio continuously.

The coupling between the belt 18 and the conical member 10 is better shown in Figure 6. The belt 18 is formed of flexible steel and cups 20 are stamped out of the belt 18. A ball 22 is received between each cup 20 and the opposite corrugation 12 in the conical member 10.

The ball is wedged between the surfaces of the cup and the corrugation and any tendency to slip out increases the grip between the belt and the conical member in a manner similar to the operation of a one way clutch.

However, it is noticed that there is now a great deal of latitude in the position of the ball 22 and on meeting the discontinuity 16 the balls can rearrange themselves in their respective cups 20 to accommodate the change in pitch. It is essential that the belt and the balls be capable of riding over the discontinuity without damage to either but it is not essential that torque be transmitted between the two at the discontinuity since there will always be a torque transmitting ball somewhere around the circumference of the conical member 10. Thus even if a part of the discontinuity should present a space twice the pitch of the corrugations, this will not have a deleterious effect on the operation of the variable transmission.

On leaving one conical member, the balls 22 are no longer prevented by the conical member from dropping away from the belt but it is possible to provide a guide disposed between the conical members 10 and 10' and movable with the belt to retain the balls.

Alternatively, the belt may itself include a retainer band operating in a manner analogous to that of a cage of a roller bearing.

In the alternative embodiment shown in Figure 7, the balls 32 are located in cups 34 which form links in a chain. The links are pivoted relative to one another about pivot pins 36 and 38. Because the links are triangular and the centres of the balls 32 are offset from the plane of the pins 36 and 38, any

Claims

CLAIMS 1. A continuously variable transmission comprises two conical members arranged adjacent one another for rotation about parallel axes and tapering in opposite directions, and a belt passing over both conical members to transmit torque from one conical member to the other, the belt being slidable axially along the two conical members so as to vary the transmission ratio continuously, wherein each of the two conical members is formed on its surface with corrugations which, in the development of the conical surface onto a flat plane, are parallel to one another and equally spaced, and wherein the belt comprises, on its side facing the conical members, cups which have a mutual spacing equal to that between the corrugations on the surface of the conical members, balls -::having a radius of curvature smaller than that of the cups of the belt and the corrugations on the surfaces of the conical members being trapped between the cups and the corrugations to transmit torque between the conical members and the belt.
2. A continuously variable transmission as claimed in claim 1, wherein two guides are provided to maintain the balls captive in the cups of the two runs of the belt between the two conical surfaces.
3. A continuously variable transmission comprises two conical members arranged adjacent one another for rotation about parallel axes and tapering in opposite directions, and a chain passing over both conical members to transmit torque from one conical member to the other, the chain being slidable axially along the two conical members so as to vary the transmission ratio continuously, wherein each of the two conical members is formed on its surface with corrugations which, in the development of the conical surface onto a flat plane, are equally spaced and parallel to one another and wherein the chain comprises a plurality of mutually pivotable links of the same mutual spacing as the corrugations on the surface of the conical members and carrying balls which engage in the grooves of corrugations of the conical members and which have a smaller radius of curvature than that of the latter grooves, the centres of the balls being offset from the plane of the pivotal axes between adjoining links whereby as the links pivot relative to one another the distance between adjacent balls measured along the chain is varied.
4. A continuously variable transmission as claimed in any preceding claim, wherein the peaks of the corrugated surface are narrow in order that a ball falling directly onto the peak should -be -naturally deflected into one or other of the adjacent grooves.
5. A continuously variable transmission as claimed in any preceding claim, wherein, adjacent the discontinuity, the corrugations are rounded to avoid any sharp edge at which ball movement may be obstructed.
6. A flexible inextensible drive element having coupling elements for engagement with a sprocket of uneven tooth spacing, each coupling element being capable of a limited degree of lost motion relative to the drive element.
7. A continuously variable transmission constructed, arranged and adapted to operate substantially as herein described with reference to and as illustrated in the accompanying drawings.