GB2159592A

GB2159592A - Variable ratio epicyclic gearing

Info

Publication number: GB2159592A
Application number: GB08413773A
Authority: GB
Inventors: Alan George Shaw
Original assignee: Individual
Current assignee: Individual
Priority date: 1984-05-30
Filing date: 1984-05-30
Publication date: 1985-12-04
Also published as: GB2159592B; GB8413773D0

Abstract

An epicyclic gearing unit comprises two coaxial and relatively rotatable members 5 and 7 with facing annular surfaces provided by a groove 10 and a nose-like projection 11, respectively. A plurality of caged ball members 12 are in frictional driving engagement with and between said surfaces, and the caging of the ball elements 12 is such that the pitch circle diameter of the balls 12 can be changed to vary the epicyclic gearing ratio. To this end the ball cage comprises two pressings 13 and 14, relative angular adjustment of which varies said pitch circle diameter. A spring 8 provides relative axial spring loading of the members 5 and 7 and thus maintains the frictional driving engagement of the balls 12. <IMAGE>

Description

SPECIFICATION Variable ratio epicyclic gearing The invention relates to variable ratio epicyclic gearing. It is particularly, but by no means exclusively, concerned with such gearing the gear ratio of which adjusts itself to output torque requirements and thus which is especially suitable for use in automotive transmission applications as an element of a CVT (constant velocity transmission) system with which the driving engine speed remains substantially constant.

The main object of the invention is to provide a compact epicyclic gearing unit which is relatively cheap to manufacture and the ratio of which is easily controlled, if desired in dependence on output torque requirements. A further object is to provide such a unit which is also usable in torque-limiting applications, for example in the drive of a machine tool.

According to the invention an epicyclic gearing unit comprises two coaxial and relatively rotatable members with facing annular surfaces, and a plurality of caged ball members which are in frictional driving engagement with said surfaces, the caging of the ball elements being such that the pitch circle diameter of the balls can be changed to vary the epicyclic gearing ratio.

Normally the two relatively rotatable members will be used as the input and output members, respectively, in the drive sense with the ball cage being non-rotatable so that the ball/cage arrangement acts as the reaction member of the epicyclic gearing. The invention will be generally described hereinafter in this sense, but it will be appreciated that alternatively the unit could be employed using the ball/cage arrangement as the input or output member of the epicyclic gearing.

Preferably the two rotatable members are disposed side by side in the axial sense, an annular groove in one providing the corresponding ball-engaging surface and a projecting radiused annular nose on the other rotary member providing the ball-engaging surface thereof. With this construction a one-to-one drive ratio is obtained between the two rotatable members when the balls engage the peak of the nose surface, radial positioning of the balls on one side of the peak or the other providing a step-up ratio or a reduction ratio as required.

The cage by which the balls are radially positioned, and which acts as the planet carrier of the epicyclic gearing, preferably comprised two relatively rotatable annular pressings positioned midway between said ballengaging surfaces on the relatively rotatable members and having crossing inclined balllocating slots. Thus the radial ball positions, and hence the pitch circle of the balls, depends upon the relative angular positioning of the cage pressings. One of these pressings may be fixed and the other anchored through a spring, so that its angular position depends on the ball reaction forces and the spring rate.

With this construction the radial ball position, and hence the transmission ratio, depends on the transmitted torque.

The frictional ball engagement is preferably ensured by relative axial spring loading of the two rotatable members, and the ball-engaging surfaces of the latter are preferably not quite parallel so that under the spring loading there is a slight wedging action of the balls between the surfaces. It will be appreciated that the epicyclic gearing unit of the invention provides a frictional rather than positive drive, and this can be turned to advantage when a limited-torque drive is required. Thus the unit can be designed to 'slip' at a predetermined torque, for example when used in the drive of a machine tool. However, in general the unit will be designed so that the torque at which it would slip is materially greater than the maximum torque which is to be transmitted in the intended application.

A prime mover, such as a vehicle engine, operates most efficiently over a narrow speed range and it has a narrow power band. Thus a variable-ratio transmission is necessary and traditionally stepped-ratio gear boxes have been used to transmit the engine power to the vehicle wheels, and automatic gear boxes have been introduced through the years for driver convenience. With the recent accent on fuel efficiency CVT transmissions have been under intensive development utilising epicyclic gearing and a transmission system which provides alternative power transmission paths between input and output shafts of the transmission. The power is shared between the two paths in accordance with output torque requirements, the overall transmission ratio varying according to the power sharing.

Generally speaking, such systems can be considered as comprising an output epicyclic gearing the reaction member of which rotates at a variable speed dependent on the output torque requirements. An epicyclic gearing unit according to the invention is suitably used in such a transmission system to provide a variable speed drive between the input shaft and the reaction member, for example an outer ring gear, of the output epicyclic gearing. In this case the unit of the invention transmits part of the power dependent on the reaction torque at the outer ring gear of the output epicyclic gearing.

The invention will now be further described with particular reference to the accompanying drawings which diagrammatically illustrate, by way of example, an epicyclic gearing unit in accordance with the invention employed in a transmission system of CVT type. In the drawings: Fig. 1 is an axial sectional view of the system; and Fig. 2 is a partial axial detail view of a bail/cage arrangement of the unit.

The transmission system illustrated has coaxial input and output shafts I and 0, the drive direction being clockwise viewed from the input side. At its input end the shaft I is integral with a sun gear 1 of the output epicyclic gearing and the output shaft 0 is integral with a planet carrier 2 of this gearing, on which planet gears 3 are mounted in the usual manner for engagement with an outer toothed annulus or ring gear 4. The ring gear 4 provides the reaction member of the output epicyclic gearing.

The epicyclic gearing unit U of the invention is mounted on the input shaft I. It comprises a rotatable input member which engages splines on the shaft I at 6, so as to be rotatable therewith but axially slidable thereon, and a rotatable output member 7 which is formed integrally with the ring gear 4. The output member 7 is axially located, by bearing means which are not illustrated, and the input member 5 is urged towards the output member 7 by a helical compression spring 8 which surrounds the shaft I and engages a spring abutment 9 on the shaft I.

The input and output members 5 and 7 are of annular form. The inner side face of the member 5 has an annular groove 10 of generally semi-circular cross-section and the adjacent face of the output member 7 has an annular nose-like projection 11 also of generally semi-circular cross-section which projects centrally into the groove 10. The complementary facing surfaces of the groove 10 and nose 11 are in frictional engagement with a ring of spherical balls which transmit the drive between the members 5 and 7 and act as planet elements of the unit U. The frictional drive engagement of the balls 1 2 is maintained by the spring 8, and the groove 10 and nose 11 surfaces are so formed that they are not exactly parallel and this provides a slight wedging engagement with the balls 1 2.

A two-part cage 12, which acts as the planet carrier of the unit U, consists of two dished steel pressings 1 3 and 14 positioned centrally in the ball space between the members 5 and 7. The cage 12 determines the pitch circle diameter of the balls, this being adjustable to vary the drive ratio of the unit U.

Any desired number of balls 1 2 may be employed, but they are typically four in number.

Referring to the partial detail view of the cage 1 2 shown in Fig. 2, the pressing 1 3 has a projecting peripheral ear 1 5 which is anchored to a casing of the unit at 1 6. The reaction torque on the cage 1 2 is in the direction shown by arrow 17, and the pressing 14 has a projecting peripheral ear 1 8 which as shown is anchored through a spring 1 9 which opposes the reaction torque on the cage 1 2. The pressings 1 3 and 14 have crossing ball-locating slots, respectively 20 and 21, in which the balls 12 are caged.As can be seen clearly from Fig. 2 the radial positioning of the balls, ie the pitch circle diameter thereof, depends upon the relative angular positioning of the pressings 1 3 and 14.

In the maximum operative torque position illustrated in the drawings the balls 12 are in their innermost positions, at a pitch circle 22 shown in Fig. 2. As the output torque at the shaft 0 decreases the decreased reaction torque at the ring gear 4 results in a decreased reaction torque at the cage 1 2. As the pressing 1 5 is fixed in angular position only the pressing 1 4 can move against the spring 19, this movement being determined by the torque reaction. Thus the radial positioning of the balls 12, and hence the drive ratio of the unit U, depends on the output torque. The radially outermost position of the balls 1 2 is shown in broken lines in Fig. 2, this corresponding to a ball pitch circle 23.

As can be seen from Fig. 1, when the balls 1 2 are positioned midway in their adjustment range and exactly on the peak of the nose 11 the drive ratio of the unit U is unity. For a lesser pitch circle diameter the ratio is.less than unity, and for a greater pitch circle diameter the drive ratio is greater than unity.

The compactness of the illustrated transmission system is clearly shown in Fig. 1, and although the invention is well suited to automotive applications a generally similar system can be made small enough for lower power requirements. For example for use as a motorcycle transmission, a hub gear of a pedal cycle or the gearing of a fishing rod reel.

Other possible applications of the invention are to variable rate steering mechanisms, variable rate suspension systems and variable rate lever systems. In fact units in accordance with the invention are of application wherever a compact variable ratio drive is required, particularly with automatic control of the effective drive ratio. The unit can be designed so that it will slip at a pre-determined torque for torquelimiting drive applications.

Claims

1. An epicyclic gearing unit comprising two coaxial and relatively rotatable members with facing formed annular surfaces, and a plurality of caged ball members which are in frictional driving engagement with and between said surfaces, the caging of the ball elements being such that the pitch circle diameter of the balls can be changed to vary the epicyclic gearing ratio.

2. An epicyclic gearing unit according to claim 1, wherein said two relatively rotatable members provide the input and output members, respectively, of the unit in the drive sense.

3. An epicyclic gearing unit according to claim 1 or claim 2, wherein the ball cage is non-rotatable so that the ball/cage arrangement acts as the reaction member of the epicyclic gearing.

4. An epicyclic gearing unit according to claim 1, wherein the ball/cage arrangement provides the input or output member of the epicyclic gearing, and one of said relatively rotatable members is non-rotatable and acts as the reaction member of the epicyclic gearing.

5. An epicyclic gearing unit according to any one of the preceding claims, wherein said two relatively rotatable members are disposed side by side in the axial sense, an annular groove in one of them providing the corresponding ball-engaging surface and a projecting annular nose on the other providing the ball-engaging surface thereof.

6. An epicyclic gearing unit according to claim 6, wherein said annular groove is of generally semi-circular cross-section and said annular nose which projects centrally into the annular groove also has a semi-circular crosssection, whereby a one-to-one drive ratio is obtained between the two rotatable members when the balls engage the peak of the nose surface and radial positioning of the balls on one side of the peak or the other providing a step-up ratio or a reduction ratio as required.

7. An epicyclic gearing unit according to any one of the preceding claims, wherein the cage by which the balls are radially positioned, and which acts as the planet carrier of the epicyclic gearing, comprises two relatively rotatable annular pressings positioned midway between said ball-engaging surfaces on the relatively rotatable members and having crossing inclined ball-locating slots so that the radial ball positions, and hence the pitch circle of the balls, depends upon the relative angular positioning of the cage pressings.

8. An epicyclic gearing unit according to claims 2 and 7, wherein one of said cage pressings is fixed in the rotation sense and the other is anchored through a spring, so that the angular position of the latter pressing depends on the ball reaction forces and the spring rate whereby the radial ball position, and hence the transmission ratio, depends on the transmitted torque.

9. An epicyclic gearing unit according to any one of the preceding claims, wherein the frictional driving engagement of the balls with said facing annular surfaces is ensured by relative axial spring loading of the two relatively rotatable members.

10. An epicyclic gearing unit according to claim 9, wherein the ball-engaging surfaces of the relatively rotatable members are not quite parallel so that under said axial spring loading there is a slight wedging action of the balls between the surfaces.

11. An epicyclic gearing unit according to any one of the preceding claims, wherein the unit is designed to 'slip' at a predetermined torque, for example for use in the drive of a machine tool.

1 2. A CVT transmission system for a vehicle which provides alternative power transmission paths between input and output shafts of the transmission and comprising an output epicyclic gearing the reaction member of which rotates at a variable speed dependent on the output torque requirements, and wherein an epicyclic gearing unit according to any one of claims 1 to 10 provides a variable speed drive between the input shaft and said reaction member of the output epicyclic gearing.

1 3. A transmission system according to claim 12, wherein said reaction member is an outer ring gear of the output epicyclic gearing.

1 4. An epicyclic gearing unit according to any one of claims 1 to 11, wherein the unit is used as an ancillary control to an epicyclic transmission system.

1 5. An epicyclic gearing unit constructed and arranged substantially as hereinbefore described with reference to the accompanying drawings.

1 6. A vehicle transmission system constructed and arranged substantially as hereinbefore particularly described with reference to the accompanying drawings.