GB2183752A - Variable speed transmission comprising variable eccentricity one way gearing and planetary gearing - Google Patents

Abstract

A variable speed transmission, primarily for a cycle hub gear, comprises a variable-stage (18) and a planetary stage (14). The planetary stage has a sun gear (13) driven directly by a rotational input from the chain sprocket (12) of the cycle and a ring gear (17) responsive to added motion provided at the output (32) of the variable stage (18). The variable stage (18) includes ratchet gearing (24) mounted on a fixed carrier (23) and rocked sequentially by integral followers (22) cooperating with a cam track (21) of a circular eccentric cam (20) which rotates with the input drive. The arrangement is capable of providing an output drive to the hub (10) which is geared upwardly or downwardly from the chain sprocket input drive by selectively varying the eccentricity of the cam (20). <IMAGE>

Description

SPECIFICATION Transmission This invention relates to a variable transmission.

The invention was devised particularly for use as a hub gear in a cycle but may have application in other fields. However the following description will refer to a bicycle hub gear by way of example.

Conventional forms of cycle gear have had certain disadvantages in use. For example, the derailleur type of gearing using a series of external sprockets on the driven wheel has the disadvantage that the gears can only be changed while the cycle is in motion and hence difficulties can be encountered when a cyclist is starting from rest after making a sudden stop, for example at traffic lights.

It has been proposed, for example in British patent specification number 21302 of 1907, to provide a variable speed gear mechanism for a cycle in which a cage carries a pair of one way clutches or ratchet arrangements having followers engageable with an eccentric cam, the arrangement being such that when a relative rotation takes piace between the cage and the cam, each follower undergoes a rocking movement and intermittently transmits drive to the associated one way clutch. This added motion is then transmitted to the cycle wheel.

It is an object of the present invention to provide a variable transmission of a new or improved form.

According to the invention there is provided a variable transmission comprising a differential stage and a variable stage cooperating to provide a final output rotational drive from an input rotational drive, the variable stage comprising; a carrier generally concentric with an axis, a cam having a selectively variable lateral offset relative to the axis, a plurality of one way rotational drive means mounted on the carrier for individual rotational movement, each one way rotational drive means having follower means cooperable with said cam on relative rotation between the cam and the carrier caused by said input rotational drive, and a variable-stage output element with which said one way rotational drive means cooperate in sequence to transmit a variable rotational drive related to the input drive and to the selected cam offset from the axis;; the differential stage comprising an epicyclic gear train having a sun gear, planet gears rotatably mounted on a planet carrier and meshing respectively with the sun gear and with a ring gear, one of the sun gear or the ring gear being drivingly connected to the variable-stage output element, and the other receiving said input rotational drive directly, the planet carrier providing said final output rotational drive.

The expression "cam" is used for convenience of description to cover both true cams and eccentrics.

The expression "one way rotational drive means" is used to cover drive arrangements such as a iatchet and pawl or one way clutch.

The carrier of the variable stage may be fixed relative to the axis.

The cam may be circular. It may be rotated directly by said input rotational drive.

The variable-stage output element may comprise an extension of the ring gear of the differential stage. In this case, the sun gear is driven directly by said input rotational drive.

The one way rotational drive means may comprise toothed pinions, each having a pawl engageable with ratchet teeth on the pinion, each pawl being associated with a respective one of said follower means and being adapted to rotate the respective pin ions intermittently to drive said variable-stage output element.

The variable stage output element may then comprise a toothed element in driven engagement with the pinions, for example a sun gear or ring gear.

Alternatively, the one way rotational drive means may each comprise a rocking crank or lever including a respective follower means, each crank or lever having a direct clutched drive to a variable-stage output element.

A variable transmission embodying the invention will now be described in more detail by way of example only with reference to the accompanying drawings in which: FIGURE 1 is a section through the hub of a cycle wheel incorporating the transmission.

FIGURE 2 is an end elevational view on the arrow 2 of Figure 1 of a variable-stage of the transmission.

Referring firstly to Figure 1 of the drawings, a hub casing 10 is mounted for rotation about an axlell and is driven, via the transmission to be described, by a chain sprocket 12.

The chain sprocket 12 is rotatably mounted on the axle 11 and is keyed for rotation with a sun gear 13 of a differential stage generally indicated at 14 comprising a conventional epi cyciic gear train. The sun gear 13 meshes with a plurality of planet gears 15 which are rotatably mounted on stub axles 16 on a planet carrier, integral with the hub casing 10.

The planet gears also mesh with an external annulus or ring gear 17.

The differential stage 14 has a driving connection with a variable-stage generally indicated at 18, through an extension 19 of the ring gear 17. The variable-stage 18 also comprises a circular eccentric cam 20 which is keyed for rotation with the sun gear 13 and drive sprocket 12 by means of a sleeve jour nalled on the main axle 11.

The circular cam 20 is positioned about the axle 11 in such a way as to be movable be tween a co-axial position and an eccentric position, in which it is illustrated in Figure 1.

This lateral adjustment of the cam 20 can be carried out in any generally known manner.

The circular cam 20 provides a cam track 21 on which a plurality of followers 22 are arranged to ride. The exact configuration of the cam track 21 and followers 22 is not critical and any suitable low friction arrangement can be used. Specifically, the track 21 can be internal or external of the cam 20.

A ratchet carrier 23 is fixed on the main axle 11 and is generally co-axial with the axle.

The ratchet carrier 23 carries a plurality of ratchet arrangements generally indicated at 24, each of which includes a crank or angled lever pivoted to the carrier, one arm of the crank including the follower 22.

The arrangements of the cranks will be seen in more detail from Figure 2 of the drawings.

Each crank includes a follower arm 25 and a ratchet drive arm 26 which are fixed together at a fixed angle connection 27, at which they are also pivoted to the ratchet carrier 23.

The ratchet drive arm 26 of the crank is pivoted at 28 to a link arm 29 carrying a pivoted pawl arrangement 30. The pawl acts on internal ratchet teeth 31 of the variable stage output which is in the form of a ring 32 having a rigid interconnection with the extension 19 of the ring gear 17 of the differential stage 14.

To summarise, the variable-stage therefore consists of a circular but generally speaking eccentric cam which rotates with the input drive from the sprocket 12 and a plurality of ratchets driven by cam followers which transmit added motion to the variable-stage output 32 and hence to the ring gear of the differential stage 19.

When the circular cam 20 is concentric with the ratchet carrier 23 and fixed axle 11 of the wheel, no added motion is given to the variable stage output 32 because the crank arrangements 24 do not have any oscillatory movements imparted to them by the cam 20.

Thus, the variable stage 18 of the transmission does not give any added motion to the differential stage 14. This simply acts as a conventional epicyclic gear train with the input through the sun gear 13 and the output through the planet carrier to the hub. The planet gears 15 "walk" round the interior of the ring gear 17 which acts as a reaction member and the nett output rotation to the hub 10 is geared down from the input received from the chain sprocket 12.

If the circular cam 20 is then moved to a slightly eccentric position relative to the fixed axle 11, each crank arrangement 24 is, in turn, rocked by the associated follower 22 as relative rotation occurs between the circular cam 20 and the ratchet carrier 23. As each crank rocks, the rocking movement is transferred to the ratchet pawl 30 which alternately slips on the teeth 31 or pushes against them in driving engagement.

This results in the ratchets sequentially driving the variable-speed output member 32, those ratchets which are not in driving engagement with the teeth 31 being allowed to slip as their respective cranks are rocked in the reverse sense of motion. In order to smooth this drive, it is desirable to provide a large number of ratchet teeth to cooperate with the pawls. This avoids any jerkiness in the final output from the variable-stage 18.

At a certain degree of eccentricity of the cam 20, the sPeed of rotation of the variablestage output matches the speed of rotation of the input sun gear. When this point is reached, the differential stage locks and the drive is transmitted at a 1:1 ratio between the input sprocket 12 and the hub 10.

By causing the variable-stage output 32 to move faster than the input sun gear in terms of rotational speed, the drive to the hub 10 can be geared up compared with the original input drive from the chain sprocket.

It will therefore be seen that by combining the ratchet type variable-speed transmission described with a differential stage in the manner outlined, a range of output gear ratios can be achieved which may include both increased and decreased gear ratios.

Variations can be made to the arrangement described with reference to the drawings. For example, the cranks and their ratchet pawls can be replaced by rotatable pinions having ratchet pawls, the pawls being associated with cam followers. The arrangement would then be that relative rotation between the ratchet carrier and the circular cam would intermittently drive each pinion, the remaining pinions slipping on their ratchets. All of the pinions would be in engagement with a variable-stage output element which might be in the form of a sun gear or a ring gear for example.

In a further alternative, the variable-stage output could be fed to the sun gear of the differential stage and the main drive from the input could be fed to the ring gear of the differential, that is reversing the arrangement shown in Figure 1.

It is preferred that the cam should be circular so that a ball race can be used to give a smooth action to the cam followers. It will be appreciated that, particularly for a pedal cycle, it is important to obtain minimum friction in the drive.

Instead of a circular cam which is moved laterally to increase or decrease its lateral offset or eccentricity relative to the axis, an alternative possibility is to use a cam having a complex three dimensional surface which is moved axially along the axis so that the cam followers engage a different part of the surface to impart greater or lesser amounts of added motion to the variabie-stage output.

This may be used to compensate for any nonlinearity in the relationship between the cam eccentricity and the amount of added motion provided at the variable-stage output.

As a still further alternative, it would be possible to rotate the ratchet carrier and use a fixed cam having variable eccentricity.

Claims (10)

1. A variable transmission comprising a differential stage and a variable-stage cooperating to provide a final output rotational drive from an input rotational drive, the variable stage comprising; a carrier generally concentric with an axis, a cam having a selectively variable lateral offset relative to the axis, a plurality of one-way rotational drive means mounted on the carrier for individual rotational movement, each one way rotational drive means having follower means cooperable with said cam on relative rotation between the cam and the carrier caused by said input rotational drive, and a variable-stage output element with which said one way rotational drive means cooperate in sequence to transmit a variable rotational drive related to the input drive and to the selected cam offset from the axis;; the differential stage comprising an epicyclic gear train having a sun gear, planet gears rotatably mounted on a planet carrier and meshing respectively with the sun gear and with a ring gear, one of the sun gear or the ring gear being drivingly connected to the variable-stage output element, and the other receiving said input rotational drive directly, the planet carrier providing said final output rotational drive.

2. A transmission according to Claim 1 wherein the carrier of the variable-stage is fixed relative to the axis.

3. A transmission according to Claim 1 wherein the cam is circular.

4. A transmission according to any preced ing claim wherein the cam is rotated directly by said input rotational drive.

5. A transmission according to any preceding claim wherein the variable stage output element comprises an extension of the ring gear of the differential stage.

6. A transmission according to Claim 5 wherein the sun gear is driven directly by said input rotational drive.

7. A transmission according to any preced ing claim wherein the one way rotational drive means comprise tooth-ed pinions each having a pawl engageable with ratchet teeth on the pinion, the pawls each being associated with a respective one of said follower means and be ing adapted to rotate the respective pinions intermittently to drive said variable stage out put element.

8. A transmission according to Claim 7 wherein the variable-stage output element comprises a toothed element in driven engagement with the pinions.

9. A transmission according to any one of Claims 1 to 6 wherein the one way rotational drive means comprise rocking cranks or levers including respective follower means, each crank or lever having a direct clutched drive to a variable-stage output element.

10. A variable transmissiqn substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.