GB2249361A - An epicyclic transmission - Google Patents

Abstract

The transmission comprising a drive input shaft (10), first and second independently operable clutches (15, 17) drivable by the drive input shaft, epicyclic gearing including first and second epicyclic gear trains (46, 56), independently operable band brakes (61a, 65a, 66a) for braking elements of the epicyclic gearing to provide reaction, and an output shaft (53) which receives drive directly from one of the first and second epicyclic gearing arrangements. The first clutch (15) is associated with a driven member (20) for imparting drive to a planet gear carrier (45) of the first gear arrangement (46), the plant gear carrier (45) being drivably fast with an annulus (55) of the second gear arrangement (56). The second clutch (17) is associated with a driven member (28) for imparting drive to a planet gear carrier (57) of the second gear arrangement (56), the planet gear carrier (56), being drivably fast with a sun gear (54) of the first gear arrangement (46). Either train may have intermeshing planets and/or a carrier with holes at different radii for various sizes of planet gears. <IMAGE>

Description

AN EPICYCLIC TRANSMISSION The invention relates to an epicyclic transmission suitable for use in the field of motor vehicles, machine tools or other applications where a change speed transmission is required.

Hitherto, there has been considerable development in the field of epicyclic transmissions for providing forward and reverse drive ratios particularly in the field of motor vehicle automatic transmissions.

In motor vehicle applications, it is becoming increasingly more desirable to construct transmission gearings so that they take up minimum space within the vehicle whilst providing a wide range of ratios.

Earlier epicyclic transmission proposals are described in US-A-4,509,489 and US-A-4,658,672. Such transmissions provide a number of forward ratios using trains of planetary gearing and a plurality of independently operable clutches drivable from an input and having driven members drivably connected to elements of some or all of the epicyclic gear trains.

In 4,509,389 three epicyclic gear trains are required along with two input clutches but provide only four forward ratios and one reverse. The use of three epicyclic trains lengthens the transmission which is undesirable where space is at a premium. No.4,658,672 provides four forward speeds but, in this case, two reverse ratios are selectable. However, in this case also, three trains of epicyclic gears are required which, again, is undesirable where a transmission of minimum size is required.

An object of the present invention is to provide an epicyclic gearing having only two sets of epicyclic gear assemblies thereby minimising space but which provides a drive input arrangement from independently operable clutches arranged to impart drive the epicyclic arrangement in such a way that a multiplicity of ratios can be achieved.

According to one aspect of the invention there is provided an epicyclic transmission comprising a drive input, first and second independently operable clutches drivable by the drive input, epicyclic gearing including first and second epicyclic gear arrangements brake means for braking elements of the epicyclic gearing to provide reaction, and an output which receives drive from the epicyclic gearing, the first clutch being associated with a driven member for imparting drive to the first gear arrangement, the first gear arrangement including an element drivably connected to an associated element of the second gear arrangement and the second clutch being associated with a driven member for imparting drive to the second gear arrangement the second gear arrangement including a further element drivably connected to an associated element of the first gear arrangement.

Preferably, driven member associated with the first clutch is drivably connected to said element of the first gear arrangement whereby said element forms an input element of the first gear arrangement.

Preferably, the driven member associated with the second clutch is drivably connected to said further element of the second gear arrangement whereby the further element forms an input element of the second gear arrangement.

According to a second aspect of the invention there is provided an epicyclic transmission comprising a drive input, first and second independently operable clutches drivable by the drive input, epicyclic gearing including first and second epicyclic gear assemblies for receiving drive from the clutches, brake means for braking elements of the epicyclic gearing to provide a reaction, and an output which receives drive from the epicyclic gearing, the first clutch having a driven member connected to an input element of the first gear arrangement and an associated element of the second gear arrangement, and the second clutch having a driven member drivably connected to an input element of the second gear arrangement an associated element of the first gear arrangement.

The interconnection between the epicyclic gear arrangements can be used advantageously to provide a substantial number of drive ratios, the use of only two epicyclic gear arrangements enabling the axial length of the transmission to be maintained at a minimum.

The input member of the first gear assembly may correspond to the input member of the second gear assembly. For example, the input member of the first gear assembly may be a planet wheel carrier and the input member of the second gear assembly may be a similar planet wheel carrier.

The associated element of the second gear arrangement may comprise an annulus gear and the associated element of the first gear arrangement may comprise a sun gear.

The sun gears and annulus gears preferably mesh with a plurality of planet wheels carried by the planet carriers.

In order to increase further the number of ratios obtainable by the transmission, a third clutch drivable by the drive input may be provided. The third clutch may have a driven element drivably connected to a third or further input element which forms part of one of said first and second gear arrangements.

The third or further input element may be a sun gear.

The aforesaid output may comprise an output shaft drivably connected to an output element, preferably an annulus gear, of one of the first and second gear arrangements.

In order to provide adequate steps between ratios, provided by the transmission at least one of the first and second gear arrangements may include compound planetary gearing. In a preferred embodiment, both epicyclic gear arrangements will include compound planetary gearing.

The aforesaid brake means may include a brake for braking the input element of the first gear arrangement. In such a case, the brake may also brake the associated element of the second gear arrangement.

With the driven element of the first clutch drivably connected to the input element and associated element of the second gear arrangement, braking of the input element may also brake the driven member of the first clutch.

Similarly, the brake means may include a brake for braking the input member of the second gear arrangement and operation of the brake may also brake the associated element of said second gear arrangement.

Again, as the driven member of the second clutch is drivably connected to the input element of the second gear arrangement and the associated element of the second gear arrangement, operation of the brake may also brake the driven member of the second clutch.

Where the aforesaid third clutch is provided, a brake may also be provided for braking the third or further input element. In that case, operation of the brake may also be operable to brake the driven member of the third clutch.

US-A-4,658,672 describes the way in which input torque can be divided within the epicyclic gear arrangement.

The present invention may also provide a division of torque between the two epicyclic gear arrangements.

In one drive condition, drive is preferably directed to the input element of the first gear arrangement and the associated element of the second gear arrangement with another element of the second gear arrangement providing a reaction whereby the second gear arrangement provides an output which drives an element of the first gear arrangement relative to the input member of the first gear arrangement.

In another drive condition, drive may be directed to the input element of the second gear arrangement and the associated element of the first gear arrangement with an element of the second gear arrangement providing a reaction whereby the second gear arrangement provides an output which drives an element of the first gear arrangement relative to the input element of the first gear arrangement.

Preferably, the output from the second gear arrangement is, in each case, provided by the input element thereon which drives its associated element in the first gear arrangement.

A neutral condition may be provided when the clutches are in a non-drive transmitting condition. Preferably, the clutches will be capable of transmitting drive directly from a prime mover such as an engine so that engagement of any two of the clutches (to provide a direct drive ratio) or the selection of a suitable clutch/brake combination will cause drive to be transmitted through the transmission to the output.

Preferably, at least one of the clutches is of such a capacity to permit a stationary vehicle to pull away using that clutch.

A transmission in accordance with the invention enables simplified selection of ratios to be achieved.

For example, where drive in one ratio involves utilising one of the clutches and one of said brake means, de-selection of one ratio and selection of another ratio can be effected by a change over from the one clutch to another clutch with said one brake means remaining operative.

Alternatively, where drive in'one ratio involves utilising one of the clutches and one of said brake means, de-selection of one ratio and selection of another ratio can be effected by a change over from the one brake means to another of the brake means with said one clutch remaining operative to transmit drive.

In another instance a ratio change can be made by releasing a brake to de-select one ratio in which drive is transmitted using one of said clutches, maintaining said one clutch operative to transmit drive, and engaging second and third clutches to select the next ratio.

In yet another instance a ratio change can be made by releasing two said clutches to de-select a ratio in which three clutches are operative, maintaining one of said clutches operative to transmit drive and applying a said brake means to select the next ratio.

Where a transmission uses a number of epicyclic gear arrangements the required ratios normally makes it impossible to use common components for the sets of epicyclic gears. We propose to reduce this particular problem by providing a planet wheel carrier having mounting means thereon for enabling the planet gear or different sized planet gears to be carried thereon in number of different radial positions for meshing with sun and annulus gears of different sizes.

According to another aspect of the invention there is provided an epicyclic transmission comprising a sun gear, a planet gear mounted for rotation on a planet carrier, and an annulus gear, the sun, planet and annulus gears being in mesh and the planet carrier having mounting means thereon for enabling the planet gear or different sized planet gears to be mounted at different radial positions for meshing with sun and annulus gears of different sizes.

With the aforesaid arrangement, the same planet carrier can be used to mount a planet gear at one radial position for meshing with a given sun gear and a given annulus gear and will permit the same planet gear to be mounted at a different radial position for meshing with a different sun gear and a different annulus gear. On the other hand, an alternative planet wheel may be mounted on the carrier for meshing with alternative sun and annulus gears. If desired, the circumferential spacing of the mountings may be such as to permit certain of the mountings to be used for the mounting of meshing compound planet gears. In such a case, one planet gear will mesh with the annulus gear and the other with the sun gear.

Preferably, a plurality of sets of mountings may be provided, the mountings of each set being a the same radial position and the mountings in one set being arranged at radial positions different from those in another set.

The mountings means may include apertures in an annular plate of the planet wheel carrier, e.g., for receiving planet carrier pins.

Where there are holes formed in the annular plate, the holes not occupied by planet pins in a particular application could be utilised for any fixings necessary for assembly of a cage forming the planet wheel carrier.

In order to enable either end of the transmission to receive drive from an engine or any other prime mover, a drive input shaft may extend through the transmission from both ends, the drive input shaft being drivable connected to the clutches.

An epicyclic transmission in accordance with the invention will now be described by way of example with reference to the accompanying drawings in which: Fig.l is a diagrammatic cross-section through an upper half of a transmission in accordance with the invent ion, Fig.2 is a diagrammatic end view of a planet carrier, Fig.3 is a diagrammatic end view of the planet carrier in Fig.2 supporting compound planet gears, Fig.4 is a view similar to Fig.3 showing the planet carrier and planet gears arranged mirrorwise, when compared to Fig.3 and Figs.5, 6 and 7 are diagrammatic end views of alternative epicyclic gearing arrangements using the planet carrier of Fig.2.

In Fig.1 a drive input shaft 10 (shown partly in cross section) of a transmission in a casing 1 is drivably connected to a clutch housing 12 having internal splines 13. The splines 13 slidably engage axially movable clutch plates 14 of a first clutch 15 and axially movable plates 16 of a second clutch 17. The clutch plates 14 intercalate with driven clutch plates 18 which are slidable on splines 19 of a first hollow driven shaft 20. The splines 13 carry a reaction plate 21 held axially fast between two snap rings 22. A piston 23 is slidable in a cylinder 24 formed in the clutch housing 12. The piston 23 is movable to the left as viewed in Fig.1 to clamp the clutch plates 14, 18 together between the piston and the reaction plate 20 by means of fluid under pressure fed to the cylinder 24 through a duct 25 from a source (not shown).

The driving clutch plates 16 intercalate with driven clutch plates 26 carried by splines 27 on a second hollow driven shaft 28.

The driven shafts 20, 28 constitute the aforesaid driven members.

A piston 29 is slidable in a cylinder 30 formed in the clutch housing 12. The piston is movable to the right to clamp the clutch plates 16, 26 together between the piston 29 and the reaction plates 21. The piston 29 is movable by fluid under pressure fed to the cylinder 30 through a duct 32 from a source (not shown).

A third clutch 33 is provided and includes a drive input member 34 having splines 35. Three driving clutch plates 36 are axially slidable on the splines 35 and intercalate with driven clutch plates 37 mounted on splines 38 on a hollow driven member 39. The driven member 39 is integral with a third hollow driven shaft 40.

A piston 42 is slidable in a cylinder 43 formed in the driven member 39. The piston 42 is movable to the right as viewed in the drawings to clamp the clutch plates 36, 37 between the piston 42 and a reaction plate 31 by fluid under pressure introduced through a port 44.

The shaft 20 is drivably connected to a planet gear carrier 45 of a first epicyclic gear arrangement 46.

The planet gear carrier 45 includes spaced apart annular plates 47, 48 held together by fixings (not shown). Planet pins 49 are mounted in aligned apertures in the plates 47, 48 and rotatably support sets of compound planet gears 50, 52. Such an arrangement is shown diagrammatically in end view in Fig.3. The planet gears 50 mesh with an annulus 51 which is drivably fast with a hollow output shaft 53.

The planet gears 52 mesh with the planet gears 50 and with a sun gear 54 drivably fast with the shaft 28.

The annular plate 48 of the planet gear carrier 45 is drivably fast with a brake drum 61 concentric with the first gear arrangement 46 and with an annulus 55 of a second epicyclic gear arrangement 56. The planet gear carrier 45 and annulus 55 constitute the aforesaid input element of the first gear arrangement and the associated element of the second gear arrangement respectively. The second gear arrangement 56 includes a planet gear carrier 57 comprising an annular plate 58 drivably fast with the sun gear 54 and an annular plate 59 connected to the annular plate 58 by suitable fixings (not shown). The plates 58, 59 support a plurality of planet pins 60 on which respective compound planet gears 62, 63 are rotatably supported.

The planet gears 62, 63 are arranged in a manner similar to the planet gears 50, 52, the planet gears 62 meshing with annul us 55 and the planet gears 63 meshing with planet gears 62 and a sun gear 64 drivably fast with a shaft 40. The annular plate 59 is integral with a brake drum 65.

The hollow driven member 39 hays its radially outer portion formed as a brake drum 66.

The brake drums 61, 65 and 66 are engageable by independently operable band brakes 61a, 65a and 66a respectively.

The construction of the annular plate 47, of the planet gear carrier 45, is shown diagrammatically in Fig.2.

The annular plates 48, 58 and 59 are of identical construction.

In Fig.2, the plate 47 is formed with three sets of apertures 70, 71 and 72. The apertures 70 are on a pitch circle of radius a, the apertures 71 on a pitch circle of radius b and the apertures 72 on a pitch circle of radius c. The radial distances a, b, c, are all different. The opposite annular plate 48 of the planet gear carrier 45 will be formed with apertures coaxial with apertures 70, 71 and 72. In use, planet pins can be located in the appropriate apertures to provide the different positioning of the planet gears of the epicyclic gear arrangement. Fig.3 shows the way in which planet gears 50 are mounted on pins 49 located in apertures 72 and planet gears 52 are mounted on pins located in the apertures 70. That leaves the apertures 71 which can be used for suitable fixings which interconnect the opposing annular plates of the planet gear carrier to form a cage.Preferably, the annular plates 47, 48 will be identical and suitably drivable connectable to the associated parts 20 and 54, 55 respectively. In that way, the plates 47, 48 could be reversed so as to re-orientate the intermeshing planet gears 50, 52 mirrorwise according to the direction of rotation of the first hollow driven shaft 20. Such a reverse arrangement is shown in Fig.4.

The planet gear 50 may have a greater or lesser number of teeth than the planet gear 52 or both gears may be the same size. It is envisaged that there will be two different sized planet gears 50, 52, two different sizes of annuli 51 and two different sizes of sun gears 54.

Fig.5 shows the larger annulus 51, larger sun gear 54 and smaller planet gears, say 50, in a simple epicyclic arrangement. Fig.6 shows the larger annulus 51, the smaller sun gear 54 and the larger planet wheels, say 52, in a simple epicyclic arrangement and Fig.7 shows the smaller annulus 51, smaller sun gear 54 and smaller planet gears 50 in a simple epicyclic arrangement.

Each of the arrangements uses the same set of annular plates 47, 48 with the appropriate apertures 70, 71 and 72 being used in each case, at least three of the remaining apertures being used for fixing the two plates together to form the cage. This particular aspect of the present invention makes possible the manufacture of a family of transmissions with a choice of ratios for various applications based on common parts and tooling. In that respect the invention is most advantageous.

In operation, reference is made to the ratio selection chart at the end of the following description. In the following description, the numbers given to the ratios i.e., 1, 2, 3 etc. are used to indicate an available ratio but do not necessarily indicate that the ratios correspond to first, second, third gears etc. in ascending order, of say, a motor vehicle transmission.

In order to effect a change over from one ratio to another, to select a starting ratio or to select neutral, a suitable control system would be used operable in response to, e.g., operating parameters of the vehicle such as vehicle speed, throttle position or other suitable parameters commonly used for controlling automatic transmissions in motor vehicles.

Alternatively, the ratios may be selectable manually instead of or in addition to being selectable automatically. Manual operation will be more appropriate where the transmission is being used in applications such as machine tools where an operator or programmed control system could select a particular ratio.

In a neutral condition, the first, second and third clutches 15, 17 and 33 will be disengaged and the brake bands 61a, 65a and 66a will also disengage their respective brake drums. Drive transmitted to the clutch housing 12 will simply rotate the clutch driving plates but no drive will be transmitted to epicyclic gearing. When any one brake or clutch only is engaged, no drive will be transmitted but by engaging the complementary clutch or brake, the selected starting ratio will be taken up. For example, to move from neutral into selected ratio 1 clutch 33 and brake 65a is engaged.

To select ratio 1, clutches 33 and 65a are disengaged and clutches 17, 54a are engaged. Planet gear carrier 57 is thereby held stationary and drive is transmitted through third hollow driven shaft 40, sun gear 54, planet gears 63, 62, annulus 55, planet gear carrier 45, planet gears 52, 50 reacting against stationary sun gear 54 and finally through annulus 52 to output shaft 53.

To select ratio 2 from neutral, clutch 17 and brake 61a is engaged. Drive is then transmitted through clutch 17, second hollow input shaft 28 to rotate sun gear 54.

Brake 61a holds the planet gear carrier 45 stationary so that drive is transmitted from the sun gear 54 through planet gears 50, 52 to annulus 51 and thence to output shaft 53.

To select a third ratio from neutral, clutch 15 is engaged and brake 65a is engaged. Drive is then transmitted through the first hollow input shaft 20 so as to drive planet gear carrier 45. The brake 65a holds the planet gear carrier 57 and hence sun gear 54 stationary, the latter acting as a reaction member for the planet gears 50, 52. Rotation of the planet gear carrier 45 then causes drive to be transmitted from the planet gears 50, 52 to the annulus 51 and thence to the output shaft 53.

To select fourth ratio from neutral, clutch 17 and brake 66a are engaged. Drive is then transmitted through the second hollow input shaft 28 to planet gear carrier 57 and sun gear 54. Brake 66a holds sun gear 64 stationary to act as a reaction member for the planet gears 62, 63. The rotation of the planet gear carrier 57 causes the planet gears 62, 63 to transmit drive to the annul us 55 thereby causing the annulus and planet gear carrier 45 to rotate at a speed different from the rotational speed of sun gear 54. Sun gear 54 then acts as a reaction member for planet gears 50, 52 so as to transmit drive to the annulus and thence to the output shaft 53. It will be noted that, in this ratio, the torque is split between two epicyclic gear arrangements 46, 56.

To select ratio 5 from neutral, all three clutches are engaged and a direct drive is effected between the drive input shaft 10 and the drive output shaft 53.

Instead of engaging all three clutches for direct drive, the engagement of any two clutches would effect a direct drive through the transmission. For example, engagement of clutches 15, 17 will cause two elements of each of the epicyclic gear arrangements 46, 56 to be turned about the common sun gear axis at the same speed thereby creating a direct drive condition.

To select ratio 6 from neutral, clutch 15 and brake 66a are engaged. Drive is then transmitted through first hollow drive input shaft 20 so as to rotate the planet carrier 45 and annulus 55. Brake 66a holds sun gear 64 stationary which acts as a reaction element for planet gears 62, 63. Rotation of the annulus 55 then causes the planet gear carrier 57 and hence and sun gear 54 to rotate at a speed different from the rotational speed of the planet carrier 45. Therefore, the planet carrier 45 or sun gear 54 act as a reaction member for the planet gears 50, 52 thereby causing drive to be transmitted from the planet gears 50, 52 to the annulus 51 and thence to the output shaft 53. Therefore, it will be seen that in ratio 6 torque is split between the two epicyclic gear arrangements 46, 56.

To select a reverse ratio from neutral, clutch 33 and brake 61a are engaged. The brake 61a holds the annulus 55 and planet gear carrier 45 stationary. Drive is then transmitted through the sun gear 64 and planet gears 62, 63 react against the stationary annulus 55 to impart drive to the planet gear carrier 57 and sun gear 54. As brake 61a also holds stationary the planet gear carrier 45, the rotation of planet gears 50, 52 impart a drive in the reverse direction to the annulus 51 thereby causing the output shaft 53 to be driven in reverse.

As mentioned above, the ratios 1 - 6 are not necessarily in ascending order but simply serve to illustrate the way in which six ratios can be achieved with the transmission described.

It will be seen that a simple shift of ratio can be achieved between ratios 1 and 3 where brake 65a can remain engaged and a simple change effected from clutch 33 to clutch 15.

A simple shift can be made between ratios 2 and 4 where clutch 17 remains engaged and a simple change effected from brake 61a to brake 66a.

A simple ratio change can be made between ratios 3 and 5 where clutch 15 remains engaged and it simply requires brake 65a to be released and clutches 33 and 17 to be engaged.

A simple shift can be achieved between ratios 5 and 6 where clutch 15 can remain engaged, brake 66a applied and clutches 33 and 17 released.

A simple shift between ratios 1 and 5 can be achieved as clutch 33 can remain engaged, brake 65a released and clutches 15 and 17 engaged.

Finally, a simple shift can be achieved between ratios 3 and 6 by leaving clutch 15 engaged, releasing brake 65a and engaging brake 66a.

The timing of changes between clutches and brakes in the above simple shifts can be selected to enable ratio changes to be made during transmission of power by the prime mover.

The simple shift patterns lend themselves to the possibility of providing a choice between two ranges each of four speeds each having the simple shift arrangement as described above. One range may comprise ratios 1, 3, 5, 6 and reverse and the other 2, 4, 5, 6 and reverse. The range required could be selected according to the type of terrain or driving style to which the vehicle having the transmission would be subjected.

If desired, the transmission could be limited to four forward speeds available by simple shifting leaving out the complex shifts between ratios 1 and 2 above and ratios 2 and 3.

By suitable choice of gearing, some of the ratios may be duplicated in a particular application resulting in a reduced number of ratios but simplifying the transitional conditions by introducing gear shifts during which no change of ratio takes place. The speed of the prime mover would remain substantially constant in such a case thus minimising the acceleration or deceleration of heavy rotating masses in the prime mover. That would also reduce impulses of torque in the transmission itself by pre-selection of appropriate brakes and clutches prior to a change to the next required ratio.

Where gears are selected automatically, it is envisaged that an electrical control arrangement may be connected, say, to an ignition or injection system of the prime mover. In that case, the ignition or injection could be momentarily interrupted so as to disable the prime mover during a change of ratio thereby avoiding any undesirable rise in input shaft speed where the transmission passes through a transitional neutral condition when releasing and engaging the clutches/brakes, particularly when making complex shifts between ratios 1 and 2 and ratios 2 and 3.

It is envisaged that the transmission will provide four ratios greater than un-ity, a direct drive (unity) and one ratio less than unity (i.e. an overdrive).

If less than six forward ratios are required, one of the clutches could be omitted. For example by omitting clutch 15, ratios 3 and 6 would no longer be available. Omission of one clutch could, therefore leave four forward speeds and reverse or five forward speeds and no reverse (omission of clutch 33).

It will be noted that the input shaft 10 extends completely through the transmission to enable the transmission to receive drive from either end. Any increased loads in the planetary gearing due to the effective reversal of rotation by changing the end to which drive is applied could be compensated by the mirrorwise reversal of the planet gears as shown in Fig.4.

If desired either of the first and second gearing arrangements 46, 56 could incorporate a simple rather than a compound planet gear configuration.

RATIO SELECTION CHART

CLUTCH BRAKE 33 15 17 - 61a 65a 66a I x 2 3 x x 4 x 5 x f R > (

Claims (5)

1. CLAIMS 1. An epicyclic transmission comprising a drive input and a drive output, a first and second epicyclic gear arrangements each comprising an annulus gear, a sun gear and a planet gear mounted on a respective planet carrier, the annulus gear of the first epicyclic gear arrangement being connected to the drive output, the planet carrier of the first epicyclic gear arrangement being connectable to the drive input through a first clutch and the sun gear of the first epicyclic gear arrangement being connectable to the drive input through a second clutch, the annulus gear of the second epicyclic gear arrangement being connected to the planet carrier of the first gear arrangement and connectable to the drive input through the first clutch, the planet gear carriet of the second eipicyclic gear arrangement being connected to the sun gear of the first epicyclic gear arrangement and connectable to the drive input through the second clutch and the sun gear of the second epicyclic gear arrangement being connectable to the drive input through a third clutch, a first brake means operable to prevent rotation of the planet gear carrier of the first epicyclic gear arrangement and of the annulus gear of the second epicyclic gear arrangement, a second brake means operable to prevent rotation of the planet gear carrier of the second epicyclic gear arrangement and of the sun gear of the first epicyclic gear arrangement and a third brake means operable to prevent rotation of the sun gear of the second epicyclic gear arrangement.
2. 2. An epicyclic transmission according to Claim 1 in which at least one of the first and second gear arrangements includes compound planetary gearing.
3. 3. An epicyclic transmission according to Claim 2 in which each of the first and second gear arrangements includes compound planetary gearing.
4. 4. An epicyclic transmission according to any preceding Claim in which at least one of said gear arrangements includes a planet wheel carrier having mounting means therein for enabling the or different sized planet gears to be carried thereon in a number of different radial positions for meshing with sun and annulus gears of different sizes.
5. 5. An epicyclic transmission substantially as described herein with reference to the accompanying drawings.