GB2024344A - Improvements in variable ratio gearing - Google Patents

Abstract

The invention relates to variable ratio gearing which is particularly adapted for use in automobiles, although it could be employed for other purposes, and the object is to provide an infinitely variable ratio device. Essentially, the invention comprises two epicyclic trains, the two sun wheels being connected together, the two sets of planet wheels being mounted on a common planet carrier and the two annulii being connected together but the gear ratios of the two epicyclic trains differ from each other and all three elements of each train are free to rotate so that the velocity of the driven element is the algebraic sum of the indicated velocities obtained by considering the two trains separately. <IMAGE>

Description

SPECIFICATION Improvements in variable ratio gearing Variable ratio mechanisms are available, but these are either only variable in steps, as in the case of an ordinary automobile change-speed gearbox, with its three, four or five forward ratios, or they are more complex than toothed gearing, in that they employ friction drives such as V-belts working in variable pitch diameter pulleys or in that they employ hydraulic fluid as in the fluid flywheel.

The epicyclic gear arrangement has much to commend it, particularly its compactness, but a simple epicyclic gear arrangement which comprises a sun wheel, planet gears and annulus provides a fixed velocity ratio device. Thus for example, in a case where the sun wheel is the driver, the planet carrier is held stationary and the annulus is the driven element, then the velocity ratio is always equal to the number of teeth on the sun wheel divided bythe number of teeth on the annulus.

It is the object of the present invention to provide an epicyclic gear arrangement which has a velocity ratio which can be varied in infinitely small steps over a wide working range.

According to this invention a variable ratio gearing comprises two epicyclic trains, the two sun wheels being connected together, the two sets of planet wheels being mounted on a common planet carrier and the two annulii being connected together, the gear ratios of the two epicyclic trains differing from each other, all three elements (sun wheel, planet carrier and annuius) of each train being free to rotate, so that the velocity of the driven element is the algebraic sum of the indicated velocities obtained by considering the two trains separately.

Preferably, the ratios of the two epicyclic trains are such that at certain speeds, one train would produce rotation of the driven element in a forward direction and the other train would produce rotation of the driven element in the reverse direction. If the indicated forward and reverse speeds of the driven element were equal, this would give a zero output speed, and if they are only slightly different, this gives a very low output speed (i.e. a high velocity ratio of the complete arrangement).

It is essential to allow the planet carrier to rotate (although the planet carrier could be the driver or the driven element of the arrangement) because it is necessary for the planet wheels to rotate on their own axles at different angular velocities - and even at times in opposite senses-whilst the carrier itself is rotating.

In a conventional epicyclic gear train, one of the three elements must be locked against rotation, so that one of the other two elements acts as the driver and the other as the driven element. In the arrangement in accordance with the invention, all three elements of each train are free to rotate, so that if there is no load on the driven element, the entire assembly will rotate as a unit with all the elements locked together. At the other end of the operative range, if the load on the driven element is such as to almost prevent rotation of that element, then the planet wheels will rotate at velocities which produce as near as possible to zero speed of the driven element.

A variable ratio gearing in accordance with the invention will automatically decrease its own ratio towards a 1/1 ratio (i.e. a through drive in which the sun wheel, planet carrier and annulus all rotate at the same speed about the sun wheel axis) as the load on the driven element decreases. This feature of the invention makes it particularly applicable to use in the transmission of an automobile. In fact, a variable ratio gearing in accordance with the invention may be used in place of the change speed gear box of an automobile.

According to a preferred feature of the invention the variable ratio gearing is connected to the input of two sets of transmission gearing which are coupled to an output in such a manner that for a given input one produces forward motion of the output and the other produces reverse motion of the output. The two sets of transmission gearing may themselves comprise epicyclic gear trains, in which case they can be arranged coaxially with the variable ratio gearing.

Although reference has been made to toothed gearing, it is to be understood, that the epicyclic gear trains do not necessarily comprise toothed gears, since they could also take the form of friction gears.

However, if substantial loads are to be transmitted, as would be the case in an automobile, then it is clearly desirable to employ toothed gearing.

One embodiment of the invention, will now be described by way of example only, with reference to the accompanying drawing, which is a diagrammatic representation of part of the transmission of an automobile.

A driven shaft 10 is coupled directly to the engine crankshaft of the automobile, so that it rotates at engine speed, and always in one direction, as determined by the design of the engine. The shaft 10 is journalled in bearings which are not shown in the diagram, and indeed it will be apparent that various elements of the arrangement illustrated require to be mounted in bearings, but the bearing and housing arrangement has been omitted for simplicity.

A first sun wheel 12 is keyed on to the driving shaft 10, and in this particular arrangement, this sun wheel has 39 teeth. A planet carrier 14 carries a series of planet wheels 16 which mesh with the sun wheel 12 in conventional manner. An annulus 18 has a first set of internal gear teeth 20, which engage with the planet wheels 16. In this particular arrangement, each of the planet wheels 16 has 19 teeth, and the gearing 20 of the annulus 18 has 78 teeth.

The arrangement comprising the sun wheel 12, planet wheels 16 and gearing 20, can be considered as a first epicyclic gear train, and this gear train would function in the conventional manner if the annulus 18 were locked against rotation. In that case, the planet carrier 14 would be the driven element, and would rotate at half the angular velocity of the sun wheel 12.

However, in this arrangement, the annulus 18 is free to rotate in bearings (not shown) and this forms a significant feature of the invention. A second sun wheel 22 is keyed on to the driving shaft 10, but this sun wheel has 36 teeth, as compared with the 39 teeth of the sun wheel 12. A set of planet wheels 26 is provided on the carrier 14, and these mesh with the sun wheel 22, and each has 21 teeth. The annulus 18 is formed with a gear ring 30 which meshes with the planet wheels 26, and the gear ring 30 has 78 teeth (i.e. the same number as the gear ring 20).

Because the sun wheels 12 and 22 have different numbers of teeth, whilst the planet wheels 16 and 26 have the same number of teeth, the pitch circle diameters of the planet wheels in the carrier 14 are different.

The carrier 14 provides the driven element of the gear assembly so far described, and is coupled as indicated by the chain-dotted line 32 to an input shaft 34 of two epicyclic gear transmissions indicated diagrammatically at 36 and 38. The transmission 36 provides a forward drive to an output shaft 40, and the transmission 38 provides a reverse drive to the output shaft 40. The output shaft itself is connected to the cardan shaft of the motor vehicle, and through that shaft, to the rear axle drive of the vehicle.

The input shaft 34 is connected to a planet carrier 42 of the spicyclictrain 36, this train also including a sun wheel 44 keyed to the output shaft 40, and an annulus 46. A brake 48 is provided for engagement with the annulus 46, and when the brake 48 is applied, the annulus is held stationary, so that drive is transmitted from the planet carrier 42 to the sun wheel 44, which in turn drives the output shaft 40.

This is the normal condition for forward transmission through the epicyclic gear trains 36 and 38.

A sun wheel 50 of the reverse epicyclic gear train 38 is carried on a boss of the annulus 46, and hence rotates with that annulus. The sun wheel 50 is free to rotate relatively to the output shaft 40. A planet wheel carrier 52 has its planet wheels engaging with the sun wheel 50, and is keyed on to the output shaft 40. The planet wheels engage with an annulus 54, and there is a brake 56 which can be employed to arrest motion of the annulus 54. When the drive is being transmitted through the forward epicyclic train 36, the brake 56 is not applied, and hence, when the planet carrier 52 rotates around the sun wheel 50 (being driven by the output shaft 40) this motion is simply accommodated by rotation of the annulus 54.

However, if the vehicle is to be put in reverse, the brake 48 is released, and the brake 56 is applied.

Motion is then transmitted from the input shaft 34 through the planet carrier 42, and since there will be a load opposing rotation of the sun wheel 44, this causes the annulus 46 to rotate. This in turn rotates the sun wheel 50, and since the annulus 54 is arrested by the brake 56, the planet carrier 52 rotates in the reverse direction, taking the output shaft 40 with it.

The two epicyclic gear trains 36 and 38 which have been described, merely illustrate possible ways of converting output from rotation of the planet carrier 14 into forward or reverse rotation of an out put shaft from the entire system. In practice, the two epicyclic trains could be replaced by alternative gear ing arrangements, permitting a change in the direc tion of rotation, but epicyclic gear trains have been illustrated, since these provide a particularly compact arrangement, which can be coaxial with the driving shaft 10 of the arrangement.

Supposing that the vehicle is stationary, then there will be a load applied from the vehicle road wheels to the shaft 34. This load will resist rotation of the planet carrier 14. When the engine is rotated, the drive shaft 10 rotates, and the two sun wheels 12 and 22 rotate with it. Initially, this causes the planet wheels 16 and 26 to be rotated about their own axles, but as they attempt to apply differing velocities to the annulus 18, that annulus is rotated at a speed which is the algebraic sum of the indicated velocities of the two epicyclic gear trains mounted on the common driving shaft 10.In practice, this can only be achieved, if there is some rotation of the planet carrier 14, and if there is adequate torque supplied by the engine, the planet carrier will in fact rotate very slowly, the gear arrangement providing in effect a very high velocity ratio between the driving shaft 10 and the output shaft 34.

Supposing that the driving shaft 10 rotates through one revolution, then the sun wheel 22 would revolve the planet carrier 14 through 0.35 revolution (assuming that the annulus 18 were held stationary).

At the same time, the sun wheel 12 would attempt to rotate the annulus 18 through -0.5 revolution, assuming the planetary 14 were held stationary. If the annulus 18 rotates through one revolution, it will rotate the planet carrier 14 through 0.65 revolution.

So, as the annulus 18 has in fact revolved through -0.5 revolution, it will revolve the planet carrier 14 through -0.5 x 0.65 = -0.325 revolution. At the same time, the sun wheel 12 will have driven the planet carrier 14 through 0.350 revolution. Hence, the annulus 18 will be turned through 0.350 + (-0.0325) = 0.025 revolution.

At each succeeding revolution of the driving shaft instead of the annulus 18 remaining stationary (which would produce a fixed gear ratio) the annulus creeps forward another 0.025 revolution and the overall gear ratio is reduced.

As the load on the output shaft 34 decreases, due to increased vehicle speed, the overall ratio of the double epicyclic arrangement will increase, towards a 111 ratio. Eventually, the 1/1 ratio will be attained, and this is equivalent to the top gear position of a conventional change speed gear box.

At all times, the ratio of the compound epicyclic arrangement, will match itself to the engine speed and the load applied through the vehicle road wheels. Hence, it is unnecessary to have any change speed gearbox, and it is unnecessary to operate any kind of clutch between the engine and the following transmission elements.

In the specific example described above, the numbers of teeth on the gears 12,16,20,22 and 30 have been specified, but it is to be understood that gear wheels having different numbers of teeth could be used if a different overall gear ratio is required.

Also, whilst the specific gear train has been described for use in an automobile, it will be appreciated that the invention is not limited in its application to automobiles.

Claims (5)

1. A variable ratio gearing comprising two epicyclic trains, the two sun wheels being connected together, the two sets of planet wheels being mounted on a common planet carrier and the two annulii being connected together, the gear ratios of the two epicyclic trains differing from each other, all three elements (sun wheel, planet carrier and annulus) of each train being free to rotate, so that the velocity of the driven element is the algebraic sum of the indicated velocities obtained by considering the two trains separately.

2. A variable ratio gearing as claimed in Claim 1, in which the ratios of the two epicyclic trains are such that at certain speeds, one train would produce rotation of the driven element in a forward direction and the other train would produce rotation of the driven element in the reverse direction.

3. A variable ratio gearing as claimed in Claim 1 or Claim 2, connected to the input of two sets of transmission gearing which are coupled to an output in such a manner that for a given input one produces forward motion of the output and the other produces reverse motion of the output.

4. Avariable ratio gearing as claimed in Claim 3, in which the two sets of transmission gearing themselves comprise epicyclic gear trains arranged coaxially with the variable ratio gearing.

5. Avariable ratio gearing constructed and arranged substantially as herein described with reference to the accompanying drawing.