GB2238090A - Power transmission system comprising two sets of epicyclic gears - Google Patents

Abstract

The power transmission comprises two sets of epicyclic gears, there being two transmissions of rotation, either directly or through intermediate gearing, between the sets. In one configuration, input is applied by rotation of shaft 9 causing rotation of the planet carrier 4 of a first gear set, thereby transmitting power through the movement of the planet gears, such as 3, and causing the rotation of sun gear 1 and ring gear 2, these rotations being transmitted directly and respectively to sun gear 5 and ring gear 6 of a second gear set, with the subsequent motion of the planet gears, such as 7, causing rotation of the planet carrier 8 and output shaft 10. A brake 11 is preferably provided to give a fixed ratio when necessary. The transmission of rotations between the trains is alternatively provided through gearing such as a bevel gear or step up or step down gearing. The input, output and rotation transmission may be to various other elements of the gear sets. <IMAGE>

Description

POWER TRANSMISSION SYSTEMS WITH COiSTINUOUSLY VARIABLE RATIO This invention relates to a method of transmitting power between two rotating axles, with the ratio of input to output rotational speeds being continuously variable and naturally selected.

Whenever power needs to be transmitted between rotating axles, the varying conditions of operation which can occur generally dictate a requirement that the transmission system should provide different ratios of input to output rotational speeds. The traditional method of providing this is via a gearbox which can provide a number of discrete ratios, selected manually or automatically. Such transmission systems do have disadvantages and inefficiencies, one example being that, in any particular ratio, acceleration cannot generally be achieved on the output side without varying the power delivery on the input side. Also, in many situations, the most suitable ratio for efficient operation may not be available.It has for some time been understood that the ideal transmission system should provide a continuously variable ratio which could permit output acceleration with constant power delivery at input, as well as free selection of a range of ratios.

Many attempts have been made to design continuously variable transmission systems, usually avoiding the use of gears, which may seem to provide only discrete ratios, and often employing some form of frictional contact between moving surfaces and methods of varying the relevant radii of rotation at contact. Such designs have created problems of excessive frictional losses or limited power handling capacity which have usually prevented their full development or widespread implementation. Other designs based on hydraulics lead to problems of excessive pressure and high pumping losses. What is probably required is a design based on the action of meshing gears, as this is a well established and reliable technology which presents few problems in frictional losses due to meshing and can cope with high power transmission.

Epicyclic or planetary gear trains are regularly employed in the automatic gearboxes of some motor vehicles, usually offering three or four discrete forward ratios, plus reverse. Such gearing is known to be quite reliable and durable and does permit high power transmission. The present invention utilises this existing technology in a different way and provides a transmission system with a continuously variable ratio which will naturally select a ratio to suit prevailing conditions, as well as allowing the ratio to vary during operation.

According to the present invention there is provided a power transmission system with continuously variable ratio comprising two sets of epicyclic or planetary gears, each set of epicyclic gears consisting of a central sun gear and an outer ring gear which can rotate about the same axis of rotation as that of the sun gear, the outer teeth of the sun gear and the inner teeth of the ring gear meshing with the teeth of at least one but preferably two or more planet gears which are radially disposed around the sun gear and have axles supported by a planet carrier which can itself rotate about the comm.Dn axis of rotation of the sun and ring gears, power being transmitted via the rotation of the sun gear, ring gear and planet carrier for each set of epicyclic gears, wherein for the first set of epicyclic gears one such rotation is used for the input of power with the other two rotations being transmitted, either directly or via suitable intermediate gearing, to two of the three coaxially rotating elements, the sun gear, ring gear and planet carrier, in the second set of epicyclic gears with the third such rotation being used for output from the transmission.

Many variations are possible within the scope of the invention as claimed. Power input could be via any one of the three axles in the first set of epicyclic gears and, equally, output could come from any of the three axles in the second set of epicyclic gears.

Similarly the transmission links between the sets of epicyclic gears do not have to be between corresponding axles, nor do the dimensions of corresponding elements of the sets of epicyclic gears need to be the same.

A specific embodiment of the invention will now be described in greater detail, by way of an example, with reference to the accompanying drawings in which: Figure 1 is a diagrammatic view of a transmission in accordance with the invention, seen as a side view in a direction perpendicular to the axes of rotation, with some gears sectioned; Figure 2 is a diagrammatic view of part of the epicyclic gear arrangement on the input side of the transmission of Figure 1, as seen along the axis of rotation X'X in Figure 1; and Figures 3 to 6 illustrate diagrammatically some other possible configurations which could be used in a transmission designed in accordance with the invention.

Referring firstly to Figure 1, a power transmission device in accordance with the invention comprises two sets of epicyclic gears, the first, on the input side of the transmission, consisting of a sun gear 1 and a ring gear 2, both constrained by suitable bearings (not shown) to be able to rotate about a common axis of rotation X'X.

Meshing with these gears is a set of planet gears, such as 3, which can rotate about axles supported by a planet carrier 4, this planet carrier being able to rotate about the common axis of rotation of the sun gear and ring gear. The second set of epicyclic gears, on the output side of the transmission, consists of a sun gear 5, a ring gear 6, a set of planet gears, such as 7, and a planet carrier 8, the sun gear, ring gear and planet carrier being constrained by suitable bearings (not shown) to be able to rotate about the common axis X'X.

In a similar manner, the planet gears mesh with both the sun and ring gears in this set of epicyclic gears. In this preferred embodiment of the invention, the two ring gears 2 and 6 are directly connected so that they can rotate as one unit. Similarly, the two sun gears 1 and 5 are directly connected so that they can rotate at the same angular speed.

The planet gears 3 and 7 could be in any orbital position around their respective sun gears but, for clarity, they have been placed at the top in Figure 1. Also, two planet gears are shown here in each epicyclic set, but the number used can be varied. In this embodiment of the transmission the dimensions of the corresponding elements of the two sets of epicyclic gears are not necessarily the same. In this example, the ratio of the pitch radius of the ring gear to the pitch radius of the sun gear in the first set of epicyclic gears is less than the corresponding ratio for the second set of epicyclic gears.

In operation, power is delivered via rotation of the input shaft 9 which connects directly and coaxially with the planet carrier 4.

Rotation of the planet carrier causes motion of the planet gears and the meshing of the planet gears with the sun gear 1 and the ring gear 2 causes them to rotate, the input power thereby being split into two main parts by the resulting rotation of the sun and ring gears. These two rotations are transmitted directly and respectively to the sun gear 5 and ring gear 6 of the second set of epicyclic gears and their rotation causes rotation of the planet gears whose axles will correspondingly move to cause rotation of the planet carrier 8.

Rotation of this planet carrier is transmitted directly to the output shaft 10 which is connected to the planet carrier and is able to rotate coaxially with it. By this action, the input power, which was split into two main parts, is now recombined into one unit and becomes output via rotation of the output shaft. There would be expected to be some comparatively small power losses in such a transmission, caused by effects such as the meshing of the gears and the energy losses occurring in the motion of the planet gears, but the system of gears does provide efficient power transmission, whatever ratio its operation provides, as will be shown.

Although it is evident that such an arrangement of epicyclic gears will allow motion of the various components, the concept of efficient power transmission needs to be justified, as does the concept of a naturally selected and continuously variable ratio.

In the following mathematics, consideration will only be given to the forces involved in transmitting the power via the rotation of the various gears. Hence the forces shown in Figure 2 are the components of the reaction forces which are tangential to the circles of rotation. The radial components of these reaction forces may be present but they do no work in causing rotation and do not effect the mathematics which follows. Similarly forces such as rolling friction and the weight of the planet gears can be regarded as being negligible in comparison to the forces used in transmitting the power. For these reasons, the derived equations are close to being accurate, accepting some power losses.

Figure 2 shows some of the forces acting on one planet gear, such as 3 in Figure 1. For convenience, the meshing gears will be treated mathematically as if they are effectively laminar structures making rolling contact at points A and B where the pitch circles touch. Also it is assumed that the transmission is operating in a steady state with each of the six concentrically rotating elements in the epicyclic gear train having constant angular velocity. Planet gear 3 has pitch radius r and meshes with sun gear 1 and ring gear 2 which have pitch radii of a and a + 2r respectively. The planet carrier has angular velocity w and the sun and ring gears have angular velocities wl and w2 respectively. In the process of transmitting power through the train of gears, reaction forces will exist between the meshing gears.F is the tangential component of the force acting on the planet gear 3 form its axle, caused by the driven rotation of the planet carrier. F1 and F2 are the tangential components of the reaction forces from the sun and ring gears acting on the planet gear. Reaction forces of the same magnitude act on the sun and ring gears in their direction of motion, thereby transmitting power through their rotation.

The planet gear 3 will have an instantaneous centre of rotation which is assumed to be at some point C, where CA = x. As the transmission is assumed to be in a steady rotational state with no acceleration at input or output, then taking moments about C for the forces acting on the planet gear gives F(x + r) = Flx + F2(x + 2r) .... (1).

Point A has a tangential velocity wla and hence the angular velocity of planet gear 3 about its instantaneous centre of rotation is given by Q = wla/x.

Similarly, considering w2 and w, this angular velocity is also given by Q = w2(a + 2r)/(x + 2r) and Q = w(a + r)/(x + r).

Using these equations in equation (1), and eliminating x, leads to Fw(a + r) = Flwla + F2w2(a + 2r) ..... (2).

A similar equation will apply for all such planet gears in this set of epicyclic gears.

If T,T1 and T2 are the torques transmitted via the planet carrier, sun gear and ring gear, in their direction of rotation, then T = The sum, for all such planet gears, of the terms F(a + r), T1 = The sum, for all such planet gears, of the terms Fla, T2 = The sum, for all such planet gears, of the terms F2(a+2r).

For the last two equations the forces used are those acting on the sun and ring gears respectively, and hence the torque values T1 and T2 are in the direction of rotation of these gears.

Adding each equation of type (2) and using the above equations gives Tw = T1w1 + T2w2 .... (3).

This equation illustrates the fact that the power at input is split up into two main parts and transmitted via the rotation of the sun and ring gears.

When the directioris of angular rotation of the planet carrier, sun gear and ring gear are not all the same, the mathematics is similar and the same result is obtained with power being transmitted via rotation in whichever direction it occurs. Also, if the power is input via the rotation of the sun gear or ring gear it is split up in a similar fashion via the other two rotations.

For the second set of epicyclic gears the power is transmitted directly to the rotating sun gear 5 and ring gear 6 which in turn cause rotation of the planet carrier 8. If this rotation has angular velocity w3 and torque transmission T3, then, by similar mathematics to the above, the equation T3W3 = Tlwl + T2W2 .... (4) will apply.

Combining equations (3) and (4) gives Tw = T3w3 .... (5) and this equation represents the fact that the power at input is the same, except for some natural losses, as the power at output from the transmission, regardless of the ratio which is selected.

This result is, perhaps, no more than would be expected from a gear based transmission system and it still needs to be justified that such a system of gears does provide a continuously variable ratio. Referring again to Figure 2, the axis of the planet gear 3 has a speed of w(a + r) which is the average of the speeds of points A and B, where the meshing occurs.

Hence (wla + w2(a + 2r))/2 = w(a + r) and wla + w2(a + 2r) = w(2a + 2r) In this preferred embodiment, r has a value in the region of a/3, and the above equation becomes 3wl + 5w2 = 8w On the output side of the transmission, the pitch radius of the planet gear is in the region of twice that of the sun gear, in this example, which leads to the equation w1 + 5w2 = 6w3 Eliminating wl from these two equations gives w3 = (4w + 5w2)/9 It can be seen that, for a given value of the input rotational speed w, the output rotational speed w3 can vary by virtue of the fact that w2 can vary. When w2 = 0, w3 = 4w/9 and when wl = 0, W2 = 8w/5 and w3 = 4w/3. These two situations correspond to two w2 typical gear ratios, one low and one high, but the actual ratio may vary continuously over a wide range.

When applied to a road vehicle, such as a car, the value of w will be selected by the driver in accordance with prevailing conditions and requirements, this value being directly related to the engine speed. Similarly w3 is dictated by the speed of the vehicle. In a typical situation, a car could be cruising on a level road with the output torque T3 being just adequate to balance the resistances to motion. Also the power output from the engine and through the transmission would be just adequate to maintain the steady speed. If the driver were then to increase the engine speed, this would first cause a change in w with, perhaps, a small but unimportant change in T, assuming a reasonably flat torque curve for the engine in question.Momentarily w3 would change little and hence the ratio w : W3 would have increased, but, due to equation (5), T3 would increase, causing the car to accelerate. As acceleration continues the ratio w : w3 decreases, if w remains constant whilst w3 climbs. Such an acceleration would be achieved with substantially constant power delivery and with a ratio which would vary during acceleration causing T3 to reduce until it becomes, once again, just sufficient to maintain a new steady speed.

Effectively, in whatever driving conditions were to prevail, the driver of a vehicle equipped with such a transmission could select a specific power delivery to achieve the desired response in terms of acceleration, and the transmission would naturally select a suitable ratio. As conditions vary so will the ratio vary, in a continuous and unobtrusive fashion. In most situations no control over the ratio selection should be necessary.

It is to be expected, however, that situations will occur where control over the ratio selection may be desirable. A high input to output ratio, often called a low gear, can be held by applying a suitable brake to ring gear 2. Equally an intermediate ratio may be held by causing the sun gear 1, ring gear 2 and planet carrier 4 to share a common angular velocity, perhaps by the action of a suitable clutch which, when engaged, connects and causes common rotation of two of the three rotating elements, such as the sun gear and planet carrier.

The holding of an intermediate ratio may be desirable in situations such as cruising downhill in a motor vehicle, and such a configuration could provide it. The holding of a high ratio, or low gear, may be frequently used when a motor vehicle has to start from rest, moving forward, and there is a connection here with the possible provision of a reverse ratio.

An unusual feature of such a transmission system is that it can just as easily provide a continuously variable reverse ratio. All that is required is that w2 is given an appropriate negative value, and in this configuration w2 needs to be less than -4w/5. Let it be assumed that a clutch exists between the engine and the transmission.

When the vehicle is at rest with the engine running this clutch will be disengaged and the transmission will be at rest. If starting in a forward gear is required, this can be achieved by a brake, such as 11, holding the ring gear 2 at rest as the clutch is engaged, until the vehicle is moving above some minimal speed. The disengagement of this brake can then occur, perhaps operating in conjunction with the clutch, to permit the continuously variable transmission system to take over. If, alternatively, starting in reverse is required, the ring gear 2 can be engaged, whilst the transmission is at rest, to a suitable shaft or gear which, when rotating, will constrain the ring gear 2 to rotate in the opposite direction to the input rotation.

When power is applied to the transmission, by engagement of the clutch, the angular velocity of the ring gear 2 will be negative, perhaps with w2 = -w in this configuration, and the output angular velocity w3 will be negative. This would provide a fixed reverse ratio, but it could be made variable if ring gear 2 becomes released to find its own angular velocity. To switch between forward and reverse gears will normally require that the vehicle and the transmission will be brought to rest with the clutch disengaged, but this would be the usual practice in the driving of a motor vehicle.

In normal driving in a forward ratio it should not be possible for reverse to become engaged whilst the vehicle is still moving.

Even in extreme conditions where a very high ratio of input to output rotational speeds is selected the output torque would correspondingly increase and the barrier between forward and reverse could only be crossed if the output torque T3 became effectively infinite, with w3 effectively zero. This situation would seem to be impossible, with the vehicle in motion, and no driving conditions should be so severe as to even approach it.

Once a vehicle is in motion with the continuously variable transmission in operation, the ratio of the input speed to output speed will be dictated by the engine's rotational speed, chosen by the actions of the driver, and the vehicle's speed. If, with this ratio, the torque transmitted causes a traction force which is not in balance with the overall resisting forces then the vehicle will accelerate or decelerate, altering the ratio in the process. If required, the driver will be able to prevent such speed changes by adjusting the engine speed. By controlling the engine speed the driver will be able to control the behaviour of the vehicle in various driving conditions, with the ratio varying to suit existing conditions and requirements.

Many variations are possible within the scope of the invention as claimed. In the preferred embodiment the connections between the two sets of epicyclic gears are simple and direct with the two ring gears rotating as one unit, as do the two sun gears. Different configurations are possible in which the power is transmitted between the two sets of epicyclic gears in other ways, and these are discussed later. The relative pitch radii of the various gears in the epicyclic sets are not confined to the values which have been used here by way of example. Varying the pitch radii of the gears may change the way in which the gear ratio varies but the basic method of operation is unchanged. Similarly the design of the gear teeth and the style of meshing can be varied, as can the number of planet gears used.The bearings used to support the rotating elements of the transmission could vary in both design and location, and the supporting structure or casing can take many forms. Details of the gears, bearings and casing are omitted here.

An alternative embodiment is suggested in Figure 3 in which the sun gear 21 of the first set of epicyclic gears is connected directly to the ring gear 26 of the second set of epicyclic gears, and can rotate coaxially with it as one unit. Similarly the ring gear 22 on the input side connects with the sun gear 25 on the output side via a shaft 30 which passes through shaft 31 connecting sun gear 21 and ring gear 26. As in the previous embodiment, the planet carrier 23 on the input side is connected directly to the input shaft 29 and can rotate coaxially with it as one unit, but the connecting structure does, in this design, go around the first set of epicyclic gears.

The planet carrier 27 on the output side connects directly with the output shaft 32, and can rotate with it about the common axis and with the same angular speed.

In operation the rotation of the input shaft 29 causes the planet carrier 23 to rotate and thereby causes motion of the planet gears, such as 24. The movement of the planet gears causes rotation of the ring gear 22 and sun gear 21 and these rotations are respectively transmitted to the sun gear 25 and ring gear 26 on the output side of the transmission. These two rotations cause motion of the planet gears, such as 28, in the second set of epicyclic gears, thereby causing rotation of the planet carrier 27 and the output shaft 32.

It can be seen that the power will be transmitted in a similar manner, but via a different route, to that which was outlined in the preferred embodiment. Different equations for the angular velocities will apply and assuming that for each set of epicyclic gears the pitch radii of the sun gear, ring gear and planet gear are a, 2a and a/2 respectively, then the equations 3w = w1 + 2W2 and 3w3 = 2wl + w2 are obtained, which lead to W3 = 2w - w2.

Here, as in the preferred embodiment, w, wl and w2 are the respective angular velocities of the planet carrier, sun gear and ring gear on the input side, and w3 is the output angular velocity.

Such an arrangement does permit a more uniform design with the use of similarly dimensioned sets of epicyclic gears, and a wide range of ratios is more readily provided with less comparative variations in the other main rotations. This configuration does, however, involve the more complicated design of the link between the input shaft 29 and the planet carrier 23, which effectively goes around the first set of epicyclic gears.

The basic principle of the invention, that of using one set of epicyclic gears to split up a power delivery and a second set of epicyclic gears to recombine the power transmitted into one output rotation, can be applied in other ways as well as the two configurations suggested so far. Any one of the three coaxially rotating elements in the first set of epicyclic gears, the planet carrier, sun gear or ring gear, can be used for the input of the power delivery and, equally, any one of the three coaxially rotating elements in the second set of epicyclic gears can be used for the output rotation. In the examples given so far, each of the other two rotations on the input side is connected directly, via a suitable axle or shaft, to one of the other two rotations on the output side, so that each linked pair can rotate as one unit.This gives a further two options and hence there are eighteen configurations of this type involving direct transfer of rotations between the two epicyclic sets. Some of these configurations may present practical difficulties but they fall within the scope of the invention as claimed.

Figure 4 shows a possible configuration, as an example of the design options mentioned above, in which power is applied by rotation of the sun gear 41. Planet carrier 42 connects directly to sun gear 44 on the output side, and ring gear 43 connects directly to planet carrier 45 on the output side, each such connected pair sharing a common rotation about the common axis. Power is output via the rotation of ring gear 46.

The examples mentioned so far employ a direct transfer of rotations across the transmission but many further configurations are possible in which the transfer of rotations between the two epicyclic sets may not always be direct and in which some form of intermediate gearing is employed. If, for example, the two ring gears are linked by suitable intermediate gearing then they can be made to rotate in opposite directions. Equally they could be geared together so that the angular speed of the ring gear on the output side is some constant multiple of the angular speed of the ring gear on the input side. Similarly any one rotation on the input side could be multiplied by some constant factor, via suitable gearing, and transferred to become the angular speed of a rotation on the output side.Figure 5 shows a configuration of the invention in which an intermediate gear-wheel 50, typically a bevel-gear meshing with bevel gearing on the edge of each ring gear, is used to cause the two ring gears 51 and 52 to rotate in opposite senses.

It is not essential that the coaxially rotating elements of one set of epicyclic gears should share the same axis as the coaxially rotating elements in the second set of epicyclic gears.

Suitable intermediate gearing can provide parallel but different axes, or non-parallel axes. This intermediate gearing may involve one or more extra gears, similar to 50 in Figure 5, or two rotating elements, one from each of the epicyclic sets, may be geared together by means of the meshing of gearing on the corresponding axles of rotation. Figure 6 shows a configuration of the invention in which outer gearing on each ring gear meshes with another gear-wheel which rotates as one unit with the sun gear in the other set of epicyclic gears. Hence ring gear 62 transmits the power of its rotation to sun gear 63, via the meshing of the outer gearing on the ring gear with gear-wheel 65. Similarly sun gear 61 transmits its power of rotation to ring gear 64, via the meshing of the outer gearing on the ring gear with gear-wheel 66.In each case there may be a change in angular velocity involved, depending on the relative pitch radii of the meshing gears.

In all these configurations power is transmitted by rotation of a planet carrier, ring gear or sun gear in the direction in which the rotation occurs. Whether using direct transfer of rotations between the sets of epicyclic gears or using intermediate gearing, the power equation will apply with good accuracy, allowing for some natural losses. Also, in any of the numerous configurations of the invention, variations are clearly possible in such details as the type of gears used, the pitch radii of the gears, the number of planet gears used, the type and location of the bearings and the design of the casing. All such variations are possible, for each type of configuration, within the scope of the invention as claimed.

Many options in the design of such a transmission would be decided in the course of development. Methods have been suggested for providing a reverse gear ratio and for holding a specific ratio, such as a ratio suitable for starting a motor vehicle from rest.

Whether employing the use of brakes or using other methods to constrain the rotating elements to behave in a desired manner, so as to provide a specific ratio, such decisions would be made during the development stage and much would depend on the configuration used. In view of the fact that epicyclic gearing is widely used in automatic gearboxes, existing methods for providing lubrication should be applicable in any transmission designed in accordance with the invention.

Although reference has been made to the way in which such a transmission could operate in a motor vehicle, there are many other applications. Any device which requires power to be transmitted between rotating axles, with the provision of a variable ratio of input to output rotational speed, could use the transmission of the invention.

Whether in the form of the preferred embodiment or in one of the many possible configurations of the invention, such a transmission fulfils the need for a continuously variable transmission system with very efficient power delivery and natural selection of the ratio in various conditions of operation. Perhaps most importantly, it provides the facility for the output rotational speed to vary, if conditions so dictate, during constant or substantially constant power delivery. Such a transmission should provide much more efficient use of the available torque and power than that which is provided by conventional gearboxes or existing variable transmission systems. In application in a motor vehicle, improvements in economy and performance would be expected and similar improvements in efficiency could be gained in any application where power is to be transmitted between rotating axles and a need exists for variation of the ratio of input rotational speed to output rotational speed.

Claims (4)

1. A power transmission system with continuously variable ratio comprising two sets of epicyclic or planetary gears, each set of epicyclic gears consisting of a central sun gear and an outer ring gear which can rotate about the same axis of rotation as that of the sun gear, the outer teeth of the sun gear and the inner teeth of the ring gear meshing with the teeth of at least one but preferably two or more planet gears which are radially disposed around the sun gear and have axles supported by a planet carrier which can itself rotate about the common axis of rotation of the sun and ring gears, power being transmitted via the rotation of the sun gear, ring gear and planet carrier for each set of epicyclic gears, wherein for the first set of epicyclic gears one such rotation is used for the input of power and in the second set of epicyclic gears one such rotation is used for the output of power, the other four rotations being linked in pairs between the two sets of epicyclic gears, thereby transmitting two such rotations across the transmission system with each such link either being direct, with unchanged angular velocity, or via intermediate gearing to provide a multiplying factor of the angular velocity during transfer between the two sets of epicyclic gears.

2. A power transmission system as claimed in Claim 1 in which the planet carrier in the first set of epicyclic gears is used for the input rotation and the planet carrier in the second set of epicyclic gears is used for the output rotation, with the two sun gears being connected so as to rotate at the same angular velocity on a common axis of rotation and with the two ring gears being connected so as to rotate at the same angular velocity on a common axis of rotation.

3. A power transmission system as claimed in Claim 1 or Claim 2 in which a brake can be applied to one of the three rotating elements, the sun gear ring gear or planet carrier, in one of the sets of epicyclic gears, so as to provide a fixed ratio of input to output rotational speeds when necessary.

4. A power transmission system substantially as described herein with reference to Figures 1 to 6 of the accompanying drawings.