GB2229509A - Epicyclic friction gearing - Google Patents

Abstract

Epicyclic friction gearing utilizing a double pair of epicyclically driven wheels and comprising first and second wheel B, F eccentrically mounted on a shaft D and friction contacting surrounding wheels of the pairs is characterized by one wheel of each pair being provided with a compliant layer L of elastic material which when compressed produces the friction force necessary to transmit torque. Various wheel constructions embodying compliant layers are disclosed (see Figs 5-7). For example the wheel may comprise inner and outer rings connected together by flexible side walls with the cavity therebetween filled with compressed air (Fig 5). Alternatively the wheel may incorporate a solid star-shaped layer of compliant material (Fig 6); the peaks and troughs of the star may comprise cylinders filled with compressed air (Fig 7). Arrangements utilizing magnetic fields (permanent or electromagnetic) between the wheels of each pair are disclosed (see Fig 8). <IMAGE>

Description

THE GEARLESS EPICYCLIC DRIVE AND ROLLING RESISTANCE BRAKE This invention relates to an epicyclic drive system which, combines the geometry of epicyclic movement with static friction or magnetic force instead of conventional gears. In particular the invention relates to eccentrically driven epicyclic movement and in so doing offers both a system of speed reduction and a system of braking that does not rely on sliding friction but rather "Rolling Resistance".

Epicyclic gear drives are in common use, they normally rely on "planetary" systems rather than eccentrically driven systems because the latter are less efficient an can be prone to uneven output due to erratic take up of backlash in the gears.

Friction drives have been produced using the "planetary" system but these have relied on interference fits between surfaces to generate the friction force and this requires very high levels of precision in all the component parts to achieve functionality.

According to the present invention a static friction force is generated between rotating pairs by introducing a compliant layer in one of them and dimensioning their diameters so that the assembly produces a permanent offset which compresses the compliant layer. The compression of the compliant layer produces a fixed friction force at the point of contact of the pair.

This friction force may be employed in two ways. If the offset is designed to be the minimum possible to guarantee sufficient force is applied then the work done in continuously compressing the compliant layer will also be kept to a minimum and the resultant drive will be as energy efficient as possible. If however the offset is designed to be as large as possible the work done in continually compressing the compliant layer will be greatly increased. The resistance to rolling of the driven wheel will produce a braking effect. The effect is the same as trying to roll a loaded wheel in a soft medium such as sand or gravel.

The principle of the offset compliant wheel need not be confined to epicyclic drives but normal gear systems usually have multiple shafts in parallel, the problem of applying loads to friction wheels does not occur, indeed pulleys may be used instead. In the case of the eccentrically driven epicyclic drive the use of friction wheels overcomes the inherent problem of backlash uptake and makes available the "no runback" characteristic. It also offers, by virtue of its limitations, an inherent overload protection feature at the output.

The resistance to movement necessary to generate epicyclic movement may also be produced by mangetic forces. This may be done in several ways, by attraction, repulsion, with continuous or segmented fields, permanent or electro magnetism and with or without compliant layers.

A specific embodiment of the invention will now be described by way of examples with reference to accompanying drawings in which: Part A incorporating fig. 1 illustrates the principle of an eccentrically driven epicyclic speed reducer (gear driven) and fig. 2 illustrates a planetary equivalent.

Part B incorporating fig. 3 illustrates the "offset" principle and describes its method of application.

Part C incorporating figs. 4,5,6,7,8 and 9 illustrates several types of compliant layers an arrangement of magnetic polarity and an example of a dynamic brake together with their applications.

PART A A DESCRIPTION OF AN ECCENTRICALLY DRIVEN EPICYCLIC GEAR DRIVE AND ss PLANETARY EQUIVALENT The gear drive upon which this device is based is one of the most basic of the large family of epicyclic systems.

The input shaft is fitted with an eccentric (5) which is free to rotate in a gear (1) with say 50 teeth. The high point of the eccentric causes this free gear to engage with a fixed outer gear (2) say 55 teeth. As the eccentric rotates gear (1) rolls around inside gear (2) causing a relative movement of five teeth between them for each revolution. Gear (3) is mounted on the same eccentric and connected to gear (1). Gear (3) rotates inside gear (4) which is connected to the output shaft and free to move. Gear (3) generates a rotation in the output shaft which is in the same direction as the eccentric.

It should be noted that the eccentric causes the gears (1) and (3) to precess rather than rotate and true epicyclic movement is induced i.e.

in the opposite direction to the eccentric for the free gear (1) and opposite to gear (3) for the free gear (4). The combination of the eccentric and the epicyclic movement stop the output shaft from "running back" i.e. the output shaft cannot be rotated by applying a torque, the eccentric can only be rotated via the input shaft. The output shaft is naturally an automatic brake.

However if the load on the output shaft exceeds the limit of the static friction force available the shaft will slip thereby offering overload protection.

This is not true for the planetary equivalent shown at fig. 2. In this case the connected gears (l) and (3) actually roll around their mating gears. In both cases a point on the gears will describe a roseate as the input shaft rotates but the planetary gear generates this roseate by completing several revolutions1 the precessing eccentrically driven gear does not, and it will describe its roseate in the opposite sense.

A planetary system would "runback".

The reductIon ratio for such a drive is as follcws: The inverse of 1 - i2) x 43 = 1 - 55 x 40 (l) t4) 50 45 i.e. the inverse of .C222 = 45:1 When applied te the friction drive alternative the effective radius of the equivalent wheels should be used. It should be noted that in this case the reduction ratio need not be a function of the "whole number of teeth". The number of ratios available is infinite.

Herein after both versions of the input drive shown in figures 1 and 2 i.e. planetary and eccentrically will be referred to as eccentrically driven and the term "eccentric" may be substituted by "radius arm".

PART B THE OFFSET RING PRINCIPLE (Fig. 3) In an idealised situation the diameter of the circle described by any wheel driven in an epicyclic manner would be the same as the diameter of the circle in which it is running. if the described circle is larger movement cannot take place the components would lock.

The offset ring principle is based on this characteristic. The pair of wheels are dimensioned so that the described circle is two large. One half of the mating pair is supported by a compliant layer, this layer must allow a small amount of radial movement but must be as rigid as possible in the torsional sense to transmit torque. On assembly this layer is compressed at the point of contact of the pair. The compliant layer must act as a spring and thus a static friction force is generated between the pair of wheels at the point of contact.

Provided sufficient force is available to compress the compliant layer rotation can occur and epicyclic movement will be generated by the rotating wheel.

Fig. 3 illustrates this arrangement. A is a fixed outer case, E is the eccentric driving the wheel B, C the mating wheel is held in place by a compliant layer D. The circle described by B is slightly larger in diameter than the inside diameter of C. On assembly C is offset from the centre line by an amount equal to half the difference between the two diameters (or the difference between the radii).

The compliant layer may be placed in either half of any pair of wheels assemblied to run in an epicyclic manner i.e. for fig 3 the compliant layer could be part of wheel B. A major advantage is gained over the use of gear wheels when the principle is applied to eccentrically driven epicyclic movements. In these cases the absence of backlash greatly improves the quality of movement. The precessional movement of eccentrically driven epicyclic gears takes up backlash causing breaks in the output rotation. The absence of any backlash in friction drives avoids this problem.

The primary disadvantage of the compliant layer is the amount of energy consumed in continuously compressing this layer. For speed reducing drives this may be mitigated by ensuring the deflection induced at assembly is the minimum necessary to ensure the friction force is generated. The force applied then travels the minimum distance possible reducing the work done to a minimum. If however a mating pair of wheels are assembled to produce a high level of deflection and thus a high rolling resistance they may be employed as a brake.

Neaative Offset For magnet applications the wheels would be dimensioned to produce a small gap between them on assembly instead of a deflection. When one wheel is then attracted to the other the gap closes giving a Ñegative Offset". In the absence of a compliant layer this deflection would be very small amounting to any clearance between the rotating components.

In this case the component tolerancing would have to be more rigorous.

PART C APPLICATIONS Fig 4 illustrates in cross section a simple eccentrically driven epicyclic friction drive employing a compliant layer to generate the friction force needed to produce epicyclic motion and a useful output torque.

An eccentric B is mounted on an input shaft D. This drives wheel C inside E which is mounted on an outer case A via a compliant layer L which is bonded to A and E. This layer may consist of any suitably elastic material i.e. rubber, polyurethane foam, silastic etc. and may take any of the forms shown below. Likewise this layer could be incorporated in wheel C rather than between E and A.

The circle described by C is slightly larger in diameter than the internal diameter of E. On assembly this causes E to be offset from its natural position. This relationship is repeated between wheel F and G.

As D rotates epicyclic movement is generated in C. C is connected to F.

The rotation of F inside G generates epicyclic movement in G. G is free to rotate and it is connected to the output shaft H. As in each case the epicyclic movement generated is in the opposite sense to that of its generator the output shaft rotates in the same direction as the input shaft. In all cases it is important for the rolling surfaces to be rigid to avoid any scrubbing action.

This basic format may be used with any suitable combination of techniques for generating the epicyclic movement. If for instance C and E are driven using electro magnetism this would allow the drive to be switched on and off and F and G could be a magnetic or friction type assembly to produce the output torque.

Unlike a gear drive a speed reducer of this nature will not convert a high speed low torque input into a low speed high torque output. The torque generated at the output shaft will be a fixed value set by the friction force established between the output pair of wheels and the mechanical advantage produced will be the ratio between the input and output torques. This is dependant on the relative dimensions of the eccentric and the driven wheels.

Fig. 5 illustrates another type of compliant layer. Ring B is attached to ring A via two flexible side walls (C) rather in the manner of a car tyre. The side walls are held in place and sealed by rings D which are a press fit onto A and B. The cavity produced is inflated with compressed air thus making B resistant to any movement away from concentricity with A. Whilst the side walls are flexible vertically they are resistant to radial "twisting" motion and can therefore transmit torque. The assembly could be an eccentrically driven wheel or a wheel could be fitted inside B.

Fig. 6 illustrates and alternative assembly with a solid layer of compliant material arranged in a "star" shape. This would be best employed as an eccentrically driven wheel. the compliant layer will be compressed on one side when assembled. This arrangement has the advantage of transmiting torque by compressing the compliant medium rather than putting it in shear.

Fig. 7 illustrates a variation to fig. 6 which employs cylinders of compliant material (C) fitted at the top and bottom of each star".

These cylinders may be tubes filled with compressed air. This has the advantage of generating less heat as the compliant component is continually compressed and released.

Fig. 8 illustrates in very simple terms some possible magnet arrangements. The top half of the part section shows a polarity arrangement across the width of wheels A and B such that they repel.

The bottom half shows a polarity through each wheel such that they again repel. Either case could be reversed such that A and B attract.

Without a compliant layer the assembly would have to be made to relatively high working tolerances to avoid interferance.

The magnetic fields could be continous or segmented. They may be permanent or electromagnetic, the latter allows switching and variable force to be employed. By employing electromagnetism and "Stepper Motor" techniques a localised magnetic field could be generated opposite the high point of the eccentric (E) at the point where a mating pair of wheels meet. This field could be made to follow the eccentric helping to reduce the required input torque and improve overall energy efficiency.

Fig. 9 illustrates a simple brake assembly. Input shaft D mounted on bearings I rotates eccentric B. Wheel C is mounted on this eccentric and rotates in contact with wheel E which is bonded to compliant layer L. This in turn is bonded to an outer case A. The only difference between this and the arrangement in fig. 4 is that the offset is designed to be at a practical maximum. As a result a great deal of energy is required to rotate D due to the high rolling resistance.

If the arrangement shown in fig. 5 is used the braking effect on D may be turned on or off by applying or releasing compressed air or fluid to alter the stiffness of the compliant layer.

Similarly the braking effect may be induced by applying a magnetic field as described under fig. 8. All of these systems would generate a high rolling resistance and therefore a high load on the input shaft.

Work would have to be done to produce epicyclic motion in the driven wheel thereby absorbing energy.

Claims (8)

1. A fixed ratio speed reducer consisting of a two part eccentrically driven epicyclic assembly with pairs of plain wheels in frictional contact generating useful torque. The friction force being produced by the presence of a compliant layer of elastic material in one wheel of each pair. The layer is compressed on one side at assembly due to the inside diameter of the outer wheel being slightly less than that of the circle described by the eccentrically driven wheel. The compression thereby producing a static friction force at the point of contact of each pair.

2. A fixed ratio speed reducer as claimed in 1 employing compliant layers consisting of flexible side walls in the manner of a car tyre which are clamped onto two plain rings in such a manner as to keep the rings apart concentrically. compressed gas or fluid introduced into the cavity between the rings stiffens the assembly in direct ratio to the pressure applied thereby introducing a degree of control over the applied friction force.

3. A fixed ratio speed reducer as cliamed in 1 employing driven wheels made up of two interlocking "star" shapes the space between which is filled with an elastic material. The material is bonded to each half of the wheel and provides the friction force when compressed at assembly.

The "star" shape ensures the resiliant layer is always under compression when transmiting torque rather than shear.

4. A fixed ratio speed reducer as claimed in 3 employing "star" wheels held apart be resiliant cylinders placed axially at the top and bottom of each arm of the "star". These cylinders may be inflatable thus reducing heat build up due to hysteresis.

5. A fixed ratio speed reducer consisting of a two part eccentrically driven epicyclic assembly with pairs of plain wheels in frictional contact generating useful torque. The friction force being produced by magnetic attraction at the point of contact of the mating pair of wheels. Contact may be induced by the magnetic attraction taking up any clearance in the system or by inducing "negative offset" in a compliant layer as described in claims 1 to 4. The magnetic attraction may be generated by permanent or electromagnets and may be continuous or segmental.

6. A fixed ratio speed reducer consisting of a two part eccentrically driven epicyclic assembly with pairs of plain wheels repelling each other by magnetic force. The force of repulsion will be at a maximum at the point of near contact of the mating pair of wheels. The magnetic fields may be continuous or segmented and may be permanent or electromagnetic.

7. A fixed ratio speed reducer consisting of a two part eccentrically driven epicyclic assembly with pairs of plain wheels repelling each other by magnetic force as claimed at 6 but employing a rotating magnetic field corresponding to the point of near contact and in synchronous rotation with the eccentric.

8. A fixed ratio speed reducer assembly employing only one eccentrically driven wheel producing unusable output torque. The wheels are assembled, using any of the methods claimed in 1 to 7, such that the resistance to rolling of the driven wheel is maximised. This increases the load on the drive eccentric causing a breaking effect.