GB2594492A

GB2594492A - Damped coupling

Info

Publication number: GB2594492A
Application number: GB2006345.9A
Authority: GB
Inventors: Parker Simon
Original assignee: Punk Couplings Ltd
Current assignee: Punk Couplings Ltd
Priority date: 2020-04-30
Filing date: 2020-04-30
Publication date: 2021-11-03
Also published as: GB202006345D0

Abstract

A flexible coupling 1, to join a rotatable input to a rotatable output, comprises a first end plate 5 and a second end plate 6 parallel and opposed to one another. An axle (52, fig 4) extends from the first end plate 5, and a tube 62 extends from the second end plate 6. The tube 62 is separated from the axle (52) by one or more bearings 42 to maintain concentricity of the coupling. Elastomeric members 32 between fixed dividers (34, 36) on end plates 5, 6 absorb energy pulses in the input, avoiding transfer of the pulses to the output. The flexible coupling 1 is connected to a coupling 10 which allows misaligned rotational drives to be connected to one another through the flexible drive 1. Coupling 10 comprises a plurality of annular members one nesting inside another, an outer convex surface of an inner member 111 nesting in an inner concave surface of an intermediate member 121 and an outer convex surface of an intermediate nesting in an inner concave surface of an outer member 131.

Description

Damped coupling Technical Field

[0001] This invention relates to a coupling to join two misaligned shafts or shaft to a fixed structure.

Background Art

[0002] It is known to utilise both the torsional damping properties of a rubber ring element to absorb the engine pulses when coupling misaligned shafts, but this is achieved by utilising the out of plane 'twist' capability inherent in rubber ring design. However, this creates an imbalance in the system, with extra forces that need to be overcome therefore inducing inefficiencies and unnecessary stresses and, therefore noise/vibrations in the system. An example of such a coupling is seen in EP0063022A (Dunlop Limited) published 20 Oct 1982. In consequence of the limitations of such a coupling, the angular misalignment is limited to 2° or 3°.

Summary of invention

[0003]According to the present invention a flexible coupling to join an rotatable input to a rotatable output comprises a first end plate and a second end plate parallel and opposed to one another, disposed around a central axis, the first end plate being connectable to the rotatable input and the second end plate to the rotatable output, the one of said end plates having an axle centred on the central axis extending therefrom towards the other of said end plates, the other of said end plates having a tube extending therefrom around the axle and forming a housing for a plurality of bearings between the tube and the axle, an even number of dividers disposed equidistantly from one another around the tube, the dividers having elastomeric material between them; alternate dividers being fixed to the first end plate from which the axle extends, the other dividers being fixed to the co-axial tube.

[0004] In one embodiment the dividers comprise blocks, alternate blocks being fixed to the end plate from which the axle extends, the other block being fixed to the co-axial tube and forcing the elastomeric members into contact with the co-axial tube.

[0005] In this embodiment the blocks are preferably of a wedge shape.

[0006] Preferably, the wedge-shaped blocks have curved sides adjoining the elastomeric members, the curvature of the sides increasing towards axial tube. This arrangement imposing greater pressure on the resilient members towards their edges ensuring good contact with axial tube.

[0007] In another embodiment the first end plate has a cylindrical flange extending towards the second end plate co-axially with the central axis, with a plurality of dividers projecting radially inwardly from the cylindrical flange, and the tube has an equal number of dividers projecting radially outwardly, the inward projecting dividers and outwardly projecting dividers being alternately distributed around the axis, with equal spaces between pairs of dividers, the spaces between the dividers being occupied by wedges of elastomeric material.

[0008] The presence of the bearings between the axle and the co-axial tube forces the flexible joint to flex only around the central axis and not radially thus preventing imbalances associated with the known flexible joint.

[0009] In one embodiment, the rotatable output is connected to the flexible joint though a coupling, said coupling comprising an inner member and an outer annular member, and one or more intermediate members, the members being arranged in pairs, each pair being a first member and a second annular member and having a common first centre on the central axis; a first member having an outer convex spherical periphery; the second annular member having an inner spherical concave periphery in which the outer convex periphery of the first member is received; the outer convex periphery and the inner concave periphery being concentric about the first centre and complementary to one another and co-acting with one another to transmit axial loads between them; the first and second members being constrained to be rotatable one relative to the other an axis orthogonal to central axis, and in which the inner member and the second annular member forming the pair of members with the inner member comprise spherical segments, the segments having a common centre coincident with the first centre.

[0010] Examples of suitable couplings for use with this invention and for enabling misaligned shafts to be connected to one another through the flexible coupling of the invention are described in PCT/GB2015/087080 and PCT/GB2015/087081.

[0011] A further example of a coupling for use with this invention and for enabling misaligned shafts to be connected to one another through the flexible coupling of the invention comprises: * an inner member and an outer annular member and an intermediate annular member which when aligned share the central axis and have a common centre on the central axis; * the inner member having an outer convex periphery, the intermediate annular member having an inner concave periphery in which the outer convex periphery of the inner member is received; the outer convex periphery of the inner member and the inner concave periphery of the intermediate annular member having the same central axis when the coupling is aligned; one or a diametrically opposed pair of first axle(s) disposed radially of the common centre coupling the inner member and the intermediate annular members, and the inner member and the intermediate annular member being constrained by the first axle(s) to rotate, one with respect to the other about a second axis perpendicular to the central axis; * the intermediate member having an outer convex periphery, the outer annular member having an inner concave periphery in which the outer convex periphery of the intermediate member is received; the outer convex periphery of the intermediate member and the inner concave periphery of the outer annular member having the same central axis when the coupling is aligned; a diametrically opposed pair of second axles disposed radially of the common centre coupling the intermediate annular member and the outer annular member, the intermediate annular member and the outer annular member being constrained by the second axles to rotate, one with respect to the other about a third axis perpendicular to both the central axis and the second axis; and * in which the inner member, intermediate annular and outer annular member are spaced apart to leave a gap between each of the members.

[0012] In a further development of coupling as described in the preceding paragraph is one of two such couplings sharing a common centre and in that the outer annular member of one coupling is the inner member of the other coupling.

[0013]As a further alternative the flexible joint of the invention may be joined to a constant velocity joint comprising: * a first coupling and a second coupling, the couplings having a common axis which is the central axis; * each coupling having an inner annular member being disposed around the central axis, an outer annular member and an intermediate member in which the members comprise pairs of members one of the pair being contained within the second of the pair, with the inner member of a pair having an outer convex spherical periphery and the outer member of the pair having an inner spherical concave periphery into which the outer convex periphery of the first inner member of the pair is received and in which the rotation of the outer one of a pair of second members is constrained to be about an axis perpendicular to the common central axis and in which the inner annular member; * a common shaft supported by the inner annular members of the couplings; * the outer member of the second coupling is constrained to move about axes orthogonal to the central axis in the mirror image of the first coupling; * the outer member of one the said first and second couplings being bolted to the second end plate, the other outer member being connectable to the rotatable output.

Brief description of drawings

[0014] Figure 1 is a perspective view of a flexible coupling according to the invention with the output connected to a second coupling to allow a rotatable input to be connected to misaligned rotatable output; [0015] Figure 2 is a perspective view from another angle of the flexible joint of figure 1; [0016] Figure 3 is a perspective view of the flex element of figures 1 and 2 in isolation; [0017] Figure 4 is an exploded diagram showing the component part of the flexible coupling of figures 1 and 2, the second coupling is omitted in this figure; [0018] Figure 5 is a side view of the flexible coupling of figures 1 and 2 with the output connected to a second coupling to allow a rotatable input to be connected to misaligned rotatable output showing the second coupling misaligned; [0019] Figure 6 is a section on the plane A-A of figure 5; [0020] Figure 7 is a section on the line B-B of figure 6 but showing the second coupling aligned; [0021] Figure 8 is a section on the line C-C of figure 6 but showing the second coupling aligned; [0022] Figure 9 illustrates a double concentric coupling suitable for employment with the flexible coupling of the present invention; and [0023] Figure 10 is a vertical section through the couplings of figure 9; [0024] Figure 11 is a section of a second example of the invention, the section being similar to figure 8; [0025] Figure 12 is a section of the second example of the invention on the line DD of fgure11; [0026] Figure 13 is an exploded perspective view of the second example of the invention; and [0027] Figure 14 is another exploded perspective view of the invention from different angle to that of figure 13.

Examples of the Invention [0028] In figures 1 to 8, the output of a flexible coupling 1 is the input of coupling 10 to connect misaligned rotating drives.

[0029] The flexible coupling 1 comprises a first end disc shaped plate 5 connectable to a rotatable input drive and a second disc shaped end plate 6 parallel and opposed to the first end plate 5 and both end plates disposed around a central axis X. [0030] The first end plate 5 has an axle 52 centred on the central axis extending therefrom towards the other of said end plates 6. The axle has an end plate 57, which is bolted by bots 58 to the end plate 5.

[0031] The second end plate 6 has a tube 62 ending therefrom co-axially around the axle 52 and forming a housing for a pair of bearings 42 between the tube 62 and the axle 52. The tube is stepped 64 to allow the bearings 42 to be seated. The bearings 42 are held in place at one end of tube 62 by an annular locking nut 55 having an internal screw thread 56 co-operating with an external screw thread 54 around the axle 52. A flared washer 60, is placed between the annular member 55 and one of the pairs of bearings 42. At the other end of tube 62 the bearings are held in place by a stepped portion 53 of axle 52.

[0032]Around the tube 62 is disposed a flex-ring 3 shown in more detail in figure 3. The flex-ring 3 comprises an even number (six in this case) of wedge blocks 34 and 36, forming dividers between elastomeric members 32 disposed equidistantly from one another around the tube 62. The blocks 34 and 36 are separated from one another by elastomeric members 32. Alternate blocks 36 are bolted to the first end plate 5 by bolts 7. In the embodiment of figures 1 to 8, blocks 34 and 36 are of a wedge shape, with slightly curved side walls as seen in the figures.

[0033] Elastomeric material components can be considered 'hydraulic' in nature, in that they are not compressible, merely deformable in that their volume cannot change only their shape. By altering their initial designed shape and/or selecting different elastomeric materials the vibration or shock absorbing qualities and reactivity can be altered or tuned to particular applications.

[0034] The blocks 34 between those bolted to the first end plate are bolted with bolts 8 to the co-axial tube 62 (see figures 2 and 6). The bolts 8 through the blocks 34 cause the elastomeric members 32 to bear against the coaxial tube 62. This arrangement forces the flex-ring 3 to flex only around the central axis and not radially thus preventing imbalances associated with the known flexible joint. As can be seen in figures 1 to 4, the wedge blocks 34 have curved sides adjoining the resilient members the curvature of the sides increasing towards axial tube. This arrangement imposing greater pressure on the resilient members towards their edges ensuring good contact with axial tube.

[0035] The compliant connecting elastomeric sections 32 of ring 3 absorb the power pulses delivered from, for example, internal combustion engines by continuously and alternately compressing and extending in between the fixed blocks 34 and 36 during operation. Ring 3 is an over-moulded component with fixed blocks 34 and 36 restrained in place by the contiguous moulded elastomeric material, over-moulding also prevents overexpansion radially of the elastomeric material in use.

[0036] Pins 38 help position and retain the blocks 34 in place with respect to the co-axial tube 62. Further pins 59 locate and assist in keeping the blocks 36 in place with respect to the first end plate 5.

[0037] The wedge blocks are normally made of aluminium or aluminium alloy for lightness to minimise inertial forces in the flexible coupling [0038] In this example the flexible joint is connected to a coupling 10 allowing misaligned rotating drives to be connected to one another through the flexible drive 1.

[0039] The coupling 10 comprises an inner annular member 111, an intermediate annular member 121, and an outer annular member 131. The inner member 111 is centred on the central axis X. The inner member 111 has a convex outer peripheral surface 112 which is convexly spherical centred on a point C on the central axis X. Each inner member 111 has a central cylindrical bore 100 respectively, with splines or keyway 101 to engage with longitudinal splines on a shaft or key on a shaft (not shown).

[0040] The intermediate member 121 has an inner concave peripheral surface 123 which is a spherical segment complementary to the convex outer peripheral surface 122 of the inner member 111. The concave surface 123 is also centred on point C. Diametrically opposed axles 114 extend radially of the axis X along second axes Y, which are orthogonal to the central axis X, second axes Y pass through the point C (see figure 8).

[0041] The pairs of axles 114 couple the inner member 111 to the intermediate member 121. The axles 114 constrain the intermediate member 121 to rotate with respect to the inner member 111 about the second axes Y. [0042] The intermediate member 121 has an outer periphery 122 which is convexly spherical centred on point C. The outer member 131 has an inner concave peripheral surface 133 which is concavely spherical and centred on point C complementary to the convex outer periphery 122 of the intermediate member 121. A second pair of diametrically opposed axles 134 extend radially of the central axis X on third axis Z (see figure 7), the third axis Z being orthogonal to the axis Y and axis X. The second pair of diametrically opposed axles couple the intermediate member 121 to an outer member 131. Third axis Z passes through the same centre point C as the central axis X and second axes Y. [0043] The pairs of axles 134 constrain intermediate member 121 and outer member 131 to rotate one relative to the other about axis Z. The pairs of axles 134 allow relative rotation of intermediate member 121 and outer member 131 independently of the relative rotation of the inner member 111 and intermediate member 121.

[0044] The axles 114 are supported in bearings 125 mounted in bores 124 in intermediate member 121 and fixed in bores 115 in the inner member 111. The axles 134 are supported in bearings 129 in bores 128 in the intermediate member and fixed in bores 135 in the outer member 131. The ends 164 of the axles 114 and 134 in bearings 125 and 129 act against the inner end faces 166 of the bearings 125 and 129 to maintain concentricity of the annular members 111, 121,131 [0045] The thicknesses of the members 111, 121 and 131 are chosen so that a small gap 103 is left between the convex and concave outer and inner peripheries 112 and 123 of the inner member 111 and the intermediate member 121; and similarly a small gap 103 is left between the convex and concave outer and inner peripheries 122 and 133 of the intermediate member 121 and the outer member 131. Therefore, the inner and outer surfaces of the annular members 111, 121, 131 do not touch one another.

However, should a failure occur the inner member and the intermediate member will be retained within the structure of the coupling 10.

[0046] Loading slots 102, as described in WO 2015/087081A are provided to allow the inner members 111 to be inserted within the intermediate annular member 121 and the intermediate annular members 121 to be inserted in the outer annular members 131.

[0047] The outer member 131 has a flange190 to one side, which is bolted by bolts 194 engaging the second end plate 6.

[0048]As an alternative to the coupling 10 shown in the figures, a double concentric constant velocity coupling as shown in figures 9 and 10 can be used.

[0049] In figures 9 and 10, a double concentric coupling 20 comprises an inner coupling 21 and an outer coupling 22 having a common centre C. Each coupling 21 and 22 is an example of a coupling according claim1 below.

[0050] The outer member of the first coupling 21 and the inner member of the outer coupling 22 is the same member shared with both couplings. In this description it is called the common annular member and is designated 231 in figures 24 to 26.

[0051] Coupling 21 comprises an inner member 211, which, in this example, is annular, having a central bore 200 with splines 201 around the bore to receive a splined drive or driven shaft (not shown), an outer annular member -namely the common annular member 231 and an intermediate member 221.

[0052] Coupling 22 comprises an inner annular member -namely the common annular member 231, an intermediate member 241 and an outer annular member 251.

[0053] The inner member 211, in this example is annular, the intermediate members 221, 241 and the common annular member 231 comprise spherical segments. Each of the members (211, 221, 231, 241, 251) is disposed around a common central axis X and has a common centre C on the common axis X. [0054] The outer annular member 251 has bolt holes 257 on its side 256 to receive bolts passing through a flange of a shaft (not shown) to be coupled to the outer annular member.

[0055] The inner member has an outer convex spherical periphery 212 and the intermediate annular member 221 of coupling 21 has an inner spherical concave periphery 223 in which the outer convex periphery 212 of the inner member 211 is received.

[0056] The intermediate annular member 221 has an outer convex spherical periphery 222 and the common annular member 231 has an inner spherical concave periphery 233 in which the outer convex periphery 222 of the intermediate annular member 221 is received.

[0057] The common annular member 231 has an outer convex spherical periphery 232 and the intermediate annular member 241 of coupling 22 has an inner spherical concave periphery 243 in which the outer convex periphery 232 of the common annular member 231 is received.

[0058] The intermediate annular member 241 of the second coupling 22 has an outer convex spherical periphery 242 and the outer annular member 251 has an inner spherical concave periphery 253 in which the outer convex periphery 242 of the intermediate annular member 241 is received.

[0059] The outer convex peripheries (212, 222, 232, 242) and the inner concave peripheries (223, 233, 243, 253) are concentric about the centre C and are complementary to one another.

[0060] A pair of diametrically opposed axles 214 extend from opposed bores 215 in the inner member 211 into bearings 225 fixed in bores 224 in the intermediate annular member 221. The axis Y of the axles 214 is perpendicular to the common central axis X. The intermediate annular member 221 of coupling 21 is constrained to rotate about the inner member 211 about the second axis Y perpendicular to the common axis X. [0061] A pair of diametrically opposed axles 234, whose common axis Z is perpendicular both to the common central axis X and the second axis Y, is fixed in opposed bores 236 in the common annular member 231 and mounted in bearings 227 and 247 fixed in bores 226 in the intermediate annular member 221 of coupling 21 and bores 246 in the intermediate annular member 241 of coupling 22. The common annular member 231 thus is constrained to rotate about the intermediate annular member 221 and the intermediate annular member 241 about the common annular member 231 on a third axis Z perpendicular to both the common axis X and the second axis Y. [0062] A pair of diametrically opposed axles 254 extend from opposed bores 255 in the outer annular member 251 into bearings 245 fixed in bores 244 in the intermediate fourth annular member 241. The axis of the axles 254 is aligned with the second axis Y perpendicular to the common central axis X. The outer annular member 251 thus is constrained to rotate about the intermediate annular member 241 about the second axis Y and perpendicular to the common axis X. [0063] The bearings 225, 245, 227, 247 can be plain bearings or capped roller needle bearings.

[0064] The dimensions of the members 211, 221, 231, 241 and 251 and axles 214, 234 and 254 are chosen to provide a small gap 203 between each convex outer periphery and each concave inner periphery. The gaps 203 between the members and concentricity of the members is maintained by the ends of the axles 214 and 254 being fixed by an interference fit in bores 215 and 255 respectively in inner and outer members 211 and 251 and supported by bearings 225 and 245 in the intermediate annular members 221 and 241, and also by axles 234 being fixed in bores 236 in the common annular member 234 by having an interfering fit therein and at either end of the axles by bearings 227 and 247 in the intermediate members 221 and 241.

[0065] The members 211, 221, 231 and 241 are loaded in turn within the members 221, 231, 241 and 251 using opposed loading slots 202 by inserting in a direction parallel to the common central axis, then rotating the smaller member into position. The axles are then located in position.

[0066] The double concentric coupling 20 of figures 9 and 10 form two joints 21 and 22 with the common annular member transmitting rotational motion and torque from one coupling 21 to the other 22 or vice-versa. It acts instead of the joints, shafts, or gears in prior art constant velocity joints to transmit rotational motion from one coupling to the other [0067] In the coupling 1, if a less compliant torsionally damped coupling is required, a slightly embodiment can be used. Such an embodiment is shown in figures 11 to 14.

[0068] In figures 11 to 14, the output of a flexible coupling 1 is the input of coupling 10 to connect misaligned rotating drives.

[0069] The flexible coupling 1 comprises a first end plate 5 connectable to a rotatable input drive and a second end plate 6 parallel and opposed to the first end plate 5 and both end plates disposed around a central axis X. [0070] The first end plate 5 has an axle 52 centred on the central axis extending therefrom towards the other of said end plates 6. The axle has an end plate 57, which is bolted by bots 58 to the end plate 5.

[0071] The second end plate 6 has a tube 62 ending therefrom co-axially around the axle 52 and forming a housing for a pair of bearings 42 between the tube 62 and the axle 52. The tube is stepped 64 to allow the bearings 42 to be seated. The bearings 42 are held in place at one end of tube 62 by an annular locking nut 55 having an internal screw thread 56 co-operating with an external screw thread 54 around the axle 52. A flared washer 60, is placed between the annular member 55 and one of the pairs of bearings 42. At the other end of tube 62 the bearings are held in place by a stepped portion 53 of axle 52. Equidistantly disposed around the tube 62 are dividers 61 projecting in a radial direction form the tube 62.

[0072] The first end plate 5 has a cylindrical flange 50 extending from the periphery of end plate 5 to close to the periphery of end plate 6. Projecting inwardly towards the central axis from the cylindrical flange 50 are further dividers 51, equal in number to the dividers 61 disposed around tube 62. In this case a total of eight dividers 51 and 61 are shown, four of each. The dividers 51 and 61 are disposed around the axis X alternately, with equal separation between each of the dividers.

[0073] Between each pair of dividers 51 and 61, is a wedge-shaped elastomeric member 31.

[0074] In use, radial over-expansion of the elastomeric members 31 is prevented by the flange 50 on one side and the tube 62 on the other.

[0075] The compliant connecting elastomeric members 31 absorb the power pulses delivered from, for example, internal combustion engines by continuously and alternately compressing and extending in between the dividers 51 and 61 during operation.

[0076] As in figures 1 to 8 the flexible joint 1 is connected to a coupling 10 allowing misaligned rotating drives to be connected to one another through the flexible coupling 1.

[0077] The coupling 10 comprises an inner annular member 111, an intermediate annular member 121, and an outer annular member 131. The inner member 111 is centred on the central axis X. The inner member 111 has a convex outer peripheral surface 112 which is convexly spherical centred on a point C on the central axis X. Each inner member 111 has a central cylindrical bore 100 respectively, with splines or keyway 101 to engage with longitudinal splines on a shaft or key on a shaft (not shown). The detail of coupling 10 is as described with respect to figures 7 and 8. As before, the outer member 131 of coupling 10 has a flange190 to one side, which is bolted by bolts 194 engaging the second end plate 6.

Claims

Claims 1. A flexible coupling to join an rotatable input to a rotatable output comprising a first end plate and a second end plate parallel and opposed to one another, disposed around a central axis, the first end plate being connectable to the rotatable input and the second end plate to the rotatable output, the one of said end plates having an axle centred on the central axis extending therefrom towards the other of said end plates, the other of said end plates having a tube extending therefrom around the axle and forming a housing for a plurality of bearings between the tube and the axle, an even number of dividers disposed equidistantly from one another around the tube, the dividers having elastomeric material between them; alternate dividers being fixed to the first end plate from which the axle extends, the other dividers being fixed to the co-axial tube.
2. A flexible coupling according to claim 1 in which the dividers comprise blocks, alternate blocks being fixed to the fixed to the end plate from which the axle extends, the other block being fixed to the co-axial tube and forcing the elastomeric members into contact with the co-axial tube.
3. A flexible coupling according to claim 2 in which alternate blocks are bolted to the coaxial tube, and the remaining blocks being bolted to the end plate from which the axle extends.
4. A flexible coupling according to claim 2 or 3 in which the blocks are wedge shaped.
5. A flexible coupling according to claim 2, 3 or 4 in which the wedge-shaped blocks have curved sides adjoining the elastomeric members, the curvature of the sides increasing towards axial tube.
6. A flexible coupling according to any one of claims 2 to 5, in which the blocks are over-moulded with the elastomeric material to form a single ring structure.
7. A flexible coupling according to claim 1 in which the first end plate has a cylindrical flange extending towards the second end plate co-axially with the central axis, with a plurality of dividers projecting radially inwardly from the cylindrical flange, and the tube has an equal number of dividers projecting radially outwardly, the inward projecting dividers and outwardly projecting dividers being alternately distributed around the axis, with equal spaces between pairs of dividers, the spaces between the dividers being occupied by wedges of elastomeric material.
8. A flexible coupling according to any preceding claim in which the rotatable output is connected to the flexible joint though a second coupling, said second coupling comprising an inner member and an outer annular member, and one or more intermediate members, the members being arranged in pairs, each pair being a first member and a second annular member and having a common first centre on the central axis; a first member having an outer convex spherical periphery; the second annular member having an inner spherical concave periphery in which the outer convex periphery of the first member is received; the outer convex periphery and the inner concave periphery being concentric about the first centre and complementary to one another and co-acting with one another to transmit axial loads between them; the first and second members being constrained to be rotatable one relative to the other an axis orthogonal to central axis, and in which the inner member and the second annular member forming the pair of members with the inner member comprise spherical segments, the segments having a common centre coincident with the first centre.
9. A flexible coupling according to any one of claims 1 to 8 in which the rotatable output is connected to the flexible joint though a second coupling, said second coupling comprising: * an inner member and an outer annular member and an intermediate annular member which when aligned share the central axis and have a common centre on the central axis; * the inner member having an outer convex periphery, the intermediate annular member having an inner concave periphery in which the outer convex periphery of the inner member is received; the outer convex periphery of the inner member and the inner concave periphery of the intermediate annular member having the same central axis when the coupling is aligned; one or a diametrically opposed pair of first axle(s) disposed radially of the common centre coupling the inner member and the intermediate annular members, and the inner member and the intermediate annular member being constrained by the first axle(s) to rotate, one with respect to the other about a second axis perpendicular to the central axis; * the intermediate member having an outer convex periphery, the outer annular member having an inner concave periphery in which the outer convex periphery of the intermediate member is received; the outer convex periphery of the intermediate member and the inner concave periphery of the outer annular member having the same central axis when the coupling is aligned; a diametrically opposed pair of second axles disposed radially of the common centre coupling the intermediate annular member and the outer annular member, the intermediate annular member and the outer annular member being constrained by the second axles to rotate, one with respect to the other about a third axis perpendicular to both the central axis and the second axis; and * in which the inner member, intermediate annular and outer annular member are spaced apart to leave a gap between each of the members.
10. A flexible coupling according to claim 10 in which the second coupling is connected to the output though a third coupling which is concentric with the second coupling, said third coupling being similar to the second coupling, but the outer member of the third coupling is the inner member of the second coupling or vice-versa.
11. A flexible coupling according to any one of claims 1 to 8 in which the output is connected to the flexible joint through a coupling comprising: * a first coupling and a second coupling, the couplings having a common axis which is the central axis; * each coupling having an inner annular member being disposed around the central axis, an outer annular member and an intermediate member in which the members comprise pairs of members one of the pair being contained within the second of the pair, with the inner member of a pair having an outer convex spherical periphery and the outer member of the pair having an inner spherical concave periphery into which the outer convex periphery of the first inner member of the pair is received and in which the rotation of the outer one of a pair of second members is constrained to be about an axis perpendicular to the common central axis and in which the inner annular member; * a common shaft supported by the inner annular members of the couplings; * the outer member of the second coupling is constrained to move about axes orthogonal to the central axis in the mirror image of the first coupling; * the outer member of one of the couplings being connected to the second end plate and the outer member of the other said coupling being connected to the rotatable output.