WO2009106840A2

WO2009106840A2 - Apparatus for converting between rotational and linear movement

Info

Publication number: WO2009106840A2
Application number: PCT/GB2009/000549
Authority: WO
Inventors: James Macnaghten; Ronald John Dennis; Jonathan Sebastian Howes
Original assignee: Isentropic Limited
Priority date: 2008-02-29
Filing date: 2009-02-27
Publication date: 2009-09-03
Also published as: WO2009106840A3; GB0803796D0

Abstract

Apparatus (10) for converting between rotational and linear movement, comprising: a body (20) configured to rotate about a first axis (A1-A2), the body (20) defining a track (51) having a surface which is asymmetric about the first axis (A1-A2); first and second rollers (33, 34) located on opposed lateral sides of the first axis (Al- A2), the first and second rollers (33, 34) each being configured to rotate about at least a second axis (CBl- CB2) inclined relative to the first axis (A1-A2) and each comprising an outer contact surface configured to engage the surface of the track (51) defined by the body (20) when the body (20) rotates, whereby in use rotation of the body (20) relative to the first and second rollers (33, 34) results in oscillating displacement of the first and second rollers (33, 34); and a pivotable rocker (30) extending between the first and second rollers (33, 34), the rocker (30) being configured to maintain contact between the surface of the track (51) defined by the body (20) and the outer contact surfaces of the first and second rollers (33, 34).

Description

TITLE: APPARATUS FOR CONVERTING BETWEEN ROTATIONAL AND

LINEAR MOVEMENT

DESCRIPTION

The present invention relates to apparatus for converting between rotational and linear movement and particularly, but not exclusively, to apparatus for use with piston and cylinder assemblies.

A swashplate is a device used in Mechanical Engineering to translate the motion of a rotating shaft into a linear back and forth motion. Conversely it can used to translate a reciprocating motion into a rotating one and can be used to replace use of a crankshaft in engine designs. Swashplates are used in a wide variety of applications that include but are not restricted to internal and external combustion engines, compressors, pumps and other reciprocating machinery.

A conventional swashplate comprises a rotatable shaft and a disk coupled to the shaft, the disk having a surface which is asymmetric about the axis of the rotatable shaft.

In this way, rotation of the shaft will cause the disk edge to appear to describe an oscillating linear path when viewed from a non-rotating point of view away from the shaft. The greater the angle of the surface of the disk to the shaft the more exaggerated the apparent linear motion will be. The apparent linear motion can be turned into an actual linear motion by means of a follower, stationary with respect to the shaft but which presses against the top or bottom edge of the plate.

The swashplate device has many similarities to a cam.

Like a cam, the follower can experience high unbalanced loads and in a conventional design there is often significant friction between the follower and the plate. There must also be a means to ensure that the follower remains in contact with the plate. This is often achieved via a spring, but this means that there is an additional force on the plate at all times. The present applicant has identified the need for apparatus which overcomes, or at least alleviates, some of the above-mentioned problems associated with conventional swashplate mechanisms .

In accordance with a first aspect of the present invention, there is provided apparatus for converting between rotational and linear movement, comprising: a body configured to rotate about a first axis, the body defining a track having a surface which is asymmetric about the first axis (e.g. with the surface of the track being inclined relative to an axis normal to the first axis) ; first and second rollers located on opposed lateral sides of the first axis, the first and second rollers each being configured to rotate about at least a second axis inclined relative to the first axis and each comprising an outer contact surface configured to engage the surface of the track defined by the body when the body rotates, whereby in use rotation of the body relative to the first and second rollers results in oscillating displacement of the first and second rollers (e.g. with a component of the displacement parallel to the first axis) ; and a pivotable rocker extending between the first and second rollers, the rocker being configured to maintain contact between the surface of the track defined by the body and the outer contact surfaces of the first and second rollers.

In this way, an improved swashplate apparatus is provided in which contact between the first and second rollers and the track surface may be maintained without requiring biasing means for applying a force on the rollers. When used with pairs of pistons and cylinder assemblies, the apparatus can provide a mechanism that is both mass and force balanced such that the device will in use exhibit minimal vibration. In addition, when used in a balanced configuration opposing forces acting on the mechanism may be aligned and in balance resulting in a mechanism capable of handling very high loads relative to the size of the mechanism. The body and rollers may be coupled to first and second couplings respectively. The first and second couplings may be configured to provide an input/output force depending upon whether the apparatus is to be used to convert rotational movement to linear movement or vice versa. The rollers may be indirectly coupled to the second coupling via the rocker. The angular position of the rollers may each be fixed (or substantially fixed) relative to the first axis. In one embodiment, the first and second rollers are rotatably coupled to the rocker at points on opposed lateral sides of the first axis. In this way, the first and second rollers are constrained to pivot with the rocker . In one embodiment, the surface of the track defined by the body at least notionally extends through an intersection point (P) on the first axis. The rocker may be pivotable about the intersection point (P) . In this way, the rocker may be configured to maintain a predetermined orientation relative to the body as it rotates .

In one embodiment, the first and second rollers are substantially cylindrical rollers or spheroidal rollers.

In another embodiment, the outer contact surfaces of the first and second rollers may each define a substantially frusto-conical profile (e.g. conical profile) centred around a respective axis of rotation extending through the intersection point (P) . In this way, an improved swashplate apparatus is provided in which the surface (or notionally extending surface) of the track and the rotational axis of the roller intersect the first axis at a common intersection point (P) . Advantageously, such an arrangement substantially minimises the potential for scuffing between the roller and the track surface during movement of the body relative to the roller.

In one embodiment, the surface of the track defines a further frusto-conical profile. In the case of rollers each having a surface defining a substantially conical profile, the surface of the track may also define a conical profile. However, in the case of rollers each having a surface defining a substantially frusto-conical profile, the profile of the track surface need only be frusto-conical . In another embodiment, the surface of the track is substantially planar.

The outer contact surface of the rollers may be continuous (i.e. the rollers themselves may each comprise a frusto-conical part configured to define their respective outer contact surfaces) . In another embodiment, the outer contact surface of each roller may be discontinuous. For example, each roller may comprise a plurality of spaced frusto-conical parts configured to define its respective outer contact surface. In one embodiment, the surface of the track and the outer contact surface of each roller may comprise interengageable teeth.

In one embodiment, opposed lateral edges of the outer contact surfaces of the rollers are curved. For example, the opposed lateral edges of each roller may have a slight crowning. In this way, the presence of sharp lateral edges capable of cutting into the track surface may be avoided. The first and second rollers may be diametrically opposed to one another relative to the first axis. In one embodiment, the rocker is constrained to pivot around a single axis of rotation. In this way, a rocker is provided having only one degree of freedom. Advantageously, such an arrangement may substantially avoid unwanted transmission of side loads through the mechanism.

The apparatus may further comprise third and fourth rollers as previously defined and a further pivotable rocker extending between the third and fourth rollers, the further rocker being configured to maintain contact between the outer contact surfaces of the third and fourth rollers and the surface of the track defined by the body.

The further rocker may be pivotable about an axis (e.g. constrained to pivot about a single axis) perpendicular to the axis of the first-mentioned rocker.

The further rocker may be pivotable about an axis extending through the pivot axis of the first-mentioned rocker. For example, the further rocker and first- mentioned rocker may each be pivotable about axes extending through intersection point (P) . The further rocker may be pivotalIy coupled to the apparatus at pivot points spaced from the pivot points of the first-mentioned rocker (e.g. spaced from the intersection point (P)) .

In another embodiment, the rocker is configured to pivot around two axes of rotation (e.g. two orthogonal axes) . For example, the rocker may be coupled to the apparatus by means of gimbal means. In this embodiment, the apparatus may comprise a third roller as previously defined coupled to the rocker. The first, second and third rollers may be substantially equally spaced around the track. The two axes of rotation may each extend through the intersection point (P) on the first axis

In a further embodiment, the apparatus comprises a further mechanism as defined in any of the embodiments above located an opposed side of the body. In this way, apparatus which is both force and mass balanced may be achieved.

In accordance with a second aspect of the present invention, there is provided apparatus for converting between rotational and linear movement, comprising: a body configured to rotate about a first axis; the body defining a track having a surface at least notionally extending through an intersection point (P) on the first axis, the surface being asymmetric about the first axis (e.g. with the surface of the tracking being inclined relative to an axis normal to the first axis) ; and a roller configured to rotate about a second axis inclined relative to the first axis, the roller comprising an outer contact surface configured to engage the surface of the track defined by the body when the body rotates, whereby in use rotation of the body relative to the roller results in oscillating displacement of the second axis relative to the intersection point (P) (e.g. with a component of the displacement parallel to the first axis) ; wherein the outer contact surface of the roller defines a substantially frusto-conical profile (e.g. conical profile) centred around the second axis, and the second axis extends through the intersection point (P) .

In this way, an improved swashplate apparatus is provided in which the surface (or notionally extending surface) of the track and the rotational axis of the roller intersect the first axis at a common intersection point (P) . Advantageously, such an arrangement substantially minimises the potential for scuffing between the roller and the track surface during movement of the body relative to the roller.

When used with pairs of pistons and cylinder assemblies, the apparatus can provide a mechanism that is both mass and force balanced such that the device will in use exhibit minimal vibration. In addition, when used in a balanced configuration opposing forces acting on the mechanism may be aligned and in balance resulting in a mechanism capable of handling very high loads relative to the size of the mechanism. The body and roller may be coupled to first and second couplings respectively. The first and second couplings may be configured to provide an input/output force depending upon whether the apparatus is to be used to convert rotational movement to linear movement or vice versa. The roller may be indirectly coupled to the second coupling via a rocker as defined below. The angular position of the roller may be fixed (or substantially fixed) relative to the first axis.

In one embodiment, the surface of the track defines a further frusto-conical profile. In the case of a roller having a surface defining a substantially conical profile, the surface of the track may also define a conical profile. However, in the case of a roller having a surface defining a substantially frusto-conical profile, the profile of the track surface need only be frusto- conical . In another embodiment, the surface of the track is substantially planar. The outer contact surface of the roller may be continuous (i.e. the roller itself may comprise a frusto- conical part configured to define the outer contact surface) . In another embodiment, the outer contact surface of the roller may be discontinuous. For example, the roller may comprise a plurality of spaced frusto- conical parts configured to define the outer contact surface. In one embodiment, the outer contact surface of the roller and the surface of the track may comprise interengageable teeth. In one embodiment, opposed lateral edges of the outer contact surface are curved. For example, the opposed lateral edges may have a slight crowning. In this way, the presence of sharp lateral edges capable of cutting into the track surface may be avoided.

The apparatus may further comprise a rocker pivotable about an axis extending through the intersection point (P) and including a part extending to one lateral side of the first axis, the part being configured to maintain contact between the outer contact surface of the roller and the surface of the track defined by the body. The roller may be rotatably coupled to the part of the rocker. In this way, the rocker may be configured to maintain the correct orientation of the roller relative to the track surface.

In one embodiment, the rocker is constrained to pivot around a single axis of rotation extending through the intersection point (P) . In this way, a rocker is provided having only one degree of freedom. Advantageously, such an arrangement may substantially avoid transmission of side loads through the mechanism. In one embodiment, the rocker includes biasing means for maintaining the part of the rocker in a predetermined orientation relative to the body at a point on said one lateral side of the first axis. The biasing means may comprise spring means.

The apparatus may comprise a further roller as previously defined, the further roller and first-mentioned roller being diametrically opposed to one another relative to the first axis. The rocker may include a further part extending along a second lateral side of the first axis opposed to said one lateral side, the further part being configured to maintain contact between an outer contact surface of the further roller and the surface of the body. In one embodiment, the first-mentioned roller and further roller may each be rotatably coupled to the rocker at points on opposed lateral sides of the first axis. In this way, contact between each of the rollers and the track surface may be maintained without requiring biasing means .

The apparatus may comprise a yet-further roller as previously defined and a further rocker pivotable about an axis extending through the intersection . point (P) and including a part extending to one lateral side of the first axis, the part being configured maintain contact between an outer contact surface of the yet- further roller and the surface of the body. The further rocker may be pivotable about an axis perpendicular to the axis of the first-mentioned rocker. The further rocker may be pivotally coupled to the apparatus at pivot points spaced from the intersection point (P) .

In another embodiment, the rocker is configured to pivot around two axes of rotation extending through the intersection point (P) on the first axis. For example, the rocker may be coupled to the apparatus by means of gimbal means. In this embodiment, the apparatus may comprise a further roller as previously defined and a yet- further roller as previously defined. Each of the further roller and yet-further roller may be coupled to the rocker. The first-mentioned, further and yet-further roller may be substantially equally spaced around the track. In a further embodiment, the apparatus may comprises a further mechanism as defined in any of the embodiments above located an opposed side of the body. In this way, apparatus which is both force and mass balanced may be achieved.

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings in which:

Figure 1 shows a schematic cross-sectional view of a modified swashplate mechanism according to a first embodiment of the present invention;

Figures 2A-2D show schematic cross-sectional views of the modified swashplate mechanism of Figure 1 during operation of the mechanism; Figure 3 shows a schematic view of the modified swashplate mechanism of Figure 1;

Figure 4 shows a schematic cross-sectional view of a modified swashplate mechanism in accordance with a second embodiment of the invention; Figure 5 shows a schematic view of the modified swashplate mechanism of Figure 4 ;

Figure 6 shows a further schematic view of the modified swashplate mechanism of Figure 5;

Figure 7 shows a schematic view of a modified swashplate according to a third embodiment of the present invention;

Figure 8 shows a schematic view of a modified swashplate mechanism according to a fourth embodiment of the present invention;

Figure 9 shows a schematic view of a modified swashplate mechanism according to a fifth embodiment of the present invention; and Figure 10 shows a schematic view of a modified swashplate mechanism according to a sixth embodiment of the present invention.

Figure 1 shows a cross-section through a modified swashplate mechanism 10 comprising: swashplate 20; rocking beam assemblies 30, 31. The rocking beam assembly 30 comprises: beam 32; frusto-conical rollers 33,34; pivot 35; drive fork 36; bearing 37 & 39; con rod 38 and drive rod 40. The rocking beam assembly 31 comprises: beam 42; frusto-conical rollers 43,44; pivot 45; drive fork 46; bearing 47 & 49; con rod 48 and drive rod 50. The swashplate 20 comprises a pair of tracks defining running surfaces 51 & 53 each defining a frusto-conical profile, and swashplate body 52.

To achieve the required geometry the apex of the frusto-conical surface D2-P-D1 must fall at the pivot point P. This surface is at an incidence θl to the plane B1-B2. The running surface 51 must be coincident with this frusto-conical surface. The roller wheel 34 must comprise of part of the conic surfaces made up of CA2-P- CC2, where the centre of rotation of the wheel is along the axis CB2-P. The roller wheel 33 must comprise of part of the conic surfaces made up of CAl-P-CCl, where the centre of rotation of the wheel is along the axis CBl-P. The beam 32 rotates about pivot 45, where the centre of the axis of rotation passes through the pivot point P. The beam 32 is laterally constrained so that it can only move in one vertical plane that is perpendicular to the plane B1-B2 and the axis of rotation of the beam 32.

Connected to the beam 32 is the roller wheels 33 & 34 and also the drive fork 36. The drive fork controls the position of the drive rod 40 via the bearings 37&39 and the con rod 38. The geometry of the rocking beam assembly 31 is effectively a mirror of that above the plane B1-B2.

In operation the swashplate mechanism rotates about the axis A1-A2. This rotates the conic plane D2-P-D1 such that the surface appears to oscillate between a high and a low position once each cycle. In the figure the roller 33 is in a high position and the roller 34 in a low position. As the swashplate rotates the rollers 33 & 34 roll on the running surface 51 with minimal scuffing or slippage. As the roller 33 or 34 moves from the low point to the high point they force the beam 32 to oscillate between the low and the high point with a sinusoidal motion. The roller that is moving from the low to the high point is normally the roller that is seeing the majority of the load when the swashplate is driving the shafts, for example as a compressor. The reverse is true when the swashplate is being driven by the shafts, such as in the case of an engine .

The basic factors for determining the geometry of the mechanism in this embodiment are as follows:

A reference angle (θl) is picked that will control the inclination of the circular plate D1-D2. It is assumed that Dl the highest point and D2 is currently at the lowest point that it can reach relative to the plane Bl-

B2.

D1-D2 is located such that a perpendicular taken from it's centre C will intersect the pivot P.

The Pivot P is set on the axis of rotation A1-A2 and allows beam 32 to rotate in one axis only. When Dl and D2 are in the configuration described (highest-lowest) then the axis of rotation of D1-D2 will be perpendicular to the page and passing through point P.

A conic surface CA is formed between CA1-P-CA2. A conic surface CC is formed between CC1-P-CC2

An conic surface (in this case with zero cone angle) is formed between CB1-P-CB2 such that the angle between CA and CB (Θ2) is equal to the angle between CB and CC (Θ3) .

Rollers 33, 34 are located at either end of beam 32. The centre of rotation of the rollers 33, 34 is along the axis formed by CB. The rollers 33, 34 are ground such that their faces are parallel and touching the conies CA and

CC, which they must be if Θ2 and Θ3 are equal.

The beam 32 can be connected to shafts (in this case El and E2) via a short con rod or other suitable means. It is preferable that the axis of the shafts be perpendicular to the plane B1-B2.

In one embodiment, it is preferable that the mechanism should be reflected in the plane B1-B2 to create a mirror image. In another embodiment it is preferable that the mechanism is mass balanced about the axis of rotation A1-A2. Depending upon the configuration of the shafts the reciprocating parts can now be mass balanced. Mass balance can be easily achieved by the attachment of a counterweight to an opposing shaft. For example if a single piston/cylinder configuration was chosen then a mass of equal reciprocating weight could be attached to the shaft mirrored in the B1-B2 plane such that its motion is always 180 degrees out of phase. However, if there are are multiple piston/cylinders then it is preferable that they are attached as pairs and balance weights added to ensure that their reciprocating mass is equal.

In certain configurations the con rod can introduce a small secondary imbalance. This can be easily eliminated if the con rod is extended such that it is mass balanced about its connection with the drive shaft . Depending upon the configuration of the shafts the mechanism can now be force balanced. Force balance can be only be achieved with pairs of, for example, piston/cylinders. For perfect force balance it is necessary that each pair of cylinders be matched such that the forces they experience are equal and opposite relative to the plane B1-B2.

The mechanism illustrated in Figure 1 shows a pair of actuated shafts in line and in direct opposition to each other. Four shafts are easily accommodated by attached another opposed pair to the opposite end of the rocking beam to the shafts illustrated.

If the rocking beam is replaced by a rocking ring mounted via a gimbal at point P, the ring may be supported against the swashplate track by three or more rollers . In this configuration a multiplicity of shaft pairs may be attached at points around the rocking ring.

The compound rocking of the ring in this configuration precludes the use of simple pins in the rod to ring attachments and so ball end, or universal rod end joints can accommodate this motion.

A second rocking beam can be added that is perpendicular to the first rocking beam and it can be attached as long as the centre of the pivot passes through the point P as well. In this configuration the mechanism could drive up to 8 shafts. The advantage of this mechanism over the gimballed ring is that ball or universal joints are not required. Power can be taken out/added in a number of different ways. In the configuration shown it would normally be done using a drive belt. It could also be done with an integral motor or generator, such as a pancake motor. This would obviously negate any losses that might be suffered in a transmission system.

As a general rule in this embodiment:

Reducing θl reduces forces, but requires the D1-D2 to increase in length in order to achieve the same stroke length. This can mean higher rotation speeds for the roller.

Increasing roller size by increasing Θ2 and Θ3 will reduce roller speeds, which can be beneficial for the bearing selection.

The geometry of this device is very simple, in contrast to a desmodromic cam, which means that it is possible to machine it accurately on simple machinery. This mechanism can also be simply pre-loaded to ensure that the rollers never break contact with track under any design running conditions.

Figure 2 shows a schematic view of rocking beam assembly 30 and running surface 51 during operation of the device 10. In Fig 2a) roller 33 is in the highest position and roller 34 in the lowest position. In Fig 2b) the swashplate has now rotated 90 degrees from its original position and both roller 33 and 34 are in a middle position equidistant from the highest and lowest positions. In Figure 2c) the swashplate has rotated a further 90 degrees and roller 33 is in the lowest position and roller 34 is in the highest position. In Figure 2d) the swashplate has rotated a further 90 degrees and both roller 33 and 34 are in a middle position equidistant from the highest and lowest positions A further rotation of 90 degrees will return the rollers to the position they are in Fig 2a) .

Figure 3 shows another view of device 10 in which it is possible to see the hub 60, which is rigidly fixed (fixings not shown) and about which the swashplate body 52 and swashplate running surfaces 51 and 53 can rotate on bearings attached to the hub 60, but not shown. The additional pivot 45is also shown in this Figure. Figure 4 shows a cross-section through a modified swashplate mechanism 10' comprising swashplate 20' ; rocking beam assemblies 30' , 31' and hub 60' . The hub 60' comprising hub flanges 61' and 64', hub inners 62' and 63', connecting bolts 65' & 66', spacers 67' and ball bearings 68'. The rocking beam assembly 30' comprising beam 32', frusto-conical rollers 33', 34', pivot 35', drive fork 36', bearing 58', end plate 56' and bolts 57'. The rocking beam assembly 31' comprise beam 42', frusto- conical rollers 43' ,44', pivot 45', drive fork 46', bearings 59', end plate 76' and bolts 77' . The swashplate 20' comprises toothed drive wheel 70', upper body 71', lower body 72', running surfaces 51' & 53' .

The central hub 60' is constructed by bolting hub flange 64' to hub inner 63' using bolts 65' . Hub inner 62' is sandwiched between hub flange 61' and hub inner 63' using bolts 66' . There are 3 spacers 67' which are used to control the accuracy of the assembly. The hub 60' is rigidly fixed by both flanges to additional structure that is not shown. The swashplate 20' is assembled by bolting (bolts not shown) the upper body 71' through the toothed drive wheel 70' to the lower body 72' . In this way the toothed drive wheel 70' is securely held between the upper and lower body. The running surfaces 51' and 53' are both securely attached to the upper body 71' and lower body 72' .

The swashplate 20' is assembled prior to the hub 60' and the hub 60' is assembled with the bearings 68' used to separate the two bodies. These bearings allow the swashplate 20' to freely rotate about the hub 20', which is rigidly secured to another structure (not shown) . If a toothed belt is attached to the toothed drive wheel then the swashplate 20' can either be driven by the belt or drive the belt.

The rocking beam assemblies 31' and 32' are constructed in the same manner. To assemble rocking beam assembly 31' the frusto-conical roller 33' is constrained via bearings 58' on a shaft that is part of the end plate 56' . The end plate 56' is secured to the beam 32' by bolts 57' . The frusto-conical roller 34' is constrained via bearings 58' on a shaft that is part of the drive fork 36' . The drive fork 36' is secured to the beam 32' by bolts (not shown) . Where the swashplate is converting rotary motion to reciprocating motion power is delivered to the swashplate 10' via the toothed drive wheel. This causes the swashplate 20' to rotate about the hub 60' and to imposes a force on the rollers 33',34', 43' and 44' via the running surfaces 51' and 53'. This force causes the rocking beams assemblies 31' and 32' to rotate in an oscillatory manner about the pivot 35' and 45' respectively. This 'rocking' motion can then be used to drive shafts that can be attached to the forked ends 36' and 46' .

When the swashplate is converting reciprocating motion to rotary motion the process is simply reversed and power is delivered out via the toothed drive wheel.

Figure 5 is a further view of the mechanism shown in Figure 4. In addition the cover plate 80' can be seen that is used to constrain the pivot 35' .

Figure 6 is a plan view of the mechanism shown in Figure 4. Here is can be clearly seen that the centre of rotation of the toothed wheel 70' is the centre of the pivot 35' . However it is also clear that the running surface 51' is offset from this centre. The level of offset depends upon the angle that the running surface 51' is inclined at.

Figure 7 shows a plan view of a device 110 according to another embodiment of the present invention in which with two rocking beams using the same modified swashplate principle. Double beam swashplate 110 comprises rocking beam assembly 30'' comprising beam 32'', frusto-conical rollers 33' ',34'', pivot 35'', drive fork 36'' and end plate 56'' . The rocking beam assembly 130 comprising beam 132, frusto-conical rollers 133, 134, pivot 135 & 138, drive fork 136 and end plate 156. Hub flange 64'' and swashplate 20'' comprising running surface 51'', toothed drive wheel 70' ' and upper body 71' '.

The rocking beam assembly 30'' acts in a manner that is identical to that described in Figures 4,5 & 6. The rocking beam assembly 130 also acts in a similar manner, where the centre of rotation of the pivots 135 & 138 passes through the pivot point P that is also the apex of the frusto-conical surface on which the running surface 51' ' is located and the apex of all the frusto-conical surfaces on which the frusto-conical rollers 33' ' , 34' ' , 133 & 134 are located. In this embodiment there are two drive forks shown, but in another embodiment this number could be three or four drive forks, with a similar number mirrored in the opposite side of the mechanism, allowing in total a maximum of 8 drive forks and hence 8 different driving rods .

In a further embodiment there could be more than one drive fork on each side of the rocking beam and in addition the attachment points can be both inboard and outboard of their current locations allowing for varying stroke lengths within the same mechanism.

Figure 8 shows a rocking ring swashplate 200 comprising: gimballed rocking ring 250; frusto-conical rollers 233,234 & 235; gimbal 240; rocking ring 250; running surface 251; upper body 271; toothed drive wheel 270; and drive attachment shafts 236.

In this embodiment the rocking ring 250 replaces the rocking beam used in previous embodiments . The rocking ring 250 is supported by three frusto-conical rollers 233,234,235 that run on running surface 251 supported by upper body 271 and driven by toothed drive wheel 270. In this way the rocking ring 250 now has two degrees of freedom, as opposed to the single degree of freedom of the rocking beam. This necessitates that there is some form of gimbal 240 at the pivot point P that supports the rocking ring 250, for example a ball joint. The pivot point P that is also the apex of the frusto-conical surface on which the running surfaces 251 is located and the apex of all the frusto-conical surfaces on which the frusto-conical rollers 233, 234 & 235 are located. As the running surface rotates the rocking ring moves to follow it. This creates a reciprocating motion at the edge of the plate and through the drive attachment shafts various mechanisms can be driven. There are two axis of rotation occurring so it may be necessary to use, for example, a ball joint to drive the appropriate rod. More than three frusto-conical rollers can be used, but this may require additional accuracy. Six drive attachment shafts are shown, but as many drive attachment shafts as are required can be added provided they fit within the space allowed. Attachment points can be both inboard and outboard of their current locations allowing for varying stroke lengths within the same mechanism.

Figure 9 shows a further swashplate mechanism 300 in accordance with the present invention. Mechanism 300 comprises: a swashplate 320 defining a planar surface 350, the swashplate 320 being rotatable about axis of rotation A1-A2; and a rocking beam assembly 330. Rocking beam assembly 320 comprises: a beam 332; frusto-conical rollers 333,334 rotatably mounted on opposed ends of the beam 332; a pivot 335; drive fork 336; bearings 337 & 339; con rod 338 and drive rod 340. In accordance with the present invention, frusto-conical rollers 333, 334 are rotatable about an axis extending through intersection point P (the point at which the surface 350 intersects axis of rotation A1-A2.

Figure 10 shows a further swashplate mechanism 400 in accordance with the present invention. Mechanism 400 comprises: a swashplate 420 defining a planar annular surface 450, the swashplate 420 being rotatable about axis of rotation A1-A2; and a rocking beam assembly 430. Rocking beam assembly 420 comprises: a beam 432; a pair of substantially cylindrical rollers 433,434 rotatably mounted on opposed ends of the beam 432; a pivot 435; drive fork 436; bearings' 437 & 439; con rod 438 and drive rod (not shown) . In this embodiment, pivot 435 is located at a point on axis of rotation A1-A2 spaced from the point at which the surface 450 notionally intersects axis A1-A2.

Claims

1. Apparatus for converting between rotational and linear movement, comprising: a body configured to rotate about a first axis, the body defining a track having a surface which is asymmetric about the first axis; first and second rollers located on opposed lateral sides of the first axis, the first and second rollers each being configured to rotate about at least a second axis inclined relative to the first axis and each comprising an outer contact surface configured to engage the surface of the track defined by the body when the body rotates, whereby in use rotation of the body relative to the first and second rollers results in oscillating displacement of the first and second rollers; and a pivotable rocker extending between the first and second rollers, the rocker being configured to maintain contact between the surface of the track defined by the body and the outer contact surfaces of the first and second rollers.

2. Apparatus according to claim 1, wherein the first and second rollers are rotatably coupled to the rocker at points on opposed lateral sides of the first axis.

3. Apparatus according to claim 1 or claim 2, wherein the surface of the track defined by the body at least notionally extends through an intersection point (P) on the first axis.

4. Apparatus according to claim 3, wherein the rocker is pivotable about the intersection point (P) .

5. Apparatus according to any of claims 1 to 4, wherein the - first and second rollers are substantially cylindrical rollers or spheroidal rollers.

6. Apparatus according to any of claims 1 to 4 , wherein the outer contact surfaces of the first and second rollers each define a substantially frusto-conical profile centred around a respective axis of rotation extending through the intersection point (P) .

7. Apparatus according to any of the preceding claims, wherein the first and second rollers are diametrically opposed to one another relative to the first axis.

8. Apparatus according to any of the preceding claims, wherein the rocker is constrained to pivot around a single axis of rotation.

9. Apparatus according to any of the preceding claims, wherein the apparatus further comprises third and fourth rollers as previously defined and a further pivotable rocker extending between the third and fourth rollers, the further rocker being configured to maintain contact between the outer contact surfaces of the third and fourth rollers and the surface of the track defined by the body.

5 10. Apparatus according to claim 9, wherein the further rocker is pivotable about an axis perpendicular to the axis of the first-mentioned rocker and extending through the pivot axis of the first-mentioned rocker.

10 11. Apparatus according to claim 10 (when dependent upon claim 5) , wherein the further rocker and first-mentioned rocker are each pivotable about axes extending through intersection point (P) .

15 12. Apparatus according to any of claims 1-7, wherein the rocker is configured to pivot around two axes of rotation.

13. Apparatus according to claim 12, wherein the rocker is coupled to the apparatus by means of gimbal means.

20

14. Apparatus according to claim 12 or claim 13, wherein the apparatus further comprises a third roller as previously defined coupled to the rocker.

25 15. Apparatus according to claim 14, wherein the first, second and third rollers are substantially equally spaced around the track.

16. Apparatus according to any of the preceding claims wherein the apparatus comprises a further mechanism as previously defined located an opposed side of the body.