GB2282205A

GB2282205A - Rotary to linear motion conversion

Info

Publication number: GB2282205A
Application number: GB9319668A
Authority: GB
Inventors: David Wager
Original assignee: Individual
Current assignee: Individual
Priority date: 1993-09-23
Filing date: 1993-09-23
Publication date: 1995-03-29
Anticipated expiration: 2013-09-23
Also published as: GB9319668D0; GB2282205B

Abstract

A system for converting rotary to linear motion comprises a differential gear arrangement (6, 7) for transmitting rotary motion from primary drive means (1) through first and second drive shafts (2, 3) and a linearly movable output member (8) mounted concentrically with, and acting between, the first and second drive shafts (2, 3). The direction of relative rotation of the first drive shaft (2) with respect to the second drive shaft (3) determines the direction of linear movement of the output member (8). The member (8) has an external screw thread (22) engaging an internal screw thread (17) of the first drive shaft (2) and a flat (23) engaged by a screw (12) to lock the member (8) against rotation relative to the second drive shaft (3). The system is controlled by respective disc brake arrangements (9, 25 and 10, 25) for each drive shaft. A number of such systems can be arranged around a common rotary input gear (33, Figure 3) meshing with respective ring gears (13) of each primary drive means (1). <IMAGE>

Description

ROTARY TO LINEAR MOTION CONVERSION The invention relates to the conversion of rotary to linear motion.

In control engineering it is often required to perform rotary to linear conversion so as, for instance, to position a load accurately. However, many of the conventional systems currently in use exhibit limitations, particularly when overall size and weight are design factors, for example in portable systems. The main contributor to large size and weight is the power source which is usually an electric motor. If the system requires multiple axis control then a number of rotary power sources must be provided, one for each axis, resulting in an even more bulky and cumbersome system.

It would be desirable to enable a single rotary power source to drive a number of independent linear actuators.

However, in order to do this the direction of motion of any one of the actuators would need to be independent of the direction of rotation and speed of the power source.

If linear speed and direction are not to be controlled by the power source itself, then some other method must be provided so that independent action of each actuator is possible.

Preferred embodiments of the present invention aim to provide a system for converting rotary to linear motion whereby linear output motion is controllable substantially independently of rotary input motion.

It is another aim of preferred embodiments to provide such a system whereby a number of independently controllable linear outputs are provided driven by a single rotary input.

A first aspect of the present invention provides a system for converting rotary to linear motion, the system comprising: a primary drive means for connection to a rotary input power source; a differential gear arrangement for transmitting rotary motion from the primary drive means through first and second cylindrical drive shafts; and an output member mounted concentrically within said first and second drive shafts so as to connect them to one another; wherein, relative rotation in a first direction of said first drive shaft with respect to said second drive shaft causes the output member to move in a first linear direction, and relative rotation of the first and second drive shafts in a second, rotationally opposite, direction causes the output member to move in a second linear direction, opposite to said first linear direction.

Preferably, said first drive shaft is provided with an internal thread which mates with an external thread formed on at least part of said output member.

Preferably, said second drive shaft is provided with locking means to prevent rotation of said output member relative to said second drive shaft.

Preferably, said locking means comprises a locking screw which projects radially inward into said second drive shaft such that an end portion of the screw contacts a flattened region of the output member, wherein the flattened region extends along at least part of the length of the output member.

Preferably, control means are provided for altering the relative rotational speeds of said first and second drive shafts.

Preferably, the control means comprises a braking arrangement.

Preferably, the braking arrangement comprises a first disc brake mounted on said first drive shaft and a second disc brake mounted on said second drive shaft, and braking pads operable to slow down either said first or second drive shaft.

Preferably, when either one of the first or second drive shafts is retarded by its respective brake discs and pads, the other drive shaft is not retarded.

Preferably, said differential gear arrangement comprises a first bevel gear attached to a driven end of said first drive shaft, a second bevel gear attached to a driven end of said second drive shaft, and first and second idler bevel gears attached to the primary drive and interposed between the first and second bevel gears, so as to mesh therewith.

According to a second aspect of the invention, a plurality of systems according to the first aspect are provided, each arranged so that rotary power is distributed between them.

Preferably, the periphery of said rotary power source forms a ring gear which meshes with a corresponding ring gear of each primary drive means.

For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings which show exemplary embodiments of the system according to the present invention, in which: Figure 1 is an exploded view showing the main elements of an embodiment of a system according to the first aspect of the invention; Figure 2 is a cut-away view of an assembled system, in which means for delivering rotational drive, brake actuation and other minor assemblies are omitted for clarity; Figure 3 is a perspective view showing a multiple axis drive arrangement; and Figure 4 is a cross-sectional view of the assembled system of Figure 2.

Referring initially to Figure 1, the main elements of an exemplary embodiment of the present invention will now be described.

The system of Figure 1 comprises a primary drive 1, two drive shafts, hereinafter referred to as a nut drive 2 and a screw drive 3, idler adjusters 4, 5 together with associated idler bevel gears 6, 7, an actuator screw 8, brake discs 9, 10 and brake location screws 11, 12.

In more detail, the primary drive 1 consists of a hollow cylindrical member having a ring gear 13 at one end for engagement with an input rotary drive means (not shown in this Figure). Half way along the length of the primary drive 1, two drilled and tapped holes 14, 15 are positioned radially opposite one another. The holes 14, 15 have an internal thread which meshes with an external thread formed on the idler adjusters, 4, 5. Each idler adjuster 4, 5 has a protruding peg which acts as a bearing spigot for the idler bevel gear 6, 7, so that with the primary drive assembled with the idler adjusters 4, 5 screwed into the body of the primary drive, the idler bevel gears protrude within the hollow cylindrical region of the unit 1 and are able to freely rotate on the bearing spigots.

The nut drive 2 comprises a hollow cylinder having a central collar 16. The external diameter of the cylinder is arranged so as to be slightly less than the internal diameter of the hollow primary drive 1. In this manner, the nut drive 2 may be inserted into the primary drive 1 so that the central collar, in combination with the outer diameter of the primary drive 1, forms a thrust bearing.

(This is more clearly shown in Figure 2, to be described hereinafter). The hollow interior of the nut drive 2 is tapped, so as to form an internal thread 17. Towards the outside of the nut drive (to the left, as viewed in Figure 1) there is provided a radially extending aperture 18 for the acceptance of a brake location screw 11. The location screw 11 does not extend into the internally threaded region 17, but simply acts as a key for the brake disc 9 which is mounted thereon.

The screw drive 3 is similar in construction to the nut drive 2. However, the hollow internal region of the screw drive is not tapped with an internal thread, and is of a slightly larger internal diameter. The screw drive features a central collar 19 (identical to the nut drive central collar 16) and a hole 20 for acceptance of the brake location screw 12. When the location screw 12 is screwed into the hole 20, the screw is designed so as to penetrate into the hollow internal region, for reasons which will become evident later in the description. In similar manner to the brake location screw 11, the location screw 12 acts as a key for the brake disc 10.

The left hand end of the screw drive (as viewed in Figure 1) has a bevel gear arrangement 21. The nut drive 2 also has a bevel gear attached to the inner end thereof, however due to the perspective view of Figure 1 this bevel gear cannot be seen. It is to be understood that the bevel gear of the nut drive 2 is of an identical formation to the bevel gear 21.

The actuator screw 8 has an external thread 22 which is arranged to cooperate with the internal thread 17 of the nut drive. The screw 8 has a flat portion 23 formed along at least part of the length of the screw 8, this is best seen by referring to Figure 4, to be described later.

It should be understood that the screw 8 extends throughout the entire length of the apparatus of Figure 1 and beyond. The flat region 23 of the actuator screw is designed so that, with the apparatus in an assembled form, the brake location screw 12 of the screw drive 3 impinges thereon. In this manner, the screw drive and the actuator screw are fixed so that rotational movement of the actuator screw with respect to the screw drive is not possible. Conversely, since the brake location screw 11 of the nut drive does not extend into the threaded region 17, the actuator screw and nut drive cooperate such that, by virtue of the cooperating threads the screw 8 is able to move in and out of the nut drive 2, whenever there is relative rotation of the nut drive with respect to the actuator screw.

Figures 2 and 4 show the drive train of Figure 1 in an assembled form. Means of delivering rotational drive, brake actuation and some other units are omitted in Figure 2 for clarity. As can be seen, the assembled unit is mounted to a mounting assembly 24, bearings 34, 35 being formed by the nut and screw drives 2, 3 where they pass through the mounting assembly 24. Brake pads 25, 26 are provided. These pads 25, 26 can be arranged to come into contact with their respective disc brakes 9, 10.

In Figure 2, the actuator screw is not shown.

However, as shown in Figure 4, it can be seen that the screw extends centrally through the nut and screw drives 2, 3.

Operation of an assembled unit will now be described with reference to Figures 1, 2 and 4. In normal operation, the centrally mounted primary drive is rotating at a speed which is dependent on the gear ratio of the power source delivery system. This gear ratio is determined by the ring gear 13 of the primary drive 1 which engages with ring gear of a rotational power source.

Rotation of the primary drive 1 is transmitted via the idler gears 6, 7 meshed into the bevel gears 21 of the nut and screw drives 2, 3 so as to form a differential gear train. Under normal circumstances, both nut and screw drives will rotate at the same rate and there will be no rotation of the actuator screw 8 with respect to the nut drive 2. Therefore no linear motion occurs.

If, by means of the braking arrangements 9, 25, a frictional force is applied to the nut drive 2, the nut drive 2 will tend to slow down and, through the action of the differential gear train, the screw drive 3 will tend to speed up. There is therefore relative rotation of the screw drive 3, with respect to the nut drive 2 and the actuator screw 23 will be driven by the relative rotation so as to undergo linear motion in a first direction.

Conversely, if a braking force is applied to the screw drive 3, by means of the other braking arrangement 12, 26, relative rotation of the screw drive 2 and nut drive 3 will occur in an opposite direction to that mentioned above. Linear motion of the actuator screw will therefore occur in an opposite sense.

In practise, the braking assemblies are linked so that braking force is not applied to both drives at the same time. If simultaneous braking were permitted, damage to the parts of the differential gear train could occur.

It can therefore be appreciated, that control of the linear motion of the actuator screw 8 can be achieved largely independently of the running speed of a rotary drive source, by selectively braking either one of the screw or nut drives by a required degree. In this manner, multiple access drives can be constructed as shown schematically in Figure 3. In the figure, 6 independent differential drive trains 27 to 32 are shown clustered around a single rotary power source 33. Linear motion of each of the corresponding actuator screws (not shown) is achievable by selective retardation of the brake discs.

Obviously, different numbers of differential gear trains can be provided, and those trains may have different gear ratios, according to requirements. In this manner, an extremely versatile system may be built up giving multiple access linear control, all being driven from a single rotary power source.

The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

Each feature disclosed in this specification (including any accompanying claims, abstract and drawings), may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

Claims

1. A system for converting rotary to linear motion, the system comprising: a primary drive means for connection to a rotary input power source; a differential gear arrangement for transmitting rotary motion from the primary drive means through first and second drive shafts; and an output member mounted concentrically with respect to said first and second drive shafts so as to connect them to one another; wherein, relative rotation in a first direction of said first drive shaft with respect to said second drive shaft causes the output member to move in a first linear direction, and relative rotation of the first and second drive shafts in a second, rotationally opposite, direction causes the output member to move in a second linear direction, opposite to said first linear direction.

2. A system according to claim l, wherein said first drive shaft is provided with an internal thread which mates with an external thread formed on at least part of said output member.

3. A system according to claim 1 or 2, wherein said second drive shaft is provided with locking means to prevent rotation of said output member relative to said second drive shaft.

4. A system according to claim 3, wherein said locking means comprises a locking member which projects radially inward into said second drive shaft such that an end portion of the locking member contacts a flattened region of the output member, wherein the flattened region extends along at least part of the length of the output member.

5. A system according to claim 4, wherein said locking members is a screw.

6. A system according to any of the preceding claims, wherein control means are provided for altering the relative rotational speeds of said first and second drive shafts.

7. A system according to claim 6, wherein the control means comprises a braking arrangement.

8. A system according to claim 7, wherein the braking arrangement comprises a first disc brake mounted on said first drive shaft and a second disc brake mounted on said second drive shaft, and braking pads operable to slow down either said first or second drive shaft.

9. A system according to claim 8, wherein when either one of the first or second drive shafts is retarded by its respective brake discs and pads, the other drive shaft is not retarded.

10. A system according to any of the preceding claims, wherein, said differential gear arrangement comprises a first bevel gear attached to a driven end of said first drive shaft, a second bevel gear attached to a driven end of said second drive shaft, and first and second idler bevel gears attached to the primary drive and interposed between the first and second bevel gears, so as to mesh therewith.

11. A system substantially as herein described, with reference to the accompanying drawings.

12. An arrangement for converting rotary to linear motion, wherein a plurality of systems according to any of the preceding claims are provided, each arranged so that rotary power is distributed between them.

13. An arrangement according to claim 12, wherein the periphery of said rotary power source forms a ring gear which meshes with a corresponding ring gear of each primary drive means.