GB2142410A - Improvements in or relating to linear actuators - Google Patents

Abstract

A linear actuator comprises a shaft 41 of D-shaped cross-section having a rack 39 formed on the flat thereof. A pinion 36 engages with rack 39 and is driven by a worm 32 and wormwheel 35, the worm being driven by a DC motor and the wormwheel being mounted on a shaft to be rotationally fast with the pinion 36. A roller 43 straddles the rack 39 and bears against the flats on either side of the rack 39 to prevent twisting of the shaft 41. A clutch may be provided in the drive train between the drive motor and the shaft 41 to allow slip and an encoder may be driven by the rack 39 to indicate the position of the shaft 41. <IMAGE>

Description

SPECIFICATION Improvements in or relating to linear actuators The present invention relates to linear actuators.

A first aspect of the present invention provides an actuator comprising a worm, a wormwheel, and a driven member drivably connected to the wormwheel and mounted for substantially linear motion, the arrangement being such that rotation of the worm causes rotation of the wormwheel which drives said driven member in a substantially linear direction.

Very preferably, the lead on the worm is sufficiently small to prevent reciprocal drive between the worm and wormwheel.

Drive to the wormwheel may be provided by an electric motor, preferably a D-C motor.

In a preferred form the driven member is connected to the wormwheel via a friction drive allowing slip between the driven member and the wormwheel.

Preferably the drive comprises a wheel. The wheel may be mounted fast on the wormwheel.

Preferably, the wheel has a V-shaped circumferential edge which bears directly on the driven member, contacting it at two points.

In a particularly preferred embodiment, the driven member is a rod having a substantially D-shaped cross-section. The flat of the D bears against a roller, and the curved portion sits in the V-shaped edge of the wheel. The roller may be biassed towards the wheel. The degree of bias is preferably adjustableto allow control of the slip between the driven member and the wheel.

Another wheel may be provided to further support the rod. Preferably the roller(s) is(are) located opposite the wheel(s).

In another preferred form the driven member is a rod having a D-shaped cross-section and the flat of the D has a rack formed thereupon. A pinion is mounted fast with the wormwheel and engages with the rack to transmit drive from the wormwheel to the rack. The engagement of the rack and pinion will limit twisting ofthe rod, however in a preferred form, a flat or flats are provided either side of the rack and a roller bears on the flat(s). The roller may be biassed against the rod, however, in a preferred form the roller is rotatably mounted on a shaft which is eccentrically mounted to allow adjustment of the roller towards the rod. Preferably the rod is supported in a recirculating ball bushing.

An absolute encoder may be provided to indicate the degree of travel of the driven member. Preferably, the encoder is driven directly by the driven member, for example via the or a rack on the member and a pinion connected to the encoder.

Means may be provided to prevent backlash of the wormwheel.

The invention will be further described by way of example with reference to the accompanying drawings, in which: Figure 1 is a plan view, partly cut away, of a first embodiment of the invention; Figure2 is a sectional view along the line ll-ll of Figure 1; Figure 3 is a perspective view of part of the apparatus of Figures 1 and 2; Figure 4 is a side view, partially cut away, of another embodiment of the invention; Figure 5 is a sectional view along the line V-V of Figure 4; and Figure 6 is a perspective view of part of the apparatus of Figures 4 and 5.

In Figure 1,a D-C motor 1 is directly coupled two a worm 2 which rotates a wormwheel 3 which is mounted on a shaft 4 via angular contact bearings 5.

Shaft 4 is firmly mounted on base 6. A pulley wheel 7 is mounted on the wormwheel 3 to rotate therewith.

A driven rod 8 having a generally D-shaped crosssection bears against the circumferential edge of pulley wheel 7, contacting the wheel at two points, are on each side of the V-shaped groove in the edge of the wheel.

Another pulley wheel 9 is mounted on the base 6 in the same plane as pulley wheel 7 to provide further support for the rod 8. A pair of rollers 10, 11 bear against the flat surface 20 of the rod 8.

The rollers 10, 11 are mounted opposite the pulley wheels 7,9 on a support member 12. Support member 12 is mounted on a pair of stub shafts 13, 14 on wall 15 to be slidable axially of the stub shafts 13, 14. The support member 12 is urged towards the pulley wheels 7,9 by disc springs 16, so that the rod 8 is clamped between the rollers 10, 11 and pulley wheels 7,9.

Rod 8 has a rack 17 formed longitudinally thereof on a surface opposite flat 20. An absolute encoder 18 is carried by a pinion 19 which meshes with the rack 17.

In use, the motor 1 drives the worm 2 to rotate the wormwheel 3. Pulley 7, fast with the wormwheel 3, rotates to drive the rod 8 in a linear direction. On stopping the motor 1 the rod 8 stops immediately, since the wormwheel 3 is held fast against continuing its rotation by the worm 2, provided the lead on the wormwheel is sufficiently small.

Any attempt to rotate the wormwheel 3 will not initiate the necessary rotation of the worm 2 because of the angle of cut of the teeth on the worm and wormwheel, this will also depend on the friction between the teeth of the wormwheel and the thread of the worm.

Means (not shown) may be provided to avoid backlash of the wormwheel.

If sufficient force is applied to the rod 8, the rod will slip over the surface of pulley wheel 7 (rollers 10, 11 and pulley wheel 9 are free to rotate). The degree of force required may be varied by adjusting the pressure exerted by the disc springs 16 which urge the rod 8 against the pulley wheel 7 via roller 10.

By providing support for the rod 8 at three points around its circumference the rod is prevented from lateral movement; also rollers 10, 11 bearing against flat 20 prevent rotation of the rod 8.

The absolute encoder 18 is in direct contact with the rod 8 via pinion 19 which engages with rack 17 formed on the rod 8. Hence, even if slip occurs between the pulley wheel 7 and rod 8, which may occur particularly at the start and finish of a drive cycle, the position of the rod 8 is reliably known.

A D-C motor facilitates reversal of the drive and allows controlled braking.

In Figure 4, a DC electric motor 3, has a worm 32 coupled to a spindle 33 thereof by a pin 34. Worm 32 drives a wormwheel 35 which is mounted fast on an axle 36 supported in ball bearings 37. A spur gear or pinion 38 is also mounted fast on axle 36 to rotate with the wormwheel 35 and engages with a rack 39 formed on the flat surface 40 of a D-shaped crosssection rod 41. Rod 41 is supported in recirculating ball bushings 42. Preferably the rod 41 is supported on its curved surface at either side of the flat 40 and opposite the flat 40. Motor31 will rotate spindle 33 and worm 32 to drive the wormwheel 35 and hence rod 41 via pinion 38 which engages with the rack 39.

Preferably means are provided to hold the rod 41 against twisting. Engagement of the pinion 38 with rack 39 will reduce twisting to some extent however additional means are also provided.

A roller 43 has a recess 44 formed in its outer circumferential edge 45. The recess receives the rack 39 and projecting annular portions 46 bear against flats 47 formed by the flat 40 on either side of the rack 39. Roller 43 is mounted on a needle roller bearing 48 on shaft 49. Shaft 49 is eccentrically mounted in a bearing block body 50. By rotating shaft 49 in its eccentric mounting the roller 43 is brought to bear on rod 41.

Bearing block body 50 is integrally formed and houses and drive train formed by worm 32, wormwheel 35 and pinion 36, and the anti-twist roller 43, the rod 41 passing through the block 50. Motor 31 is attached to block 50 and angular contact bearings 51 and 52 support the motor shaft 33. Bearing 52 is held in place by a locking ring 53 screwed into the block body 50 Preferably an anti-backlash mechanism is provided between pinion 36 and rack 39. An encoder may be provided to indicate the position of the rod 41. Preferably the encoder is driven off the rack 39.

Even if backlash is present in the drive train, the encoder will indicate the true position of the rod 41.

Preferably a slippable clutch is provided in the drive train, preferably between the motor 31 and worm 32, to prevent damage if the rod 41 meets an obstruction whilst it is being driven and to allow rod 41 to be moved independently of motor 31.

Two rods 8, 41 may be provided substantially parallel to one another, a first of the rods being driven and the second being slidably mounted spaced from the first rod, the rods being coupled together. This will increase the transverse load carrying capacity of the device and also provide increased resistance against twisting.

Claims (15)

1. An actuator comprising a worm, a wormwheel and a driven member drivably connected to the wormwheel and mounted for substantially linear motion, the arrangement being such that rotation of the worm causes rotation of the wormwheel to drive said driven member in a substantially linear direction.

2. An actuator as claimed in claim 1, in which the lead on the worm is sufficiently small to prevent reciprocal drive between the worm and wormwheel.

3. An actuator as claimed in claim 1 or 2, in which the worm is driven by a DC motor.

4. An actuator as claimed in claim 1,2 or 3, in which the driven member has a subslantially Dshaped cross-section.

5. An actuator as claimed in any one of claims 1 to 4, in which a roller bears against the flat of the D-shaped cross-section driven member to hinder axial rotation of the member.

6. An actuator as claimed in claim 5, in which the roller is resiliently biassed towards the driven member.

7. An actuator as claimed in claim 5, in which the roller is rotatably mounted on an eccentrically mounted shaft, the shaft being rotatable to cause the roller to bear on the driven member and means being provided to lockthe shaft in position.

8. An actuator as claimed in any one of claims 1 to 7 in which the drive is transmitted from the wormwheel to the driven member by means of a wheel mounted fast for rotation with the wormwheel.

9. An actuator as claimed in claim 8, in which the wheel frictionally engages with the driven member.

X0. An actuator as claimed in claim 8 in which the wheel is a pinion which engages with a rack provided on the driven member.

11. An actuator as claimed in claim 9 when dependent upon claim 4, wherein the wheel has a V-shaped circumferential edge which bears against the curved face of the driven member.

12. An actuator as claimed in claim 10, when dependent upon claim 4, wherein the rack is provided on the flat of the driven member.

13. An actuator as claimed in any one of claims 1 to 12, wherein a second member is slidably mounted substantially parallel to the driven member and coupled thereto for linear motion therewith.

14. An actuator as claimed in any one of claims 1 to 13, wherein an encoder cooperates with said driven member to indicate its position.

15. An actuator substantially as hereinbefore described with reference to Figures 1 to 3 or Figures 4 to 6 of the accompanying drawings.