GB2077355A

GB2077355A - Rotary actuators

Info

Publication number: GB2077355A
Application number: GB8017273A
Authority: GB
Original assignee: Worcester Controls UK Ltd
Current assignee: Worcester Controls UK Ltd
Priority date: 1980-05-27
Filing date: 1980-05-27
Publication date: 1981-12-16
Also published as: JPS57103915A; ES502492A0; AU7091581A; ES8301525A1; ZA813367B; EP0040976A1

Description

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GB 2 077 355 A 1

SPECIFICATION Rotor actuators

The invention relates to piston-rack rotary actuators.

5 A disadvantage of a piston-rack rotary actuator which includes a pair of pistons which are displaceable towards and away from one another along a pair of opposed cylinders is that the pistons have a tendency to cock or turn in the 10 cylinder thereby jamming and/or damaging the radial inner surface or surfaces of the cylinders and the seals.

In U.K. Patent 1,251,805 this was overcome by " providing guide members to constrain the piston 15 to move only in an axial direction, the piston sliding on stationary guide members. However, in some designs, it is desirable to have a large pinion on which the racked piston acts and it is not possible conveniently to provide guide members, 20 such as described in U.K. Patent 1,251,805. A solution is proposed in U.S. Patent 4,167,897 where a rack bearing portion of each piston is provided with low coefficient friction elements between that portion and an internal surface of a 25 cylinder to prevent scoring and to maintain the rack in contact with a pinion of the actuator. In U.S. specification, annuli are also provided of low friction elastomeric or synthetic resin material at each side of the pinion which slide on runways 30 provided on each side of the rack portions. Such annuli are provided to tend to urge the racked portions of the pistons towards the low friction element referred to above.

It is an object of the invention to provide an 35 improved piston-rack actuator.

According to the invention there is provided a piston rack rotary fluid pressure actuator, comprising a housing including a cylinder having a central longitudinal axis; two pistons disposed 40 coaxially within said cylinder and arranged for linear movements substantially parallel to said central longitudinal axis of said cylinder either towards or away from one another under the influence of a difference in fluid pressure created 45 on opposite sides of each of said pistons, each said piston including a rack-bearing portion having a toothed rack rigidly connected to said piston; an actuator output shaft rotatably mounted in said housing and disposed transverse to said central 50 longitudinal axis of said cylinder at the mid-length point of said cylinder; a pinion rigidly affixed to said actuator output shaft and disposed between said racks integral with said pistons so that teeth of the pinion engage with teeth of the racks; at 55 least one element having a low coefficient of friction being disposed between and in contact with both said rack-bearing portion of each piston and the respective portion of the internal surface of said cylinder whereby any forces which tend to 60 displace said rack-bearing portion of each of said pistons toward said respective portion of said internal surface of said cylinder are counteracted, spaced annuli of non-compressible material rigidly fixed for rotation with said actuator output shaft in

65 the vicinities of the opposite ends of said teeth of said pinion, said annuli having diameters equal to the pitch circle diameter of said teeth of said pinion, two spaced flat runways provided on said rack-bearing portion of each of said pistons; and 70 said annuli forming a rolling contact with the runways on the pistons during operation of the actuator to help to maintain contact between each said element and the internal surface of said cylinder to prevent rotational movement of said 75 pistons, and to prevent over-meshing of the teeth of the racks and the pinion.

A piston rack rotary fluid pressure actuator according to the invention will now be described by way of example, with reference to the 80 accompanying schematic drawing, in which:—

Figure 1 is a cut-away side elevation of the actuator;

Figure 2 is a section through A—A of Figure 1 ; and

85 Figure 3 is an isometric view of one piston and a pinion of the actuator.

Referring to the drawings, in Figure 1 a housing 10 encloses a cylinder 11 in which two pistons 12 and 13 are disposed coaxially within the cylinder 90 11 and spring biassed by springs 14 and 15

towards the centre of the cylinder 11. Each piston has a toothed rack portion 16 and 17. An output shaft 18 rotatably mounted in the housing 11 has a toothed pinion 19 keyed to the shaft 18 which 95 engages the teeth of both the racks 16 and 17. Low friction bearing material such as DELRIN (Trade Mark) is supported in grooves 20 and 21 in outer surfaces of the rack portions 16 and 17. Similarly, low friction material, formed as 100 trapezoids, is supported in grooves 22 of each piston 12 and 13.

Spaced annuli 30 and 31 (best seen in Figure 3) keyed to the shaft 18 and having a diameter equal to the pitch circle diameter of the 105 teeth of the pinion 19 are mounted at opposite ends of those teeth. Two flat runways 32 and 33 are fixed to each of the rack bearing portions 16 and 17 to contact the annuli 30 and 31 respectively when the teeth of the pinion 19 and 110 the teeth of the racked portions 16 and 17 are in correct meshing position.

In operation, pressurised air or liquid is supplied into the cylinder 11 between the pistons 12 and 13 to force the pistons 12 and 13 away from each 115 other and the centre of the cylinder 11 and so rotate the shaft 18. When the pressure is reduced the springs 14 and 15 return the pistons towards their central positions. During movement of the pistons 12 and 13, the annuli 30 and 31 rotate 120 with the shaft 18 to form rolling contacts with the runways 32 and 33. This contact maintains the contacts between the material in the grooves 20 and 21 and the walls of the cylinder 11 and at the same time prevents overmeshing of the teeth of 125 the pinion 19 and the teeth of the rack portions 16 and 17, and rotation of the pistons 12 and 13.

By forming the annuli 30 and 21 and the runways 32 and 33 of load bearing material such an anodised aluminium, or say, steel where

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corrosion is not a problem, there is no tendency for such parts to be compressed or deformed by any forces acting transversely to the axis of the cylinder 11. Further, by arranging for the contact 5 between the annuli 30 and 31 and the runways 32 and 33 to be a rolling contact any tendency for the annuli to stick or slide is overcome.

In a prior art proposal in U.S. Patent 4,167,897, the material used for the annuli being elastomeric 10 or synthetic material there is tendency for the material to become deformed in use. Further, as the contact is not a rolling contact, both the annuli or sometimes one annulus tends to jam and cause uneven wear which means that excessive wear 15 occurs on the teeth due to overmeshing at one or both sides of the pinion 19 or rack bearing portions.

Claims

1. A piston rack rotary fluid pressure actuator, 20 comprising: a housing including a cylinder having a central longitudinal axis; two pistons disposed coaxially with said cylinder and arranged for linear movements substantially parallel to said central longitudinal axis of said cylinder either towards or 25 away from one another under the influence of a difference in fluid pressure created on opposite sides of each of said pistons, each said piston including a rack-bearing portion having a toothed rack rigidly connected to said piston; an actuator 30 output shaft rotatably mounted in said housing and disposed transverse to said central longitudinal axis of said cylinder at the mid-length point of said cylinder; a pinion rigidly affixed to said actuator output shaft and disposed between 35 said racks integral with said pistons so that teeth of the pinion engage with teeth of the racks; at least one element having a low coefficient of friction being disposed between and in contact with both said rack-bearing portion of each piston 40 and the respective portion of the internal surface of said cylinder whereby any forces which tend to displace said rack-bearing portion of each of said pistons toward said respective portion of said internal surface of said cylinder are counteracted, 45 spaced annuli of non-compressible material rigidly fixed for rotation with said actuator output shaft in the vicinities of the opposite ends of said teeth of said pinion, said annuli having diameters equal to the pitch circle diameter of said teeth of said 50 piston, two spaced flat runways provided on said rack-bearing portion of each of said pistons; and said annuli forming a rolling contact with the runways on the pistons during operation of the actuator to help to maintain contact between each 55 said element and the internal surface of said cylinder, to prevent rotational movement of said pistons, and to prevent over-meshing of the teeth of the racks and the pinion.

2. A piston rack rotary fluid pressure actuator 60 according to Claim 1, in which said annuli are formed of anodised aluminium,

3. A piston rack rotary fluid pressure actuator according to Claims 1 or 2, in which said runways are formed of anodised aluminium.

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4. A piston rack rotary fluid pressure actuator according to any of Claims 1 to 3, in which said teeth of said pinion and said annuli are keyed to said actuator output shaft.

5. A piston rack rotary fluid pressure actuator 70 according to any of Claims 1 to 3, in which said annuli are attached to said actuator output shaft by locking screws.

6. A piston rack rotary fluid pressure actuator substantially as herein described with reference to

75 the accompanying drawing.

New claims or amendments to claims filed on 21st July 1981.

Superseded claims 1—6.

New or amended claims:—

80 1. A piston rack rotary fluid pressure actuator, comprising: a housing including a cylinder having a central longitudinal axis; a piston disposed coaxially within said cylinder and arranged for linear movement substantially parallel to said 85 central longitudinal axis of said cylinder under the influence of a difference in fluid pressure created on opposite sides of said piston, said piston including a rack-bearing portion having a toothed rack rigidly connected to said piston; an actuator 90 output shaft rotatably mounted in said housing and disposed transverse to said central longitudinal axis of said cylinder; a pinion rigidly affixed to said actuator output shaft and disposed so that teeth of the pinion engage with teeth of 95 the rack; at least one element having a low coefficient of friction being disposed between and in contact with said rack-bearing portion of said piston and the adjoining portion of the internal surface of said cylinder whereby any forces which 100 tend to displace said rack-bearing portion of said piston toward said adjoining portion of said internal surface of said cylinder are counteracted, spaced annuli of non-compressible material rigidly fixed for rotation with said actuator output shaft in 105 the vicinities of the opposite ends of said teeth of said pinion, said annuli having diameters equal to the pitch circle diameter of said teeth of said pinion, two spaced flat runways provided on said rack-bearing portion of said piston; and said annuli 110 forming a rolling contact with the runways on the piston during operation of the actuator to help to maintain contact between each said element and * the internal surface of said cylinder, to prevent rotational movement of said piston, and to prevent 115 over-meshing of the teeth of the rack and the pinion.

2. A piston rack rotary fluid pressure actuator, comprising: a housing including a cylinder having a central longitudinal axis; two pistons disposed 120 coaxially within said cylinder and arranged for linear movements substantially parallel to said central longitudinal axis of said cylinder either towards or away from one another under the influence of a difference in fluid pressure created 125 on opposite sides of each of said pistons, each said piston including a rack-bearing portion having a toothed rack rigidly connected to said piston; an

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actuator output shaft rotatably mounted in said housing and disposed transverse to said central longitudinal axis of said cylinder at the mid-length point of said cylinder; a pinion rigidly affixed to 5 said actuator output shaft and disposed between said racks integral with said pistons so that teeth of the pinion engage with teeth of the racks; at least one element having a low coefficient of friction being disposed between and in contact 10 with both said rack-bearing portion of each piston and the respective portion of the internal surface of said cylinder whereby any forces which tends to displace said rack-bearing portion of each of said pistons toward said respective portion of said 15 internal surface of said cylinder are counteracted, spaced annuli of non-compressible material rigidly fixed for rotation with said actuator output shaft in the vicinities of the opposite ends of said teeth of said pinion, said annuli having diameters equal to 20 the pitch circle diameter of said teeth of said pinion, two spaced flat runways provided on said rack-bearing portion of each of said pistons; and said annuli forming a rolling contact with the runways on the pistons during operation of the

25 actuator to help to maintain contact between each said element and the internai surface of said cylinder, to prevent rotational movement of said pistons, and to prevent over-meshing of the teeth of the racks and the pinion.

30 3. A piston rack rotary fluid pressure actuator according to Claim 2, in which said annuli are formed of anodised aluminium.

4. A piston rack rotary fluid pressure actuator according to Claims 2 or 3, in which said runways

35 are formed of anodised aluminium.

5. A piston rack rotary fluid pressure actuator according to any of Claims 2 to 4, in which said teeth of said pinion and said annuli are keyed to said actuator output shaft.

40 6. A piston rack rotary fluid pressure actuator according to any of Claims 2 to 4, in which said annuli are attached to said actuator output shaft by locking screws.

7. A piston rack rotary fluid pressure actuator

45 substantially as herein described with reference to the accompanying drawing.

Printed for Her Majesty's Stationery Office by the Courier Press, Leamington Spa, 1981. Published by the Patent Office, 25 Southampton Buildings, London, WC2A 1AY, from which copies may be obtained.