US3537358A

US3537358A - Rotary actuators

Info

Publication number: US3537358A
Application number: US776042A
Authority: US
Inventors: Alan Donald Bunyard
Original assignee: Individual
Current assignee: Individual
Priority date: 1967-11-25
Filing date: 1968-11-15
Publication date: 1970-11-03
Anticipated expiration: 1987-11-03
Also published as: BR6804267D0; BE724375A; DE1811427A1; SE337746B; IL31096A0; IL31096A; FR1592924A; GB1251805A; DE1811427B2

Description

United States Patent [72] lnventor Alan Donald Bunyard 2,844,127 7/1958 Steiner 92/136X Burrett Road, l-laywards l-leath, Sussex, 3,055,343 9/ 1962 Kurt 92/1 10 England 3,148,595 9/1964 Looney 92/136X [21] Appl. No. 776,042 3,150,489 9/1964 Dewar 92/136X [22] Filed Nov. 15, 1968 3,212,604 10/1965 Garnett 92/110X [45] Patented Nov. 3, 1970 3,411,409 11/1968 Bunyard... 92/166X [32] Priority Nov. 25,1967 3,447,423 6/1969 Henry 92/136X 3? g gs gr Primary Examiner-Martin P. Schwadron I 1 Assistant Examiner Leslie J. Payne Attorney Shoemaker and Mattare [54] ROTARY ACTUATORS 14 Claims, 5 Drawing Figs.

[52] US. Cl 92/136, ABSTRACT: piston rack type actuator i which the pressed air or gas is supplied either to the central chamber or [51] Int. Cl F0111 9/00, to the two end chambers, defined by two pistons located in a F 15/18 cylinder, in order to cause the two pistons to move towards or [50] Field of Search 92/50, 75, away from each other and thereby to rotate an output shaft by 69,31 140166,165,109J11i74/67, means of their integral racks meshing with a pinion on the l shaft. Two parallel guides extend through the two pistons, one piston being fixed to one guide and the other piston being [56] References Cited fixed to the other guide, end portions of each guide sliding in UNlTED STATES PATENTS bearings and serving to limit the extent of cocking or skewing 5 37 4/1953 Flak 2/1 1 X of one or both of the pistons and thus preventing fouling of the 2,828,722 4/1958 Bohnhoffet a1. 92/136X li d r th r by,

K I/ y V 7 /7 mi 8 V a up dlinplllllIplpullla '1' sheet' 1 014 ROTARY ACTUATQRS This invention relates to piston-rack rotary actuators.

A disadvantage of a piston-rack rotary actuator which includes a pair of pistons adapted to reciprocate in opposite directions along a pair .of opposed cylinders is that the pistons have a tendency to cock or turn in the cylinder, thereby jamming and/or damaging the radially inner surface or surfaces of the cylinders and thus, in time, the seats.

The principal object of the present invention is to overcome this disadvantage.

According to a first aspect, the present invention consists in a pneumatically operated piston-rack rotary actuator which includes two pistons adapted for reciprocating movements in opposite directions along chambersnunder the influence of fluid pressure, said pistons having integral racks meshingwith a common pinion which is connected to an output shaft of said actuator, two parallel guide members each of which is so connected to a different one of said two pistons that relative movement between saidguide member and said piston is inhibited, each guide member also extending through the other of said pistons in a manner such that relative movement between said guide member and said other of said pistons is permitted.

The present invention will now be more particularly described with reference to the accompanying drawings, in which:

P16. 1 illustrates, in longitudinalaxial section along the line [-1 in FIG. 3, oneembodiment of a pneumatically operated piston-rack rotary actuator constructed in accordance with the present invention and including parallel guide members;

H6. 2 illustrates a detail section on each of the lines ll-ll in FIG. 1;

F IG. 3 illustrates, in side elevation, partly in section, the actuator illustrated in FIG. 1 which includes pistons movable away from one another under the influence of a fluid under pressure and movable towards one another under the influence of compression springs;

FIG. 4 illustrates an end elevation of the actuator illustrated in FIG. 3 as seen from the left-hand end thereof; and

FIG. 5 illustrates an alternative embodiment of a pneumatically operated piston-rack rotary actuator'having a different means of attachment of end caps compared with the means used in said one embodiment. .1

in FIG. 1 there is illustrated a pneumatically operated piston-rack rotary actuator which includes two

pistons

10, 11 adapted for reciprocating movements in opposite directions along cylindrical chambers under the influence of fluid pressure, said pistons having

integral racks

12, 13, respectively, meshing with a common mutilated pinion 14 which is connected to or is integral with an output shaft of said actuator, of which shaft the axially outermost end is provided with spanner flats 15 and of which the opposite end is recessed (not illustrated) to accommodate the free end of the stern of a valve .which is to be operated by the actuator. Said

pistons

10, 11 are mounted upon tubular

parallel guide members

16, 17 and are keyed thereto through

grub screws

18, 19, respectively. The cylindrical chambers, in the embodiment illustrated are coaxial, being opposed parts of a continuous cylindrical chamber 20 which is defined by a cylindrical body 21- and ,endcaps'22, 23, the end caps being spaced apart by the cylindrical body 21 and said

end caps

22, 23 and said body 21 being held together by open-ended tubes 24, 25 each of which is exteriorly screw threaded at each end thereof. lnteriorly screw threaded nuts 26 are screwed'on to the left-hand ends (as seen in FIG. 1) of the parallel tubes 24, 25 and inter-iorly screw threaded nuts 27 are screwed on to the right-hand ends of said tub es, the nuts 26 being substantially cup-shaped in orderto close off said left-hand ends of the tubes but the nuts 27 being provided with screw threaded bores 28 for the purpose hereinafter described. It will be seen from FIG. 1 that the tubes 24, 25extend coaxially through the

guide members

16, 17, respectively, and that said tubes do not at any point touchthe respective guide members.

The

integral racks

12, 13 are formed in axially extending

sleeves

30, 31 which formparts of

said pistons

10, 11.

It will further be noted that the

pistons

10, ,11 not only have the

guide members

16, 17 extending through the

sleeves

30, 31 thereof, respectively, but also have the other of the

guide members

17, 16 extendin'g through apertures in which are located

bearing bushes

32, 33, respectively, which are made of a material having a low coefficient of friction (for example Delrin", P.T.F.E., or the like).

Sealing rings

34, 35 are provided in seats therefor.

Each of the

end caps

22, 23 is provided with axially spaced seats for the accommodation of a bearing bush 36 and a sealing ring 37 each of which encircles the guide member 16 and also witha seat for the accommodation of a bearing bush 38 which encircles the guide member 17. Each of the

bearing bushes

3,6, 38 is made of the same material as are the

bearing bushes

32, 33 and each of the

fourbearing bushes

32, 33, 36, .38 is discontinuous not only to enable them to be fitted and to allow for thermal expansion but also for the purpose of providing a passageway 39 (see FIG. 2) for the passage of air (or of any other ambient gas in which the actuator is Working) expelled from or drawn into the cylindrical chamber as the

pistons

10, 11 make their respective movements along said chambers, said air or other gas passing through the annular space between the guide member 17 and the tube 25, through right-hand open end (as seen in FIG. 1) of the tube 25 and through the bore 28 of that out 27 which is screwed on to said right-hand end of the-tube 2,5.

The tube 24 is similarly providedwithat least one hole 41 and the guide member 16 (unlike the guide member 17) is provided with at leastonehole 42.

Each

end cap

22, 23 is provided with a plurality of axially extending recesses 43 (FIG. 3 and 4), each recess accommodating one end of at least one helical compression spring. in fact, two compression springs 44, 45 are illustrated in Fig. 3, one within the other and the convolutions of one extending in a helix of opposite sense to that of the helix of the other.

In order to operate the actuator described above, a hose or other conduit (not illustrated) has one end thereof screwed into the bore 28 of that nut which is screwed onto the tube 24, the other end of said hose or conduit being connected directly or indirectly to a supply of fluid (for example air) under pressure. When said fluidis admitted into the tube 24, said fluid passes through the hole 41 but is unable to pass the sealing rings 37 and so passes through the hole 42 in the guide member 16 into the space between thepistons l0, 11 with the result that said pistons move away from one another (against the influence of the plurality of springs 44, 45 which act against the axially outer faces of the pistons 10, 11) into the relative positions thereof which have been illustrated in FIG. 1. Such movements of said pistonscause the integral racks 12, '13 to'turn the output shaft by means of the-two sets of teeth of the mutilated pinion 14 and also displace air past the bushes 38, through the hole ,40 in the tube 25 to atmosphere. When the supply of said fluid under pressure is interrupted, the

pistons

10, 11 will be moved under theinfluence of the power stored-in the compressed springs 44, 45 towards one'another from the relative positions thereof illustrated in H6. 1, thereby rotatingthe outputshaft in the opposite direction to that in which itiwaspreviously rotated and thereby alsodisplacing the fluid between the

pistons

10, 11 through the hole 42,- through the, hole 41,'and so on, to atmosphere.

It will be appreciated that the actuator described above can be operated by omission of the sets of springs 44, 45 and the connection to that one of the nuts 27 which isscrewed on to the tube 25 of another hoseor other conduit. in this case, the two hoses or conduits are connected to a valve and a supply of fluid under pressure, the arrangement beingsuch that, as the valve is usedto operate the actuator, to rotate the output shaft, the fluid previously applied to one side of each of the pistons is vented to atmosphereto enable the fluid being applied to the pistons simultaneously in the relative directions.

It may be desirable, for a number of reasons, to do without the parallel tubes 24, 25, which are, in effect, tie rods, and the

ancillary nuts

26, 27. A pneumatically operated actuator which is devoid of said tubes 24, 25 and of the

ancillary nuts

26, 27 is illustrated in FIG. and includes the same (or closely similar) cylindrical body 21, the same pistons l0, 11 with their .integral racks l2, 13, respectively, whose teeth mesh with the two sets of teeth of the mutilated pinion 14, and so on, the parts common to the two embodiments being indicated by the same reference numerals. The principal differences stem from the omission of the tubes which acted as tie rods, and include

end plates

50, 51.

The end plate 50 has formed therein spaced cylindrical cavities 52, 54 which are open at only one end and the end plate 51 has formed therein spaced cylindrical cavities 53, 55 which are open at both ends. The cavities 52 and 53 are in axial alignment with one another when the end caps are properly assembled with the body 21 as also are the cavities 54 and 55, and the cavities 52, 53 accommodate the end portions of the guide member 17 (which is plugged at 17') whilst the cavities 54, 55 accommodate the end portions of the guide member 16. Each of the cavities 52, 53 is provided with a seat for a bearing bush 36 and each of the cavities 54, 55 is provided with a bearing bush 38 which creates the passageway 39 which has been described above with reference to FIG. 2, the only difference being that in the embodiment now being described the bushes 38 encircle the guide member 16 whereas in the other embodiment they encircle the guide member 17.

It will be noted that, adjacent the seat for the bearing bush 38 which encircles the left-hand end (as seen in FIG. 5) of the guide member 16, there is another seat 56 in the end cap 50 and that said seat does not accommodate a sealing ring whereas the corresponding seat in the end cap 51 accommodates a sealing ring 37 which encircles the right-hand end of the guide member 17. The reason for such differentiation is that fluid under pressure is intended to reach the axially outer faces of the

pistons

10, 11 by passing through the passageways 39 and that such passage of said'fluid would be prevented by sealing rings encircling the two ends of the guide member 16. However, the

end caps

50, 51 are initially made identical with one another in order to simplify production and to keep production costs as low as possible.

Each of the

end caps

50, 51 isfurther provided with a radially outwardly facing seat portion which, when the cap has been inserted into the respected end of the cylindrical body 21, is in radial alignment with a complementary radially inwardly facing seat portion, said two seat portions constituting a seat for the accommodation of a square section metal (preferably stainless steel) wire 57 which is formed with a loop or the like (not illustrated) at one end. The other end of said wire is inserted into an enlarged circumferential slit (not illustrated) in the cylindrical body 21, said slit being in radial register with the seat formed by the said two seat portions and being in communication with said seat, and said wire is pushed through said slit into said seat until the whole or substantially the whole of said seat has been occupied by said wire. The loop or the like protrudes from said slit to an extent which is such as to enable any person wishing to remove the respective one of the

end caps

50, 51 to grip the said loop with a pair of pliers and to extract the wire from its seat. Said wire, being a tight push fit in said seat, keeps the end cap firmly in place and the end cap is made pressure-tight by a sealing ring 58 accommodated in a seat which is formed in the cylindrical body 21 and which is located adjacent to and axially inwardly of the seat which accommodates the wire 57.

It has been stated above that the cavities 53, 55 are open at both ends. Thus, the end wall of each of said cavities 53, 55 is drilled and tapped to provide a bore 59 whereby an exteriorly screw threaded union (not illustrated) on one end of a hose or other conduit (not illustrated) can be connected to the actuator, the other end of said hose or other conduit being connected to a supply of fluid (for example air) under pressure by way of the customary three-way valve.

The sleeve 31 of the piston 11 and also the guide member 17 are drilled to provide a port 60 by which the interior or bore of the tubular guide member 17 and the space between the axially inner faces of the

pistons

10, 11 are placed in communication with one another.

In operation, assuming that connection of said hoses or other conduits to the actuator has already been made, the relative positions of the

pistons

10, 11 are altered from those illustrated in FIG. 5 in order to rotate the output shaft in an anticlockwise direction of rotation (as seen in FIG. 5) by admitting fluid under pressure into the cavity 55 and by simultaneously venting the cavity 53 to atmosphere. The fluid under pressure will pass along the bore of the guide member 16 and also through the passageways 39 provided by the breaks in the bushes 38, and will thus be applied to the axially outer faces of the two pistons.l0, 11. The

pistons

10, 11 will consequently move simultaneously towards one another, displacing the fluid which is in the space between their axially inner faces through the port 60, through the end part of the bore of the guide member 17, through the cavity 53 and the hose or conduit which is screwed into the bore 59 which communicates with said cavity 53, and so to atmosphere via the three-way valve and rotating the output shaft in an anticlockwise direction. Conversely, in order to restore the

pistons

10, 11 to the relative dispositions thereof which are illustrated in FIG. 5 from the relative dispositions thereof in which they have been moved as close to one another as it is possible for them to be, fluid under pressure will be supplied to the cavity 53 from which it will pass through the port 60 to the space between the axially inner faces of the pistons l0, 11. The

pistons

10, 11 will thereupon move away from one another, thereby displacing the fluid which is in the space between the axially outer face of each piston and the axially inner face of the respective end cap past the bushes 38 into and through the cavity 55, through the hose or conduit which is screwed into the bore 59 which communicates with said cavity 55, and so to atmosphere via the three-way valve and rotating the output shaft in a clockwise direction.

When it is desired to change the operating condition of the actuator from a first condition thereof to the second condition thereof by fluid pressure applied in a single sense (namely, to rotate the output shaft of the actuator in a clockwise direction) with automatic restoration of the actuator from the second operating condition thereof to the first operating condition thereof whenever the supply of said fluid pressure is discontinued, each of the

end caps

50, 51 can be modified in the manner which has been indicated on and which will now be described with reference to the end cap 50. Said end cap 50 is shown by the dotted line 61 to have been increased in thickness over the greater part of its area when seen in plan; thus modified, the end cap 50 will not consist of a flat plate having two protuberances which will provide the cavities 52, 54 but will consist of a thick end cap having an outwardly directed flange whose thickness is considerably less than that of the remainder of said cap. The increased thickness will enable the requisite number of recesses 43 (only one recess is shown in FIG. 5) to be formed in the end cap for the accommodation of one end of at least one compression spring.

The two exemplary embodiments of an actuator according to the present invention which have been described above with reference, respectively, to FIGS. 1 to 4 and to FIG. 5 as supplemented or modified by any of FIGS. 1 to 4 operate very satisfactorily because the

bearing bushes

36, 38 are spaced apart by substantially the greatest distance which is possible in the actuator and because each of the

bearing bushes

36, 38 is also spaced from the point of loading by substantially the greatest distance which is possible in the actuator. Thus, considering FIG. 5 without the modification which involves the introduction of the springs accommodated in recesses 43, as fluid under pressure is supplied to the axially outer faces of the

pistons

10, 11, the

guide members

16, 17 bow outwardly and in so doing ultimately make contact with the

bearing bushes

33, 32, respectively.

Iclaim:

l. A fluid operated piston-rack rotary actuator comprising, an open-ended cylinder, end caps sealingly closing each end of the cylinder, a pair of axially adjacent pistons reciprocably mounted on said cylinder, a rotatably mounted output shaft extending radially through the wall of said cylinder between said pistons, gear teeth on said shaft, a toothed rack on each of said pistons cooperatively mating with the gear teeth on said shaft for rotating said shaft upon reciprocation of said pistons, said pistons in sealing sliding engagement with the inner surface of said cylinder and defining with said cylinder and with said end caps a pair of chambers at opposite ends of said cylinder between said pistons and said end caps and an intermediate chamber between said pistons, a pair of spaced guide bores in the inner surface of each said end cap in axial alignment with the spaced guide bores in the other end cap, a

reciprocable elongated guide member keyed to each of said pistons and reciprocable therewith and slidably received through the other of said pistons, said guide members slidably received at opposite ends thereof in said guide bores in said end caps during the entire stroke of said pistons, and means for introducing fluid under pressure into at least one of said chambers for moving said pistons in at least a first direction,

I saidguide members and said guide bores cooperating with said pistons to prevent canting and binding of said pistons in said cylinder due to lateral forces imposed thereon caused by the engagement between the gear teeth on said shaft and the toothed rack on said pistons.

2. An actuator as claimed in claim 1, wherein an axially extending sleeve encircles each guide member, said sleeve being integral with the respective piston and the respective toothed rack being integral with said sleeve.

3. A fluid operated piston-rack rotary actuator comprising a cylinder having closedends, a pair of pistons reciprocably mounted within said cylinder, a shaft extending radially through the side of said cylinder, cooperating gear means on said shaft and on said pistons for causing said shaft to' rotate upon reciprocation of said pistons, a pair of guide bores in each end of the cylinder in axial alignment with the guide bores in the other end of the cylinder, a pair of parallel reciprocable guide members each fixed to a different one of said pistons and slidably received through the other of said pistons, the opposite ends of said guide members extending beyond said pistons and slidably received in said guide bores in said cylinder ends throughout the entire stroke of said pistons, and means for introducing fluid under pressure to said cylinder and pistons to cause said pistons to reciprocate, said guide members and said guide bores cooperating with said pistons to prevent canting and binding of said pistons in said cylinder due to lateral forces imposed on said pistons caused by engagement of said cooperating gear means on said shaft and said pistons.

4. An actuator as claimed in claim 3, wherein each guide member is tubular and wherein there is a bearing made of a material having a low coefficient of friction carried byeach of said pistons, said one of said guide members extending through that bearing which is carried by said other of said pistons and said other of said guide members extending through that bearing which is carried by said one of said pistons.

5. An actuator as claimed in claim 4, wherein is included apertured means permissive of the supply to and the exhaust from the respective guide member of said fluid pressure, means defining an aperture in said respective guide member, an intermediate chamber in said cylinder, said intermediate chamber being defined by said pistons in first rest positions thereof in said cylinder, said one aperture in said respective guide member communicating with said intermediate chamber, spring means'located between each piston and the respective cylinder end, supply of said fluid pressure to said intermediate chamber by way of said respective guide member and said one aperture therein effecting simultaneous movements of said pistons from said first rest positions thereof away from each other towards said cylinder ends into second rest positions thereof and simultaneous compression of said spring means to store power therein, exhaust of said fluid pressure from said respective guide member enabling simultaneous movements of said pistons toward one another from said second rest positions thereof to said first rest positions thereof under the influence of the power stored in said spring means, locating means carried by each cylinder end, the spring means located between each piston and the respective cylinder end consisting of a set of springs, one end of each spring being in contacting relationship with the respective piston and the other end of each spring being in engagement with one of said locating means carried by the respective cylinder end.

6. An actuator as claimed in claim 5, wherein there are two tubes parallel to one another, one of said tubes extending concentrically through said one of said tubular guide members and th'e other of said tubes extending concentrically through said other of said guide members said tubes being of lengths such that the two ends of each tube extend through said guide bores and protrude beyond said cylinder ends, the protruding ends of each of said tubes being externally threaded, means defining an aperture in each of said tubes whereby the bores of the two tubes are placed in communication with the bores of the respective guide members, four nuts screwed on to the ends of said two tubes whereby said cylinder ends are caused to close the ends of said cylinder in a sealed manner, means defining an aperture in each of two of said nuts, one of said apertured nuts being screwed onto one end of one of said tubes and constituting said apertured means permissive of the supply to and the exhaust from the respective guide member of said fluid pressure, the other of said apertured nuts being screwed on to one of the other of said tubes.

7. An actuator as claimed in claim 5, wherein said locating means consists of an axially extending recess in said cylinder end.

8. An actuator as claimed in claim 7, wherein each spring of each set consists of a single helical compression spring.

9. An actuator as claimed in claim 7, wherein each spring of each set consists of a pair of helical compression springs nested one within the other, the helix of said one spring of said pair of springs being wound in a sense opposite to that in which the helix of said other spring of said pair of springs is wound.

10. A pneumatically operated piston-rack rotary actuator which includes means defining two chambers, two parallel guide members having a nonfixed mounting relationship with said means, a rotatably mounted output shaft located between said two chambers, gear teeth carried by said shaft, :1 piston located in each of said twochambers, atoothed rack connected to each piston, the teeth of each rack being in permanent engagement with some of the gear teeth carried by said shaft, one of said pistons being fixed to one of said guide members for reciprocating movements in opposite directions with said one of said guide members and the other of said pistons being fixed to the other of said guide members for reciprocating movements in opposite directions with said other of said guide members, bearing means made of a material having a low coefficient of friction and disposed between said means defining two chambers on the one hand and the respective guide members on the other hand, said one of said guide members also extending through said other of said pistons with freedom for sliding movement therealongand said other of said guide members also extending through said one of said pistons with freedom for sliding movement therealong, means permissive of the application of fluid pressure to said pistons to effect simultaneous movements of said pistons along the respective chambers and consequent rotation of said shaft, each guide member comprising a tube, a bearing made of a material having a low coefficient of friction carried by each of said pistons, said one of said guide members extending through that bearing which is carried by said other of said pistons and said other of said guide members extending through that hearing which is carried by said one of said pistons, said two chambers constituting opposed parts of a continuous cylindrical chamber, said means defining said two chambers being constituted by an open-ended cylindrical body, end caps closing the ends of said cylindrical body in a pressure-tight manner, means in each end cap defining two spaced bores which are disposed in axial alignment with the two spaced bores in the other end cap, the end portions of said guide members extending into the bores in said end caps, apertured means permissive of the supply to and the exhaust from each of the respective guide members of said fluid pressure, means defining an aperture in said one of said guide members, an intermediate chamber is said continuous cylindrical chamber, said intermediate chamber being defined by said pistons in first rest positions thereof in said two chambers, said aperture in said one of said guide members communicating with said intermediate chamber, said bearing means being disposed between said end caps and the respective end portions of said guide members and also being in contact with said end portions over less than the entire periphery of said end portions whereby a passageway is formed in each bearing means, the passageways in those bearing means which are disposed between the end caps and the respective ends of said other of said guide members being in communication both with the bore of said other of said guide members and with said two chambers, supply of said fluid pressure to said intermediate chamber by way of one of said apertured means and said one of said guide members and the aperture therein effecting simultaneous movements of said pistons from said first rest positions thereof away from each other towards said end caps into second rest positions thereof and simultaneous displacement to atmosphere of the fluid in said two chambers by way of said passageways and the bore of said other of said guide members and the other of said apertured means, and

supply of said fluid pressure thereafter to said two chambers by way of said other apertured means and the bore of said other of said guide members and said passageways effecting simultaneous movements of said pistons from said second rest positions thereof towards each other away from said end caps into said first rest positions thereof and simultaneous displacement to atmosphere of the fluid in said intermediate chamber by way of the aperture in and the bore of said one of said guide members and said one apertured means.

11. A pneumatically operated piston-rack rotary actuator which includes means defining two chambers, two parallel guide members having a nonfixed mounting relationship with said means, a rotatably mounted output shaft located between said two chambers, gear teeth carried by said shaft, a piston located in each of said two chambers, a toothed rack connected to each piston, the teeth of each rack being in permanent engagement with some of the gear teeth carried by said shaft, one of said pistons being fixed to one of said guide members for reciprocating movements in opposite directions with said one of said guide members and the other of said pistons being fixed to the other of said guide members for reciprocating movements in opposite directions with said other of said guide members, bearing means made of a material having a low coefficient of friction and disposed between said means defining two chambers on the one hand and the respective guide members on the other hand, said one of said guide members also extending through said other of said pistons with freedom for sliding movement therealong and said other of said guide members also extending through said one of said pistons with freedom for sliding movement therealong, means permissive of the application of fluid pressure to said pistons to effect simultaneous movements of said pistons along the respective chambers and consequent rotation of said shaft, each guide member comprising a tube, a bearing made of a material having a low coefficient of friction carried by each of said pistons, said one of said guide members extending through that bearing which is carried by said other of said pistons and said other of said guide members extending through that bearing which is carried by said one of said pistons, said two chambers constituting opposed parts of a continuous cylindrical chamber, said means defining said two chambers being constituted by an open-ended cylindrical body, end caps closing the ends of said cylindrical body,

and vice versa of said fluid pressure, means defining an aperture in said one of said guide members, an intermediate chamber in said continuous cylindrical chamber, said intermediate chamber being defined by said pistons in first rest positions thereof in said two chambers, said one aperture in said one of said respective guide members communicating with said intermediate chamber, said bearing means including two bearings disposed one in the vicinity of each end of said other of said guide members and also disposed between said end and the respective end cap, each of said bearings being in contact with said other guide member over less than the entire periphery thereof whereby a passageway is formed which is in communication both with the bore of said other guide member and with the respective one of said two cavities on the one hand and with the respective one of said two chambers on the other hand, supply of fluid pressure to the cavity which communicates with the intermediate chamber by way of the bore of and the aperture in said one of said guide members effecting simultaneous movements of said pistons away from one another towards the end caps to second rest positions thereof in said two chambers and displacement to atmosphere of the fluid in said two chambers by way of said passageways, the bore of said other of said guide members and the cavity with which said bore is in communication.

12. An actuator as claimed in claim 11, which further includes recesses in each end cap, at least one spring having one end thereof located in each recess, the other end of the springs bearing against the axially outer faces of said two pistons, interruption of the supply of said fluid pressure to said intermediate chamber permitting the power which was stored in said springs during travel of the pistons from said first to said second rest positions thereof to restore said pistons to said first rest positions thereof.

13. An actuator as claimed in claim 11, wherein the supply of fluid pressure to the cavity which communicates with the said two chambers effects simultaneous movements of said pistons towards one another to said first rest positions thereof in said two chambers and displacement to atmosphere of the fluid an said intermediate chamber by way of the aperture in and the bore of said one of said guide members and the cavity which communicates with said bore.

14. An actuator as claimed in claim 11, which further comprises two seats for a locking device, said seats being constituted by complementary groove portions which are in radial register with one another and which are formed one in the vicinity of each end of said cylindrical body in a radially inner face thereof and the other in each end cap in a radially outer face thereof, means defining a cut-away portion in the vicinity of each end of said cylindrical body, each cut-away portion being in communication with the respective seats, and a deformable locking device having resistance to shear forces located in each seat and occupying substantially the whole thereof.