NZ237720A - Fluid pressure linear or rotary actuator; piston in cylinder with internal tie rods for end caps - Google Patents

Description

237720 PATENTS FORM NO. 5 Fee No. 4: $260.00 PATENTS ACT 1953 V ** **+ COMPLETE SPECIFICATION 'D After Provisional . _"r, No: 237720 Dated: 5th April 1991 ?/ r* •«> ACTUATORS WE NORTRAC ENGINEERING LIMITED, a New Zealand company of Newton Place, Hamilton, New Zealand hereby declare the invention for which we pray that a patent may be granted to me/us, and the method by which it is to be performed to be particularly described in and by the following statement: 1 237720 This invention relates to actuators.

The invention may be particularly applicable to pneumatically operated actuators for fluid control valves.

Pneumatically operated actuators for fluid control valves generally comprise a cylinder closed at one or both ends by end caps and a piston able to move freely in the bore of the cylinder. The piston is connected by a connecting device to a valve shaft so that the valve is moved when the piston moves. The connecting device can be a direct connection so that the valve is moved linearly with the piston, as is the case with a poppet valve. Alternatively the connecting device can be a translation device which converts linear movement of the piston into rotational movement for rotating the valve as with a rotary valve. With this type of pneumatically operated actuator the valve to which it is connected can be moved in either direction ie. open or closed by supplying air pressure to either side of the piston of the actuator. However it is often desirable, for reasons such as safety in the event of air pressure failure, to have the piston biased towards an open or closed position using a heavy spring. In this case air pressure is used to overcome the spring force to open (or close) the valve and spring pressure is used to close (or open) the valve. In the event of air pressure failure, the spring force acts to securely close (or open) the valve and hold it in that condition.

Conventionally with such actuators a base end cap is formed integral with the cylinder and the actuating device passes through a hole in the base end cap to connect to a valve. Furthermore, the spring is encapsulated and held under a predetermined pre-load in such a way that the spring may be further compressed but is prevented from becoming fully unloaded^? A f ; *N> Such an encapsulated spring is placed inside the cylinder of the actuator between the piston and the base end cap and further compressed by an adjusting device fitted between the valve and actuator so as to apply a force to the valve to hold it under load in a closed or open condition.

Typical means of assembling the actuator and valve may involve placing the encapsulated spring in the cylinder, inserting the piston shaft through the spring to pass out through a hole in a base end cap of the cylinder, and fitting a top end cap over the top end of the cylinder, the top end cap sealing the top end of the cylinder. The valve position is then adjusted relative to the actuator to apply a load to the valve poppet or gate and the valve is then fixedly held in the adjusted position by clamping to the base end cap of the actuator.

In the case of the actuator being used for rotary actuation, a translation device is fitted between the piston and the actuator shaft. The translation device converts linear motion of the piston into rotary motion of the actuator shaft which is connected to the gate of a rotary type valve. The rotary reaction from the translation device acts on the piston and is resisted by support pins which are cantilever mounted to the base end cap of the cylinder and slidable in axial bores in the piston.

In operation, pressurized air is supplied to the region between the top end cap and the piston so that the piston is forced to move against the spring load, and the actuator rod is moved to operate the valve. The axial loads resulting from the operating pressure of the valve are transferred through the end cap connection to the walls of the cylinder. Consequently the end cap connection carries the main operating load of the valve and its attachment to the cylinder must be carefully, designed to ensure the load 3 257720 can be safely carried and also to ensure a seal. One means of attaching the end cap utilizes a spring steel rod threaded between peripheral grooves in the cylinder bore and in the end cap, and an O ring is used for sealing. Other means may include having a flanged connection between the top end cap and the top end of the cylinder.

With a valve of this construction several problems such as the following exist; (1) Since the actuator base end cap and cylinder sides are formed as one, repair or replacement of either the base end cap or cylinder involves removal of the combined base end/cylinder assembly. Consequently components not requiring attention are unnecessarily removed leading to inefficiencies in maintenance. (2) The method of attaching the top cover such as by use of a spring steel rod is complicated and time consuming. Furthermore damage of the connection elements may require complete replacement of the cylinder. It is also difficult with this method to check if the connection has been done properly. (3) Use of a flange to attach the top end cap to the cylinder results in increased manufacturing costs, and assembly costs and also requires additional space to accommodate the increased diameter of the flange portion.

A V- ^ i- t g /i..',, />. ©V. % 3 % $ 237720 (4) In the case of a configuration for a a rotary actuator, since the support pins for preventing rotation of the piston are cantilever mounted they must be of rigid construction to prevent bending, and their attachment to the base end cap must be carefully designed resulting in high cost.

It is an object of the present invention to provide a valve which addresses the above problems and difficulties, or at the very least offers a useful choice.

According to a broadest aspect of the present invention there is provided an actuator that is operated by fluid pressure, comprising a cylinder provided with a means for supplying fluid pressure thereto closed at one end by a top end cap and an other end by a base end cap, and having a piston slidable in said cylinder and connected by a connection device to an actuator rod passing to the outside through either said base end cap or said top end cap such that when fluid pressure is applied to said cylinder, said piston slides in said cylinder and drives said actuator rod, said top end cap and said base end cap held in sealing abutment with said cylinder by a cylinder tie rod passing between said top end cap and said base end cap, through at least one aperture in said piston, said aperture being such as to allow said piston to move freely in an axial direction relative to said cylinder tie rod without allowing any appreciable fluid flow therethrough.

According to another aspect of the present invention there is provided an actuator that is operated by fluid pressure and biased by a spring, comprising a cylinder closed at one end by a top end cap and an other end by a base end cap, and having a piston slidable in said cylinder with a spring disposed between said piston and said base end cap, said spring 237720 being held by a retaining means in a compressed state such that said spring can be further compressed by movement of said piston towards said base end cap due to an increase in fluid pressure.

According to another aspect of the present invention said retaining means comprises a pre-load tie rod connected between said base end cap and said piston rod.

According to yet a further aspect of the present invention said pre-load tie rod is attached to either said base end cap or said piston and slides freely through a hole in either said piston or said base end cap respectively.

According to a yet further aspect of the present invention said hole is sealed so that said pre-load tie rod can pass freely through said hole without allowing any appreciable fluid flow.

According to yet a further aspect of the present invention said piston is connected directly to said actuator rod so that a linear movement of said piston results in a substantially linear movement of actuator rod.

According to yet a further aspect of the present invention said piston is connected to a translation device which is connected to said actuator rod the arrangement and construction of the translation device being such that a linear movement of the piston results in a rotational movement of the actuator rod.

According to yet a further aspect of the present invention said translation device may comprise a hollow block on the base of said piston having a slot at an incline to the axis of said piston so that said actuator rod can pass 6 I o % r 237720 into the hollow portion of the block and a pin on the end of said actuator rod can mesh with said slot, the arrangement and construction being such that linear movement of said piston is translated into rotational movement of said actuator rod by the sliding of said pin in said slot, and said piston is restrained from rotation by said cylinder tie rod.

According to yet a further aspect of the present invention means for supplying fluid pressure to said cylinder is disposed on either or both of said top end cap and said bottom end cap.

According to yet a further aspect of the present invention said actuator rod passes through a hole in said bottom end cap.

According to yet a further aspect of the present invention said hole in said bottom end cap is sealed so as to prevent any appreciable flow of fluid.

According to yet a further aspect of the present invention said actuator can be assembled and disassembled as three components, namely; (i) a base component comprising base end cap, spring, piston, pre-load tie rod, translation device, actuator shaft and cylinder tie rod, (ii) a cylinder component comprising the cylinder and (iii) a top end component comprising the top end cap. 7 Due to the top and base end caps being attached to the cylinder by clamping action of one or more cylinder tie rods passing through the piston a variety of sealing means is possible. Possible means include but are not limited to; (i) having cylindrical protrusions on the end caps which fit into each end of the cylinder bore and are sealed by 0 rings between the bore and protrusions, (ii) having circular grooves around the peripheries of the end caps of such dimensions that the ends of the cylinder can fit in the grooves and be sealed by O rings fitted in the bottoms of the grooves.

It is generally envisaged that at least two cylinder tie rods would be provided between the two end caps. This may simplify the provision of even clamping of the end caps thereby ensuring positive sealing with the cylinder. It may also result in a more balanced support of the piston providing both a vertical guide and a restraint to rotation.

The cylinder tie rods may be preferably made of a high strength steel with a polished wear resistant surface so as to slide smoothly in the holes in the piston and provide a seal with the piston holes. One end of the cylinder tie rods may be fixedly attached to either the base end cap or the top end cap. However the cylinder tie rods are preferably attached to the base end cap so as to make up a complete base component assembly.

The piston may be made of a metal or plastic material. The sealing with the internal bore of the cylinder may be by means of an 0 ring fitted into a circumferential groove in the piston, although with precision machining v a ;■ 8 of the bore and piston an 0 ring may not be necessary. Similarly sealing between the cylinder tie rod and piston may be by means of 0 rings fitted inside bores in the piston or by providing precision machined mating surfaces.

For linear actuators the actuator rod may be attached to one side of the piston by any conventional means and protrude through the base end cap with a sealing device fitted if necessary. Alternatively it may pass through the piston and protrude through both the base end cap and the top end cap, the passage through the top end cap being sealed to prevent fluid leakage. The portion of the actuator rod protruding through the top end cap can be used for an element in a device such as a valve controller or position indicator and the like.

For rotary actuators, the translation mechanism is connected between the piston and the actuator rod which mechansim may be made from high density plastic and formed integrally with the piston. Rotation of the translation mechanism due to reaction force when turning the actuator rod may be restrained by the cylinder tie rods passing through the piston. Since the cylinder tie rods are under tension and supported at each end, they may be of relatively small diameter and yet have sufficient strength to prevent rotation, thereby reducing costs and reducing friction due to a small surface contact area. The translation mechanism may provide linear to radial translation by means of a hollow block on the base of the piston which has a slot at an incline to the axis of the piston so that the actuator shaft may pass into the hollow portion of the block, and a pin on the end of the actuator rod may mesh with the slot, so that linear movement of said piston may be translated into rotational movement of the actuator rod by the sliding of t h^ev 9 !* \ 2^77£Q pin in the slot. Alternatively the actuator rod may be formed with a spiral spline at an end portion which slides in a mating spiral spline in a hole in a protrusion on the piston such that linear motion of the piston relative to the actuator rod causes the rod to rotate as the splined surfaces mesh. With either of these methods the actuator rod may pass through the base end cap and be supported by a bushing therein which allows rotational motion of the actuator shaft but restricts linear motion thereof. Alternatively linear motion of the actuator shaft may be restricted by a thrust bearing in a device to which the actuator is connected. The actuator rod may also pass slidably through the piston and out through the top end cap, and sealing at the piston and the top end cap may be by means of a sealing device such as an 0 ring. The portion of the actuator rod protruding through the top end cap may be used for an element in a device such as a valve controller or position indicator and the like.

The actuator may have a return device to operate the actuator in a direction opposite to that when operated under fluid pressure or to shut off the actuator in the event of operating fluid pressure failure. It is generally envisioned that the return device would comprise a coil compression spring disposed between the piston and the base end cap. However the return device may be any suitable device that would allow movement of the piston under fluid pressure and provide a returning force to move the piston to its original position when the fluid pressure was reduced.

To ensure that the actuator will positively hold the device to which it is connected when fluid pressure is removed, the return device may be set to a predetermined pre-load. This may be achieved by a loading device. With an arrangement wherein the return device is disposed between the piston ■' ? a y v V 237720 and the base end cap, the loading device may consist of one or more preload tie rods connecting between the piston and the base end cap whereby a load can be applied to the return device by tensioning the pre-load tie rods.

With the actuator in a configuration for linear operation and connected to a valve device such as a poppet type valve, the poppet of the valve may be loaded against the valve seat by linearly moving the valve device relative to the actuator assembly, and locking the valve device to the actuator assembly with the poppet pre-loaded against the valve seat. With the actuator in a configuration for rotary operation and connected to a valve device such as a rotary valve, the gate of the valve may be loaded in the closed condition by rotation of the actuator assembly with respect to the valve device and locking the actuator assembly to the valve device with the gate of the valve pre-loaded in the closed condition. In both configurations, a banjo type connection may be used for connecting the valve to the actuator. This would enable quick removal and installation and provide a simple means for adjusting alignment between the valve and actuator.

The pre-load tie rods may comprise metal rods fixed at one end to either the piston or the base end cap and provided with a means whereby the distance between the piston and the base end cap can be adjusted so that the load on the return device can be set to a predetermined value. Possible means include but are not restricted to a threaded end with a nut, or one or more circlip grooves machined in the pre-load tie rod with a circlip fitted in the appropriate groove.

With an arrangement wherein the pre-load tie rod passes through a hole in the piston, this passage may be sealed from fluid flow with an O ring fitted into a groove in the passage or in the pre-load tie rod. Alternatively...

V *■ 11 1- \\ ? V ft f • - if the pre-load tie rod is fixed relative to the piston, the end away from the piston may be used for length adjustment and loading and this end will pass through the base end cap and slide relative thereto. If required, a seal such as an 0 ring fitted into a groove in either or both the base end cap and the pre-load tie rod may be used to provide a fluid seal.

Aspects of the present invention will now be described by way of example only with reference to the accompanying drawings in which: Figure 1: is a partially cut away side view of an actuator in a configuration for linear operation in accordance with one possible embodiment of the present invention, and Figure 2: is a partially cut away side view of an actuator in a configuration for rotary operation in accordance with another possible embodiment of the present invention, and Figure 3: is a partially cut away view of the actuator of Fig. 2 in the configuration for rotary operation viewed at 90 degrees to the view of Fig. 2 and fitted with a return spring.

With respect to Figure 1 of the drawings there is provided an actuator generally indicated by arrow 1. The actuator comprises a cylinder 2, a piston 3, top end cap 4, base end cap 5, cylinder tie rods 6 with end nuts 6a and an actuator rod 7. A top end port 8 and base end port 9 are provided in the top end cap 4 and bottom end cap 5 for connection to an operating fluid supply (not shown). A fluid seal between the internal bore of the cylinder 2 and the piston 3 is provided by an O ring 10 fitted into a circumferential groove in the piston, and a fluid seal between the end caps 4 and 5 Lis */„ •; I /> o 1 2 237720 provided by O rings 11 and 12 respectively. The cylinder tie rods 6 pass through holes 13 in the piston 3 and a fluid seal between the cylinder tie rods 6 and the piston 3 is provided by O rings 14 fitted into grooves in the holes 13. The actuator rod has a flange 15 and threaded end 16. The threaded end 16 passes through a hole 17 and is fixedly attached to the piston 3 by nut 18. The connection is sealed by 0 ring 19 fitted in a groove in the hole 17. The other end of the actuating rod 7 passes through a hole 20 in the bottom end cap 5 and is sealed with an O ring 21 to prevent fluid flow. The actuating rod 7 and cylinder tie rods 6 are made of a corrosion resistant steel and polished to a high finish to ensure a fluid tight seal with the O rings 19, 21 and 0 ring 14 respectively. The cylinder 2 is also made of a corrosion resistant steel with a highly polished bore, while the piston 3 is made of a high density plastic.

The actuator is assembled by fitting the actuator rod 7 of the piston/actuator rod assembly into the hole 20 and then passing the cylinder tie rods 6 with nuts 6a attached to the lower end thereof, through the bottom end cap 5 and the piston 2, 0 rings 22 having been fitted beforehand. The cylinder 2 is then placed over the piston 3 so that the piston 3 is housed in the cylinder bore, and the bottom end of the cylinder fits over the O ring 12 of the end cap 5. The top end cap 4 is then fitted over the top end of the cylinder 2 with the cylinder tie rods 6 passing through holes 22 fitted with O rings 23 and slid down so that the O ring 11 fits inside the bore of the cylinder 2. The end caps are then clamped in position by tightening the nuts 6a.

To operate the actuator, air supplies (not shown) are connected to port 8 and port 9 via a control valve (not shown) so that a controlled differential air pressure can be applied across the piston 3. By adjustment of the , v , v' /> < 13 i % * \ ^ - difference in air pressure supplied to the ports 8 and 9, the piston 3 can be made to move either up or down (as viewed in Fig. 1) thereby operating a device such as a valve connected to the end of the actuating rod 7.

With the above construction the actuator can be easily disassembled. Removal of the two nuts 6a on the top end cap enables the unit to be easily disassembled in its main components for inspection and replacement. The actuator can also be easily reassembled by tightening the two nuts 6a, and correct assembly can be verified by noting the length of cylinder tie rod 6 protruding from the nut 6a. The method is thus superior in several ways to the conventional method of attachment using a spring steel rod.

The fact that the cylinder 2 can be removed from the base end cap 5 offers advantages in inspection and component replacement in cases where the base end cap 5 is attached to a valve and is difficult to remove from the valve.

The provision of cylinder tie rods 6 passing through the piston 3 provides additional support of the piston 3 compared to conventional methods where the piston is only supported by the bore. The cylinder tie rods 6 maintain the piston 3 concentric with the bore and prevent uneven wear on the bore. This is a particular advantage in the case that there is a transverse load •• V 1 4 237720 on the actuator rod 7 due to a misalignment such as can occur with incorrect valve connection.

With respect to Figure 2 of the drawings there is provided an actuator generally indicated by arrow 20. For the sake of brevity components of the actuator of similar construction and having a similar function to those of the actuator of Fig. 1 are denoted by the same numbers and will not be described. The actuator 20 differs from the actuator 1 of Fig. 1 in that it is designed for operation of a device requiring rotary motion such as a rotary valve. The piston 3 is therefore provided with a translation device generally indicated by arrow 21. The translation device 21 comprises a cylindrical block 22 moulded as one with the piston 3 and protruding from the base of the piston 3. A hole 23 is bored inside the block 22 concentric with the piston 3 and of a diameter so as to slidably accommodate an actuator shaft 24. The actuator shaft 24 passes through the base end cap 5 and is connected to a rotary valve (not shown) the rotary valve being fixedly connected to the base end cap 5 and having suitable thrust bearings so as to prevent axial movement of the actuator shaft relative to the base end cap 5. The piston end portion of the actuator shaft 24 is fitted with a translation pin 25 which is fixed perpendicular to actuator shaft 24 and protrudes from opposite sides thereof. Opposite ends of the translation pin 25 fit into respective slots 26 machined in the block 22 and are inclined at angles of identical magnitude to the axis of the actuator shaft 24 so as to cause rotation of the actuator shaft 24 when the opposite ends of the translation pin 25 slide in the respective slots 26 as the actuator shaft 24 is slid axially with respect to the piston 3. 1 5 v m c- '"rr, To operate the actuator assembly, an air pressure differential is applied across the piston 3 so that the piston 3 is moved axially in the bore of the cylinder 2. This results in the translation pin 25 sliding in the slot 26 producing a force that tends to rotate the actuator shaft 24 relative to the piston 3. The piston 3 is restrained from rotation in the bore by the cylinder tie rods 6. Consequently the actuator shaft 24 is forced to rotate relative to the cylinder 2 and base end cap 5. The rotation of the actuator rod 24 is transmitted to a rotatable element such as a rotatable gate of a rotary valve thereby providing rotary motion for operation of the valve.

With such a rotary actuator as described above the rotary reaction acting on the piston 3 is opposed by transverse loading on the cylinder tie rods 6. This transverse load is opposed by bending moment forces in the cylinder tie rods 6. However since the cylinder tie rods 6 are under tension, there is an additional restraining force due to the tension in the cylinder tie rods 6 and hence the diameter of the cylinder tie rods 6 can be reduced. This enables a reduction in cost and lower friction between the piston 3 and cylinder tie rod 6 as compared to conventional restraining methods where restraining rods are cantilever mounted to the base end cap with their free ends fitted into holes in the base of the piston.

With respect to Figure 3 of the drawings there is provided an actuator generally indicated by arrow 30. For the sake of brevity components of the actuator of similar construction and having a similar function to those of the actuator of Fig. 2 are denoted by the same numbers and will not be described. The actuator 30 differs from the actuator 20 of Fig. 2 in that it is provided with a return device in the form of a spring 31. The spring 31 is disposed between the piston 3 and the base end 5 and pre-loaded by means of pre-load tie rods , v 1 6 23772o 32. The pre-load tie rods 32 are threaded at both ends and fixedly attached to the base end cap 5 by screwing into threaded holes 33 in the base end cap 5. The other ends pass through holes 34 in the piston 3 and are threaded into nuts 35 which are seated in recesses in the face of the piston 3. The spring 31 can be pre-loaded to a predetermined load by tightening or loosening the nuts 35. The holes 34 in the piston 3 are fitted with O rings 36 to prevent any appreciable fluid flow therethrough, and the piston is free to slide along the pre-load tie rods 32 so that the piston can move axially relative to the base end 5.

An indicator rod 37 is formed as an extension to the actuator rod 24 and passes through a hole 39 in the piston and out through a bushing 40 in the top end cap 4. The hole in the bushing 40 is sealed with an O ring 41 which allows rotation of the indicator rod 37, while the hole 39 in the piston 3 is sealed by an O ring 42 which allows both rotation and axial motion of the indicator rod 37 relative to the piston 3.

To operate the actuator assembly, air pressure is applied to the top of the piston 3 until the force on the piston 3 exceeds the force due to the pre-load spring 31. The piston 3 then moves axially in the bore of the cylinder 2 towards the base end cap 5. As with the actuator of Fig. 2, this results in the translation pin 25 sliding in the slot 26 producing a force that tends to rotate the actuator shaft relative to the piston. The piston 3 is restrained from rotation in the bore by the cylinder tie rods 6. Consequently the actuator rod 24 is forced to rotate relative to the cylinder 2 and base end cap 5. The rotation of the actuator rod 24 is transmitted to a rotatable element such as a rotatable gate of a rotary valve thereby providing rotary motion for operation of the valve.

I* 17 « ZH 1 ,n - CV ' O TV ' % - 237720 When the air pressure is reduced the force due to the spring load exceeds the pressure force on the top of the piston 3 so that the piston 3 is moved away from the base end cap 5. This results in the translation pin 25 sliding in the slot 26 producing a force that tends to rotate the actuator shaft 24 in an opposite direction relative to the piston 3. Further reduction in air pressure results in the piston 3 moving away from the base end cap 5 until the spring 31 is restrained by the end nuts 35 or until the actuator shaft 24 is prevented from further rotation due to a limit in a device to which it is connected.

When the actuator 30 is connected to a device such as a rotary valve, this limit could be the condition wherein the gate of the valve is fully closed. This limit can be set by rotation of the rotary valve relative to the actuator 30 until the gate is held closed at the required loading by the spring 31, and then securing the rotary valve to the actuator 30 in this condition.

It is also possible to have an arrangement with the return spring 31 of Fig. 3 fitted to the actuator 1 of Fig. 1. In this case the actuator rod 7 is moved axially due to fluid pressure on the piston 3. Reduction in air pressure acting on the piston 3 results in the piston 3 moving away from the base end cap 5 until the spring 31 is restrained by the end nuts 35 or until the actuator shaft 7 is pfev tinted from further axial movement due to a limit in a device to which it is connected.

When the actuator is connected to a device such as a poppet valve this limit could be the condition wherein the poppet of the valve is fully closed against the valve seat. This limit can be set by axial positioning of the valve relative to the actuator until the valve is held closed at the required loading - I-*'* by spring, and then securing the valve to the actuator in this condition* « 5 18 % V 1-s 23 7720 We believe the advantages of our invention to be as follows, however it should be appreciated that all such advantages may not be realized on all embodiments of the invention, and the following list is given by way of example only as being indicative of potential advantages of the present invention. Furthermore, it is not intended that the advantages be restricted to those of the list which follows: 1. Attachment of the top end cap the cylinder is simplified and the external dimensions of the end of the cylinder and top end can be minimized. 2. The condition of attachment of the end caps to the cylinder can be verified by checking the amount of thread protruding from the nut of the cylinder tie rods, thereby improving safety and reliability. 3. The actuator rod or indicator rod can be extended through the top end cap for use in indicator or control devices. 4. O rings can be used through out for sealing, enabling procedures related to seals to be standardized.

. The bottom end cap can be made separate from the cylinder simplifying manufacture and enabling the cylinder and bottom end cap to be serviced separately.

The pre-load spring can be easily assembled and disassembled and the pre- load can be easily adjusted. 1 °'1 ' % £■; 1 9 \ .o * . * v :■ 7. The pre-load spring can be safely tensioned and untensioned. 8. Faulty components of the actuator can be easily replaced without removing other components. 9. The cylinder tie rods are used for both clamping the end caps and restraining the piston from rotation, thereby providing efficiency of use. The tension in the tie rods enables them to carry a large side load without the need for a large cross sectional area thereby enabling a weight reduction and lower cost.

. The piston is guided in the bore by the cylinder tie rods thereby reducing uneven loading and uneven wear of the bore.

Aspects of the present invention have been described by way of example only and it will be appreciated that modifications and additions thereto may be made without departing from the scope thereof as defined in the appended claims. c

Claims (9)

WHAT WE CLAIM IS:

1. An actuator operated by fluid pressure, comprising a cylinder provided with means for supplying fluid pressure thereto closed at one end by a top end cap and an other end by a base end cap, said actuator including a piston slidable in said cylinder and connected by a connection device to an actuator rod passing to the outside through either said base end cap or said top end cap such that when fluid pressure is applied to said cylinder, said piston slides in said cylinder and drives said actuator rod, said top end cap and said base end cap being held in sealing abutment with said cylinder by a cylinder tie rod passing between said top end cap and said base end cap, through at least one aperture in said piston, said aperture being such as to allow said piston to move freely in an axial direction relative to said cylinder tie rod without allowing any appreciable fluid flow therethrough.

2. An actuator as claimed in claim 1 biased by a spring, the spring being disposed between said piston and said base end cap, said spring being held by a retaining means in a compressed state such that said spring can be further compressed by movement of said piston towards said base end cap due to an increase in fluid pressure.

3. An actuator as claimed in claim 2 wherein said retaining means comprises a pre-load tie rod connected between said base end cap and said piston. y A t \ £37720

4. An actuator as claimed in claim 3 wherein said pre-load tie rod is attached to either said base end cap or said piston and slides freely through a hole in either said piston or said base end cap respectively.

5. An actuator as claimed in any one of claims 1 to 4 wherein said piston is connected directly to said actuator rod so that a linear movement of said piston results in a substantially linear movement of said actuator rod.

6. An actuator as claimed in any one of claims 1 to 5 wherein said piston is connected to a translation device which is connected to said actuator rod the arrangement and construction of the translation device being such that a linear movement of the piston results in a rotational movement of the actuator rod.

7. An actuator as claimed in claim 6 wherein said translation device comprises a hollow block on the base of said piston having a slot at an incline to the axis of said piston so that said actuator rod passes into the hollow portion of the block and a pin on the end of said actuator rod meshes with said slot, the arrangement and construction being such that linear movement of said piston is translated into rotational movement of said actuator rod by the sliding of said pin in said slot, and said piston is restrained from rotation by said cylinder tie rod. 22 A fl-V /^v c % % % *- 237720

8. An actuator rod as claimed in any one of claims 1 to 7 wherein said means for supplying fluid pressure to said cylinder is disposed on either or both of said top end cap and said base end cap.

9. An actuator as claimed in any one of claims 1 to 8 wherein said actuator rod passes though a hole in said base end cap. NORTRAC ENGINEERING LIMITED By their Attorneys JAMES & WELLS 23 *''*AT4 ■ y ^ I 'Sa - % / ■ f ? I* J*" JRj