GB2431215A - Linear actuator - Google Patents

Abstract

A linear actuator (eg. screw mechanism) for moving a member, the actuator comprising a motor 34, a gear assembly 32, and a converting device, eg., screw 29 and nut 28 for converting the rotary output from the gear assembly to a linear output so as to move the member between first and second end positions, said actuator having a plurality of modular casings 14,15,16, which are connected together, for the component parts of the actuator, there being respective casings 16,14 for at least the gear assembly and the converting device 28,29. The actuator may have spring biasing means 17, a locking mechanism 37, rotation specific device 39, a torque limiter 38, and a damper mechanism (25, fig.2) associated with it. The actuator preferably serves to operate a valve. An independent claim is directed to a linear actuator which does not have modular casings, but has spring bias means and a locking mechanism.

Description

Improvements in linear actuators This invention relates to linear

actuators, for example, for use as valve actuators in the oil and gas industry.

The invention provides a linear actuator for moving a member such as a valve stem, the actuator comprising a motor, a gear assembly for delivering drive from the motor, a device for converting the rotary output from the gear assembly into linear movement, means for delivering the linear output from the drive converting device to the member to move the member between first and second end positions, and a housing assembly for housing the component parts of the actuator, the housing assembly comprising a plurality of modular casings connected together, there being respective casings for the gear assembly and the drive converting device.

The invention also provides a linear actuator for moving a member such as a valve stem, the actuator comprising a motor, a gear assembly for delivering drive from the motor, a device for converting the rotary output from the gear assembly into linear movement, means for delivering the linear output from the drive converting device to the member to move the member between first and second end positions, spring biassing means for urging the member towards its first end position, and a locking mechanism for holding the member in its second end position against the force of the spring biassing means, the locking mechanism being arranged to act on the rotary drive transmission between the motor and the drive converting device at a position of low torque.

By way of example, an embodiment of the invention will now be described with reference to the accompanying drawings, in which: Fig. I is a sectional view of a form of linear actuator according to the present invention, Fig. 2 is an alternative sectional view of the actuator of Figure 1, Fig. 3 is a sectional detail showing the locking device, and Fig. 4 is a sectional detail showing the spline drive mechanism.

A linear actuator according to the present invention is seen in Figures 1 and 2.

The actuator here is designed to open and close a valve (not shown) by linear reciprocal movement of its valve stem 13 between end positions.

At one end (the lower end in Figures 1 and 2), the actuator has an adaptor 10 for mounting the actuator to the bonnet of the valve (not shown). An interface plate 11 connects an output shaft 12 of the actuator to the valve stem 13. The interface plate 11 is designed to allow adjustment, to enable the strokes of the actuator and the valve to be matched. Once set, this is locked in position.

As will be seen, the actuator comprises a modular housing assembly, consisting of a lower casing 14, an intermediate casing 15 and an upper casing 16. The lower casing 14 houses a return spring 17, a bottom stop plate 18, which is attached to the output shaft 12, and a spring compression plate 19.

The bottom stop plate 18 is movable axially within a recessed bore in the lower casing 14 and a slide ring 20 is fitted to it to ensure a low friction sliding engagement with the lower casing 14. The spring compression plate 19 is movable axially within the bore of the intermediate casing 15 and a slide ring 21 is fitted to it to ensure a low friction sliding engagement. The top end of the output shaft 12 is bolted to the spring compression plate 19.

The spring compression plate 19 forms part of the connection between the rotary drive from the motor and the linear movement of the output shaft 12.

An assembly of thrust bearings 22 connects between the spring compression plate 19 and the lower end of a drive shaft 23. A bearing retention plate 24 secured to the spring compression plate 19 by cap screws captures the thrust bearing assembly 22. It will be understood that this arrangement means that the spring compression plate 19 will move axially together with the drive shaft 23. However, the drive shaft 23 will remain free to rotate independently of the spring compression plate 19, which does not rotate. The bearing retention plate 24 also provides the lower mounting points for a series of self contained hydraulic damper units 25 (see Figure 2).

The intermediate casing 15 houses the splined drive assembly. This comprises a drive nut 26, shell spline drive 28, splined nut 29 and top drive 30. The drive nut 26 is secured to a mounting plate 27 using cap screws and the mounting plate is itself secured to the intermediate casing 15 by cap screws. The drive nut 26 is thus fixed with respect to the casing, ie it does not move axially or rotate. The splined nut 29 is attached to the drive shaft 23 by means of a key and bolt. The two thus rotate and move axially together. The top drive 30 and shell spline drive 28 are secured together using cap screws (see Figure 2).

The shell spline drive 28 cooperates with the splined nut 29 so that the two are rotationally linked together, whilst remaining free to move axially with respect to each other. The drive shaft 23 is threaded, as is the drive nut 26, and the two together form a planetry roller screw. By action of the roller screw, rotation of the drive shaft 23 relative to the drive nut 26 (which is fixed) causes the drive shaft to move axially. The axial movement of the drive shaft 23 is transmitted directly to the spring compression plate 19, to which it is connected, and thence to the output shaft 12, to which the spring compression plate is connected, and thence to the valve stem 13, to which the output shaft is connected. In the opening movement of the valve, the axial movement of the drive shaft 23 is downwards in Figure 1, and it will be seen that this movement has the effect of compressing the return spring 17.

The lower and intermediate casings 14 and 15 are secured together by cap screws and the joint is sealed to prevent ingress of contaminates.

A closure plate 31 fixed to the upper end of the intermediate casing 15 by cap screws provides a mounting for a fixed differential drive assembly 32. The closure plate 31 also provides the upper mounting points for the damper units 25 (see Figure 2).

The damper units 25 are sealed and balanced. A device to adjust their rate of closure is incorporated. The actuator seen in Figures 1 and 2 is fitted with two damper units 25, but any number of damper units could be used. Also, different damping devices could be used, such as rotary dampers. The purpose of the damper units 25 is to control the speed of movement of the valve stem 13 and thus avoid damage being done to the valve and/or the actuator components by excessively violent movement.

The differential drive assembly 32 is connected to the top drive 30 of the splined drive assembly via a shaft 33. The drive assembly 32 provides speed reduction and torque multiplication for the output from the drive motor 34.

The drive assembly 32 in this case uses spur gears, but it could equally well use helically cut gears instead. Such devices are well known and need not be described in any more detail here. The output gear of the drive assembly 32 is keyed to the shaft 33.

The drive assembly 32 is housed in the upper casing 16, which is attached to the intermediate casing 15 radially using cap screws. The connection between the two housing 15 and 16 is sealed to prevent ingress of contaminates.

The upper casing 16 provides a mounting for an explosion proof enclosure 35.

The explosion proof enclosure 35 houses a primary drive pinion 36, an electrically operated locking device 37 and a torque limiter 38. The torque limiter 38 serves to protect the motor 34 by ensuring that the load on it does not exceed an upper limit. The locking device 37 has a rotation specific device 39 to allow rotation in one direction, but not the other. The torque limiter 38 also has a rotation specific device 40 to allow rotation in one direction, but not the other. The rotation specific devices 39 and 40 here are in the known form of a freewheel sprung roller mechanism.

The drive motor 34 is mounted to the explosion proof enclosure 35. The drive motor 34 in this case is electric and powered by AC voltage. However, it could equally well be powered by DC voltage instead. A drive shaft 41 from the primary drive pinion 36 to the differential drive assembly 32 extends between the explosion proof enclosure 35 and the upper casing 16 via a sealed boss 42.

Electrical connections to the actuator are made through explosion proof certified connectors mounted in the explosion proof housing 35.

Conveniently, the explosion proof housing 35 contains all the electronic switch gear and computer control systems for the actuator. A position sensing system is incorporated within the explosion proof housing 35 and is connected to a range of sensors (not shown) mounted within the actuator. One of these sensors is a rotary sensor which is contained within the explosion proof housing 35 and which is arranged to sense the rotational position of the primary drive pinion 36. Since there is a direct relationship between the primary drive pinion 36 and the valve stem 13, this sensor is able to provide an accurate indication of the position of the valve.

The electrically operated locking device 37 is seen in more detail in Figure 3.

The device 37 is in the well known form of a toothed brake with two opposed sets of teeth. When energised, the opposed sets of teeth are held together in engagement. This locks an idler gear 53 against rotation. This in turn locks the primary drive pinion 36 and hence the drive assembly 32. This serves to hold the valve in its operative end position, in this case, in its open condition, against the force of the compressed return spring 17 and the line pressure in the valve.

To allow the valve to return to its other end position (its closed condition), the locking device 37 is de-energised, releasing the two opposed sets of teeth from engagement with each other. This frees the idler gear 53 for rotation and consequently allows the primary drive pinion 36 and the drive assembly 32 the freedom to rotate. With the locking effect on the transmission thus released, the power of the return spring 17 is able to force the spring compression plate 19 back up to its starting position seen in Figure 1. This in turn causes the drive assembly 32 to back-drive, by the roller screw action between the drive shaft 23 and nut 26. The rotation specific device 40 ensures, however, that the reverse drive from the drive assembly 32 is not transmitted from the idler gear 53 back to the motor 34.

The rotation specific device 40 also serves to disconnect drive between the motor 34 and the primary drive pinion 36 in the event that the motor for some reason rotates in the wrong direction on start-up. This is therefore a safety feature to protect the motor.

The purpose of the rotation specific device 39 is to allow the locking device 37 to be energised, ie set in its locked condition, at any stage of movement of the valve from its closed to its open position. In effect, therefore, the idler gear 53 is able to be driven during this opening movement with or without the locking device 37 engaged. However, the rotation specific device 39 serves to prevent movement in the opposite direction when the locking device 37 is engaged and the motor drive is discontinued.

The splined drive mechanism is seen in more detail in Figure 4. The shell spline drive 28 is fixed axially in the drive nut mounting plate 27 between a bearing 60 and the top drive 30. As the shell spline drive 28 rotates, the splined nut 29 rotates the drive shaft 23, which is caused to move linearly by engagement of the drive nut 26. This causes the output shaft 12 to move axially.

It will be seen that the locking device 37 is able to act as a failsafe mechanism in the event of a power failure. If power should fail when the valve is in its open position, the locking device 37 will be de-energised, automatically freeing the idler gear 53, releasing the primary drive pinion 36 and thus allowing the return spring 17 to back-drive the drive assembly 32, forcing the valve back into its (closed) end position. The set of balanced damper units 25 ensure that this takes place at a suitably controlled rate.

It will be noted that the locking device 37 performs its locking function at the point of lowest torque in the drive transmission. This has the advantage that the locking device 37 itself can be relatively low powered and hence less prone to wear and/or malfunction. It will further be noted that the locking device 37 is located within the explosion proof enclosure 35. This has the advantage that the device is readily accessible for checking, maintenance and, if necessary, replacement.

The design of the actuator makes it particularly adaptable. It will be seen that the actuator can be mounted in any orientation, whether vertically, horizontally or at an angle, and function equally well in any position. This can be a very important advantage when space is at a premium.

Also, there are significant advantages in providing a housing assembly in the form of a number of dedicated casing units. This greatly facilitates maintenance, for example, as it is not necessary to dismantle the entire actuator in order to inspect and work on particular component parts. If the drive assembly 32 requires attention, for example, it is only necessary to lift off the explosion proof enclosure 35. This modularity also brings important benefits in terms of adaptability. Because the various components of the actuator are essentially housed in their own casing unit, it is a relatively straightforward task to modify the actuator. To accommodate an installation with a long-travel valve, for example, the basic actuator essentially needs only to be modified with a longer intermediate casing 15 and a longer drive shaft 23. Or, the drive assembly 32 can be quite easily replaced as a unit by an alternative assembly, if more power or speed is required.

It will be appreciated that the actuator according to the present invention could be used in any situation requiring a member to be moved between an operative position and an inoperative position, with an automatic failsafe return facility in the event of a loss of electrical power.

Claims (22)

1. A linear actuator for moving a member, the actuator comprising a motor, a gear assembly for delivering drive from the motor, a device for converting the rotary output from the gear assembly into linear movement, means for delivering the linear output from the drive converting device to the member to move the member between first and second end positions, and a housing assembly for housing the component parts of the actuator, the housing assembly comprising a plurality of modular casings connected together, there being respective casings for the gear assembly and the drive converting device.

2. An actuator as claimed in claim 1 wherein the actuator further comprises spring biassing means for returning the member to its first end position, the housing assembly comprising a further respective casing for housing the spring biassing means.

3. An actuator as claimed in claim 2 wherein the casing for the spring biassing means also houses the means delivering the linear output from the drive converting device to the member.

4. An actuator as claimed in claim 2 or claim 3 wherein the actuator further comprises a locking mechanism for holding the member in its second end position against the force of the spring biassing means, the housing assembly comprising a further respective casing for housing the locking mechanism.

5. An actuator as claimed in any preceding claim wherein the actuator further comprises a rotation specific device for delivering rotational drive in one direction but not in the opposite direction, the rotation specific device being interposed between the motor and the gear assembly and being housed in one of the casings.

6. An actuator as claimed in claim 5 wherein the rotation specific device is housed in the same casing as the locking mechanism.

7. An actuator as claimed in any preceding claim wherein the actuator further comprises a torque limiting device, the torque limiting device being interposed between the motor and the gear assembly and being housed in one of the casings.

8. An actuator as claimed in claim 7 wherein the torque limiting device is housed in the same casing as the locking mechanism and the rotation specific device.

9. An actuator as claimed in claim 8 wherein the casing which houses the locking mechanism, the rotation specific device and the torque limiting device is an explosion proof enclosure.

10. An actuator as claimed in claim 9 wherein the explosion proof enclosure further contains electronic switch gear and computer control systems for the actuator.

11. A linear actuator for moving a member, the actuator comprising a motor, a gear assembly for delivering drive from the motor, a device for converting the rotary output from the gear assembly into linear movement, means for delivering the linear output from the drive converting device to the member to move the member between first and second end positions, spring biassing means for urging the member towards its first end position, and a locking mechanism for holding the member in its second end position against the force of the spring biassing means, the locking mechanism being arranged to act on the rotary drive transmission between the motor and the drive converting device at a position of low torque.

12. An actuator as claimed in claim 11 wherein the locking mechanism is electrically operated and is arranged to be energised when holding the member in its second end position and to automatically release the member upon becoming de-energised.

13. An actuator as claimed in claim 11 or claim 12 wherein the actuator further comprises a rotation specific device for delivering rotational drive in one direction but not in the opposite direction, the rotation specific device being interposed between the drive converting device and the motor and being arranged to allow the member to return to its first end position without the gear assembly back-driving the motor.

14. An actuator as claimed in claim 13 wherein the actuator comprises a plurality of modular casings for housing the component parts of the actuator, and the locking mechanism and the rotation specific device are housed in the same casing.

15. An actuator as claimed in claim 14 wherein the actuator further comprises a torque limiting device, the torque limiting device being interposed between the motor and the gear assembly and being housed in the same casing as the locking device and the rotation specific device.

16. An actuator as claimed in claim 14 or 15 wherein the casing that houses the locking mechanism and the rotation specific device is an explosion proof enclosure.

17. An actuator as claimed in claim 16 wherein the explosion proof enclosure further contains electronic switch gear and computer control systems for the actuator.

18. An actuator as claimed in any preceding claim wherein the actuator further comprises means for damping the return movement of the member towards its first end position.

19. An actuator as claimed in claim 18 wherein the damping means is adjustable to allow the rate of return movement of the member to be adjusted.

20. An actuator as claimed in claim 19 wherein the damping means comprises a plurality of damping units, the damping units being adjustable whereby to enable their damping action to be balanced.

21. A linear actuator substantially as described herein with reference to the accompanying drawings.

22. A valve assembly comprising a linear actuator as claimed in any preceding claim, wherein the member is a valve stem.