GB1599634A

GB1599634A - Compressed-air actuators

Info

Publication number: GB1599634A
Application number: GB5645/77A
Authority: GB
Original assignee: Telehoist Ltd
Current assignee: Telehoist Ltd
Priority date: 1977-02-10
Filing date: 1977-02-10
Publication date: 1981-10-07
Also published as: BE863846A; SE7801540L; NL7801447A; FR2380452A1; FR2380452B1

Description

(54) IMPROVEMENTS IN COMPRESSED-AIR ACTUATORS (71) We, TELEHOIST LIMITED, a British Company, of Manor Road, Cheltenham, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to compressed-air actuators such as are used for effecting opening and closing of a ball valve or butterfly valve. The actuators are used for operating a variety of mechanisms and may include spring means operative in the event of failure of the compressed air supply to ensure that the ball valve or other mechanism operated by the actuator is then biased into a closed or safety position.

It is an object of the invention to provide a compressed-air actuator which has improved output torque characteristics.

According to the invention there is provided a compressed-air actuator having a rotatable output shaft, said actuator comprising a cylinder within which are disposed two pistons arranged for movement in opposite directions relative to each other between inner and outer limiting positions, a drive element fixed to said output shaft and having an axis extending transversely of the axis of rotation of the output shaft, and means providing a drive-transmitting relationship between each of the pistons and said drive element whereby the drive element and output shaft are caused to rotate through a predetermined angle during movement of the pistons between their inner and outer limiting positions, the drive element passing through an intermediate position in which its axis is at right angles to the directions of movement of the pistons, the angular displacement of the drive element from said intermediate position, as the pistons move to their outer limiting positions, being less than its angular displacement from said intermediate position as the pistons move to their inner limiting positions.

The torque applied to the output shaft is dependent on the angular relationship of the axis of the drive element and the directions of movement of the pistons and the particular arrangement set out above is such as to increase the torque that is applied during final closing movement of a ball valve or butterfly valve operated by the output shaft.

Preferably the drive element and output shaft are caused to rotate through substantially 90" during movement of the pistons between their inner and outer limiting positions. For example, the angular displacement of the drive element from said intermediate position, as the pistons move to their outer limiting positions, may be substantially 41", and its angular displacement from said intermediate position as the pistons move to their inner limiting positions may be substantially 49".

The drive element may be formed with two diametrically opposed slots extending away from the axis of rotation thereof, each slot receiving a member connected to one of said pistons respectively, whereby, during reciprocating movement of the pistons, said members effect angular movement of the drive element.

For example, the drive element may be in the form of a plate, said slots being openended at the edges of the plate remote from the output shaft. Each said member received in a slot may comprise a roller mounted on its associated piston for rotation about an axis parallel to the axis of rotation of the output shaft.

At each end of the cylinder there may be a spring which is compressed during movement of the pistons away from one another and towards their outer limiting positions.

The springs act to bias the pistons towards one another into their inner limiting positions.

Alternatively the pistons may be moved by air pressure in both directions.

The invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a longitudinal part-sectional view of a compressed-air actuator, Figure 2 is a section along the line 2-2 of Figure 1, Figure 3 is a diagrammatic view of the actuator with the various parts thereof in one limiting position, Figure 4 is a view corresponding to Figure 3 but with the parts of the actuator in their other limiting position Figure 5 is a graph illustrating the torque output characteristics of a known form of compressed-air actuator, fitted with spring return, Figure 6 is a graph illustrating the torque output characteristics of a compressed-air actuator as shown in Figure 1, fitted with spring return, and Figure 7 is a graph showing the comparative torque output characteristics of a double-acting actuator in accordance with the invention and a known double-acting actuator.

The actuator shown in Figure 1 comprises two opposed pistons 10 which are arranged for free sliding movement in a cylinder afforded by a bore of a housing 11. Each piston comprises a portion 12 of cylindrical form which fits closely within the bore of the housing, and has a peripheral groove which receives an O-ring 13 which sealingly engages the bore. Each piston also comprises a projecting operating arm 14. A driving element in the form of a yoke plate 15 is mounted on an output shaft 16 and is held against rotation relative thereto by means of a drive key 17.

Cylindrical rollers 18 encircle the shaft 16 on opposite sides of the plate 15.

Each of the operating arms 14 is of bifurcated form and the end portions of the yoke plate 15 run within a slot 19 provided between the two limbs of the associated arm 14. A pin 20 extends between the two limbs of each bifurcated portion adjacent the free ends thereof and the pins 20 carry rollers 21 which run in slots 22 formed in the ends of the yoke plate 15. Additional guidance of each of the pistons 10 is provided bv flat inwardly facing surfaces 23 on the operating arms 14 which run on the rollers 18 carried on the output shaft 16.

Concentric helical compression springs 25 are located within chambers 26 at the ends of the housing 11, each chamber 26 being provided by an end cap 27 attached by fasteners to the housing 11. Each concentric pair of springs is disposed between the bottom of the end cap 27 and an annular flange 28 which abuts the end face of the adjacent piston 10 and is formed on a cupshaped member 29. The cup-shaped member 29 is formed at its bottom with a hole through which passes a retaining assembly 30 which serves to retain the cocentric springs 25 on the end cap when the end cap is removed from the housing 11. Each retaining assembly comprises a round-headed screw 31 which passes through a sleeve 32 and engages a threaded hole in the end cap 27. A washer 33 is disposed between the head of the screw and the end of the sleeve 32.

When compressed air is supplied to the space 24 between the pistons 10 through a suitable port (not shown) the pistons are displaced relatively outwardly away from each other and the yoke plate 15 is caused to rotate in an anticlockwise direction as viewed in Figure 1. When the pistons 10 are displaced outwardly away from each other, the springs 25 are compressed and, upon release of the air pressure, the springs 25 bias the pistons inwardly towards each other thus rotating the yoke plate 15 in a clockwise direction as viewed in Figure 1. The arrangement is such that the shaft 16 is caused to rotate through 90" upon movement of the pistons between their inner and outer limiting positions so that, by means of a drive socket 34 in the shaft 16 (see Figure 2), a ball valve, or butterfly valve (not shown) can be rotated between its open and closing positions.

Figures 3 and 4 illustrate diagrammatically the positions of the yoke plate 15 when the valve is in its fully open position and when the valve is in its fully closed position. It is to be noted that, when one considers an intermediate position in which an axis of the yoke plate passing through the centres of the rollers 21 and through the axis of the ouput shaft 16 is at right angles to the directions of movement of the pistons, the yoke plate 15 is displaceable from this intermediate position through an angle (p, which is less than 45 , in the direction of opening of the valve and is displaceable therefrom through an angle 0, which is more than 45 , during closing of the valve.

Known compressed-air actuators are, on the other hand, of symmetrical construction in that the drive element is turned through 45" in each direction from its intermediate position so that, during outward travel of the pistons, the springs 25 must be capable of storing sufficient energy to enable the actuator to overcome the frictional and other restraints within the driven mechanism to obtain full 90" reverse rotation. The curves shown in Figure 5 illustrate the output torque available and it will be seen that, in order to achieve the required closing torque A by means of spring return, it is necessary to have a larger than necessary torque output B at the commencement of the return stroke.

Turning next to Figure 6, the curves shown illustrate the output torque available in a arrangement according to the invention in which the outward stroke is effectively limited to lessthan 45" from the intermediate position in compressing the spring and is greater than 45" during the inward spring stroke from the intermediate position. It will be noted that the closing torque C is much closer to the torque D at the commencement of the closing stroke than is the case in the conventional symmetrical arrangement. It is therefore possible, with an asymmetrical arrangement in accordance with the invention to provide a higher closing torque C without increasing the physical size of the actuator. Similarly, it is possible, to obtain the same closing torque from a smaller actuator.To date the optimum results have been obtained with an outward stroke so of 41 and in inward stroke 8 of 49".

The above brief decription is in relation to an actuator which is single-acting in the sense that the ball valve, butterfly valve or the like is opened by the action of the compressed air and is closed by the springs 25. However, the advantages derived from the invention are also obtained in the case of a doubleacting actuator in which air may also be introduced into the spaces 26 containing the springs 25 to urge the pistons 10 inwardly towards each other to effect valve closure.

In this case the springs may be omitted.

The torque output characteristics of a known symmetrical double-acting arrangement and of an asymmetrical double-acting arrangement in accordance with the invention are illustrated in Figure 7 and it can be seen that, in the known arrangement (shown in solid line), the torque is the same at each limit of the drive of the pistons whereas, with the asymmetrical arrangement provided by the invention (shown in broken line), the torque is greatest when the valve is either moving into or out of its fully closed position. It is obvious that the maximum torque is required during initial opening and closing of the valve so that, again, it is possible to obtain improved torque characteristics without any increase in the size of the actuator.

WHAT WE CLAIM IS: 1. A compressed-air actuator having a rotatable output shaft, said actuator comprising a cylinder within which are disposed two pistons arranged for movement in opposite directions relative to each other between inner and outer limiting positions, a drive element fixed to said output shaft and having an axis extending transversely of the axis of rotation of the output shaft, and means providing a drive-transmitting relationship between each of the pistons and said drive element whereby the drive element and output shaft are caused to rotate through a predetermined angle during movement of the pistons between their inner and outer limiting positions, the drive element passing through an intermediate position in which its axis is at right angles to the directions of movement of the pistons, the angular displacement of the drive element from said intermediate position, as the pistons move to their outer limiting positions, being less than its angular displacement from said intermediate position as the pistons move to their inner limiting positions.

2. A compressed-air actuator according to claim 1, wherein the drive element and output shaft are caused to rotate through substantially 90" during movement of the pistons between their inner and outer limiting positions.

3. A compressed-air actuator according to claim 2, wherein the angular displacement of the drive element from said intermediate position, as the pistons move to their outer limiting positions, is substantially 41", and its angular displacement from said intermediate position as the pistons move to their inner limiting positions is substantially 49".

4. A compressed-air actuator according to any of claims 1 to 3, wherein the drive element is formed with two diametrically opposed slots extending awey from the axis, of rotation thereof, and each slot receives a member connected to one of said pistons respectively, whereby, during reciprocating movement of the pistons, said members effect angular movement of the drive element.

5. A compressed-air actuator according to claim 4, wherein the drive element is in the form of a plate and said slots are openended at the edges of the plate remote from the output shaft.

6. A compressed-air actuator according to claim 4 or claim 5, wherein each said member received in a slot comprises a roller mounted on its associated piston for rotation about an axis parallel to the axis of rotation of the output shaft.

7. A compressed-air actuator according to any of the preceding claims wherein at each end of the cylinder there is a spring which is compressed during movement of the pistons away from one another and towards their outer limiting positions.

8. A compressed-air actuator substantially as hereinbefore described with reference to Figures 1 to 4 of the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. symmetrical arrangement. It is therefore possible, with an asymmetrical arrangement in accordance with the invention to provide a higher closing torque C without increasing the physical size of the actuator. Similarly, it is possible, to obtain the same closing torque from a smaller actuator. To date the optimum results have been obtained with an outward stroke so of 41 and in inward stroke 8 of 49". The above brief decription is in relation to an actuator which is single-acting in the sense that the ball valve, butterfly valve or the like is opened by the action of the compressed air and is closed by the springs 25. However, the advantages derived from the invention are also obtained in the case of a doubleacting actuator in which air may also be introduced into the spaces 26 containing the springs 25 to urge the pistons 10 inwardly towards each other to effect valve closure. In this case the springs may be omitted. The torque output characteristics of a known symmetrical double-acting arrangement and of an asymmetrical double-acting arrangement in accordance with the invention are illustrated in Figure 7 and it can be seen that, in the known arrangement (shown in solid line), the torque is the same at each limit of the drive of the pistons whereas, with the asymmetrical arrangement provided by the invention (shown in broken line), the torque is greatest when the valve is either moving into or out of its fully closed position. It is obvious that the maximum torque is required during initial opening and closing of the valve so that, again, it is possible to obtain improved torque characteristics without any increase in the size of the actuator. WHAT WE CLAIM IS:

1. A compressed-air actuator having a rotatable output shaft, said actuator comprising a cylinder within which are disposed two pistons arranged for movement in opposite directions relative to each other between inner and outer limiting positions, a drive element fixed to said output shaft and having an axis extending transversely of the axis of rotation of the output shaft, and means providing a drive-transmitting relationship between each of the pistons and said drive element whereby the drive element and output shaft are caused to rotate through a predetermined angle during movement of the pistons between their inner and outer limiting positions, the drive element passing through an intermediate position in which its axis is at right angles to the directions of movement of the pistons, the angular displacement of the drive element from said intermediate position, as the pistons move to their outer limiting positions, being less than its angular displacement from said intermediate position as the pistons move to their inner limiting positions.