Failsafe valve actuator

1. A failsafe valve actuator, the valve actuator comprising:

a drive mechanism;

a drive shaft comprising an externally threaded portion extending along at least a portion of the length of the drive shaft,

the drive shaft being coupled to the drive mechanism, such that the drive shaft is rotatable by the drive mechanism when the drive mechanism is energised, and wherein

the drive shaft is further axially moveable between a retracted position and an extended position;

a drive shaft bias member arranged to axially bias the drive shaft into a failsafe position;

a clutch arranged adjacent to the threaded portion of the drive shaft; and comprising at least a partial internal thread complementary to the external thread of the drive shaft,

a clutch actuator coupled to the clutch and arranged to move the clutch between an engaged position in which the clutch engages with the threaded portion of the drive shaft and a disengaged position in which the clutch is disengaged from the drive shaft;

wherein in response to energisation of the clutch actuator the clutch is moved to the engaged position such that threaded engagement between the thread of the drive shaft and clutch transfers rotational movement of the drive shaft by the drive mechanism into axial movement of the drive shaft and holds the drive shaft against the drive shaft bias member when the drive mechanism is inactive;

wherein movement of the clutch to the disengaged position, causes the drive shaft bias member to automatically return the drive shaft to the failsafe position;

wherein the actuator further comprises a clutch body defining a passage extending through the clutch body, the externally threaded portion of the drive shaft extending through the passage;

wherein the clutch comprises a first shell having an arch shaped cross section and a semicircular exterior of a first diameter; and a second shell having an arch shaped cross section and a semicircular exterior of a second diameter and an internal thread defined by a threaded portion on an inner surface;

wherein the first shell and second shell are arranged in the passage concentrically around the at least a portion of the externally threaded portion of the drive shaft; and

wherein the second diameter is smaller than the first diameter.

2. A valve actuator as claimed in claim 1 , wherein movement of the clutch to the disengaged position is the result of de-energising of the clutch actuator.

3. A valve actuator as claimed in claim 1 , wherein the first shell has a smooth inner surface proximal to the drive shaft.

4. A valve actuator as claimed in claim 1 , wherein the clutch actuator is arranged to move the second shell into engagement with the at least a portion of the externally threaded portion of the drive shaft in response to energisation of the clutch actuator, such that the threaded portion of the second shell engages with the threaded portion on the drive shaft to cause the drive shaft to be moved axially from one of the retracted and extended position to the other of the retracted and extended position.

5. A valve actuator as claimed in claim 1 , wherein the first and second shells are separated by a clutch bias member, and the clutch actuator is arranged to move the second shell into engagement with the drive shaft against the clutch bias member.

6. A valve actuator as claimed in claim 5 , wherein the clutch bias member comprises at least one spring located in corresponding spring pockets in at least one of the first and second shells.

7. A valve actuator as claimed in claim 1 , wherein the drive shaft bias member comprises at least one spring arranged to automatically return the drive shaft to one of the retracted and extended position when the clutch disengages from the drive shaft.

8. A valve actuator as claimed in claim 1 , wherein the clutch actuator comprises a solenoid having a plunger, the plunger being linearly moveable to move the clutch into engagement with the drive shaft in response to energisation of the solenoid.

9. A valve actuator as claimed in claim 8 , wherein the solenoid acts in a direction substantially perpendicular to the axial direction of the drive shaft.

10. A valve actuator as claimed in claim 1 , wherein the drive mechanism is automatically de-energised and the drive shaft is held in one of the retracted and extended position against the drive shaft bias member by the clutch after one or more of: a pre-determined length of operating time; a position sensor indicating that the full stroke of the drive shaft has been reached; a sensor measuring the current and indicating that a pre-determined current limit has been reached.

11. A valve actuator as claimed in claim 1 , wherein the drive mechanism comprises an electrical motor.

12. A valve actuator as claimed in claim 11 , wherein the electrical motor is driven through an adjustable current limited supply, such that when a pre-determined current limit is reached, the motor automatically de-energises and the drive shaft is held in one of the retracted and extended position by the clutch against the drive shaft bias member.

13. A valve actuator as claimed in claim 1 , wherein the valve actuator further comprises a drive pin and a sliding coupling for engagement between the drive mechanism and the drive shaft.

14. A valve actuator as claimed in claim 1 , wherein the clutch actuator requires less power to hold the drive shaft in one of the retracted and extended position by the clutch compared to the amount of power required to hold the drive shaft in the same position through operation of the drive mechanism.

15. A valve apparatus comprising a valve and a valve actuator, the valve actuator comprising:

a drive mechanism;

a drive shaft comprising an externally threaded portion extending along at least a portion of the length of the drive shaft, the drive shaft being coupled to the drive mechanism, such that the drive shaft is rotatable by the drive mechanism when the drive mechanism is energised, and wherein the drive shaft is further axially moveable between a retracted position and an extended position;

a drive shaft bias member arranged to axially bias the drive shaft into a failsafe position;

a clutch arranged adjacent to the threaded portion of the drive shaft; and comprising at least a partial internal thread complementary to the external thread of the drive shaft,

a clutch actuator coupled to the clutch and arranged to move the clutch between an engaged position in which the clutch engages with the threaded portion of the drive shaft and a disengaged position in which the clutch is disengaged from the drive shaft;

wherein in response to energisation of the clutch actuator the clutch is moved to the engaged position such that threaded engagement between the thread of the drive shaft and clutch transfers rotational movement of the drive shaft by the drive mechanism into axial movement of the drive shaft and holds the drive shaft against the drive shaft bias member when the drive mechanism is inactive; and

wherein movement of the clutch to the disengaged position, causes the drive shaft bias member to automatically return the drive shaft to the failsafe position;

wherein the actuator further comprises a clutch body defining a passage extending through the clutch body, the externally threaded portion of the drive shaft extending through the passage;

wherein the clutch comprises a first shell having an arch shaped cross section and a semicircular exterior of a first diameter; and a second shell having an arch shaped cross section and a semicircular exterior of a second diameter and an internal thread defined by a threaded portion on an inner surface;

wherein the first shell and second shell are arranged in the passage concentrically around the at least a portion of the externally threaded portion of the drive shaft and

wherein the second diameter is smaller than the first diameter.