Motors: spring – weight – or animal powered – Spring – With winding means
Patent
1996-11-22
1999-02-02
Marmor, Charles A.
Motors: spring, weight, or animal powered
Spring
With winding means
744248VA, F16H 2520
Patent
active
058652728
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
This invention relates to actuators of the type comprising an output shaft which, in use, effects an axial movement, and drive means therefor. In particular the output shaft is intended to effect a controlled, slow moving, extending or retracting axial movement for operating an associated mechanism. A particular application for such actuators is for driving the valve spindle of a valve, where the spindle is extended or retracted in order to force a valve sealing element against the valve seat.
In most existing designs of actuators of the type referred to above, the axial movement of the output shaft is obtained by arranging for a power source to drive the worm of a worm and worm wheel set to effect a single stage reduction in speed, varying typically between 15 to 1 to 80 to 1. The worm wheel rotates a drive nut which is threaded on to the valve spindle so that rotation of the drive nut extends or retracts the output shaft.
There are two applications where this type of actuator introduces installation and operating problems. Firstly, there are applications where the plan area for the equipment is restricted to a relatively small diameter. This occurs in under-sea equipment where the gear box and motor has to be encased in chambers subjected to high external pressures. Also in nuclear reactor engineering and similar sites producing ionising radiation the equipment has to pass through the biological shielding and hence apertures in the shield must be of the smallest possible diameter.
Secondly, there are applications where the equipment must be of the fail-safe type--in the case of a valve, the spindle must move under spring force to one or other terminal position upon failure of the power supply. The conventional actuator is not, normally, suitable for this duty as the drive nut will not normally "back drive" when a force is applied to the spindle.
SUMMARY OF THE INVENTION
According to the present invention there is provided an actuator of the type discussed above, and comprising an output shaft having a pair of driven wheels mounted thereon, one being rotatably mounted in a fixed plane and having a drive nut for a thread on said output shaft, the other being rotatably fixed to said output shaft, and an input shaft in side-by-side relationship with said output shaft adapted to be rotated by a suitable power source, said input shaft providing a drive wheel for each of the driven wheels, the ratio between each drive and driven wheel set being chosen to rotate the driven wheels at different speeds in the same rotational direction to produce a controlled axial movement of the output shaft in a direction depending upon the relative rotation of said driven wheels, wherein said other driven wheel and the drive wheel of its set are in the form of meshing helical gear wheels, and wherein a fail-safe arrangement is provided comprising a clutch between the drive wheels of the input shaft, a back-drive for said output shaft, and biasing means for effecting a back-drive, the arrangement being such that the meshing helical gears impart an axial force to engage the clutch only while power is supplied and the biasing means are operative to back-drive the output shaft to a fail-safe position when the power is terminated.
Preferably, an electromagnet or other suitable force applying means is provided and adapted to maintain the clutch in engagement when power is terminated.
In one arrangement, said other driven wheel may be axially fixed on said output shaft, whereby the speed difference between the driven wheels will cause this wheel and hence the output shaft to move axially towards or away from said one driven wheel depending upon the relative rotation of said driven wheels.
In another arrangement, said other driven wheel may also be in a fixed plane and said output shaft may be axially movable relative to said other driven wheel via an axially extending key/keyway arrangement, whereby the speed difference between the driven wheels will cause movement of the output shaft in an axial directi
REFERENCES:
patent: 3505888 (1970-04-01), Denkowski
patent: 3640140 (1972-02-01), Gulick et al.
patent: 4090589 (1978-05-01), Fitzwater
patent: 4256065 (1981-03-01), Hirt
patent: 5129273 (1992-07-01), Fukui et al.
patent: 5195721 (1993-03-01), Akkerman
patent: 5472065 (1995-12-01), Vergin
Sanders Peter G.
Wiggins Christopher R.
Grabow Troy
Marmor Charles A.
Rotork Controls Limited
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