A mechanical device ensuring irreversibility in a limiter assembly for electric actuators coupled through a reversible kinematic chain

Abstract: A mechanical device forming part of the limiter assembly (1) associated with reversible electric actuators, typically for controlling pipeline valves, comprises a mechanism by which one or more limiters (3) can be held in the engaged position even after the trigger has been deactivated; the undesirable phenomenon of pumping (the effect of the actuator motor cutting in and out repeatedly) is prevented by a simple flexible ribbon (15) with teeth (16) positioned in such a way as to engage corresponding sprockets (17) that are capable of rotation on their respective shafts (12) in one direction only.

Description: The present invention relates to a mechanical device which ensures irreversibility in a limit switch assembly for electric actuators coupled through a reversible kinematic chain. Electric actuators or motors of the type utilized for operating valves are equipped generally with electrical control units by which the movement of the actuator is arrested in positions corresponding to limit positions of the valve. The arrest position is triggered in most instances by a plurality of limit. Limit switches preferable in the form of microswitches which are operated by moving elements such as flexible tongues, which are independent of each other. The moving elements in their turn are operated by one or more cams carried by a shaft that is connected to a mechanical linkage forming part of the actuator system. The requisite torque of an electric actuator is generated by a motor, a speed reducer, and a worm gear driving a related wheel by means of which motion is transmitted to the valve. In transmission of the torque, a movement produced by the worm gear against a related spring brings about the rotation of a shaft, the shaft in turn bringing about the operation of the limit switches by moving trigger elements mentioned above. In irreversibly coupled systems on arrival, the worm gear remains in position at the limit even though the motor ceases to produce torque, and returns only when rotated in the opposite direction; thus, the limiter stays engaged. On the other hand, in reversible systems, the worm gear will bring about the operation of the microswitches and duly stop the motor.However the worm gear can then return under the influence of the spring, thereby releasing the switches and allowing the motor to restart if it is still able to do so. The result is that the motor is taken through a succession of stops and starts, giving rise to an undesirable phenomenon referred to as hunting or 'pumping' by those skilled in the art. Such repeated stopping and starting will ultimately damage the mechanical parts associated with the motor. Hitherto the problem of hunting or pumping has been corrected by means of electrical type of devices which, however, are somewhat costly and which require a small control unit associated with the actuator. Moreover, such devices are often sold separately and must therefore be installed by the purchaser. From US-A-4,171,472 is known a limit switch for the starting and stopping of an electric motor which operates a valve which can be opened or closed by rotatory movement of the stem through 90 degrees. The stem ends in a bushing rotatably journalled in the fixed body of the valve to which is fixed a circular switch housing having an internal ring gear which can rotate together with the bushing. Three pins or elongated poles, which are parallel to the axis of the stem and can rotate about their own axis, carry in their lower end a pinion gear designed to be meshed with the above internal gear and to be biased downwardly by a compression spring. The coupling between each gear and the internal gear may be disengaged if a human operator grasps and lifts the upper portion of the pole against the bias of its spring. Threaded transversely in the upper portion of each pole are one or more screws which can operate operating buttons of switches. Said device relates to a conventional geared limit switch which is operated manually and is not able to control a torque. The object of the present invention is to overcome the drawbacks aforementioned, and in particular to provide a mechanical device suitable for emplacement directly into a limiter assembly and able to prevent the occurrence of hunting or pumping. The stated objects are realized in a mechanical device for ensuring irreversibility in the limiter assembly of an electric actuator coupled through a reversible kinematic chain, according to the following claims. Mechanical means are provided that are capable of retaining one or more limiters in position, i.e. in engagement, even following a termination of the action of the trigger element by which engagement is brought about. Such means are connected,mechanically,on the one hand to elements by which the limiters are operated, and on the other to rotary mechanical transmission components associated with the actuator. The invention will now be described in detail, by way of example, with the aid of the accompanying drawings, in which: fig 1 illustrates a limiter assembly for electric actuators, seen in perspective and in its entirety; fig 2 is a plan view of the device according to the invention, seen in the at-rest condition; fig 3 is a front elevation of the device; fig 4 shows a detail of a shaft of the device, viewed in section through a vertical plane; fig 5 is a plan view of the device, seen in the operative condition. With reference to the figures, 1 denotes a limiter assembly for electric actuators, comprising two cams 2 controlling a plurality of limit switches or other suitable limiters 3. The cams 2 are rigidly associated with a common shaft 4 carrying two gears 20, rotatable as one, at the bottom thereof. The gears 20 permit the assembly to be coupled to an electric actuator (not illustrated). Each limit switch or other suitable limiter 3 comprises a plurality of micro switches, each microswitch having a related actuator button (41, 41a and contacts 6. The limit switches have well known normal and off-normal positions which relate to whether the switches are or are not operated. For example, if a set of microswitch contacts are closed in the normal position, they are open in the off-normal position, an vice versa; The cams 2 have a substantially stepped profile designed to engage a roller 7 or 5 in such a way as to enable two-stage operation of the microswitches. A first step of the profile sets the system in such a way that the torque microswitch controlling the opposite direction of movement will not cut in during the first part of the cam travel (release of the valve at maximum permissible torque). The second step operates the microswitches to stop the motor without the torque control system cutting in. Each roller 5, 7 is carried by a corresponding arm 9 connected via a shaft 10 to flexible strips 40 which serve to operate the microswitch actuator buttons 41 of the limit switches or limiters 3 by applying a mechanical pressure. The arms 9 are positioned in such a way as to strike against a substantially Y-shaped element 11 which is capable of movement internally within a slot 14 and designed to operate one or more buttons of the limiters by applying a mechanical pressure thereto. The number 8 denotes screws for adjusting the interaction between each arm 9 and the moving element 11. By conditioning the system to prevent the torque control from cutting in when the motor is operated in the opposite direction, the first step of the cam profile produces the so-called "mechanical by-pass". The number 15 denotes a ribbon, fashioned from a rectangular band of metal which is bent and shaped in such a way that the ends can be screwed or otherwise anchored to the moving element 11, and affording two teeth or projections 16 issuing from one face. The teeth 16 are positioned in such a way as to engage the teeth of two corresponding and freely rotatable sprockets 17 mounted relative to shafts 12. Each sprocket 17 is partly enveloped by a spring 18 coiled in such a way that when the corresponding tooth 16 projecting from the ribbon 15 is engaged between the teeth of the sprocket 17, the shaft 12 is able to rotate in one direction only, namely, the direction in which the spring is coiled around the sprocket. More exactly, the topmost end of each shaft 12 carries the sprocket 17, and a seating element 13 for the spring 18, the spring being wound tightly around this same element 13 and around a part of the sprocket 17 in such a way as to allow movement of the sprocket 17 in only one direction, relative to the shaft 12. At the ends farthest from the ribbon 15, the two shafts 12 carry respective pinions 19 in mesh with a gear 20 coupled to the actuator by way of further mechanical transmission components. With reference to figs 2 and 5, which respectively illustrate an at-rest configuration and a position in which the microswitches are activated, the two arrows denoted 21 indicate the directions in which the sprockets 17 are rotatable. It will be clear from fig 5, in which the sprocket denoted 17a is indicated as being rotatable only anticlockwise in relation to the relative shaft 12, that when the tooth 16 of the ribbon 15 engages the teeth of the sprocket 17a and the moving element 11 operates the microswitch of the limit switches or limiters 3, the moving element 11 is prevented from retracting by reason of the sprocket 17a being rotatable relative to the shaft 12 in one direction only. The ultimate release of a device according to the invention occurs automatically when the actuator rotates in the direction opposite that which caused the device to operate initially. While the shafts 12 and the relative sprockets 17 are capable of rotation in either direction, the engagement of a tooth 16 of the ribbon with one of the sprockets has the effect of inhibiting the return movement of that sprocket 17, given that the corresponding pinion 19 at the far end of the shaft is in constant mesh with the gear 20 The mechanical device according to the invention is connected to a torque control system (not shown) and serves to prevent the occurrence of hunting or 'pumping' throughout the entire opeating stroke of motors or actuators coupled through a reversible kinematic chain, as used for valves. The ribbon 15, the moving element 11, sprockets 17, shafts 12, springs 18 and seatings 13 and the gear 20 together constitute mechanical means by which to retain one or more travel limiters in position, i.e. engaged, even after the action of the trigger element by which engagement is brought about (the moving element 11 through cams 2 and the element 42 of figure 1) has been discontinued.

Claims: A mechanical device for ensuring an irreversibility in a limiter assembly (1) of an electric actuator coupled through a reversible kinematic chain, said limiter assembly, designed to control a torque, having at least one limit switch or limiter (3), where in the device comprises mechanical means (11, 15, 16, 17, 18) capable of retaining one or more limiters (3) in position i.e. in engagement, even following a termination of the action of a trigger element (2, 42) by which engagement has been brought about, said mechanical means comprising a ribbon (15) of material which is rigidly associated with a moving element (11) operating said one or more limiters, said ribbon (15) being shaped in such a way as to interact with the teeth of sprockets (17) to allow the rotation of a shaft (12) of the sprocket in only one direction, such mechanical means being connected mechanically on the one hand to operating elements (40) by which the limiters (3) are operated, and on the other hand to rotary mechanical transmission components associated with the actuator. Device as in claim 1, wherein the ribbon (15) has two projections (16) designed to selectively engage teeth of two corresponding toothed sprockets (17) mounted to freely rotate on two individually associated shafts (12), each of said shafts being partially enveloped by an individually associated spring (18) coiled in such a direction that when either projection (16) engages the teeth of the corresponding sprocket (17), the respective shaft (12) is allowed to rotate in only one direction, concurrently with the direction in which the spring is coiled around the sprocket. . A device as in claim 2, further comprising two pinions (19) mounted on the ends of the shafts (12) remote from the sprockets (17) and the ribbon (15), said pinions (19) being in mesh with a common gear wheel (20) connected to the actuator by way of further mechanical transmission components. A device as in claim 2, further comprising two elements (13) on which the springs (18) are seated, said elements being mounted on the relative shafts (12) and enveloped together with the freely revolving sprockets (17) by the springs (18).

Inventors: LUNATI MAURIZIO, LUNATI, MAURIZIO
Applicants: BIFFI ITALIA, BIFFI ITALIA S.R.L.
Application year: 1991
Main classification: F16K31
IPC codes: H01H3