The present invention relates to a device which converts a linear motion in a rotary one. In particular, said device can be advantageously used for actuators with “Scotch yoke” kinematism.
As it is known, in case of actuators for quarter turn hydraulic or pneumatic valves, the Scotch yoke kinematism is a common mechanism used for converting the linear motion in a rotary motion of a quarter turn. Therefore, the actuator with Scotch yoke kinematism allows the valve to carry out a rotation of about 90°, with a torque with the maximum value at ends, and the lowest values in the middle of the trajectory. This feature is positive for those valves which have the need of a high torque when they are in their closed position. Moreover, the actuators with Scotch yoke are normally cheap to be produced with respect to other actuators, above all when it is needed to work with high output torques. In other words, it is possible to say that the Scotch yoke kinematism is very competitive for great dimensioned valves.
The typical design for a Scotch yoke kinematism, with reference to
A drawback of the design of actuators with Scotch yoke kinematism is linked to the fact that, during a significant portion of the 90° rotation, the force exerted by the stem is not orthogonal to the arm of the Scotch yoke (arm of Scotch yoke indicating the imaginary line joining the rotation axis of the Scotch yoke with the pin of the block, which is constrained to the stem). This causes a transverse load stress, made up of reaction forces applied orthogonally to the pressure rod and rotating shaft. Such side stresses have to be balanced so that the misalignment of the stem and/or the pin of the block is prevented.
In case of significant side forces combined with the continuous sliding of the stem in the bushings, an excessive wear of the bushings occurs up to the possible breaking of the same element. Moreover, the side forces can be transmitted through the bushings to other elements of the actuator group, which guide the stem. Finally, the friction between bushings and stem causes irregular or stepping starts and runnings of the same, thus making the use of the actuator with Scotch yoke kinematism little suitable for applications, such for example the regulation valves, which require exact and well controlled movements. A way to obviate this problem is to use a guide bar 15, auxiliary to the stem, which prevents the stem deflection by supporting transverse loads.
Actuators with Scotch yoke kinematism require that the pin of the guide block is always perfectly parallel to the rotation axis of the Scotch yoke. According to a traditional design, the Scotch yoke is made up of a central pin on which two wings are fixed, in whose slots the pads of the guide block joined to the stem slide. According to this design, the hole through the guide block is responsible for keeping the pin in the correct alignment with the rotation axis of the Scotch yoke and is helped thereto by the slots of the Scotch yoke. Therefore, it is very important that the coupling of the guide pads of the block and the slots of the Scotch yoke is almost free of any clearance which can cause irregular movements during the rotation of the Scotch yoke or the inversion of the angular motion. On the other hand, it is rather impossible to realize elements with null coupling clearances, since any mechanical working is not free of working tolerances.
Moreover, in the mounting step it is required a clearance, even a minimal one, between the guide pads of the block and the slots in the slotted link. But even if, hypothetically, it were possible to realize perfect couplings with null clearance between slots and pads of the block, the motion transmission forces would generate friction between mechanical portions in movement, which could lead to wear between the guide pads of the block and the seats of the slotted link.
Therefore, there exists the need for defining a device which allows to overcome the above cited problems, which is in particular indifferent to the coupling clearances of the device, when new and during its life cycle, owing to the wear occurred.
Therefore, the present invention is a device which converts a linear motion in a rotary one, in particular suitable for actuators with Scotch yoke kinematism, and which is provided with at least a guide pad configured so that the coupling clearances can be balanced both when it is new, and when induced by the wear which occurs during the life cycle of the actuator.
Therefore, object of the present invention is a device for converting a linear motion in a rotary one, as described in the appended independent claim.
The dependent claims describe particular and further advantageous aspects of the invention.
These and other advantages of the invention will be described in detail in the following, with reference to the appended drawings, in which:
As it is shown in
The device 10 is shown in exploded view in
Advantageously, on the guide pads there are introduced two metal sheets 4 coated with self-lubricating material to lower and reduce the wear of the elements.
Elastic means 6, preferably a Belleville washer, generate a force on the guide pads 5 such that possible clearances are balanced between the device 10 and the wings of Scotch yoke 9, which form the seat of the device, as it is shown in
During the life cycle of the actuator possible clearances due to wear will be equally balanced by the device, thanks to the translation of the guide pads 5, i.e. the sliding of the inclined surfaces 5′ of the guide pads on corresponding inclined surfaces 3′ of the guide block 3. So, as a consequence the balancing of coupling clearances will cause an increasing distance A between the outer side surfaces 5″ of guide pads 5, which will remain always parallel to each other, keeping the mechanical coupling without clearances with the wings of the Scotch yoke.
As it is shown in
This embodiment of the invention is described as a way of example, clearly it is possible to eliminate the coupling clearances between guide pads of the pin of the Scotch yoke and the wings of the Scotch yoke also in the opposite manner, i.e. by reducing the distance between the surfaces of the wings of the Scotch yoke.
In addition to the embodiments, as above described, it is to be intended that there exist many other variants. Further, it is to be intended that said embodiments are only example and do not limit in any way the object of the invention and its applications or configurations. On the contrary, even if the above described description makes possible for the experts in the field to provide the present invention in at least an embodiment, it is to be intended that many variations of the elements described can be made, without departing from the object of the invention, as it is encompassed by the appended claims and/or technical/legal equivalents thereof.
Filing Document | Filing Date | Country | Kind |
---|---|---|---|
PCT/IT2016/000130 | 5/19/2016 | WO | 00 |