Abstract:
A load limiting device to transmit an axial motion to operating mechanisms, comprising: an engaging element (6, 68) which can be connected to a mechanism (4); self-bearing means (2,5) transmitting in axial direction the motion of a driving group (1); a disengagement group (7) which is integral with the element transmitting the axial motion and comprises one or more elastic disengagement means (72) which are connected with the engaging element (68); means (73,74) to adjust the threshold value of the axial load causing said elastic disengagement.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a load limiting device intended to limit the axial force which can be exerted to move operating elements. 
     PRIOR ART 
     At present, several devices to axially displace operating elements (for example shafts) are already known. 
     These devices comprise axially concentric transmission elements which are connected at a predetermined point of a shaft through a load limiting device, possibly of the adjustable type. 
     By the limiting device, a threshold value of the applied load can be fixed. Over this value the shaft is disengaged from the limiting device in order to avoid any over loads which could damage the structure. 
     It is also known that once the limiting device has performed its function it is important that the connection between the shaft and the transmission element can be automatically re-established in order to allow the following operation to start without that a manual maintenance is required. 
     To solve this kind of problem it is possible to install the limiting device on the shaft upstream of the direction of the possible disengagement of the same shaft. In this way the limiting device is supported by the shaft and can coaxially slide without coming out from the shaft after the relative motion following their disengagement. Unfortunately, in case of bi-directional operating shafts, the latter solution involves the need to provide a shaft having a length which has to be at least twice the operational stroke of the shaft. This limitation is often unacceptable due to design specifications. 
     AIM OF THE INVENTION 
     A first aim of the invention is to overcome the limits and the drawbacks of the already known limiting devices. 
     A second aim of the invention is to provide an high-reliable load limiting device able to displace operating mechanisms suitable for a bi-directional functioning. 
     SUMMARY OF THE INVENTION 
     These aims have been reached according to the invention by a load limiting device able to axially displace operating mechanisms, which comprises: an engagement member which can be connected to an operating element; a self-bearing member transmitting the axial motion and preferably consisting of a nut engaging an endless screw; a load limiting group integral to the above said member transmitting the axial motion and comprising one or more elastic means connected to the engagement member (for example one or more radial inserts); means to adjust the threshold value of the elastic engagement, these means being preferably constituted by one or more axially arranged bucket springs; and means to contain said elastic means, consisting for example of a retaining ring. 
     According to the invention the engagement member can be disengaged from the load limiting group when an axial over load occurs on the driving elements in one of the axial directions. However, the endless screw (and then the load limiting group) does not miss a substantial reciprocal alignment with the engaging member which is then able to reach the engagement position again on the subsequent axial stroke of the nut. 
     Furthermore, the device is provided with position reference means and with suitable external contacts of the electromechanical and/or electronic type. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Further advantages will be evident for the man skilled in the art from the following description and from the annexed drawings, given as a non limitative example, in which: 
     FIG. 1 shows a schematic view of a device according to the invention; 
     FIG. 2 shows a detail of the load limiting group provided with a reference arm; 
     FIG. 3 shows a second preferred embodiment of the invention; 
    
    
     With reference to FIG. 1 a device according to the invention consists of a driving group 1 which rotationally drives a first extremity of an endless screw 2, preferably a sphere endless screw, thus determining the axial displacement of a nut 5 mounted on the endless screw 2. 
     Endless screw 2 is supported by a couple of bearings 21, 22 the first one being engaged with the driving group 1 and the second one being positioned at the second extremity of the screw. 
     Screw 2 is arranged internally and coaxially to an hollow cylindrical shaft 6 which can axially slide in respect of the screw 2 and is engaged with the nut 5 by a load limiting group 7. Group 7 is mounted on the screw by a sliding friction support element 61 into which the bearing 22 is positioned. 
     The axial motion of shaft 6 is guided by the bushings 63 of a support 62 fixed to an external case inside which the driving group 1, screw 2 and shaft 6 are also contained. 
     With reference now to FIG. 2, the load limiting group 7 connecting screw 2 to shaft 6 is described. 
     A sleeve 71 presenting a sloping surface S1 is mounted on nut 5 by screw means 76. Two frustum-pyramidal inserts 72 are in turn mounted on sleeve 71. The inserts have upper lateral sides in the shape of sloping surfaces 78, 79. Inserts 72 are closed, in the lateral direction, by the sloping surface S2 of a ring 77 and, in the radial direction, by a retaining ring 75. On ring 77 at least a bucket spring 73 exerts a force which is adjustable by a threaded bush 74. Bush 74 is engaged with a corresponding external thread of the sleeve 71 so that it adjusts the force exerted by spring 73 and at the same time is able to axially contain the various components of group 7. 
     Inserts 72 are surrounded on the upper side by a circumferential groove 68 which is present in the internal surface of shaft 6 and which acts as engaging member of the inserts with the shaft. Groove 68 present front and rear sloping edges 66, 67 (corresponding in FIG. 2 respectively to the left and right sides). Inserts 72 are slidingly inserted into slots 81, 82 which are respectively provided on surfaces S1, S2 and which act as inclined planes for the radial sliding motion of inserts 72. Between the lower surface of inserts 72 and sleeve 71 a free space 83 having at least the same height of groove 68 is provided. 
     In FIG. 2 a reference arm 8 fixed to sleeve 71 by screw means 84 is also shown. During the stroke of the shaft, arm 8 touches front and rear end-stroke reference contacts. As it will be better hereinafter described, when a disengagement between group 7 and shaft 6 occurs, the reference arm 8 slides along an axial slot of the shaft 6 which extends from the rear extremity 65 of the shaft to the front end-stroke point. 
     The motion of arm 8 is guided by a guide 85 which is parallel to the screw and avoids any possible rotational motion of the nut 5 in respect of the screw. 
     Advantageously, at the engaging point with group 7 shaft 6 presents a thickened portion which ensures the needed stiffness in spite of the presence of the groove. Extremity 65 of the shaft 6 presents a conical shape in order to facilitate the group 7 to automatically enter again into the shaft 6 after that a disengagement has occurred. 
     In functioning, driving group 1 causes a rotational motion of the screw 2 and determines the axial displacement of nut 5 and of the shaft 6 which is thereto connected through the limiting group 7. Starting from the end stroke point of FIG. 1 the forward motion of nut 5 determines the displacement of the shaft 6 until it reaches the second end-stroke point where, at the proper time, a suitable contact will give to the driving group 1 the instruction of inverting the direction of the following displacement. 
     In the regular functioning the shaft will be driven from the nut 5 from the second, back to the first end-stroke point. However, it is also possible that an over load is applied on shaft 6. In this case, the force exerted on the shaft is transmitted by the sloping surfaces 66 to the inserts 72 and then on the ring 77 and this can overcome the opposite force which is applied on the ring by the spring 73 which is then compressed. As a result, the inserts slide in the radial direction and the shaft is disengaged from group 7. In this situation, when the screw 2 moves the nut 5 back, the group 7 can slide inside the shaft and it reaches again its engagement position. 
     A second disengagement possibility occurs when the over load on the shaft forces the same shaft to go forward while the limiting group is at the initial point of the stroke. In this case the pressure of the sloping surfaces 67 on the rear sloping surfaces 78 of the inserts causes, as previously seen, the compression of spring 73 and the radial displacement of the inserts, so that a relative motion occurs and group 7 gets out from shaft 6. 
     It is evident that the device functions in a similar way when the over load is due to an unwanted stop of the shaft 6 during its normal displacement so that the predetermined load threshold value is exceeded. 
     According to the invention, the retaining ring 75 avoids that inserts can move outwardly for a distance greater than the engagement height of the same inserts with the groove 68. Furthermore, ring 75 ensures that the inserts have a free margin of radial displacement which allows the spring pressure to be adjusted. 
     Advantageously, the inclination of the sloping surfaces 78 can also be different from the inclination of the surfaces 79, to take account of the different conditions which are present in the two described cases and of the possible dynamic dissymmetry which can be caused by the frictional effects. 
     In the second case screw 2 is supported by bearings 21, 22 while shaft 6 is kept in aligned position by group 62. To re-establish the connection between the shaft and the group 7 it is then sufficient that the driving group 1 moves the nut 5 forward, so that it can enter the conical opening 65 and the inserts engage groove 68 again. 
     In the above described embodiment an engagement member consisting of a cylindrical shaft has been considered, but engagement members having different shape can be used within the scope of the invention. Such engagement members can also be free from any function of guiding the limiting group. 
     In FIG. 3 a preferred embodiment of the invention is shown, which permits to overcome the above said drawbacks due to dynamic dissymetry. 
     In this embodiment, the device comprises a second spring 73&#39; (or group of springs) which is compressed between a fixed wall of a sleeve 71&#39; and a second ring 77&#39;. Ring 77&#39; is substantially identical to ring 77 and is simmetrically mounted on the sleeve 71&#39; in respect of the inserts 72 which are comprised between the sloping surfaces S1 of ring 77&#39; and S2 of ring 77. 
     Advantageously, with this solution any risk of possible differences in the dynamic behaviour of the device in the two axial directions is avoided and it is no longer necessary to provide different inclinations of the surfaces 78, 79 of the inserts. 
     More particularly, the device according to the invention has a substantially self-bearing structure which does not depend on the engagement conditions with the element which has to be displaced. In the described solution, such self bearing structure is ensured by the presence of the endless screw (which ensures the aligned position of group 7 in any conditions) and of the retaining ring 75 (which guaranties the functioning of the inserts of the limiting group). 
     According to different embodiments, a radial arrangement of the elastic means can be also considered. However, in the described solution the axial bucket springs and the retaining ring (which is comprised inside the radial dimension of the inserts) allow the device to occupy the minimum space in the radial direction and to have a limited size in the axial direction. 
     The present invention has been described with reference to preferred embodiments, but equivalent changes can also be made by the man skilled in the art without departing form the scope on the invention