Device for limiting the axial load of an endless screw

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. DE 33 22 566, which comprises the features mentioned in the preamble of claim 1, describes a safety lead screw driving for a machine tool and workpiece slides comprising a housing defining a first circumferential groove with which an annular spring engages by means of a sloping surface coupling. The housing is fixed on a workpiece slide. The annular spring is also engaged by means of two sloping surfaces with a second circumferential groove defined by the nut of the lead screw and by a tubular sleeve which is slidably set on the lead screw. In normal working conditions the annular ring transmits the driving forces from the drive screw to the workpiece slide. When the workpiece slide hits an obstacle, or anyway its axial motion is hindered by a force greater than a threshold value, by means of a radial displacement the annular spring comes out of the first circumferential groove and disengages the workpiece slide from the drive screw. Nevertheless this mechanism can have a different behaviour in the two axial directions caused by frictional effects." Aim of the invention A first aim of the invention is to overcome the limits and the drawbacks of the already known limiting devices, with particular reference to the difference dynamical behaviour in the two axial directions. 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 for axial loads, comprising: self-bearing axial transmission means suitable to transmit in both the axial direction the axial motion generated by a driving group; an engagement member provided with a groove and suitable to drive an operating mechanisms; a load limiting group suitable to connect and disconnect from each other said self-bearing transmission means and said engagement member, said load limiting group being integral to said self bearing transmission means and comprising one or more frusto-pyramidal radial inserts suitable to engage elastically said groove by lateral sloping sides, said one or more frusto-pyramidal radial inserts being comprised between two lateral sloping surfaces the first one being part of a first ring axially slidable on a sleeve integral to said self-bearing axial transmission means; elastic means comprising one or more springs and able to fix and adjust a threshold value of the load required to cause a disengagement between said groove (68) and said one or more frusto-pyramidal radial inserts , said elastic means comprising a first spring suitable to force said first ring against said one or more inserts. The load limiting device according to the present invention is characterised in that said load limiting group comprises a second ring mounted on said sleeve, said second lateral sloping surface being part of said second ring, said elastic means comprising a second spring suitable to force said second ring against said one or more inserts. 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, and it is possible to avoid risks of substantial differences in the dynamic behaviour of the device in the two axial directions due to frictional effects. 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: figure 1 shows a schematic view of a device which is not part of the invention as defined in claim 1, but contains some details which are part of the preferred embodiments object of some dependent claims; - figure 2 shows a detail of the load limiting of the device of figure 1; figure 3 shows a schematic view of a preferred embodiment of the present invention; The device shown in Figure 1, 2 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 - in the present description called also end 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 endless screw. Endless screw 2 is arranged internally and coaxially to a cylindrical hollow shaft 6 which can axially slide in respect of the endless 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 end bearing 22 is positioned. The axial motion of hollow shaft 6 is guided by the bushings 63 of a support 62 fixed to an external case inside which the driving group 1, endless screw 2 and hollow shaft 6 are also contained. With reference now to figure 2, the load limiting group 7 connecting endless screw 2 to hollow shaft 6 is described. A sleeve 71 presenting a sloping surface S1 is mounted on nut 5 by screw means 76. Two frustum-pyramidal radial inserts 72 - in the present description also called 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 first ring 77 and, in the radial direction, by a retaining ring 75. On first 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 hollow 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 figure 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 figure 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 an rear end-stroke reference contacts. As it will be better hereinafter described, when a disengagement between toad limiting group 7 and hollow 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 the load limiting group 7 the hollow shaft 6 presents a thickened portion which ensures the needed stiffness in spite of the presence of the groove. Extremity 65 of the hollow shaft 6 presents a conical shape in order to facilitate the load limiting group 7 to automatically enter again into the hollow shaft 6 after that a disengagement has occurred. In functioning, driving group 1 causes a rotational motion of the endless screw 2 and determines the axial displacement of nut 5 and of the hollow shaft 6 which is thereto connected through the load limiting group 7. Starting from the end stroke point of figure 1 the forward motion of nut 5 determines the displacement of the hollow 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 hollow 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 first 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 the load limiting group 7. In this situation, when the endless screw 2 moves the nut 5 back, the loading limiting group 7 can slide inside the hollow shaft 6 and it reaches again its engagement position. A second disengagement possibility occurs when the over load on the hollow shaft forces the same shaft to go forward while the load 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 the load limiting group 7 gets out from the hollow 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 hollow shaft 6 during its normal displacement so that the predetermined load threshold value is exceeded. According to a preferred embodiment of the present 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. According to another particular embodiment of the present invention, 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 the hollow shaft 6 is kept in aligned position by group 62. To re-establish the connection between the load limiting 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 embodiments of the present invention an engagement member consisting of a hollow 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 load limiting group. In figure 3 a preferred embodiment of the present invention is shown, which permits to overcome the above said drawbacks due to dynamic dissymmetry. In this embodiment, the device comprises a second spring 73' (or group of springs) which is compressed between a fixed wall of a sleeve 71' and a second ring 77'. Second ring 77' is substantially identical to the first ring 77 and is simmetrically mounted on the sleeve 71' in respect of the inserts 72 which are comprised between the sloping surfaces S1 of second ring 77' and S2 of first 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 load limiting 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.

Claims: Load limiting device for axial loads, comprising: self-bearing axial transmission means (2,5) suitable to transmit in both the axial direction the axial motion generated by a driving group (1); an engagement member (6) provided with a groove (68) and suitable to drive an operating mechanisms; a load limiting group (7) suitable to connect and disconnect from each other said self-bearing transmission means (2,5) and said engagement member (6), said load limiting group (7) being integral to said self bearing transmission means and comprising one or more frusto-pyramidal radial inserts (72) suitable to engage elastically said groove (68) by lateral sloping sides (78, 79), said one or more frusto-pyramidal radial inserts (72) being comprised between two lateral sloping surfaces (S2, S1') the first one (S2) being part of a first ring (77) axially slidable on a sleeve (71') integral to said self-bearing axial transmission means (2, 5); elastic means (73,73',74) comprising one or more springs and able to fix and adjust a threshold value of the load required to cause a disengagement between said groove (68) and said one or more frusto-pyramidal radial inserts (72), said elastic means comprising a first spring (73) suitable to force said first ring (77) against said one or more inserts (72); characterised in that said load limiting group comprises a second ring (77') mounted on said sleeve (71'), said second lateral sloping surface (S1') being part of said second ring (77'), said elastic means comprising a second spring (73') suitable to force said second ring (77') against said one or more frusto-pyramidal radial inserts (72). Device according to claim 1, characterised in that said self-bearing axial transmission means are constituted by a nut (5) engaged with an endless screw (2) mounted on end bearings (21,22). Device according to claim 1 or 2, characterised in that said one or more frusto-pyramidal radial inserts (72) can slide along slots (81,82) of said lateral sloping surfaces (S1', S2). Device according to claim 2 and/or 3, characterised in that said engagement member comprises a hollow shaft (6) coaxial to said screw (2) and one of said end bearings (21, 22) is mounted on a first support (61) sliding inside said hollow shaft (6). Device according to claim 4, characterised in that said hollow shaft (6) is slidingly mounted on a second support (62) of a fixed structure (3). Device according to one or more of the preceding claims, characterised in that said groove (68) presents sloping edges (66,67) which oppose to said lateral sloping sides (78, 79) of said one or more frusto-pyramidal radial inserts (72). Device according to one or more of the preceding claims, characterised in that the slope of said lateral sloping sides (78) differs from the slope of said lateral sloping sides (79) in order to take account of the different conditions and of any dynamic dissymmetry which can be caused by the frictional effects acting in the two axial directions. Device according to one or more claims from 4 to 7, characterised in that said hollow shaft (6) presents a rear extremity (65) with a conical opening in order to facilitate said load limiting group (7) to enter into said hollow shaft (6) again after that a disengagement has occurred. Device according to one or more of the preceding claims, characterised in that said elastic means consist of a couple of first and second bucket springs (73,73') which are arranged co-axially to said endless screw (2) the first bucket spring (73) being compressed between said first ring (77) by a threaded bush (74) and the second bucket spring (73') being compressed between a fixed wall of said sleeve (71') and said second ring (77'). Device according to one or more of the preceding claims , characterised in that it comprises retaining means (75) consisting of a ring which is inserted inside an encircling seat (86) of said one or more frusto-pyramidal radial inserts (72). Device according to one or more of the preceding claims , characterised in that it comprises a reference arm (8) providing front and rear end stroke contact points. Device according to claims 11 and one or more claims from 4 to 10, characterised in that said reference arm (8) can slide along an axial slot of said hollow shaft (6) and it is guided by a guide parallel to said endless screw (2) and which avoid any possible rotational motion of said nut (5) in respect of said endless screw (2). Device according to one of more claims form 4 to 12, characterised in that said hollow shaft (6) presents a stiffening portion at the engagement point with said load limiting group (7).

Inventors: BIAGIOTTI MAURIZIO, SILIANI CLAUDIO, BIAGIOTTI, MAURIZIO, SILIANI, CLAUDIO
Applicants: SILIANI HARMON S P A, SILIANI HARMON S.P.A.
Application year: 1999
Main classification: F16H25
IPC codes: F16H25