Torque limiting coupling with adjustable intervention threshold

Abstract: A torque limiting coupling with adjustable intervention threshold comprises: a hollow cylindrical body (2) whose inner walls (2a) are affected by at least two slots (3) which are parallel to the longitudinal axis of the body (2); a solid cylindrical hub (5), which is rotationally retained in said hollow body (2) by means of conventional elements and forms a plurality of radial recesses (7) for the slidingly guided accommodation of corresponding pawls (8) for engaging the slots (3); a presser means (9) for the pawls (18), which is retained in forced contact therewith by means of elastic elements (10) which can be adjusted from outside through corresponding axial openings (4, 6) formed in the hollow body (2) and/or hub (5); the pawls (18) engage, in the torque transmission configuration, the corresponding slots (3) and disengage them, with a centripetal motion into the corresponding recesses (7), in the torque limiting configuration.

Description: The present invention relates to a torque limiting coupling with adjustable intervention threshold. Many torque limiters have long been known and are used to control the transmission of force between a driving element and a driven element. In particular, in the field of agricultural machines, the use of said limiters is essentially indispensable in order to protect both the motion transmission elements and the elements that receive motion therefrom against peak overloads caused by the heavy duty to which said machines are usually subjected and by the onset of resisting torques. Conventional limiters, disclosed for example in European Patent 543,142, in US Patent 2,238,583 and in French Patent 8,806,398 (FR patent 2615256), substantially consist of a cylindrical body which forms a cup-shaped seat at one end and a fork for the articulation of a universal joint at the other end. Inside the seat there is provided a plurality of pawls which are interposed radially between the motion transmission shaft so that they can slide radially with respect to the shaft under the thrust of preadjusted springs which keep them engaged with retention slots formed in the inside walls of said seat. This interlock maintains the normal coupling between the transmission shaft and the driven shaft through the joint. When the transmitted torque, i.e., the reaction torque of the driven shaft, exceeds the total value of the thrust force of the springs, the pawls are pushed back so as to retract centripetally, disengaging the slots and thus allowing one of the two shafts to rotate with respect to the other. This action produces a stepped slippage which persists until the value of the resisting torque returns below the total value of the thrust force of the springs that act on the pawls. In this case, the springs overcome the force that tends to make the pawls retract again; since the pawls restore the rigidity of the engagement with the slots, they accordingly restore the direct connection between the driving shaft and the driven shaft. The conventional limiters, though offering a substantially satisfactory performance, suffer the drawback that it is not possible to adjust their intervention threshold unless one either replaces every time all the springs with others having preset characteristics according to the operating requirements or opens the body of the limiter and readjusts the springs. Thus, both operations require the complete or partial disassembly and reassembly of the limiter in order to replace or readjust the springs, with a consequent great expenditure of time and energy. A torque limiting coupling as set forth in the preamble of claim 1, is known from document FR-A-2 429 929. The aim of the present invention is to solve the above problems of the prior art by providing a torque limiting coupling with adjustable intervention threshold which allows to perform the adjustments required to modify its intervention threshold without having to perform time-consuming disassemblies and troublesome replacements. An object of the present invention is to provide a torque limiting coupling with adjustable intervention threshold which allows to perform a particular and successive refinement of the adjustments meant to determine the value of the threshold. This aim, this object and others which will become apparent hereinafter are achieved by a torque limiting coupling with adjustable intervention threshold according to the invention and having the features of claim 1. Further characteristics and advantages of the present invention will become apparent from the description of a preferred embodiment of a torque limiting coupling with adjustable intervention threshold, illustrated only by way of non-limitative example in the accompanying drawings, wherein: Figure 1 is a vertical sectional view of the torque limiting coupling according to a first embodiment, shown for illustrating purposes and which does not form part of the claimed invention; Figure 2 is a corresponding transverse sectional view thereof, taken along the plane II-II of figure 1, in which one half illustrates the torque transmitting configuration and the other half illustrates the torque limiting configuration; Figure 3 is a schematic sectional view of a pusher means in a first embodiment, not according to the invention; Figure 4 is a corresponding plan view of the pusher means; Figure 5 is a side view of a pin which is part of the externally adjustable elastic elements; Figure 6 is another side view of a pin which is part of externally adjustable elastic elements in a second embodiment, not according to the invention; Figure 7 is a detail view of an embodiment of externally adjustable elastic elements; Figure 8 is an enlarged-scale detail view of the contact between a pawl which is part of the coupling and a presser means; Figure 9 is a sectional view, taken along a vertical plane, of one half of the coupling in the torque limiting configuration; Figure 10 is a corresponding view of figure 9, in the configuration for normal torque transmission; Figure 11 is a vertical sectional view of a further embodiment of the torque limiting coupling which constitutes the invention claimed, provided with elements for refining the intervention threshold value; Figure 12 is a corresponding transverse sectional view thereof. With particular reference to the above figures, 1 generally designates a torque limiting coupling with adjustable intervention threshold, which substantially comprises a hollow cylindrical body 2 in the inner walls 2a whereof at least two slots 3 parallel to the longitudinal axis of said body 2 are defined. The body defines, on the bottom of the cavity, a first central opening 4 which can be protected by a corresponding cover 4a; a substantially solid cylindrical hub 5 can be inserted snugly in said cavity so as to form a pack-like assembly and is retained in the body 2 so that it can rotate through adapted means 2b of a conventional type. Like the body 2, the hub 5 also defines, in turn, a second opening 6 through which it is possible to reach the adjustable elastic elements described hereinafter. A plurality of radially orientated recesses 7 is cut into the hub 5, which has, on its outward face, a set of hollow threaded seats 5a for the coupling of driving or driven means. The recesses 7 can slidingly accommodate, so that they are guided, corresponding pawls 8 which are adapted to engage the slots 3 in the configuration for normal torque transmission. The pawls 8 are in turn placed in contact with a presser means 9, which is retained in forced contact therewith under the thrust of elastic elements, designated by 10, which can be adjusted from outside through the first opening 4 and the second opening 6 after removing the respective mentioned covers 4a and 6a. The slots 3 have a cross-section shaped like an isosceles trapezoid, in which the longer parallel side is open and directed towards the center of the cylindrical body 2. The number of the slots is preferably but not necessarily equal to the number of the pawls 8 and of the corresponding recesses 7, which are formed along perpendicular diameters of the hub 5. In the pawls 8, the side 8a for engaging the slots 3 replicates, with a convex profile, the trapezoidal cross-section of the slots. Moreover, the surfaces 8b and 8c for mutual contact with the opposite surfaces 9a, 9b of the presser means 9 are shaped so as to split the resultant of the forces into components having preset directions, particularly radial ones. In greater detail, the surfaces 8b and 8c have generatrices which form an obtuse angle. The presser means 9 is substantially constituted by a circular plate 13 which is mounted, so that it can be moved coaxially, inside the hollow cylindrical body 2 and is peripherally provided with a plurality of teeth 13a, each of which is arranged at a pawl 8. The adjustable elastic elements 10 comprise a pin 14 whose head is provided with a wider rim 15 and can be associated in an axial position which is adjustable with the elastic elements 10 which can be accessed from the outside through the openings 4 and/or 6. The pin 14 is perpendicular to, and centered in, the plate 13 and has, at its head and at its base 14b, respective notches 14c which are arranged along parallel diameters. A plurality of springs 16 (in figure 1) and 115 (in figure 11) of the Belleville washer type is fitted on the pin 14; the springs are stacked in a pack between the head 15 and the plate 13, where they bear onto a ring shoulder 12 provided at the hub 5, and are part of said elements 10. Locking means 17 are also provided to prevent the unscrewing of the pin 14. Said means consist of a split pin 17a shaped like an inverted cup and interposed between the rim 15 and the uppermost Belleville washer 16. The lower wing 17b of said split pin 17a diverge slightly outwards and can be inserted in the recesses 7. Furthermore, the bottom of the split pin 17a has a plurality of protrusions 17c which are adapted to engage corresponding hollow indents 14d formed in said rim 15. The elements 10 can be constituted by a screw coupling, in which a first thread 18 is formed at the base 14b of the pin 14 to allow the screwing thereof in a complementary thread 19 defined in a hollow seat 20 centered in the plate 13 and adapted to receive said base 14b. In another alternative embodiment, the elastic elements 10 can be constituted by a ring 22 which defines, on one face, a plurality of pairs of concave sectors 22a with staggered planes, and is accommodated in a corresponding seat 23 formed centrally in the back of the plate 13. The seat 23 has a partial through opening for the base 14b of the pin 14 which can be provided, once passage has occurred, with a transverse key 24 adapted to engage the sectors 22 selected after rotating the pin 14. In the further embodiment shown in figures 11 and 12, which constitutes the invention claimed, in the cylindrical hub 5 there are provided at least two through seats 111 which are parallel to the axis A of said hub and in which it is possible to accommodate corresponding elastic means 112 which can also be adjusted from outside and are antagonists of the presser means 9 constituted by the circular plate 13. In order to provide a good balancing of the hub 5, said seats 111 are provided so as to be diametrically opposite with respect to said central axis A. The elastic means 112 consist of corresponding helical compression springs 116, each of which is snugly inserted in its own seat 111 and is provided with a corresponding end which rests directly on the perimeter of the plate 13 and with an opposite end which rests on the lower faces of screw means which can be micrometrically screwed into said seats 111 and are substantially constituted by grub screws 117. The operation of the invention is as follows. In the normal torque transmission configuration, the pawls 8 are inserted in the slots 3 and the body 2 and the hub 5 rotate rigidly. By acting on the pin 14 through one of the openings 4 or 6, it is possible to change the axial compression load on the Belleville washers 16 so as to correspondingly change the value of the force that they exchange with the plate 13, which is in contact with the pawls 8. In practice, the action on the pin 14 consists in inserting the tip of a screwdriver in one of the notches 14c and in screwing it more or less deeply into the seat 20. In this manner, the springs 16 are compressed between the head 14a and the plate 13. Once said adjustment has been completed, the pin 14 is locked through the means 17 so that it cannot unscrew accidentally. The split pin 17a, during the screwing of the pin 14, in fact remains rotationally fixed and is retained by the rims 17b inserted in the recesses 7, while the hollow seats 14d move in a ratchet-like fashion beyond the protrusions 17c, exploiting the elasticity of the bottom of said split pin. Once the chosen position has been reached, the engagement between the hollow recesses 14d and the protrusions 17c prevents the accidental unscrewing of the pin 14. In a second solution provided in order to vary the preloading of the springs 16 and therefore the intervention threshold of the torque limiting coupling 1, the pin 14, after being inserted through the seat 23 and provided with the transverse key 24, is rotated so that said key engages, by resting against it, one of the pairs of sectors 22a defined on staggered planes in the ring 22. By doing so, the pin 14 moves axially and again compresses the springs 16 between the head 14a and the plate 13 with values whose intensity can vary according to the pair of sectors 22a that has been selected. In this case too, means 17 for preventing unscrewing are used. It is also possible to micrometrically refine the value of the intervention threshold of the coupling 1, once the coarse adjustment has been set according to the above described procedure, by acting on the grub screws 117. By screwing the grub screws, the springs 116 are gradually compressed and, in this case, become antagonists of the Belleville washers 115, proportionally contrasting the action thereof as the screwing of the grub screws 117 is increased, once again by acting from outside. If the grub screws 117 are instead unscrewed, in the extreme case, the complete unloading of the springs 116, which become ineffective in this case, is achieved. Both the hub 5 and the hollow cylindrical body 2 are provided with threaded holes of their own, designated by the reference numerals 5a and 25 respectively, for the coupling of conventional driving and driven elements such as, for example, a spline for the engagement of a shaft and, at the opposite end, a fork for the articulation of a universal joint, all being commonly used in agricultural applications. If, for example, during the operation of an agricultural machine provided with a soil treatment tool and with the limiter 1 inserted between the tool and the machine, the tool encounters an obstacle, the torque that contrasts motion increases. This leads to a stress which the body 2 discharges onto the pawls 8 through contact with the slots 3 in which the pawls are inserted. The pawls 8 oppose a resistance whose absolute value is equal to the value of the radial component of the contact force that is exchanged between the surfaces 8b and 9a. Said force, as mentioned above, is adjustable by acting on the pin 14 through one of the openings 4 or 6 and preloading the springs 16, optionally refining it by acting on the grub screws 117. If the above mentioned resisting torque exceeds, in terms of absolute value, said component, the pawls 8 are repelled centripetally from the body 2 and, by retracting into the hollows 7, exit from the slots 3. In this configuration, the hub 5 and the body are uncoupled one another. In practice, the hub 5 can thus rotate inside the body 2 without transmitting any appreciable torque, thus automatically uncoupling the driving shaft of the machine and the driven transmission shaft of the tool. During this configuration for limiting transmitted torque, each time the pawls 8, by rotating together with the hub 5 inside the body 2, pass at the slots 3, an impact occurs, producing a corresponding torque impulse between the body 2 and the hub 5 which tends to spontaneously restore the coupling between the pawls 8 and the slots 3. The succession of the impulses, combined with the possible elimination of the cause that determines the resisting torque, and with a consequent gradual decrease in the rotation rate of the moving parts, facilitates a new interlock between the pawls 8 and the slots 3 and the consequent restoring of the normal transmission of torque between the machine and the tool. It has thus been observed that the invention achieves the intended aim and objects, i.e., it allows to adjust the intervention threshold of the torque limiting coupling directly from the outside, without having to disassemble the limiter for each different use for which it is to be employed. In the practical execution of the invention, the materials used, as well as the shapes and the dimensions, may be any according to requirements without thereby abandoning the scope of the protection of the appended claims. Where technical features mentioned in any claim are followed by reference signs, those reference signs have been included for the sole purpose of increasing the intelligibility of the claims and accordingly, such reference signs do not have any limiting effect on the interpretation of each element identified by way of example by such reference signs.

Claims: A torque limiting coupling with adjustable intervention threshold, comprising: a hollow cylindrical body (2) in the inner walls (2a) whereof at least two slots (3) parallel to the longitudinal axis (A) of the body (2) are defined; a solid cylindrical hub (5), which is rotationally retained in said hollow body (2) by way of retaining elements (2b) and which defines a plurality of radial recesses (7) for the slidingly guided accommodation of corresponding pawls (8) for engaging said slots (3); a presser means (9, 13) for said pawls (8), which is retained in forced contact therewith by first adjustable elastic means (10, 16, 115) which are adjustable from outside for setting a threshold value of the torque limiting coupling (1), said pawls (8) engaging, in the torque transmission configuration, said corresponding slots (3) and disengaging them, with a centripetal motion into the corresponding recesses (7), in the torque limiting configuration, characterized in that it further comprises second adjustable elastic means (112), which are antagonist of said presser means (9), for micrometrically refining the threshold value of the torque limiting coupling (1), said first (10, 16, 115) and second adjustable elastic means (112) being adjustable from outside said coupling (1) through corresponding axial openings (4, 6) formed in said hollow body (2) and/or hub (5). A coupling according to claim 1, characterized in that said hollow cylindrical body (2) and said solid hub (5) are both provided with means (5a, 25) for coupling driving and driven elements. A coupling according to claim 2, characterized in that said coupling means are constituted by a plurality of threaded hollow seats (5a, 25) for the screwing of corresponding bolts for fastening said driving and driven elements. A coupling according to claim 1, characterized in that it comprises four slots (3) defined at the four diametrical poles of said hollow body (2). A coupling according to claim 4, characterized in that said slots (3) have a cross-section shaped like an isosceles trapezoid in which the longer parallel side is directed towards the center of said cylindrical hollow body (2). A coupling according to claim 4, characterized in that the number of said slots (3) is equal to the number of the pawls (8) and of the corresponding recesses (7), said recesses (7) being formed along perpendicular diameters of said hub (5). A coupling according to the preceding claims, characterized in that in said pawls (8) the side for engagement with said slots (3) convexly replicates the trapezoidal cross-section of said slots (3) and the surfaces (8b, 8c) for contact with said presser means (9) are shaped so as to split the resultant of the mutual contact forces into components having preset directions. A coupling according to claim 7, characterized in that said contact surfaces (8b, 8c) are arranged with respect to each other so that their generatrices form an obtuse angle. A coupling according to claim 1, characterized in that said presser means (9) is constituted by a circular plate (13) mounted so that it can be moved coaxially inside said hollow cylindrical body (2) and peripherally provided with a plurality of teeth (13a) arranged at said pawls (8). A coupling according to claim 9, characterized in that said adjustable elastic elements (10) comprise: a pin (14) whose head (14a) has a wider rim (15) and which is associable, in an adjustable axial position, with corresponding axial position adjusting means (14c, 18, 19, 20, 22, 24) which can be accessed from outside through said openings (4, 6), said pin (14) being centered at right angles in said plate (13); a plurality of Belleville compression washers (16) stacked in a pack and fitted on said pin (14) between the head (14a) and the plate (13) so as to bear on a ring shoulder (12) provided at the hub (5); and a locking means (17) for preventing uncoupling of said pin (14) from said plate (13). A coupling according to claim 10, characterized in that said head (14a) of the pin (14) and the base end (14b) thereof are affected respectively by a transverse notch (14c) arranged along a diameter for the engagement of an adjustment tool through said openings (4, 6). A coupling according to claim 11, characterized in that the wider rim (15) of said pin head (14a) is affected by a plurality of concave indents (14d) for the elastic coupling of said uncoupling prevention means (17). A coupling according to claim 12, characterized in that said uncoupling prevention means (17) consist of a split pin (17a), which is shaped like an inverted cup and is interposed between said head (14a) and the uppermost one of said Belleville washers (16), its lower flaps (17b) being slightly divaricated for insertion in said recesses (7), the bottom of said split pin (17a) having corresponding raised portions (17c) for engagement in said indents (14d). A coupling according to claim 10, characterized in that said axial position adjusting means (14c, 18, 19, 20) for axially adjusting the position of said pin (14) are constituted by a screw coupling, a first thread (18) being formed at the base (14b) of said pin (14) for screwing into a second thread (19) formed in a hollow seat (20) which is centered in said plate (13) and is adapted to receive said base (14b). A coupling according to claim 8, characterized in that said axial position adjusting means (22, 24) are constituted by a ring (22) with concave sectors (22a) with staggered planes, which is located in a corresponding seat (23) formed centrally on the back of said plate (13) and having a partial through opening, said sectors (22a) being rotationally engageable with a key (24) which is inserted transversely with respect to the base (14b) of said pivot (14) which can slide axially in said through opening. A coupling according to the preceding claims, characterized in that at least two through seats (111) parallel to the axis (A) of the hub (5) are defined in said solid cylindrical hub (5) and accommodate said second adjustable elastic means (112) which can be adjusted from outside and are antagonists for said pawl presser means (9). A coupling according to claim 16, characterized in that said seats (111) are defined in diametrically opposite positions with respect to the central axis (A) of said cylindrical hub (5). A coupling according to claim 16 or claim 17, characterized in that said second adjustable elastic means (112) are constituted by helical compression springs (116) each of which is snugly inserted in a corresponding seat (111), one end of said springs (116) resting directly on said presser means (9), the opposite ends resting on the lower face of screw means (117) which can be micrometrically screwed into said seats (117). A coupling according to claim 18, characterized in that said screw means are constituted by so-called grub screws (117).

Inventors: BRUNAZZI ACHILLE, PROSPERO ROBERTO, BRUNAZZI, ACHILLE, PROSPERO, ROBERTO
Applicants: COMER GROUP S P A, COMER GROUP S.P.A.
Application year: 1998
Main classification: F16D43
IPC codes: F16D43