Cross arm body for a cardan joint and method for making it

Abstract

A cross arm body (2) for a cardan joint (1) of a delta robot (100), comprising a main body (2a) having a first and a second through holes (10, 11) whose respective axes (X, Y) are skew to each other, in particular perpendicular to and offset from each other, and a first and second shafts (5, 6), each rotatably insertable into a respective one of said through holes (10, 11). Each shaft (5, 6) is provided, at its ends, with connecting means (9) for fastening to a respective forked body (3, 4), and the main body (2a) has, on each end of each through hole (10, 11), an annular seat (15) for receiving the outer ring (7a, 8a) of a rolling bearing (7, 8) intended to engage, on the inner ring (7b, 8b) of the same rolling bearing (7, 8), a corresponding end portion of one of said shafts (5, 6).

Description

[0001] DESCRIPTION CROSS ARM BODY FOR A CARDAN JOINT AND METHOD FOR

[0002] MAKING IT

[0003] Technical field

[0004] The object of the present invention is a cross arm body for a cardan joint and a method for making it.

[0005] In particular, the invention preferentially applies to cardan joints employed for delta robots, where each arm of the robot is an articulated arm provided with articulation based on a cardan joint.

[0006] Background art

[0007] Delta robots employing articulated arms with cardan joints are well known in the prior art, for example, from WO2019 / 166923 to the Applicant.

[0008] It is known that cardan joints for this type of use require a perpendicularaxis cross arm and a pair of rolling bearings for each axis.

[0009] Typically, such bearings are of the radial type, spaced apart from each other to transmit torque between two shafts, but they are not capable of ensuring accurate positioning nor sustaining loads along their axis, which are instead required in the robotic application of the present invention. In particular, in order to obtain the correct preload, these bearings are to be mounted “back-to-back”, in a direct manner, or by interposing double spacers. In particular, it is necessary for the two spacers to have the same length since the applications of such cardan joints require increased accuracy and even a minimum difference between the distance between the inner rings of the two bearings and the distance between the outer rings can cause the formation of backlash such as to cause positioning inaccuracies.

[0010] To obtain this, it is known to machine the two spacers at the tool machines simultaneously, while they are arranged on the machine tool table, so that one single machining operation obtains both heads of the two spacers. Then the two spacers of each pair need to be connected to each other so as to prevent the spacer of one pair from being inadvertently exchanged with the spacer of another pair, with which it does not share the mechanical machining and therefore, the length with increased accuracy. This makes the production process slow and complex and requires specific interventions for preventing accidental exchanges which further complicate the process.

[0011] Object of the invention

[0012] Therefore, the object of the present invention is to provide a cross arm body for a cardan joint which allows simple and efficient manufacturing while maintaining a high quality standard such as to allow increased movement accuracy.

[0013] It is a further object of the present invention to provide a method for manufacturing the cross arm body which is simple, albeit highly reliable.

[0014] Within this context, the Applicant has observed that the outer spacer can be defined by a cross arm body, and that the inner spacer can be defined by a shaft defining an axis of the cardan joint, where shaft and cross arm body are obtained by separating a single monobloc body. This allows carrying out the mechanical machining of the (inner and outer) seats of the bearings, therefore of the abutting surfaces defining the inner and outer spacer shoulders, before the separation of the monobloc body, and the monobloc body can be separated only after such machining operations, thus obtaining the cross arm body and shaft. Thereby, the mechanical machining can be carried out simultaneously by the machining tool without changing piece reference systems and thus ensuring very narrow tolerances suitable for ensuring increased movement accuracy of the cardan joint.

[0015] Moreover, according to an advantageous aspect, the separation of the monobloc body into cross arm body and shaft (shafts) can occur by means of wire electro discharge machining. Moreover, according to a further advantageous aspect, the different components into which the monobloc body is separated (cross arm body and shafts) can be marked with suitable recognizable marks such as to make unique the association of each shaft with the relative cross arm body, preventing incorrect assemblies of shafts with non-corresponding cross arm bodies.

[0016] The object specified is substantially achieved by a cross arm body for a cardan joint and by a method for making it comprising the technical features disclosed in claims 1 and 13, respectively, and / or in one or more of the claims dependent thereon.

[0017] Further features and advantages of the present invention will become more apparent from the following indicative and thus non-limiting description of an embodiment of a cross arm body for a cardan joint and a method for making it, according to the invention.

[0018] Brief description of the drawings

[0019] Such description is set out below with reference to the accompanying drawings, provided purely by way of non-limiting example, in which:

[0020] - Figure 1 shows a perspective view of a delta robot employing cardan joints according to the invention;

[0021] - Figure 2 shows one of the arms of the robot in Figure 1 ;

[0022] - Figure 2A shows an enlarged detail of the arm in Figure 2 to better show the cardan joint;

[0023] - Figure 3 is an enlarged and partially sectioned view of the arm in Figures 2 and 2A;

[0024] - Figure 4 is a perspective view of a cross arm body employed in the cardan joint according to the invention;

[0025] - Figure 5 is a perspective and partially sectioned view of the cross arm body in Figure 4;

[0026] - Figures 6 and 7 show the cross arm body in Figure 4 and 5 according to different side views and with the rolling bearings removed; - Figure 8 is a sectional view of the cross arm body according to the invention, according to line VIII-VIII in Figure 6;

[0027] - Figure 9 is a sectional view of the cross arm body according to the invention, according to line IX-IX in Figure 7;

[0028] - Figures 10 and 11 are a sectional and front view of a first machining step of the cross arm body according to the invention;

[0029] - Figures 12 and 13 are a sectional and front view of a second machining step of the cross arm body according to the invention;

[0030] - Figures 14 and 15 are a sectional and front view of a third machining step of the cross arm body according to the invention.

[0031] With reference to Figure 1 , a robot for moving items, a so-called “delta robot”, is indicated as a whole by reference numeral 100.

[0032] Detailed description of preferred embodiments of the invention

[0033] In its general structure, robot 100 is of the known type and will not be described in detail. Only at a general level, robot 100 comprises a plurality of arms (generally three) 110 connected on one side, or at the top, to respective movement members 120, and on the other side, or at the bottom, to a common platform 130 for supporting a gripping tool or the like . Each arm 110 is connected to the respective movement member 120 and to the common platform 130 by means of cardan joints 1 , in particular of the type forming the object of the present invention.

[0034] Figure 2 shows a detail view of one of the arms 110 of robot 100. Figure 2A shows an enlarged view of one of the two cardan joints 1 of arm 110. In particular, it should be noted in Figure 2A how the cardan joint 1 comprises a cross arm body 2 and a first forked body 3, applicable to a first mechanical member (platform 130, for example) and connected to the cross arm body 2 to rotate around a first axis “X” and a second forked body 4, applicable to a second mechanical member (arm 110) and connected to the cross arm body 2 to rotate around a second axis “Y”. The two axes “X”, “Y” are skew to each other, in particular perpendicular to and spaced apart from each other.

[0035] In greater detail, as shown in Figure 3 showing the sectional view of joint 1 , the cross arm body 2 comprises the main body 2a, a first shaft 5 and a second shaft 6. The two shafts 5, 6 are rotatably mounted on the main body 2a by means of respective pairs of bearings 7, 8 so that each shaft 5, 6 is rotatable around its respective axis “X”, “Y”.

[0036] Each shaft 5, 6 is provided, at its ends, with connecting means 9, in the form of holes, intended for the connection to the respective forked body 3, 4. In greater detail, each forked body 3, 4 has a connecting portion 3a, 4a intended for a head connection with the respective mechanical member, by means of screws for example, and a pair of arms 3b, 4b connected to the connecting portion and intended to be coupled at opposite ends of a respective shaft 5, 6, also here by means of screws for example.

[0037] Figure 4 shows a perspective view of the cross arm body 2 in a configuration suitable for an installation, while Figure 5 shows a partial sectional view of the same cross arm body 2.

[0038] In greater detail, the cross arm body 2 is configured so that the first shaft 5 is received in a corresponding first through hole 10 of the main body 2a and the second shaft 6 is received in a corresponding second through hole 11 of the main body 2a, where the first and second through holes 10, 11 develop around the first and second axes “X”, “Y”, respectively. The two through holes 10, 11 are therefore skew to each other (perpendicular to and spaced apart from each other) and preferably have a circular section. Preferably, the two through holes 10, 11 are arranged at a minimum mutual distance between 1 and 10 mm (measured as the minimum distance between the inner cylindrical surfaces of the through holes).

[0039] Moreover, preferably the main body 2a has a first portion 12 containing the first through hole 10 and a second portion 13 containing the second through hole 11 , such portions 12, 13 being integral with each other. More preferably, the main body 2a is a monobloc body, in particular obtained as a monobloc body from a melting process.

[0040] More preferably, the main body 2a is made from a non-ferromagnetic material whose specific weight is less than that of steel, preferably aluminium.

[0041] Preferably, each of the two portions 12, 13 of the main body 2a ends with a rounded external surface whose profile extends around axis “X”, “Y” of the respective through hole 10, 11 and, in particular, which has a cylindrical shape arranged coaxial with said axis “X”, “Y”.

[0042] Moreover, the main body 2a has beveled outer edges, in particular a single continuous edge 14 running around the two through holes 10, 11. Moreover, as shown in Figure 5, the main body 2a is provided, on each end of each through hole 10, 11 , with an annular seat 15 for receiving the outer ring 7a, 8a of a respective rolling bearing 7, 8 intended to engage, on its inner ring 7b, 8b, a corresponding portion of a respective shaft 5, 6, and in particular, a corresponding end portion of shaft 5, 6.

[0043] Preferably, each shaft 5, 6 has a pair of shoulders 16 mutually facing away from and perpendicular to the relative axis “X”, “Y”, defined preferably by a central zone of shaft 5, 6 which is larger in diameter than the end portions of shaft 5, 6. The mutual distance between said shoulders 16, measured along axis “X”, “Y”, is identical to the distance between the annular seats 15 of the corresponding through hole 10, 11 , in particular measured on the bottom surfaces of the annular seats 15 perpendicular to axis “X”, “Y” and intended to receive the outer ring 7a, 8a of the respective rolling bearing 7, 8 in abutment against it. The term “identical” in this situation means the same value from a nominal viewpoint, with a predetermined tolerance preferably equal to a machining tolerance (preferably between ±1 pm and ±5 pm).

[0044] Preferably, the rolling bearings 7, 8 are oblique ball bearings.

[0045] Moreover, preferably the ball bearings 7, 8 of each pair are mounted in an “O” configuration. This means that the rolling track of the outer ring of each oblique bearing defines an inclined thrust on the balls and the directions of the thrusts of the four outer rings of the four bearings applied to the same shaft form a rhombus (or “O”).

[0046] Moreover, preferably the outer rings 7a, 8a of the rolling bearings 7, 8 are mounted in the seats 15 by forced or friction fitting, in particular without tightening elements. Such outer rings 7a, 8a abut against the bottom surfaces of the annular seats 15. Moreover, the inner rings 7b, 8b of the rolling bearings 7, 8 are packed closely together on respective shoulders 16 of the respective shaft 5, 6 by axial compression exerted by the respective forked body 3, 4 mounted at the ends of shaft 5, 6 via the connecting means 9 (Figure 3).

[0047] Due to the perfect identity between the distance between the shoulders 16 and the distance between the bottom surfaces of the annular seats 15, the bearings 7, 8 are perfectly abutting both on shaft 5, 6 and on the main body 2a, avoiding therefore the formation of backlash.

[0048] As shown in Figure 3 moreover, each of the arms 3b, 4b of the forked bodies 3, 4 has a protrusion 17, preferably circular in shape, extending axially (along the respective axis “X”, “Y”) towards the other arm 3b, 4b, while inserting, preferably with zero backlash, into the inner ring 7b, 8b of the respective rolling bearing 7, 8. Thereby, the forked bodies 3, 4 can be positioned correctly and in a specific manner with respect to the bearings 7, 8, and therefore with respect to the main body 2a.

[0049] Moreover, each of the arms 3b, 4b of the forked bodies 3, 4 has respective through holes that can be aligned with the connecting means 9 (holes) present on the end of shaft 5, 6 so that the use of threaded members defines a stable connection of the arms 3b, 4b to the respective shaft 5, 6. In greater detail, in the specific embodiment shown, the connecting means 9 are in the form of through holes for the entire length of shaft 5, 6 so that the threaded members intended for the connection between the forked bodies 3, 4 and the respective shaft 5, 6 can extend between the first and second arms 3b, 4b of each forked body 3, 4. Moreover, the forked body 3, 4 can be separable into two parts (preferably substantially symmetrical), each of which containing one of the two arms 3b, 4b so as to allow an easy assembly and disassembly of the forked body 3, 4 on the respective bearing 7, 8 when the bearings 7, 8 are already inserted in the annular seats 15.

[0050] In the specific embodiment, the connecting means 9 are defined by four through holes for the entire axial length of shaft 5, 6 and preferably are arranged in a square formation.

[0051] According to a method according to the invention, in order to make a cross arm body 2 of the type described above, it is provided to start from a monobloc body, for example made of a material of the type mentioned above, preferably aluminium (Figures 10-11 ).

[0052] The method provides making, on the monobloc body, two pairs of annular seats 15 so that the annular seats 15 of a first pair are coaxial around the first axis “X” and the annular seats 15 of the second pair are coaxial around the second axis “Y”. Preferably, the annular seats 15 of each pair are identical in size and made on the opposite sides of the monobloc body.

[0053] Making the annular seats 15 can also result in making, when provided, a recessed central zone 21 in proximity of axis “X”, “Y”, i.e. recessed with respect to the outer shape of the monobloc body. However, in such a situation, the annular seats 15 are more recessed than such a recessed central zone 21 , therefore defining a more internal bottom zone with respect to the zone arranged around axis “X”, “Y” (Figures 8-9; Figures 12- 13).

[0054] The method further provides making, before or after making the annular seats 15, the connecting means 9 in the form of one or more holes, preferably through-holes, arranged around the respective axis “X”, “Y” and on both sides opposite to the monobloc body, in such a position that the connecting means 9 are included again in the annular seats 15.

[0055] Then (Figures 14- 15) a through-cut is made on the monobloc body along a first circular profile extending around the first axis “S” and in the annular seats 15 of the first pair of annular seats 15 to obtain the first shaft 5, and along a second circular profile extending around the second axis “Y” and in the annular seats 15 of the second pair of annular seats 15 to obtain the second shaft 6, so that the first and second shafts 5, 6 entirely contain (on their head surfaces) the respective connecting means 9.

[0056] Such a through-cut is preferably made by means of wire electro discharge machining. In such a solution, it is possible to exploit one of the holes defining the connecting means 9 passing along the entire axial length of shaft 5, 6, as a wire insertion point. Then, the wire is moved in outward radial direction up to reaching the preset cutting position and then moved along the circular cutting trajectory (the accompanying drawings show such a solution, with radial cutting line at one of the four holes).

[0057] According to a variant embodiment not shown, it can be instead provided to make an additional little hole, sufficient in size for the wire to pass, from which the cutting line then starts.

[0058] Preferably, the step of making the annular seats 15 of each pair, and more preferably the annular seats 15 of both pairs, is carried out entirely by stock removal, using a numeric control machine without intermediate steps of moving the monobloc body. In other words, the monobloc body is positioned on a machine tool table and kept fastened for the whole time the annular seats 15 are being made. Thereby, the annular seats (which, once the cut is made, define the axial abutments of the inner ring and of the inner ring of each bearing) are made while keeping the machining references unaltered. This results in the axial abutments of the inner ring of each of the two bearings 7, 8 of the same shaft 5,6, defined by respective parts of the same annular seat 15 after the cut is made, being surely arranged at the same distance at which the axial abutments of the outer ring of each of the two bearings 7, 8 are arranged, since they are simultaneously made.

[0059] This is ensured by the fact that the method according to the invention provides making the through cut of the monobloc body, in particular by means of wire electro discharge machining, after the steps of making the pairs of annular seats 15 and making the connecting means 9.

[0060] Moreover, the cut is made on a cutting line dividing the annular seats into two annular portions, where the innermost portion (towards the axis) defines the abutment for the inner ring 7b, 8b of the respective bearing 7, 8, while the outermost portion (away from the axis) defines the abutment for the outer ring 7a, 8a of the respective bearing 7, 8 (Figure 9).

[0061] Moreover, due to the making from a monobloc body, the main body 2a and the shafts 5, 6 are made of the same material that not only is identical in terms of type, but forms part of the same melting block.

[0062] Due to the particular implementation of wire electro discharge machining, it is possible to ensure that the inner diameter of each through-hole and the outer diameter of the corresponding shaft differ from each other by an amount comprised between 0.3 mm and 0.8 mm.

[0063] Moreover, according to an aspect of the invention, each shaft 5, 6 is provided with a first graphical mark 18 obtained by engraving or printing, preferably laser engraving, while the main body 2a has, in proximity of each through hole 10, 11 or in each through hole 11 , 12, a corresponding second graphical mark 19 obtained by engraving or printing, preferably laser engraving. The first graphical mark 18 of each shaft 5, 6 is uniquely referable, preferably identical to or coincident with, the corresponding second graphical mark 19 present on the main body 2a. Thereby, it is possible to uniquely refer each shaft to the respective main body 2a simply by verifying the matching of the graphical marks 18, 19. Figures 6 and 7 show such marks, made on the head surface of shaft 5, 6 and in the annular seat 15 (for this purpose, the rolling bearings 7, 8 were omitted in these views).

[0064] Additionally, since such graphical marks can be concealed in the mounting configuration of the rolling bearings 7, 8 and of the forked bodies 3, 4, it is possible to provide an additional graphical mark 20 (different from the first and second graphical marks or identical to them) on a surface external to the main body 2a, normally visible also during the regular operability of the cardan joint 1 .

[0065] Preferably, each graphical mark is an alphanumeric code. The present invention achieves the suggested objects, overcoming the drawbacks reported in the prior art.

[0066] The making of the annular seats for the bearings, intended to define both the abutments of the inner rings and the abutments of the outer rings of the bearings, allows ensuring a perfect abutment of the bearings both on the shoulders of the shaft and in the annular seats of the main body, thus avoiding the formation of backlashes.

[0067] Moreover, the direct machining of a monobloc body without changing machining references and without the intermediate movement of the piece allows avoiding the complex magnetic blocking operations of the prior art, allowing to make the components of the cardan joint from nonferromagnetic material (aluminium, for example).

Claims

CLAIMS1. A cross arm body (2) for a cardan joint (1 ) of a delta robot (100), comprising- a main body (2a) having a first and a second through hole (10, 11 ) whose respective axes (X, Y) are skew to each other, in particular perpendicular and offset;- a first and a second shaft (5, 6), each rotatably insertable into a respective one of said through holes (10, 11 ); wherein each shaft (5, 6) is provided, at the ends of it, with connecting means (9) for fastening to a respective forked body (3, 4); and wherein said main body (2a) is provided, on each end of each through hole (10, 11 ), with an annular seat (15) for receiving the outer ring (7a, 8a) of a rolling bearing (7, 8) intended to engage, on the inner ring (7b, 8b) of the rolling bearing (7, 8), a corresponding end portion of one of said shafts (5, 6).

2. The cross arm body according to claim 1 , wherein the main body (2a) is a monolithic body.

3. The cross arm body according to claim 1 or 2, wherein the main body (2a) is made from a non-ferromagnetic material whose specific weight is less than that of steel, preferably aluminium.

4. The cross arm body according to any one of the preceding claims, wherein each shaft (5, 6) has a pair of shoulders (16), defined preferably by a central zone of the shaft (5, 6) which is larger in diameter than the end portions of the shaft (5, 6), and wherein the mutual distance between said shoulders (16), measured along the axis (X, Y) is identical to the distance between the annular seats (15) of the corresponding through hole (10, 11 ), in particular measured on the bottom surfaces of the annular seats (15), intended to receive the outer ring (7a, 8a) of the respectiverolling bearing (7, 8) in abutment against it.

5. The cross arm body according to any one of the preceding claims, wherein each shaft (5, 6) is provided with a first graphical mark (18), obtained by engraving or printing, preferably laser engraving, and wherein the main body (2a) is provided, in proximity to each through hole (10, 11 ) or in each through hole (10, 11 ), with a corresponding second graphical mark (19), obtained by engraving or printing, preferably laser engraving; the first graphical mark (18) of each shaft (5, 6) being uniquely referable, preferably identical to or coincident with, the corresponding second graphical mark (19) present on the main body (2a).

6. The cross arm body according to claim 5, wherein the graphical mark (18, 19) is an alphanumeric code.

7. The cross arm body according to any one of the preceding claims, wherein the main body (2a) has a first portion (12), containing said first through hole (10), and a second portion (13), containing said second through hole (11 ), each of said portions (12, 13) ending with a rounded external surface whose profile extends around the axis (X, Y) of the respective through hole (10, 11 ) and, in particular, which has a cylindrical shape coaxial with the axis (X, Y).

8. The cross arm body according to any one of the preceding claims, wherein the inside diameter of each through hole (10, 11 ) and the outside diameter of the corresponding shaft (5, 6) differ from each other by a quantity between 0.3 mm and 0.8 mm.

9. The cross arm body according to any one of the preceding claims, wherein the main body (2a) and said shafts (5, 6) are made from the same material and, in particular, are obtained by cutting a single monobloc body,preferably by wire electro discharge machining.

10. A cardan joint (1 ), comprising:- a cross arm body (2) according to any one of the preceding claims; - a first pair of rolling bearings (7, 8), each inserted with its outer ring (7a, 8a) in a respective annular seat (15) associated with the first through hole (10);- a second pair of rolling bearings (7, 8), each inserted with its outer ring (7a, 8a) in a respective annular seat (15) associated with the second through hole (11 );- a first forked body (3), applicable to a first mechanical member and connected to the first shaft (5) by said connecting means (9);- a second forked body (4), applicable to a second mechanical member and connected to the second shaft (6) by said connecting means (9); wherein each shaft (5, 6) is inserted in the respective through hole (10, 11 ) and inserted in the inner rings (7b, 8b) of the respective rolling bearings (7, 8).11 . The cardan joint according to claim 10, wherein the rolling bearings (7, 8) are oblique ball bearings and wherein the bearings (7, 8) of each pair are mounted in an "O" configuration.

12. The cardan joint according to claim 10 or 11 , wherein the outer rings (7a, 8a) of the rolling bearings (7, 8) are mounted in zero-backlash annular seats (15), in particular by forced or friction fitting, and more particularly without tightening elements, and wherein the inner rings (7b, 8b) of the rolling bearings (7, 8) are packed closely together on respective shoulders (16) of the respective shaft (5, 6) by axial compression exerted by the respective forked body (3, 4) mounted at the ends of the shaft (5, 6) via said connecting means (9).

13. A method for making a cross arm body (2) according to any one of claims 1 to 9, comprising the following steps:- providing a monobloc body;- making two pairs of annular seats (15) in the monobloc body, wherein the annular seats (15) of a first pair are coaxial around a first axis (X) and wherein the annular seats (15) of the second pair are coaxial around a second axis (Y), which is skew relative to the first axis (X), in particular perpendicular and offset relative to the first axis (X), and wherein the annular seats (15) are identical in size and made on opposite sides of the monobloc body;- making connecting means (9) in the form of one or more holes, preferably through holes, around each axis (X, Y) and on both of the opposite sides of the monobloc body;- making a cut through the monobloc body along a first circular profile extending around the first axis (X) and at the annular seats (15) of the first pair of annular seats (15) to obtain said first shaft (5) and along a second profile extending around the second axis (Y) and at the annular seats (15) of the second pair of annular seats (15) to obtain said second shaft (6), so that said first and second shafts (5, 6) entirely contain the respective connecting means (6).

14. The method according to claim 13, wherein the step of making the cut through the monobloc body along the first circular profile and along the second circular profile is carried out by wire electro discharge machining.

15. The method according to claim 13 or 14, wherein the step of making the annular seats (15) of each pair, and preferably the annular seats (15) of both pairs, is carried out entirely by stock removal, using a numeric control machine without intermediate steps of moving the monobloc body.

16. The method according to any one of claims 13 to 15, wherein the stepof making the cut through the monobloc body is carried out after the steps of making the pairs of annular seats (15) and of making the connecting means (9).

17. The method according to any one of claims 13 to 15, comprising a step of making, on said monobloc body, at least two first graphical marks (18), in particular by engraving or printing, and at least a corresponding second graphical mark (19), in particular by engraving or printing, said graphical marks (18, 19) being disposed in such a way that after the step of making the cut through the monobloc body along the first circular profile and along the second circular profile, said first and second shafts (5, 6) are each provided with a first graphical mark (18) and said main body (2a) is provided with said second graphical mark (19); the first graphical mark (18) of each shaft (5, 6) being uniquely referable, preferably identical to or coincident with, the corresponding second graphical mark (19) present on the main body (2a).

Images