Universal feeding frame for preassembling sizing material before substrate packaging

Abstract

The universal feeding frame comprises a frame body, an opening and closing disc, a telescopic assembly and an opening and closing assembly, the opening and closing disc, the telescopic assembly and the opening and closing assembly are installed on the frame body, and the opening and closing disc comprises a left half disc and a right half disc which are controlled by the opening and closing assembly to be separated in the back-to-back direction and clamped in the opposite direction; the left half disc and the right half disc are opened and closed to form a disc cavity which is provided with an upward opening and is used for placing a columnar sizing material, the telescopic assembly comprises two scissor fork type telescopic frames, the number of the telescopic frames is two, the telescopic frames are symmetrically installed on the two sides of the frame body, and the left half disc and the right half disc of each opening and closing disc are installed on crossed shafts of the telescopic frames on the two sides respectively. Compared with the prior art, the universal feeding frame has the advantages that the universal feeding frame for preassembling the sizing material before the substrate is packaged can adjust the spacing of the preassembled columnar sizing material according to various spacing requirements, and after the spacing is well adjusted and fixed, only emptying and blanking operations need to be executed, so that compared with the prior art, the operation procedure is simplified, and the working efficiency of sizing material filling is remarkably improved; the row pitch operation of the opening and closing disc and the opening and closing operation of the opening and closing disc are very simple.

Description

Technical Field

[0001] The invention relates to a universal loading rack for pre-installing adhesive before packaging a substrate, belonging to the technical field of chip packaging. Background Art

[0002] After the chip is mounted on the substrate, it needs to be injection molded and packaged in the mold. Generally, a number of chips are mounted in rows and columns on a substrate. During injection molding, the substrate with the chips mounted is placed in the mold cavity, and then heated and injected. The glue (epoxy resin) used for injection molding is initially a short cylindrical shape, generally with a diameter of more than ten millimeters and a height of about twenty millimeters. On the upper mold of the mold, there is a row of feed holes (POT) set at equal intervals. There are generally 8 or 9 feed holes in a row. Before heating and injection molding, 8 or 9 columnar glues need to be filled into the feed holes respectively, and then pressed into the mold runner into the mold cavity after being melted at high temperature.

[0003] Put the columnar rubber into the feed hole. In non-automated (robot) processing workshops, manual filling is required. In order to increase the discharge speed, the traditional practice in the industry is to use a pre-loading device, which is provided with pre-loading holes whose number is consistent with the number of feed holes on the upper mold, and a support plate is arranged under the pre-loading holes. Now the pre-loading device is placed above the upper mold so that each pre-loading hole is located directly above the corresponding feed hole, and then the columnar rubber is put into the pre-loading hole, and then the support plate is removed so that the columnar rubber falls into the corresponding feed holes at the same time.

[0004] The drawback of the above method is that the pre-installed holes are fixed and cannot be changed according to the spacing of the mold feed holes. Therefore, it is often necessary to prepare pre-installed devices with various pre-installed hole spacings, which undoubtedly increases the purchase cost of the pre-installed devices.

[0005] To solve the above problems, the patent document with application number 202211377354.9 discloses a dual-axis adjustable spacing glue cake picking and placing head device and a semiconductor plastic packaging machine, including a sliding seat, a guide post and a guide sleeve, a spacing adjustment component and a clamping claw group, the guide post is slidably connected to the sliding seat, and the guide sleeve is rotatably connected to the guide post, the guide post and the guide sleeve are used to realize the lifting and rotation of the picking and placing head, the spacing adjustment component includes an adjusting frame, a scissor rod group and a ball screw, the adjusting frame is rotatably connected to the guide sleeve, the ball screw is rotatably connected to the adjusting frame, the scissor rod group is connected to the nut of the ball screw, and can be extended and retracted following the rotation of the screw, the clamping claw group includes multiple pneumatic clamps, the pneumatic clamps are connected to the scissor rod group, and can be equidistantly adjusted along with the extension and retraction of the scissor rod group, so that the glue cake picking and placing head device can clamp the glue cakes on barrels with different spacings and place the glue cakes on lead frames with different placement methods.

[0006] The defect of the technical solution provided by the patent document is that multiple pneumatic grippers cannot accurately grab one from the scattered columnar rubber materials at the same time. The columnar rubber materials to be grabbed need to be placed accurately under the corresponding grippers first, and then grabbed by the grippers and moved to the corresponding position and then put down. This is actually equivalent to adding a procedure of grabbing and moving the grabbed columnar rubber materials to just above the corresponding position, which greatly reduces the loading speed. At the same time, the setting of the grippers requires the installation of a pneumatic device and multiple air supply pipelines, which complicates the device. Summary of the invention

[0007] The technical problem to be solved by the present invention is: how to realize the rapid adjustment of the material discharge spacing and simplify the filling procedure.

[0008] In view of the above problems, the technical solution proposed by the present invention is: A universal loading rack for pre-loading adhesive before packaging a substrate comprises a rack body, and an opening and closing disk, a telescopic assembly and an opening and closing assembly mounted on the rack body. The opening and closing disk comprises a left half disk and a right half disk which can be separated in opposite directions and clamped in opposite directions under the control of the opening and closing assembly. When the left half disk and the right half disk are opened and closed, a disk cavity with an opening facing upward is formed for placing columnar adhesive. The telescopic assembly comprises a scissor-type telescopic frame. There are two telescopic frames which are symmetrically mounted on both sides of the rack body. The left half disk and the right half disk of each opening and closing disk are respectively mounted on the cross axes of the telescopic frames on both sides.

[0009] The opening and closing assembly includes a rectangular frame whose number corresponds to the number of the opening and closing disks, and a left sleeve and a right sleeve passing through the frame body are respectively provided on the left and right sides of the rectangular frame, and a left telescopic rod and a right telescopic rod are respectively fixed on the left side of the left half disk and the right side of the right half disk, and the cross axis of the telescopic frame is a tubular cross axis with a tube hole, and the opening and closing disk is installed in the rectangular frame and can be separated and opened and closed in the rectangular frame, and the left telescopic rod and the right telescopic rod are respectively sleeved in the left sleeve and the right sleeve, and the left sleeve and the right sleeve are respectively sleeved in the tube hole of the tubular cross axis of the left telescopic frame and the tube hole of the tubular cross axis of the right telescopic frame, and the left telescopic rod and the right telescopic rod extend out of the tube hole of the left tubular cross axis and the tube hole of the right tubular cross axis respectively.

[0010] The frame includes a left beam and a right beam, and a left through slot and a right through slot communicating with each other are respectively provided on the left beam and the right beam. The rectangular frame of the opening and closing assembly is located between the left beam and the right beam, and the left sleeve and the right sleeve of the rectangular frame respectively pass through the left through slot to the left and the right through slot to the right and can slide back and forth in the left through slot and the right through slot.

[0011] The two telescopic frames are respectively installed on the left side of the left beam and the right side of the right beam.

[0012] The opening and closing assembly also includes a left control beam and a right control beam for respectively controlling the left and right movements of all left half-disks and all right half-disks, the right side of the left control beam and the left side of the right control beam are respectively provided with a T-shaped groove consisting of a horizontal groove and a vertical groove, the left end of the left telescopic rod and the right end of the right telescopic rod are respectively provided with a rectangular flange protruding radially outward, the left end of the left telescopic rod and the right end of the right telescopic rod are respectively inserted into the T-shaped grooves of the left control beam and the right control beam and can slide in the T-shaped grooves, and the rectangular flanges of the left end of the left telescopic rod and the right end of the right telescopic rod are located in the vertical groove.

[0013] The opening and closing assembly also includes a left opening and closing spring and a right opening and closing spring. The left opening and closing spring applies rightward pressure to the left control beam, and the right opening and closing spring applies leftward pressure to the right control beam, so that the left half disk and the right half disk are closed into an opening and closing disk.

[0014] The opening and closing assembly also includes a left outer beam arranged outside the left side of the left control beam and a right outer beam arranged outside the right side of the right control beam. The left opening and closing spring is located between the left outer beam and the left control beam, and the right opening and closing spring is located between the right outer beam and the right control beam.

[0015] The opening and closing assembly also includes electromagnets arranged on the left outer beam and the right outer beam. The left control beam and the right outer beam are iron beams. The electromagnets on the left outer beam and the right outer beam that are energized can form attractive forces on the left control beam and the right outer beam respectively, so that the left control beam and the right outer beam can simultaneously overcome the elastic resistance of the left opening and closing spring and the right opening and closing spring respectively and separate in opposite directions.

[0016] The electromagnets arranged on the left outer beam and the right outer beam are arranged on the right side of the left outer beam and the left side of the right outer beam respectively.

[0017] The telescopic assembly also includes a threaded pull rod and a wheel with a threaded hole on the axis. The rear end of the threaded pull rod is connected to the rectangular frame at the front end. The wheel is screwed onto the front end of the threaded pull rod through the threaded hole. A front limit plate and a rear limit plate are respectively provided in front and behind the wheel between the left beam and the right beam to prevent the wheel from moving forward and backward when rotating. Beneficial Effects

[0018] 1. The universal loading rack used for pre-loading adhesive before substrate packaging can adjust the spacing of pre-loaded columnar adhesive according to various spacing requirements. After the spacing is adjusted and fixed, only the material placement and blanking operations need to be performed. Compared with the existing technology, the operation procedure is simplified and the work efficiency of loading adhesive is significantly improved; 2. The spacing operation of the opening and closing disk and the opening and closing operation of the opening and closing disk are very simple. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 It is a three-dimensional schematic diagram of a universal loading rack; Figure 2 for Figure 1A partial schematic diagram showing the front portion of the universal loading rack; Figure 3 for Figure 1 A partial schematic diagram showing the rear portion of the universal loading rack; Figure 4 for Figure 1 Schematic diagram of the decomposition; Figure 5 for Figure 4 A partial schematic diagram of Figure 6 It is a cross-sectional schematic diagram of the left side of the universal loading rack; Figure 7 is a three-dimensional schematic diagram of the opening and closing disk 35; Figure 8 It is a three-dimensional schematic diagram of the opening and closing disk being installed in a rectangular frame.

[0020] In the figure: 1, frame; 101, left beam; 1011, left through slot; 102, right beam; 1021, right through slot; 2, opening and closing disk; 201, left half disk; 202, right half disk; 203, disk cavity; 3, rectangular frame; 4, telescopic frame; 401, tubular cross axis; 5, left control beam; 6, right control beam; 7, T-shaped slot; 701, horizontal slot; 702, vertical slot; 8, left sleeve; 9, right sleeve; 10, left telescopic rod; 11, right telescopic rod; 12, rectangular flange; 13, threaded pull rod; 14, wheel disc; 15, front limit plate; 16, rear limit plate; 17, left opening and closing spring; 18, right opening and closing spring; 19, left outer beam; 20, right outer beam; 21, electromagnet; DETAILED DESCRIPTION

[0021] The present invention will be further described below in conjunction with embodiments and drawings: like Figure 1 , 7As shown, a universal loading rack for pre-loading adhesive before packaging of substrates includes a frame body 1, and an opening and closing plate 2, a telescopic assembly and an opening and closing assembly installed on the frame body 1. The opening and closing plate 2 includes a left half plate 201 and a right half plate 202 which can be separated back to back and clamped toward each other under the control of the opening and closing assembly. When the left half plate 201 and the right half plate 202 are opened and closed, a plate cavity 203 with an opening facing upward is formed for placing columnar adhesive. The telescopic assembly includes a scissor-type telescopic frame 4. There are two telescopic frames 4, which are symmetrically installed on both sides of the frame body 1. The left half plate 201 and the right half plate 202 of each opening and closing plate 2 are respectively installed on the cross axes of the telescopic frames 4 on both sides. When in use, the loading rack is placed above the injection mold, and according to the spacing of a row of columnar glue feed ports on the mold, by stretching or compressing the scissor-type telescopic rack 4, the spacing between the opening and closing disks 2 can be evenly increased or decreased, so that each opening and closing disk 2 is located directly above the corresponding feed port, and then the columnar glue (or other shapes such as spherical glue) is placed in the disk cavity 203, and then the opening and closing component controls the left half disk 201 and the right half disk 202 of the opening and closing disk 2 to separate in reverse, so that the columnar glue in the disk cavity 203 falls into the feed port. In this way, the universal loading rack used for pre-loading glue before the substrate packaging can adjust the spacing of the pre-loaded columnar glue according to various spacing requirements, and after the spacing is adjusted and fixed, only the material discharge and material drop operations need to be performed, which simplifies the operation procedure and significantly improves the work efficiency of loading glue compared to the prior art.

[0022] Preferably, the inner wall of the disc cavity 203 is configured to be conical in shape, so as to facilitate the placement of rubber materials of various specifications and shapes.

[0023] Here are further improvements.

[0024] like Figure 1 , 2 As shown in Figures 3, 7 and 8, the opening and closing assembly includes rectangular frames 3 corresponding in number to the number of opening and closing discs 2, and a left sleeve 8 and a right sleeve 9 passing through the frame are respectively provided on the left and right sides of the rectangular frame 3, a left telescopic rod 10 and a right telescopic rod 11 are respectively fixed to the left side of the left half disc 201 and the right side of the right half disc 202, the cross axis of the telescopic frame 4 is a tubular cross axis 401 with a tube hole, the opening and closing disc 2 is installed in the rectangular frame 3 and can be separated and opened and closed in the rectangular frame 3, the left telescopic rod 10 and the right telescopic rod 11 are respectively sleeved in the left sleeve 8 and the right sleeve 9, the left sleeve 8 and the right sleeve 9 are respectively sleeved in the tube hole of the tubular cross axis 401 of the left telescopic frame 4 and the tube hole of the tubular cross axis 401 of the right telescopic frame 4, the left telescopic rod 10 and the right telescopic rod 11 extend out of the tube hole of the left tubular cross axis 401 and the tube hole of the right tubular cross axis 401 respectively. That is, the opening and closing of the left half plate 201 and the right half plate 202 are realized by pushing and pulling the left telescopic rod 10 and the right telescopic rod 11, and the rectangular frame 3 makes the opening and closing plate 2 composed of the left half plate 201 and the right half plate 202 have integrity.

[0025] like Figure 4 As shown in FIG. 8 , the frame 1 includes a left side beam 101 and a right side beam 102, and a left through slot 1011 and a right through slot 1021 communicating with each other are respectively provided on the left side beam 101 and the right side beam 102. The rectangular frame 3 of the opening and closing assembly is located between the left side beam 101 and the right side beam 102, and the left sleeve 8 and the right sleeve 9 of the rectangular frame 3 respectively pass through the left through slot 1011 to the left and the right through slot 1021 to the right and can slide back and forth in the left through slot 1011 and the right through slot 1021.

[0026] In this way, the spacing between the rectangular frames 3 can be arranged in the cavity between the left beam 101 and the right beam 102 , and the opening and closing of the opening and closing disk 2 can be operated outside the left beam 101 and the right beam 102 .

[0027] The two telescopic frames 4 are respectively installed on the left side of the left beam 101 and the right side of the right beam 102 .

[0028] The opening and closing assembly also includes a left control beam 5 and a right control beam 6 for respectively controlling the left and right movements of all left half disks 201 and all right half disks 202. A T-shaped groove 7 consisting of a horizontal groove 701 and a vertical groove 702 is respectively provided on the right side of the left control beam 5 and the left side of the right control beam 6. The left end of the left telescopic rod 10 and the right end of the right telescopic rod 11 respectively have a rectangular flange 12 protruding radially outward. The left end of the left telescopic rod 10 and the right end of the right telescopic rod 11 are respectively inserted into the T-shaped groove 7 of the left control beam 5 and the right control beam 6 and can slide in the T-shaped groove 7. The rectangular flange 12 of the left end of the left telescopic rod 10 and the right end of the right telescopic rod 11 is located in the vertical groove 702. By moving the left control beam 5 and the right control beam 6 to the left or right, all the opening and closing disks 2 can be opened and closed synchronously at one time. The rectangular flange 12 is located in the vertical groove 702 , and is used to limit the left and right telescopic rods 10 and 11 , and to prevent the left and right telescopic rods 10 and 11 from rotating.

[0029] To prevent the rectangular frame 3 from rotating, an anti-rotation structure is provided between the left telescopic rod 10 and the left sleeve 8 and between the right telescopic rod 11 and the right sleeve 9, such as a conventional ridge axially arranged outside the rod body and a groove axially arranged inside the tube wall to accommodate the ridge (not shown in the figure), and the ridge can slide axially in the groove.

[0030] like Figure 1 —6, further improvement: the opening and closing assembly also includes a left opening and closing spring 17 and a right opening and closing spring 18, the left opening and closing spring 17 applies a rightward pressure to the left control beam 5, and the right opening and closing spring 18 applies a leftward pressure to the right control beam 6, so that the left half disk 201 and the right half disk 202 can automatically close into the opening and closing disk 2 and maintain the open and closed state, and the elastic pressure of the left opening and closing spring 17 and the right opening and closing spring 18 can be overcome by force to make the left control beam 5 and the right control beam 6 separate back to back.

[0031] The opening and closing assembly also includes a left outer beam 19 arranged on the left side of the left control beam 5 and a right outer beam 20 arranged on the right side of the right control beam 6. The left opening and closing spring 17 is located between the left outer beam 19 and the left control beam 5, and the right opening and closing spring 18 is located between the right outer beam 20 and the right control beam 6.

[0032] like Figure 1 As shown in FIG. 6 , further improvement is made: the opening and closing assembly further includes an electromagnet 21 arranged on the left outer beam 19 and the right outer beam 20. The left control beam 5 and the right outer beam 20 are iron beams. The electromagnet 21 on the left outer beam 19 and the right outer beam 20 can respectively form an attraction force on the left control beam 5 and the right outer beam 20, so that the left control beam 5 and the right outer beam 20 can simultaneously overcome the elastic resistance of the left opening and closing spring 17 and the right opening and closing spring 18 and separate in opposite directions. The opening and closing assembly further includes a power button for controlling the on and off of the power supply circuit of the electromagnet 21. In this way, as long as the power button is pressed, the left half disk 201 and the right half disk 202 of all the opening and closing disks 2 can be synchronously separated at the moment of power on, so that the rubber material can be dropped quickly and smoothly.

[0033] Further, the electromagnets 21 provided on the left outer beam 19 and the right outer beam 20 are respectively provided on the right side of the left outer beam 19 and the left side of the right outer beam 20. In this way, the electromagnet 21 on the left side can be closer to the left control beam 5 to increase the attraction to the left control beam 5, and the electromagnet 21 on the right side can be closer to the right control beam 6 to increase the attraction to the right control beam 6.

[0034] The telescopic assembly also includes a threaded tie rod 13 and a wheel 14 with a threaded hole in the axis. The rear end of the threaded tie rod 13 is connected to the rectangular frame 3 at the front end. The wheel 14 is screwed onto the front end of the threaded tie rod 13 through the threaded hole. A front stop plate 15 and a rear stop plate 16 are respectively provided in front and back of the wheel 14 between the left beam 101 and the right beam 102 to prevent the wheel 14 from moving forward and backward when rotating. In this way, the spacing of the opening and closing discs 2 can be achieved by rubbing the wheel 14 with fingers.

[0035] The above embodiments are only used to more clearly describe the present invention and cannot be regarded as limiting the protection scope of the present invention. Any modifications in equivalent forms should be regarded as falling within the protection scope of the present invention.

Claims

1. A universal loading rack for pre-loading adhesive before substrate packaging, characterized in that: The invention comprises a frame (1), an opening and closing disk (2) mounted on the frame (1), a telescopic assembly and an opening and closing assembly, wherein the opening and closing disk (2) comprises a left half disk (201) and a right half disk (202) which can be separated in opposite directions and clamped in opposite directions under the control of the opening and closing assembly, and the left half disk (201) and the right half disk (202) form a disk cavity (203) which opens upward and is used for placing columnar rubber when the left half disk (201) and the right half disk (202) are opened and closed, and the telescopic assembly comprises a scissor-type telescopic frame (4), wherein two telescopic frames (4) are symmetrically mounted on two sides of the frame (1), and the left half disk (201) and the right half disk (202) of each opening and closing disk (2) are respectively mounted on the cross axes of the telescopic frames (4) on both sides.

2. The universal loading rack for pre-loading adhesive before substrate packaging according to claim 1, characterized in that: The opening and closing assembly comprises a rectangular frame (3) whose number corresponds to the number of the opening and closing disks (2); the rectangular frame (3) has a left sleeve (8) and a right sleeve (9) passing through the frame body on the left and right sides respectively; a left telescopic rod (10) and a right telescopic rod (11) are fixed on the left side of the left half disk (201) and the right side of the right half disk (202) respectively; the cross axis of the telescopic frame (4) is a tubular cross axis (401) having a tube hole; the opening and closing disk (2) is installed in the rectangular frame (3) and can be divided in the rectangular frame (3). The left telescopic rod (10) and the right telescopic rod (11) are respectively sleeved in the left sleeve (8) and the right sleeve (9), and the left sleeve (8) and the right sleeve (9) are respectively sleeved in the tube hole of the tubular cross axis (401) of the left telescopic frame (4) and the tube hole of the tubular cross axis (401) of the right telescopic frame (4), and the left telescopic rod (10) and the right telescopic rod (11) are respectively extended out of the tube hole of the left tubular cross axis (401) and the tube hole of the right tubular cross axis (401).

3. The universal loading rack for pre-loading adhesive before substrate packaging according to claim 2, characterized in that: The frame (1) comprises a left beam (101) and a right beam (102), the left beam (101) and the right beam (102) being respectively provided with a left through slot (1011) and a right through slot (1021) communicating with each other left and right, the rectangular frame (3) of the opening and closing assembly being located between the left beam (101) and the right beam (102), the left sleeve (8) and the right sleeve (9) of the rectangular frame (3) respectively passing through the left through slot (1011) to the left and the right through slot (1021) to the right and being able to slide forward and backward in the left through slot (1011) and the right through slot (1021).

4. The universal loading rack for pre-loading adhesive before substrate packaging according to claim 3 is characterized in that: The two telescopic frames (4) are respectively installed on the left side of the left beam (101) and the right side of the right beam (102).

5. The universal loading rack for pre-loading adhesive before substrate packaging according to claim 3, characterized in that: The opening and closing assembly further comprises a left control beam (5) and a right control beam (6) for respectively controlling the left and right movements of all the left half-discs (201) and all the right half-discs (202); a T-shaped groove (7) consisting of a horizontal groove (701) and a vertical groove (702) is respectively provided on the right side of the left control beam (5) and the left side of the right control beam (6); the left end of the left telescopic rod (10) and the right end of the right telescopic rod (11) are respectively provided with a rectangular flange (12) protruding radially outward; the left end of the left telescopic rod (10) and the right end of the right telescopic rod (11) are respectively inserted into the T-shaped groove (7) of the left control beam (5) and the right control beam (6) and are capable of sliding in the T-shaped groove (7); the rectangular flanges (12) of the left end of the left telescopic rod (10) and the right end of the right telescopic rod (11) are located in the vertical groove (702).

6. The universal loading rack for pre-loading adhesive before substrate packaging according to claim 5, characterized in that: The opening and closing assembly further comprises a left opening and closing spring (17) and a right opening and closing spring (18), wherein the left opening and closing spring (17) applies rightward pressure to the left control beam (5), and the right opening and closing spring (18) applies leftward pressure to the right control beam (6), so that the left half disk (201) and the right half disk (202) are closed to form an opening and closing disk (2).

7. The universal loading rack for pre-loading adhesive before substrate packaging according to claim 6, characterized in that: The opening and closing assembly further comprises a left outer beam (19) arranged outside the left side of the left control beam (5) and a right outer beam (20) arranged outside the right side of the right control beam (6); the left opening and closing spring (17) is located between the left outer beam (19) and the left control beam (5); and the right opening and closing spring (18) is located between the right outer beam (20) and the right control beam (6).

8. The universal loading rack for pre-loading adhesive before substrate packaging according to claim 7, characterized in that: The opening and closing assembly further comprises an electromagnet (21) arranged on the left outer beam (19) and the right outer beam (20); the left control beam (5) and the right outer beam (20) are iron beams; the electromagnet (21) energized on the left outer beam (19) and the right outer beam (20) can respectively form an attractive force on the left control beam (5) and the right outer beam (20), so that the left control beam (5) and the right outer beam (20) can simultaneously overcome the elastic resistance of the left opening and closing spring (17) and the right opening and closing spring (18) and separate in opposite directions.

9. The universal loading rack for pre-loading adhesive before substrate packaging according to claim 8, characterized in that: The electromagnets (21) arranged on the left outer beam (19) and the right outer beam (20) are arranged on the right side of the left outer beam (19) and the left side of the right outer beam (20), respectively.

10. The universal loading rack for pre-loading adhesive before substrate packaging according to claim 3, characterized in that: The telescopic assembly further comprises a threaded tie rod (13) and a wheel disc (14) having a threaded hole at its axis, the rear end of the threaded tie rod (13) being connected to the rectangular frame (3) at the front end, the wheel disc (14) being screwed onto the front end of the threaded tie rod (13) through the threaded hole, and a front limit plate (15) and a rear limit plate (16) for preventing the wheel disc (14) from moving forward and backward when rotating are respectively provided in front and behind the wheel disc (14) between the left beam (101) and the right beam (102).

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