Cylindrical battery bottom welding pin device and bottom welding pin method

Abstract

The present invention belongs to the technical field of battery manufacturing equipment, and in particular, relates to a bottom welding pin device and a bottom welding pin method for cylindrical batteries. The bottom welding pin device for cylindrical batteries includes: a hole expansion pin, including at least two needle shells, each of which has a movable part movably arranged therein; an inflation mechanism, which is used to inflate the needle shells so that the movable parts in each needle shell extend out from the needle shells and abut against each other, and each needle shell spreads out and forms a hole expansion, so that the welding pin can be inserted into the hole expansion and inserted into the bottom of the winding core for bottom welding. In the bottom welding pin device for cylindrical batteries provided by the embodiment of the present invention, after the inflation mechanism inflates the needle shells, the hole expansion pin realizes the hole expansion function in the winding core through the action of gas expansion, and after the inflation is completed, each needle shell spreads out to form a hole expansion, and the welding pin can be inserted through the hole expansion until it is inserted into the bottom of the winding core, thereby completing the bottom welding operation. The overall structure of the bottom welding device is simple, the pin insertion operation is convenient, and the pin insertion phenomenon will not occur after the hole expansion, and the bottom welding yield rate is greatly improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of battery manufacturing equipment, and in particular relates to a bottom welding pin device and a bottom welding pin method for a cylindrical battery. Background Art

[0002] With the improvement of automation, the production efficiency of lithium-ion batteries has been greatly improved. However, there are still some processes that have many problems in the process of automation. For example, after the cylindrical lithium-ion battery is wound into the shell, it needs to undergo a bottom welding process. The bottom welding process is to insert the welding needle from the center hole of the battery cell to the bottom for resistance welding. Although the existing automatic shelling machine includes an automatic pin insertion process, the winding needle of the cylindrical lithium-ion battery is generally around 3.5-4mm, the center hole diameter is relatively small, and the welding needle diameter is usually around 2.5mm, which can easily cause poor automatic pin insertion, resulting in a high bottom welding defect rate. Summary of the invention

[0003] The object of the present invention is to provide a cylindrical battery bottom welding pin device, aiming to solve the technical problem of high bottom welding defects caused by poor pin insertion of cylindrical battery bottom welding equipment in the prior art.

[0004] To achieve the above purpose, the technical solution adopted by the present invention is: a cylindrical battery bottom welding pin device, comprising:

[0005] The reaming needle comprises at least two needle shells, each of which has a movable part movably arranged therein;

[0006] The inflation mechanism is used to inflate the needle shells so that the movable parts in the needle shells extend out from the needle shells and abut against each other, and the needle shells spread out to form expanded holes, so that the welding needles can be inserted into the expanded holes and inserted to the bottom of the winding core for bottom welding.

[0007] Furthermore, the cross-section of the reaming needle is circular, and the reaming needle has a variable diameter structure and its diameter gradually decreases from top to bottom.

[0008] Furthermore, the reaming needle is provided with two needle shells, an inner cavity connected to the inflation mechanism is formed in the needle shell, and the movable part is movably arranged in the inner cavity.

[0009] Furthermore, the movable part is a slider movably embedded in the inner cavity.

[0010] Furthermore, the cross-section of the needle shell is semicircular, the slider is a tetrahedron, one surface of the slider is completely accommodated in the needle shell and forms a limiting surface, and through holes are respectively opened at corresponding positions of the inner walls of the two opposite needle shells, and the through holes are connected to the inner cavity. When the inflation mechanism inflates the inner cavity, the slider extends from the through hole, and the hole wall of the through hole is respectively matched with the remaining three surfaces of the slider. The area of ​​the limiting surface is larger than the longitudinal cross-sectional area of ​​the through hole, so that one end of the slider forms a limiting end to prevent the slider from falling off the needle shell.

[0011] Furthermore, after the needle shells are spread out, the width of the reaming needle is 4.5 mm to 5 mm.

[0012] Furthermore, the cylindrical battery bottom welding pin device also includes an elastic clamping member, which includes a pressure block and a pre-tensioning spring connected to the side of the pressure block away from the needle shell, and the pre-tensioning spring presses the pressure block against the side wall of the needle shell to ensure that the central axis of the expansion needle and the central axis of the insertion hole of the winding core remain coaxial during the bottom welding process.

[0013] Furthermore, the inflation mechanism is provided with an inflation valve and an air release valve. When the inflation valve is opened, the inflation mechanism inflates air into the reaming needle to expand the needle shells of the reaming needle. When the air release valve is opened, the needle shells gather and close under the elastic support of the elastic fastening member, and the movable member retracts into the corresponding needle shell.

[0014] Another object of the present invention is to provide a cylindrical battery bottom welding pin method, using the cylindrical battery bottom welding pin device, comprising the following steps:

[0015] Inserting the reaming needle into the central hole of the winding core;

[0016] Controlling the inflation mechanism to inflate the needle shells of the reaming needles so that the reaming holes formed between the needle shells can be suitable for the insertion of the welding needles;

[0017] Insert the welding needle into the expanded hole of the expanded hole needle until it reaches the bottom of the winding core.

[0018] Furthermore, when the welding needle is inserted into the reaming hole of the reaming needle to above the slider, the inflation mechanism is controlled to discharge a portion of the gas in each needle shell, so that when the welding needle continues to be inserted downward in the reaming hole, the welding needle presses against each slider, so that the slider can move into the corresponding needle shell, thereby allowing the welding needle to be smoothly inserted to the bottom of the winding core.

[0019] Beneficial effects of the present invention: The cylindrical battery bottom welding pin device of the present invention is provided with a reaming pin, and movable parts are movably arranged in each pin shell of the reaming pin. After the inflation mechanism inflates the pin shell, the reaming pin realizes the reaming function in the winding core through the action of gas expansion. After the inflation is completed, the pin shells spread out to form an reaming hole, and the welding pin can be inserted through the reaming hole until it is inserted into the bottom of the winding core, thereby completing the bottom welding operation. The overall structure of the bottom welding device is simple, the pin insertion operation is convenient, the pin insertion will not be defective after the hole is expanded, and the bottom welding yield is greatly improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0021] Figure 1 Schematic diagram of the structure of the winding core from a top view;

[0022] Figure 2 is a cross-sectional view of the welding pin;

[0023] Figure 3 A cross-sectional view of a reaming needle in a cylindrical battery bottom welding pin device provided by an embodiment of the present invention before reaming by gas expansion;

[0024] Figure 4 For along Figure 3 Sectional view along line AA;

[0025] Figure 5 A cross-sectional view of a reaming needle in a cylindrical battery bottom welding pin device provided in an embodiment of the present invention after the reaming needle is expanded by gas expansion;

[0026] Figure 6 For along Figure 5 Sectional view along the midline BB;

[0027] Figure 7 for Figure 3 A cross-sectional view of a cylindrical battery bottom welding pin device in which a reamer pin is inserted into a winding core;

[0028] Figure 8 for Figure 3 A cross-sectional view of the cylindrical battery bottom welding pin device after the expansion pin is inserted into the winding core and expanded by air;

[0029] Fig. 9 for Figure 3 A cross-sectional view of a cylindrical battery bottom welding pin device with a welding pin inserted into an expanded hole;

[0030] Fig.10A flowchart of a cylindrical battery bottom welding pin method provided in an embodiment of the present invention.

[0031] Among them, the reference numerals in the figure are:

[0032] 10-winding core; 11-center hole; 20-welding needle; 30-expanding needle; 31-needle tip; 32-needle tail; 301-expanding hole; 310-needle shell; 311-movable part; 41-inflating valve; 42-deflation valve; 50-elastic pressing part; 51-pressure block; 52-preload spring. DETAILED DESCRIPTION

[0033] Embodiments of the present invention are described in detail below, examples of which are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are intended to be used to explain the present invention, and should not be construed as limiting the present invention.

[0034] In the description of the present invention, it should be understood that the terms "length", "width", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", etc., indicating the orientation or position relationship are based on the orientation or position relationship shown in the drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the present invention.

[0035] In addition, the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" and "second" may explicitly or implicitly include one or more of the features. In the description of the present invention, the meaning of "plurality" is two or more, unless otherwise clearly and specifically defined.

[0036] In the present invention, unless otherwise clearly specified and limited, the terms "installed", "connected", "connected", "fixed" and the like should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium, it can be the internal connection of two elements or the interaction relationship between two elements. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0037] like Figure 1 , Figure 2As shown, the winding core 10 of the circular battery is in the center hole 11 of the innermost circle, and the end of the winding core 10 forms an S-shaped winding belt, which divides the innermost circle into two parts. The insertion hole for insertion is small, and using the welding pin 20 to directly insert the hole is easy to cause poor insertion of the pin. Figures 7 to 9 As shown, the cylindrical battery bottom welding pin device provided by the embodiment of the present invention includes a reaming pin 30 and an inflation mechanism (not shown). The reaming pin 30 includes at least two pin shells 310, each of which has an inner cavity, and a movable part 311 is movably arranged in the inner cavity of each pin shell 310. The inflation mechanism can be connected to the inner cavity of the needle shell 310 of the reaming needle 30 through a pipeline. The inflation mechanism can also be fixed on the reaming needle 30 or embedded in the reaming needle 30. The inflation mechanism is used to inflate the inner cavity of the needle shell 310 of the reaming needle 30 so that the movable parts 311 in each needle shell 310 extend from the needle shell 310 and abut against each other, thereby causing each needle shell 310 to spread out and form an expanded hole 301, thereby completing the expansion operation of the center hole 11 of the winding core 10; after completing the expansion operation, the welding needle 20 can be inserted into the expanded hole 301 in the reaming needle 30. Due to the expansion effect of the gas, the expanded hole 301 is formed between the needle shells 310 of the reaming needle 30, and the welding needle 20 can be smoothly inserted into the expanded hole 301. In this way, the welding needle 20 can be easily inserted into the bottom of the winding core 10 and complete the bottom welding operation, eliminating the expansion process of the winding machine, and also avoiding the occurrence of poor pin insertion, and the bottom welding yield is greatly improved. The insertion operation of the reaming needle 30 into the central hole 11 of the winding core 10, the inflation operation of the inflation mechanism and the insertion and reaming operation of the welding needle 20 can all be automatically completed by the driving mechanism. In this way, the automatic bottom welding operation can be conveniently realized with the help of the bottom welding device.

[0038] The cylindrical battery bottom welding pin device provided in the present embodiment is provided with a reaming needle 30, and each needle shell 310 of the reaming needle 30 is respectively provided with a movable part 311. After the inflation mechanism inflates the needle shell 310, the reaming needle 30 realizes the reaming function in the winding core 10 through the action of gas expansion. After the inflation is completed, each needle shell 310 spreads out to form an reaming hole 301, and the welding needle 20 can be inserted through the reaming hole 301 until it is inserted into the bottom of the winding core 10, thereby completing the bottom welding operation. The overall structure of the bottom welding device is simple, the pin insertion operation is convenient, the pin insertion phenomenon will not occur after the hole is expanded, and the bottom welding yield rate is greatly improved.

[0039] In one embodiment, if Figures 3 to 6 As shown, the cross section of the reaming needle 30 is circular, and the reaming needle 30 is a variable diameter structure with a diameter gradually decreasing from top to bottom. The cross section of the reaming needle 30 between the reaming holes is circular. After the reaming operation is performed by inflation, each needle shell 310 expands to form an reaming hole 301 for convenient insertion of the welding needle 20. The reaming needle 30 is a variable diameter setting, and the needle tip 31 of the welding needle 20 is smaller than the needle body of the welding needle 20. The process of inserting the welding needle 20 into the reaming hole 301 is relatively smooth.

[0040] In one embodiment, the expanded width of the reaming needle 30 is 4.5 mm to 5 mm. One end of the reaming needle 30 is a needle tip 31, and the other end is a needle tail 32. Before reaming, the diameter of the needle tip 31 of the reaming needle 30 is 1 mm, and the diameter of the needle tail 32 is less than or equal to 3.5 mm. The diameter of the welding needle 20 is 2.5 mm. In this way, after reaming, the welding needle 20 can be easily inserted into the reaming hole 301.

[0041] In one embodiment, if Figure 3 , Figure 4 As shown, the reaming needle 30 is provided with two needle shells 310, and an inner cavity connected to the inflation mechanism is formed in the needle shell 310, and a movable part 311 is movably arranged in the inner cavity. The two needle shells 310 of the reaming needle 30 can be two half needles of the same size, and the cross-sections of the two half needles are both circular. Before inflation, the two half needles are pressed against each other, and the two are in a closed state, and their respective movable parts 311 are hidden in the inner cavity; Figure 5 , Figure 6 and Figure 8 As shown, after the inflation mechanism inflates each half needle, the movable parts 311 of the two half needles extend from the inner cavity and abut against each other, and the movable parts 311 cause the two half needles to expand in the central hole 11 of the winding core 10, thereby achieving hole expansion.

[0042] In one embodiment, if Figures 3 to 6 As shown, the movable member 311 is a slider movably embedded in the inner cavity of the needle housing 310. It can be understood that the movable member 311 can also adopt other shapes, such as spherical or quasi-spherical, and the contour of the corresponding position of the inner cavity is suitable for the movable member 311, and when inflated, the movable member 311 will not completely fall out of the inner cavity of the needle housing 310.

[0043] In one embodiment, if Figures 3 to 6 As shown, the cross section of the needle housing 310 is semicircular, the two needle housings 310 have the same size and contour, the slider is a tetrahedron as a whole, each needle housing 310 has an inner cavity with the same contour, and the size of the receiving cavity of the needle housing 310 is larger than the size of the slider, so that the slider can slide in the radial direction in the receiving cavity. When the inflation mechanism inflates the needle housing 310, one end of the slider extends from the receiving cavity and abuts against one end of the other slider, thereby moving the two needle housings 310 in opposite directions, so that the insertion hole of the winding core 10 can be expanded, and the welding needle 20 can be inserted between the two needle housings 310 at the same time.

[0044] It can be understood that the slider can also adopt other regular or irregular shapes, and the inner cavity of the needle shell 310 is correspondingly set to a shape that matches the contour to facilitate the sliding in and out of the slider. When each slider slides out to a predetermined position, it will abut against each needle shell 310. When continuing to slide outward, it will abut against each needle shell 310, causing each needle shell 310 to spread out, thereby completing the hole expansion requirement.

[0045] In one embodiment, if Figures 3 to 6 As shown, the cross-section of the needle shell 310 is semicircular, and the slider is a tetrahedron. One of the faces of the slider is completely accommodated in the needle shell 310 and forms a limiting surface. Through holes are respectively opened at corresponding positions of the inner walls of the two opposite needle shells 310. When the inflation mechanism inflates the inner cavity, the slider extends from the through hole, and the hole wall of the through hole is respectively matched with the other three faces of the slider. The area of ​​the limiting surface is set to be larger than the longitudinal cross-sectional area of ​​the through hole, so that one end of the slider forms a limiting end, which can prevent the slider from escaping from the needle shell 310.

[0046] When the inflation mechanism inflates the two needle shells 310, the slider gradually slides out from the through hole on the inner wall of the needle shell 310. After the inflation is completed, when the sliders of the two needle shells 310 move to the predetermined position, the two sliders abut against each other and continue to inflate. The two sliders continue to push the needle shells 310 on both sides in opposite directions. After the inflation is completed, the two sliders also complete the hole expansion operation. At this time, the expanded hole formed by the intervals between the needle shells 310 is suitable for the insertion of the welding needle 20, making the insertion operation of the welding needle 20 easier and will not cause the welding needle 20 to be deformed or broken due to the small hole diameter.

[0047] The end of the slider away from the limiting surface can be provided with a top surface (not shown), which is helpful to improve the stability of preventing the two sliders from abutting each other. The top surface can be a surface parallel to the vertical direction, and the top surfaces of the two sliders can also be curved surfaces or arc surfaces that fit each other.

[0048] In one embodiment, if Figure 7 , Figure 8 As shown, the bottom welding device also includes an elastic clamping member 50, which is exposed on the winding core 10. The elastic clamping member 50 includes a pressure block 51 and a pre-tightening spring 52 connected to the side of the pressure block 51 away from the needle shell 310. The pre-tightening spring 52 presses the pressure block 51 against the side wall of the needle shell 310 to keep the central axis of the expansion needle 30 and the central axis of the central hole 11 of the winding core 10 coaxially arranged during the bottom welding process.

[0049] In one embodiment, if Figure 5 , Figure 7 and Figure 8 As shown, the inflation mechanism is provided with an inflation valve 41 and an air release valve 42. When the inflation valve 41 is opened, the inflation mechanism inflates air into the reaming needle 30, causing each needle shell 310 of the reaming needle 30 to expand. When the air release valve 42 is opened, the movable part 311 can be retracted into the corresponding needle shell 310, so that each needle shell 310 can be gathered and closed under the elastic resistance of the elastic fastening part 50.

[0050] When the reaming needle 30 is provided with two identical needle shells 310, two elastic fastening members 50 are also provided. After the inflation valve 41 is opened, each needle shell 310 expands in the winding core 10. After inflation, insertion of the welding needle 20 and bottom welding are completed, the deflation valve 42 is opened, and the two needle shells 310 are gathered together and closed again due to the elastic supporting effect of the elastic fastening member 50. The movable members 311 in the two needle shells 310 retract into the inner cavity, and the reaming needle 30 returns to the initial state.

[0051] Combined with reference Figures 7 to 9 and Figure 1 , Fig.10 The cylindrical battery bottom welding pin insertion method provided by the embodiment of the present invention, using the cylindrical battery bottom welding pin insertion device of the above embodiment, includes the following steps:

[0052] Step S10 : inserting the reaming needle 30 into the central hole 11 of the winding core 10 .

[0053] Step S20: Control the inflation mechanism to inflate the needle shell 310 of the reaming needle 30, so that the reaming hole 301 formed between the needle shells 310 can be suitable for the insertion of the welding needle 20. This step is the process of reaming. Specifically, after the needle shells 310 are inflated, the sliders in the needle shells 310 extend from the needle shells 310 and abut against each other, so that the needle shells 310 of the reaming needle 30 expand in the central hole 11 of the winding core 10, so that the reaming hole formed between the needle shells 310 can adapt to the size of the welding needle 20, which is convenient for the insertion of the welding needle 20 and completes the reaming requirement.

[0054] Step S30: insert the welding needle 20 into the expanded hole 301 of the expansion needle 30. After the expansion is completed, the winding core 10 moves to the station for inserting the welding needle 20, the welding needle 20 is aligned with the expanded hole of the expansion needle 30, and then the welding needle 20 is controlled to automatically descend to a predetermined position, so that the welding needle 20 is completely inserted into the expanded hole until the bottom of the winding core 10.

[0055] After the pin insertion operation is completed, the negative ear is resistance welded to the bottom shell to complete the bottom welding operation. After the bottom welding operation is completed, the welding needle 20 is pulled out from the reaming needle 30, and then the inflation mechanism opens the deflation valve 42 to exhaust the reaming needle 30. After the exhaust is completed, the reaming needle 30 is pulled out from the central hole 11 of the winding core 10. After the needle shell 310 is completely exhausted, under the action of the preload spring 52, the two needle shells 310 are held and closed, and the sliders are retracted into their respective needle shells 310. The inflation and exhaust operations of the inflation mechanism, the insertion and removal of the expansion needle, and the insertion and removal of the welding needle 20 can all be automatically operated by the driving mechanism.

[0056] In step S30, when the welding needle 20 is inserted into the expansion hole 301 of the expansion needle 30 to the top of the slider, the gas charging mechanism is controlled to discharge a part of the gas in each needle shell 310, so that when the welding needle 20 continues to be inserted downward in the expansion hole, the welding needle 20 pushes against each slider, so that the slider can move into the corresponding needle shell 310, and then the welding needle 20 can be smoothly inserted to the bottom of the winding core 10. In other words, after a part of the gas in each needle shell 310 is discharged, the air pressure in the needle shell 310 becomes smaller, and at this time, the slider will move into the needle shell 310 when it is pushed by an external force, that is, when the welding needle 20 continues to be inserted downward, it pushes each slider into the needle shell 310, so that a gap is generated between the sliders, so that the welding needle 20 can be smoothly inserted downward until it is inserted into the bottom of the winding core 10.

[0057] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A cylindrical battery bottom welding pin device, characterized in that: include: The reaming needle comprises at least two needle shells, each of which has a movable part movably arranged therein; An inflation mechanism is used to inflate the needle shell so that the movable parts in each needle shell extend out from the needle shell and abut against each other, and each needle shell spreads out to form an expanded hole, so that the welding needle can be inserted into the expanded hole and inserted to the bottom of the winding core for bottom welding; the expanded hole needle is provided with two needle shells, an inner cavity connected to the inflation mechanism is formed in the needle shell, and the movable part is movably arranged in the inner cavity; the movable part is a slider movably embedded in the inner cavity; the cross-section of the needle shell is semicircular, and the slider is a tetrahedron, one surface of the slider is completely accommodated in the needle shell and forms a limiting surface, and through holes are respectively opened at corresponding positions of the inner walls opposite to each other of the two needle shells, and the through holes are connected to the inner cavity, and the inflation mechanism When the inner cavity is inflated, the slider extends out from the through hole, the hole wall of the through hole is matched with the other three surfaces of the slider respectively, and the area of ​​the limiting surface is larger than the longitudinal cross-sectional area of ​​the through hole, so that one end of the slider forms a limiting end to prevent the slider from escaping from the needle shell; the cylindrical battery bottom welding pin device also includes an elastic clamping member, the elastic clamping member includes a pressure block and a pre-tightening spring connected to the side of the pressure block away from the needle shell, the pre-tightening spring presses the pressure block against the side wall of the needle shell, so that the central axis of the reaming needle and the central axis of the insertion hole of the winding core remain coaxial during the bottom welding process; after each of the needle shells are spread out, the width of the reaming needle is 4.5mm~5mm.

2. The cylindrical battery bottom welding pin device according to claim 1, characterized in that: The cross section of the reaming needle is circular, and the reaming needle is a variable diameter structure with a diameter that gradually decreases from top to bottom.

3. The cylindrical battery bottom welding pin device according to claim 1, characterized in that: The inflation mechanism is provided with an inflation valve and an air release valve. When the inflation valve is opened, the inflation mechanism inflates air into the reaming needle to expand the needle shells of the reaming needle. When the air release valve is opened, the needle shells are gathered and closed under the elastic support of the elastic fastening member, and the movable member retracts into the corresponding needle shell.

4. A method for welding pins at the bottom of a cylindrical battery, characterized in that: The use of the cylindrical battery bottom welding pin device according to any one of claims 1 to 3 comprises the following steps: Inserting the reaming needle into the central hole of the winding core; Controlling the inflation mechanism to inflate the needle shells of the reaming needles so that the reaming holes formed between the needle shells can be suitable for the insertion of the welding needles; Insert the welding needle into the expanded hole of the expanded hole needle until it reaches the bottom of the winding core.

5. The method for bottom welding pins of cylindrical batteries according to claim 4, characterized in that: When the welding needle is inserted into the reaming hole of the reaming needle to the top of the slider, the inflation mechanism is controlled to discharge a portion of the gas in each needle shell, so that when the welding needle continues to be inserted downward in the reaming hole, the welding needle abuts against each slider, so that the slider can move into the corresponding needle shell, thereby allowing the welding needle to be smoothly inserted to the bottom of the winding core.

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