US20110016944A1

US20110016944A1 - Method of producing metallic member

Info

Publication number: US20110016944A1
Application number: US12/840,170
Authority: US
Inventors: Jun Tamai
Original assignee: Canon Inc
Current assignee: Canon Inc
Priority date: 2009-07-24
Filing date: 2010-07-20
Publication date: 2011-01-27
Also published as: JP5383362B2; CN101961764B; CN101961764A; JP2011025277A

Abstract

A method of producing a metallic member that reduces the burden on a mold during molding is provided. The method of producing the metallic member according to the present invention includes the step of forming a cup-like member by applying a load to a metallic plate member, and the step of forming a plurality of protrusions at an opening end portion of the cup-like member by placing the cup-like member into a mold.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to a method of producing a metallic member. More particularly, the present invention relates to a method of producing a metallic member having a plurality of protrusions of, for example, a gear member or a vibration member of a vibration wave driving apparatus.
2. Description of the Related Art
As a metallic member in which a plurality of protrusions are arranged, a vibration member for a vibration wave driving apparatus discussed in Japanese Patent Laid-Open No. 07-135785 is available. The vibration member is an annular or a disc-shaped member. Many protrusions like the teeth of a comb are provided at one surface (upper surface) of the vibration member, and a base not having teeth of a comb are provided at the opposite surface (lower surface) of the vibration member. A ring-like piezoelectric element is joined to the lower surface of the base. In this type of vibration member, the many protrusions are formed by a forging method or a method of cutting many radial grooves using a grinding tool or a milling cutter having the shape of a disc (a grinding method or a cutting method). The role of the protrusions is to increase vibration displacement when rotating a moving member by transmitting vibration of the vibration member to the moving member by friction force. Depending upon use, a hole extending through an output shaft is provided in the center of the vibration member.
As another example of a metallic member having a plurality of protrusions, a gear such as that discussed in Japanese Patent Laid-Open No. 2001-205385 is available. The gear (such as a bevel gear) having teeth as the plurality of protrusions has a hole portion that does hot have a core for passing a shaft therethrough. The teeth of the gear may also be formed by the cutting method or the grinding method, or by the forging method using a pressing device.

SUMMARY OF THE INVENTION

The forging method is lower in cost than the cutting method and the grinding method because the forging method is a processing method that can be simplified.
For forming many protrusions by the forging method, it is possible to provide an annular or a disc-shaped processing object in which a pipe or a round bar is cut into round slices, and to form the protrusions on the processing object by directly subjecting the processing object to a pressing operation. FIGS. 6A and 6B schematically illustrate states in which protrusions are formed on a processing object 111 by the forging method. FIG. 6A is a sectional view, and FIG. 6B is a perspective view. As shown in FIG. 6A, the circular processing object 111 is used, and is inserted into an inside diameter portion of a die ring 123 (serving as an outer frame of a stationary mold (female mold)) to directly form the protrusions.
However, this method has the following problems. When a punch 122 (serving as a pressing mold (male mold)) contacts the processing object 111, stress applied to the female mold is instantaneously increased. As a result, the female mold tends to break. In addition, a force for reducing the thickness of the processing object 111 interposed between the punch 122 and the female mold is applied to the molds at the same time that pressure is applied. Therefore, a large total load is required for a molding operation. From this viewpoint also, the molds tend to break. In particular, the stress concentrates at a thin-walled portion 124 a of a die 124 having recesses and protrusions. The thin-walled portion 124 a is provided for forming grooves (between the protrusions) in an outer-peripheral lower surface of the disc-shaped processing object. The smaller the widths of the grooves, the thinner the thin-walled portion 124 a, thereby reducing the strength of the thin-walled portion 124 a. Therefore, during the forging, the probability with which the thin-walled portion breaks is increased. Consequently, for maintaining the strength of the thin-walled portion 124 a, the widths of the grooves between the protrusions need to be large to a certain extent, as a result of which limitations are placed on the widths in a peripheral direction of the protrusions of the gear or the vibration member.
The present invention makes it possible to easily mold a plurality of protrusions to any width on a metallic processing object, and to reduce a load on a mold during the molding of the plurality of protrusions.
According to the present invention, there is provided a method of producing a metallic member having a plurality of protrusions. The method includes forming a cup-like member by applying a load to a metallic plate member, the load applied to the plate member having a component that is perpendicular to a surface of the plate member; and forming the plurality of protrusions at an opening end portion of the cup-like member with a mold by applying a load to the cup-like member, the load applied to the cup-like member having a component that is perpendicular to a surface of a bottom portion of the cup-like member.
According to the present invention, when forming a metallic member having a plurality of protrusions, a cutting operation or a grinding operation is not required, so that a load on a mold when forming the protrusions can be reduced. In addition, it is possible form grooves between the protrusions of a molded product to any width.
Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C are schematic views showing states in which a cup-like member is formed according to a first embodiment.

FIGS. 2A to 2D are schematic views showing states in which protrusions are formed according to the first embodiment.

FIG. 3 is a schematic view of a cup-like member according to a second embodiment.

FIGS. 4A to 4C are schematic views showing states in which a preliminary molded product is formed according to the second embodiment.

FIGS. 5A to 5C are schematic views showing states of a final molding operation according to the second embodiment.

FIGS. 6A and 6B are schematic views showing a related forging molding operation.

DESCRIPTION OF THE EMBODIMENTS

Exemplary embodiments of the present invention will hereunder be described in detail with reference to the drawings. As a metallic member used in the present invention, a vibration-wave-driving-apparatus vibration member having protrusions for increasing vibration displacement, or a gear having teeth for output transmission is used as an example.

First Embodiment

In a first embodiment, as a metallic member having a plurality of protrusions protruding in the same direction, a vibration-wave-driving-apparatus vibration member having protrusions for increasing vibration formed on a surface of the vibration member is used as an example, to describe a method of producing the vibration member. The vibration wave driving apparatus includes the vibration member provided with a piezoelectric element (which is one type of electro-mechanical energy conversion element). In the vibration wave driving apparatus, an alternate signal is supplied to the piezoelectric element to generate a traveling wave at a surface of the vibration member, and the traveling wave is used to drive a moving member that contacts the vibration member.
As shown in FIG. 1A, in the embodiment, first, a disc 11-1, formed of stainless steel (SUS420J2) and being a plate member that has been punched out by a pressing operation, is placed at an inside-diameter portion of a positioning plate 26 of a mold for a drawing operation. In addition to stainless steel, the metallic member may be formed of, for example, SPC material, a low alloy steel, a high alloy steel, or a non-ferrous alloy. The drawing mold comprises a punch 22, which is a male mold, and a female mold. The female mold comprises a die 24 having a recessed portion, a die ring 23 serving as an outer frame and being a holding portion of the die 24, a knockout portion 25 for separating a molded product from the female mold, and a positioning plate 26.
As shown in FIG. 1B, by performing a drawing operation in which the punch 22 is pushed downward and a load having an out-of-plane component (that is, a component that is perpendicular to a surface of the plate member) is applied, a cup-like member 11-2 is molded as shown in FIG. 1C. In the invention, the cup-like member has the form of a container having a bottom portion 52 and a side surface 53 formed along the outer periphery of the bottom portion 52. A hole or a recess may be formed in the bottom portion 52. As shown in FIG. 1C, the cup-like member is such that the width in a radial direction of an opening end portion 51 is smaller than the width in a radial direction of protrusions (represented by reference numeral 55 in FIG. 2D) of a final molded product, and such that the height of the side surface 53 is greater than the height of the protrusions (that is, the length thereof in a direction perpendicular to the radial direction) of the final molded product. Though the details will be given later, by performing such a molding operation, since the width in the radial direction of the opening end portion 51 also increases at the same time that the height of the side surface 53 is being reduced, a load that is applied to the molds when forming the protrusions in a post-processing process is reduced. In addition, the volume of each protrusion is maintained.
Further, by molding the side surface 53 so that a slope extending outward with respect to the bottom portion 52, that is, by forming the opening end portion 51 of the cup-like member so that the outside diameter of the opening end portion 51 of the cup-like member is larger than the outside diameter of the bottom portion 52 of the cup-like member, a molding load is reduced when forming the protrusions in the post-processing process. Therefore, performing this is desirable. This is because, since, as shown in FIG. 2B, the cup-like member is deformed so as to extend outward in the molding process, a load that is applied to a thin-walled portion 34 a in a height direction is reduced.
Next, for softening the cup-like member 11-2, for example, an annealing heat treatment at a temperature of 750° C. is performed. The heat treatment is performed to reduce molding load during a forging operation performed for forming the protrusions in the post-processing process, so that the cup-like member tend to undergo plastic deformation. Depending upon the extent of drawing, the hardening of the material does not progress very much and deformation resistance does not increase very much. Therefore, the annealing heat treatment may sometimes be omitted. Thereafter, a lubricating treatment may be performed on the surface of the cup-like member 11-2. In the lubricating treatment, for example, a lubricant whose main component is molybdenum disulfide is applied.
FIG. 2A shows a state in which the cup-like member 11-2 is placed in a mold (used for molding the protrusions) and is positioned in the mold. The mold used for molding the protrusions includes a female mold (including a die ring 33, a die 34, and a knockout portion 35) and a punch 32 (serving as a male mold). A groove in which the cup-like member is inserted is formed along the circumference of the die 34. The thin-walled portion 34 a, disposed with an equal interval in a peripheral direction, is formed in the groove formed along the circumference. An inner peripheral wall of the groove has a slope. The closer to the bottom portion of the groove, the closer an outer peripheral wall and the inner peripheral wall are to each other, so that the width of the groove in the radial direction becomes smaller. By forming such a groove, as can be seen from FIGS. 2A and 2B, as the punch is moved downward, the opening end portion of the cup-like member tends to spread outward; and, if the inner peripheral wall has a slope, plastic flow is infrequently hindered. Therefore, this structure is desirable. When the shape of the mold is one that infrequently hinders plastic flow, the probability with which breakage of the mold and a defect (crack, bending, etc.) in a molded product occur is reduced.
The cup-like member is positioned at an inside-diameter portion of the die ring 33, and the opening end portion of the cup-like member is disposed so as to contact the upper end of the thin-walled portion 34 a (a portion of the die 34) for forming grooves between the protrusions.
FIG. 2B shows a state in which, as the punch 32 is moved downward, a load having a component that is perpendicular to the bottom portion of the cup-like member is applied, and the cup-like member is being deformed. At this stage, a load applied to the punch 32 and the thin-walled portion 34 a is small. However, as the entire form of the cup-like member is deformed and is compressed as indicated by reference numeral 11-3, protrusions start to form at the opening end portion of the cup-like member 11-3. Then, as the punch 32 is moved downward, the load is gradually increased, and the whole molding operation ends at the bottom dead point as shown in FIG. 2C. As a result, a final molded product 11-4 is obtained as shown in FIG. 2D.
By the compression molding performed by this forging operation, the protrusions 55 are formed at the opening end portion 51 of the cup-like member, and the width in the radial direction of the protrusions 55 becomes greater than the width of the opening end portion of the cup-like member. The thickness of the bottom portion 54 of the final molded product is less than the thickness of the bottom portion 52 of the cup-like member. Accordingly, in the embodiment, stress applied to each portion of the molds, in particular, to the thin-walled portion 34 a during the molding process is gradually increased, and a force applied to the thin-walled portion in the peripheral direction is small, so that the probability with which the thin-walled portion 34 a is broken is reduced. Since excess metal that is produced as the bottom portion of the cup-like member is made thinner is used for forming the protrusions at the outer peripheral side, the load on the die and the knockout portion is also reduced, thereby reducing the probability of deformation and breakage thereof. The shape of the side surface of the cup-like member and the shape of the protrusions are not limited to the shapes discussed in the embodiment as long as they can be deformed so that the width in the radial direction of the protrusions, that is, the plate thickness is increased during the forging operation. For example, the protrusions may be inclined not only at the inner peripheral side, but also at the outer peripheral side, or may be formed with the same width.
Further, in the embodiment, as a plate member serving as the processing object, a plate member having a width that is smaller than the width in the radial direction of the groove of the die 34 (that is, the width in the radial direction of the protrusions of the molded product) may be used, thereby reducing costs. The reason will be given below.
In the related method shown in FIG. 6, in order to form a disc (processing object), a round bar is cut in round slices with a hand saw instead of by a cutting operation. However, in this method, large variations occur in the thickness of the processing object, and the parallelism of both surfaces is not good. In addition, there are saw marks on surfaces cut in round slices, so that surface roughness thereof is high. Therefore, variations occur in the thickness of portions of the molded product, in particular, variations in the heights of the individual protrusions tend to occur. Moreover, the saw marks remain on the processing object at molded end surfaces, thereby making it necessary to perform grinding for removing the saw marks. As mentioned above, in the embodiment, as the processing object, a plate member whose diameter is larger than the diameter of the vibration member (which is the final molded product) as with the disc 11-1 shown in FIG. 1A and which is such that the thickness of the disc 11-1 is less than the thickness of the protrusions of the vibration member may be used. Accordingly, since a rolled plate that is lower in cost than, for example, a round bar may be used, it is possible to reduce material roughness and increase the parallelism of both surfaces, and to reduce costs. In addition, since the plate member is thin, little changes occur in the plate thickness when forming the bottom portion to a predetermined thickness (height), so that work hardening does not progress very much. As a result, the stress that is required for reducing the thickness of the bottom portion is reduced, so that deformation and breakage of the die and the knockout portion occur even less often.
The vibration member formed in accordance with the embodiment makes it possible for the mass of the protrusions of the vibration member to provide vibration energy, and to reduce the width of the thin-walled portion in the peripheral direction. Therefore, it is possible to cause the width of the grooves formed between the protrusions that are adjacent to each other to be as small as possible. That is, it is possible to increase the width in the peripheral direction of the protrusions, and to increase resistance to wear of the protrusions that contact the moving member.
In the vibration wave driving apparatus, if the width in the peripheral direction of the protrusions formed by the compression molding is increased, not only is the resistance to wear increased, but also undesired vibration is restricted. This is because, since natural vibration frequency of each protrusion itself is small, undesired vibration caused by the protrusions themselves infrequently occurs.
In the present invention, the method of molding the cup-like member by applying a load having a component that is perpendicular to the surface of the plate member to the plate member is not limited to drawing. As long as it is a method of three-dimensionally deforming the plate member, any other method, such as burring, stretch forming (embossing), or dish extrusion (bowl molding) may also be used.
The method of forming the protrusions by applying a load having a component that is perpendicular to the surface of the bottom portion of the cup-like member to the cup-like member is not limited to cold forging. The plate member may also be molded by hot forging, warm forging, or cold pressing.
In accordance with the embodiment, a vibration member formed of stainless steel (SUS420J2), having an outside diameter of 48 mm, whose bottom portion has a thickness of 2.5 mm, whose protrusions have a width of 5 mm in a radial direction, and whose protrusions have a height of 4.5 mm was produced. A mold used for molding the protrusions had a height of 4.5 mm and a width of 0.6 mm (at a thinnest portion) in a peripheral direction of a thin-walled portion of a die. When an attempt was made to form such a vibration member by the related method shown in FIGS. 6A and 6B, a load of at least 3 t was required, and a large pressure instantaneously acted upon the thin-walled portion, as a result of which the probability of breakage of the thin-walled portion was high. However, when the protrusions were formed using the method according to the embodiment, a load was not instantaneously applied to the thin-walled portion, or a total load was small, thereby reducing the burden on the mold during the molding.

Second Embodiment

Next, a method of molding protrusions of a vibration member according to a second embodiment will be described. The second embodiment differs from the first embodiment in that a hole is formed in the center of a bottom portion of a cup-like member and in that a preliminary molding operation is performed on the cup-like member between a step of forming the cup-like member and a step of forming a final molded product.
FIG. 3 is a cup-like member 41-1 after drawing. The cup-like member is open by press-punching a circular hole 41-1 a in the center of a bottom portion 52. The other features of the cup-like member are the same as those of the cup-like member according to the first embodiment. The circular hole may be formed when a plate member provided prior to molding the cup-like member is formed, or when the cup-like member is molded, or after the cup-like member is molded. The circular hole is formed for determining the center position in a next molding step. Therefore, instead of such a through hole, a recess may be formed.
FIG. 4A shows a state in which protrusions are molded by applying a load having a component that is perpendicular to a surface of the bottom portion 52 of the cup-like member to the cup-like member. The circular hole of the cup-like member is fitted to a positioning protrusion 42 a, provided at an end portion of a punch 42, and the center of the cup-like member 41-1 and the center of a mold coincide with each other. This prevents the cup-like member from being decentered when it is being compressed. An opening end portion 51 of the cup-like member can be restrained at an inside-diameter portion of a die ring 43, and the bottom portion 52 of the cup-like member, that is, a portion of the cup-like portion that contacts the punch 42 can be secured so as not to be shifted in a direction parallel to the surface of the bottom portion in FIG. 4A.
The reason that the circular hole is provided to restrain the center position in this way is related to the precision of the shape of the cup-like member 41-1 and uniformity in its plastic deformation. In drawing, considering the plastic deformation of a material itself, the precision of the shape of a molded product is often influenced by the degree of its anisotropy. Anisotropy occurs as a result of aggregate structures differing from each other in connection with, for example, crystal orientation according to material. For example, characteristics, such as stretching, processing hardening coefficient, an elastic limit value, and Young's modulus, in a rolling direction differ from those in a direction orthogonal to the rolling direction. As a result, differences also occur in the Lankford value (characteristic value indicating the anisotropy of a plate material) and a spring-back amount (amount by which the plate material returns to its state prior to processing by a slight opposing force after a bending operation). If the plate member having a high anisotropic property is drawn, the opening end portion 51 of the cup-like member may be curled. The opening end portion 51 of such a cup-like member infrequently has a high degree of circularity.
To overcome this problem, in the embodiment, measures are taken to allow the use of a material having a high anisotropic property. That is, in the embodiment, as mentioned above, for example, a hole that restrains the center position is formed in the cup-like member. This causes deformation in a radial direction to be uniform during the deformation of the cup-like member. As a result, variations in the thickness in a peripheral direction of a molded product that is formed by compression are reduced.
FIG. 4B shows a state 41-2 in which the cup-like member has been compressed. The molding step is a preliminary molding step. Here, the shape of a final molded product is not yet formed. Rather, the preliminary molded product 41-2 having low protrusions, that is, shallow grooves between the protrusions are formed as shown in FIG. 4C. If such a preliminary molding is performed, when final protrusions are molded, almost no sideways force is generated at the thin-walled portion, thereby making it possible to reduce the probability of breakage. This is like forming a mortar-like recessed portion at a location where the hole is previously formed with a drill to make it possible to fix an end position of the drill and prevent breakage of the drill when the drill is bent. If shallow grooves are formed at positions where the grooves are previously formed, they function as guides to make it possible to easily mold final deep grooves, that is, high protrusions.
Prior to molding the final protrusions, it is desirable that the preliminary molded product be subjected to intermediate heat treatment (annealing). By the heat treatment, the preliminary molded product is softened, so that deformation resistance is reduced in a final step. As a result, in addition to reducing the probability of seizure and the burden on the mold used in the final step, the probability of cracking of the molded product itself is reduced. The heat treatment is also effective in reducing negative effects caused by the aforementioned anisotropy as recrystallization occurs.
It is desirable that a lubricating treatment also be performed on the preliminary molded product 41-2 whose protrusions have small volumes (grooves between the protrusions are shallow). This is because, stress on the molds in the final molding step, in particular, seizure of the thin-walled portion is reduced. In the lubricating treatment, for example, a lubricant whose main component is molybdenum disulfide is applied.
FIG. 5A shows a state in which the preliminary molded product is set in the mold used to mold the shape of the final vibration member. At this time, it is desirable that the width in the peripheral direction of the grooves between the protrusions of the preliminary molded product be greater than the width in the peripheral direction of an end portion of a thin-walled portion 44 a corresponding thereto. By molding such a preliminary molded product, it becomes easier to align in the peripheral direction the grooves between the protrusions of the preliminary molded product 41-2 and the thin-walled portion 44 a of the die 44. Further, it is possible to increase the thickness of the thin-walled portion of the mold used for the preliminary molding, so that breakage occurs less frequently.
FIG. 5B shows a state in which the preliminary molded product 41-2 has been pressed until a molded product 41-3 having the final shape is formed. At this time, the circular hole 41-2 a formed in the central portion of the preliminary molded product becomes a hole 41-3 a of the final molded product shown in FIG. 5C. The diameter of the hole 41-3 a is smaller than the diameter of the circular hole 41 -2 a.
As described above, in the embodiment, the circular hole 41-2 a is formed in the center of the bottom portion of the cup-like member, and the preliminary molding step is provided prior to the step of forming the final protrusions. Forming the circular hole makes it possible to restrain the center position and to reduce variations in the width of the molded product in the peripheral direction. In addition, even in the final molding step, when a load having an out-of-plane component is applied, the circular hole makes it possible for excess metal to be used for the circular hole and to prevent excessive stress from being applied to the molds. In addition, by providing the preliminary molding step, when the final protrusions are being molded, almost no sideways force is generated at the thin-walled portion, thereby making it possible to further reduce the probability of breakage.
Although each of the embodiments is described taking as an example the case in which protrusions for increasing vibration in the vibration member are formed, the present invention is not limited thereto. That is, as long as a metallic processing object having a plurality of protrusions protruding in one direction is provided, the above-described molding method is applicable to parts having shapes similar to those of, for example, a bevel gear and a hypoid gear.
While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.
This application claims the benefit of Japanese Patent Application No. 2009-172920 filed Jul. 24, 2009, which is hereby incorporated by reference herein in its entirety.

Claims

1. A method of producing a metallic member having a plurality of protrusions, the method comprising:

forming a cup-like member by applying a load to a metallic plate member, the load applied to the plate member having a component that is perpendicular to a surface of the plate member; and

forming the plurality of protrusions at an opening end portion of the cup-like member with a mold by applying a load to the cup-like member, the load applied to the cup-like member having a component that is perpendicular to a surface of a bottom portion of the cup-like member.

2. The method of producing the metallic member according to claim 1, wherein the cup-like member is formed so that an outside diameter of the opening end portion of the cup-like member is larger than an outside diameter of the bottom portion of the cup-like member.

3. The method of producing the metallic member according to claim 1, further comprising preliminarily molding a groove that is shallower than a groove that is formed between the plurality of protrusions, the step of preliminarily molding the shallower groove being performed between the step of forming the cup-like member and the step of forming the plurality of protrusions.

4. The method of producing the metallic member according to claim 3, further comprising performing heat treatment between the step of preliminarily molding the shallower groove and the step of forming the plurality of protrusions.

5. The method of producing the metallic member according to claim 1, wherein the cup-like member has a hole or a recess at the center of the bottom portion thereof.

6. The method of producing the metallic member according to claim 1, wherein the plate member has a thickness that is less than a width in a radial direction of the protrusions.

7. The method of producing the metallic member according to claim 1, wherein the cup-like member is molded so that a width in a radial direction of the opening end portion of the cup-like member is less than a width in a radial direction of the protrusions.

8. The method of producing the metallic member according to claim 1, wherein the step of forming the cup-like member is performed by any one of drawing, burring, stretch forming, and dishing.

9. The method of producing the metallic member according to claim 1, wherein the step of forming the plurality of protrusions is performed by either forging or cold pressing.

10. A method of producing a metallic member comprising:

forming a cup-like member by applying a load to a metallic plate member; and

forming a plurality of protrusions at an opening end portion of the cup-like member by placing the cup-like member into a mold.

11. The method of producing the metallic member according to claim 10, wherein the cup-like member is formed so that an outside diameter of the opening end portion of the cup-like member is larger than an outside diameter of a bottom portion of the cup-like member.

12. The method of producing the metallic member according to claim 10, further comprising preliminarily molding a groove that is shallower than a groove that is formed between the plurality of protrusions, the step of preliminarily molding the shallower groove being performed between the step of forming the cup-like member and the step of forming the plurality of protrusions.

13. The method of producing the metallic member according to claim 12, further comprising performing heat treatment between the step of preliminarily molding the shallower groove and the step of forming the plurality of protrusions.

14. The method of producing the metallic member according to claim 10, wherein the cup-like member has a hole or a recess at the center of a bottom portion thereof.

15. The method of producing the metallic member according to claim 10, wherein the plate member has a thickness that is less than a width in a radial direction of the protrusions.

16. The method of producing the metallic member according to claim 10, wherein the cup-like member is molded so that a width in a radial direction of the opening end portion of the cup-like member is less than a width in a radial direction of the protrusions.