Fastening member, screw structure, screw fastening structure and method for fastening screw fastening structure

Abstract

To provide a fastening member that facilitates re-fastening and has excellent sealability, a screw structure, a screw fastening structure and a method for fastening a screw fastening structure.SOLUTION: A fastening member for fastening a screw having a head part and a shaft part, has a plastic washer. The plastic washer can plastically deform or is plastically deforming until an inner peripheral surface of the plastic washer is pressed against the shaft part through fastening of the screw.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to a fastening member, a screw structure, a screw fastening structure, and a fastening method for the screw fastening structure. [Background technology]

[0002] 2. Description of the Related Art A fastening member for fastening a screw is known (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 6274551 Summary of the Invention [Problem to be solved by the invention]

[0004] The above-mentioned fastening members may be removed from the fastened state and then refastened. If the fastening member separates from the screw at that time, it is time-consuming to refasten it. In addition, the above-mentioned fastening members may be required to have a sealing property.

[0005] Therefore, an object of the present disclosure is to provide a fastening member, a screw structure, a screw fastening structure, and a fastening method for the screw fastening structure that are easy to refasten and have excellent sealing properties. [Means for solving the problem]

[0006] One aspect of the present disclosure is as follows.

[0007] [1] A fastening member for fastening a screw having a head and a shaft, A plastic washer is provided. The plastic washer is a fastening member that can be or has been plastically deformed until the inner circumferential surface of the plastic washer is pressed against the shaft portion when the screw is fastened.

[0008] [2] The fastening member described in [1], wherein the plastic washer is capable of or has been plastically deformed until an inner circumferential surface of the plastic washer is pressed against the threads of the shaft portion when the screw is tightened.

[0009] [3] The fastening member according to [1] or [2], having, from the head side to the shaft side, a spring washer, a first flat washer, a second flat washer and the plastic washer, in this order.

[0010] [4] The second flat washer and the plastic washer are adjacent to each other, The fastening member described in [3], wherein the plastic washer has an outer diameter smaller than an outer diameter of the second flat washer.

[0011] [5] The fastening member according to [3] or [4], wherein a coefficient of friction between the first flat washer and the second flat washer is lower than a coefficient of friction between the second flat washer and the plastic washer.

[0012] [6] A screw structure comprising the fastening member according to any one of [1] to [5] and the screw.

[0013] [7] A screw fastening structure comprising the screw structure described in [6] and a fastened portion into which the screw is fastened.

[0014] [8] the fastened portion has a contact surface that contacts the fastening member, an insertion hole into which the shaft portion is inserted, and a chamfered portion between the contact surface and an inner circumferential surface of the insertion hole, The screw fastening structure described in [7], wherein the plastic washer can be or has been plastically deformed until it is pressed against the chamfered portion by fastening the screw.

[0015] [9] the fastened portion has a contact surface that contacts the fastening member, an insertion hole into which the shaft portion is inserted, and a chamfered portion between the contact surface and an inner circumferential surface of the insertion hole, The screw fastening structure described in [7] or [8], wherein the plastic washer is capable of plastically deforming until the inner surface of the plastic washer is pressed against the shaft portion by being pressed against the chamfered portion when the screw is fastened.

[0016]

[10] The screw fastening structure described in [8] or [9], wherein the chamfered portion is a C-chamfered portion.

[0017]

[11] The screw fastening structure described in

[10] , wherein the inclination angle of the C-chamfered portion as the chamfered portion with respect to a plane perpendicular to the shaft portion is 30° or more and 60° or less.

[0018]

[12] A medical device having a screw fastening structure described in any one of [7] to

[11] .

[0019]

[13] A method for fastening a screw fastening structure, comprising: The screw fastening structure includes a screw structure and a fastened portion to which the screw is fastened, The screw structure includes a fastening member and the screw, The fastening member is for fastening a screw having a head and a shaft and has a plastic washer; A fastening method for a screw fastening structure, comprising a fastening step of plastically deforming the plastic washer by fastening the screw until the inner circumferential surface of the plastic washer is pressed against the shaft portion.

[0020]

[14] the fastened portion has a contact surface that contacts the fastening member, an insertion hole into which the shaft portion is inserted, and a chamfered portion between the contact surface and an inner circumferential surface of the insertion hole, The fastening method for a screw fastening structure described in

[13] , wherein in the fastening step, the plastic washer is plastically deformed until it is pressed against the chamfered portion by fastening the screw.

[0021]

[15] the fastened portion has a contact surface that contacts the fastening member, an insertion hole into which the shaft portion is inserted, and a chamfered portion between the contact surface and an inner circumferential surface of the insertion hole, The fastening method for a screw fastening structure described in

[13] or

[14] , wherein in the fastening step, the plastic washer is plastically deformed until the inner circumferential surface of the plastic washer is pressed against the shank by pressing the plastic washer against the chamfered portion by tightening the screw. Effect of the Invention

[0022] According to the present disclosure, it is possible to provide a fastening member, a screw structure, a screw fastening structure, and a fastening method for a screw fastening structure, which are easy to refasten and have excellent sealing properties. [Brief description of the drawings]

[0023] [Figure 1] FIG. 1 is a partial cross-sectional external view showing a screw structure according to an embodiment of the present disclosure. [Diagram 2] 2 is a cross-sectional view showing a screw fastening structure according to one embodiment of the present invention having the screw structure shown in FIG. 1. [Diagram 3] 13 is an explanatory diagram for illustrating the pressing effect of the chamfered portion against the shaft portion when the inclination angle of the chamfered portion is 45°. FIG. [Figure 4] 13 is an explanatory diagram for illustrating the pressing effect of the chamfered portion against the shaft portion when the inclination angle of the chamfered portion is 30°. FIG. [Diagram 5] 13 is an explanatory diagram for illustrating the pressing effect of the chamfered portion against the shaft portion when the inclination angle of the chamfered portion is 60°. FIG. [Figure 6] 13 is a simulation result showing the relationship between the inclination angle of the chamfered portion and the pressing effect of the chamfered portion on the shaft portion. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings.

[0025] As shown in FIG. 1, in one embodiment of the present disclosure, the fastening member 1 is a fastening member 1 for fastening a screw 2 having a head 2a and a shaft portion 2b, and has a plastic washer 1a, and the plastic washer 1a can be plastically deformed until an inner circumferential surface 1a1 of the plastic washer 1a is pressed against the shaft portion 2b by fastening the screw 2. The fastening member 1 of this embodiment can be changed from the state shown in FIG. 1 to the state shown in FIG. 2 in which the inner circumferential surface 1a1 of the plastic washer 1a is plastically deformed until it is pressed against the shaft portion 2b by fastening the screw 2. In other words, the fastening member 1 shown in FIG. 2 is plastically deformed until the inner circumferential surface 1a1 of the plastic washer 1a is pressed against the shaft portion 2b by fastening the screw 2.

[0026] According to the above configuration, the inner peripheral surface 1a1 of the plastic washer 1a is plastically deformed until it is pressed against the shaft portion 2b by fastening the screw 2, so that the plastic washer 1a is held on the shaft portion 2b. Therefore, a fastening member 1 that is easy to refasten and has excellent sealing properties can be realized. The plastic washer 1a can be formed of, for example, a resin that can be plastically deformed. Examples of such resins include soft resins containing fluororesins such as PTFE (polytetrafluoroethylene) or ETFE (ethylene-tetrafluoroethylene copolymer). Note that in cases where a large fastening force can be applied, when operations are performed at high temperatures, or when the fastened portion 5 described later is made of a hard material such as metal, the resin is not limited to the soft resin described above, and other resins that can be plastically deformed under the fastening implementation conditions can be selected. In this case, POM (polyoxymethylene) or the like can be used appropriately depending on the conditions.

[0027] As described above, the plastic washer 1a can or has undergone plastic deformation until the inner peripheral surface 1a1 of the plastic washer 1a is pressed against the shaft portion 2b by fastening the screw 2. The portion of the shaft portion 2b against which the inner peripheral surface 1a1 of the plastic washer 1a is pressed may be the thread 2b1, the non-threaded portion 2b2 in which the thread 2b1 is not cut, or a portion consisting of both the thread 2b1 and the non-threaded portion 2b2. In the present embodiment shown in Figs. 1 and 2, the portion against which the inner peripheral surface 1a1 of the plastic washer 1a is pressed is the thread 2b1, and the plastic washer 1a can or has undergone plastic deformation until the inner peripheral surface 1a1 of the plastic washer 1a is pressed against the thread 2b1 of the shaft portion 2b by fastening the screw 2. However, the portion against which the inner circumferential surface 1a1 of the plastic washer 1a is pressed may be a portion consisting of both the thread 2b1 and the non-threaded portion 2b2, and the plastic washer 1a may be or has been plastically deformed until the inner circumferential surface 1a1 of the plastic washer 1a is pressed against the thread 2b1 of the shaft portion 2b by fastening the screw 2. In any case, according to the configuration in which the plastic washer 1a may be or has been plastically deformed until the inner circumferential surface 1a1 of the plastic washer 1a is pressed against the thread 2b1 of the shaft portion 2b by fastening the screw 2, the plastic washer 1a can bite into the thread 2b1 by plastically deforming until it is pressed against the thread 2b1 of the shaft portion 2b, and the plastic washer 1a can be held well by the shaft portion 2b. The structure for improving the holding force by a structure other than the thread 2b1 includes selecting a resin that is easily plastically deformed for the plastic washer 1a, increasing the thickness of the plastic washer 1a, reducing the weight of the spring washer 1b, the first flat washer 1c, and the second flat washer 1d when the fastening member 1 has the spring washer 1b, the first flat washer 1c, and the second flat washer 1d described later, and providing irregularities other than the thread 2b1 on the shaft portion 2b with which the plastic washer 1a can come into contact. These structures are suitable for a structure in which the inner peripheral surface 1a1 of the plastic washer 1a is pressed against the non-threaded portion 2b2. In this way, by fastening the screw 2, a screw structure 3 described later can be formed in which the spring washer 1b, the first flat washer 1c, the second flat washer 1d, and the plastic washer 1a are held by the shaft portion 2b.

[0028] The fastening member 1 has, in this order from the head 2a side to the shaft 2b side, a spring washer 1b, a first flat washer 1c, a second flat washer 1d, and a plastic washer 1a. According to the above configuration, the spring washer 1b can suppress loosening of the screw 2, the first flat washer 1c can equalize the elastic force from the spring washer 1b to the plastic washer 1a, and the second flat washer 1d, which is set to an appropriate hardness and shape, can efficiently plastically deform the plastic washer 1a. In other words, by providing the second flat washer 1d separately from the first flat washer 1c, the portion adjacent to the plastic washer 1a can be made to have a hardness and shape suitable for efficiently plastically deforming the plastic washer 1a. In addition, since the coefficient of friction between the first flat washer 1c and the second flat washer 1d is lower than the coefficient of friction between the second flat washer 1d and the plastic washer 1a, and the smoother the first flat washer 1c and the second flat washer 1d, the more efficiently the second flat washer 1d can plastically deform the plastic washer 1a, it is preferable that the coefficient of friction between the first flat washer 1c and the second flat washer 1d is lower than the coefficient of friction between the second flat washer 1d and the plastic washer 1a, as described later. Furthermore, by holding the plastic washer 1a on the shaft portion 2b, a screw structure 3 can be formed, which can hold the spring washer 1b, the first flat washer 1c, and the second flat washer 1d on the screw 2 by the head portion 2a and the plastic washer 1a. Therefore, for example, when the screw structure 3 formed after fastening the screw 2 is removed from the fastened portion 5 described below for some reason, the screw 2, spring washer 1b, first flat washer 1c, second flat washer 1d and plastic washer 1a do not come apart, which facilitates the fastening operation when fastening the screw structure 3 again to the fastened portion 5. The spring washer 1b, the first flat washer 1c and the second flat washer 1d can each be made of, for example, metal or resin.

[0029] The second flat washer 1d and the plastic washer 1a are adjacent to each other, and the plastic washer 1a has an outer diameter D1 smaller than the outer diameter D2 of the second flat washer 1d before or after plastic deformation. In a configuration in which the plastic washer 1a does not have an outer diameter D1 smaller than the outer diameter D2 of the second flat washer 1d before plastic deformation, unlike the above configuration, the plastic deformation of the plastic washer 1a when the screw 2 is fastened to the fastened portion 5 may cause the outer periphery of the second flat washer 1d to bite into the plastic deformation of the plastic washer 1a, and it is difficult to induce a sufficient amount of plastic deformation in the direction of the shaft portion 2b of the plastic washer 1a, which may cause a defective formation of the screw structure 3. In addition, at this time, there is a risk of a step being generated on the outer periphery of the plastic washer 1a. In contrast, in the case where the plastic washer 1a has an outer diameter D1 smaller than the outer diameter D2 of the second flat washer 1d before plastic deformation, the above-mentioned formation defects and the occurrence of steps in the screw structure 3 can be suppressed. In addition, in the case where the plastic washer 1a has an outer diameter D1 smaller than the outer diameter D2 of the second flat washer 1d after plastic deformation, there is no step as described above, so there is no risk of the above-mentioned formation defects and the occurrence of steps in the screw structure 3. Unlike the screw structure 3 of this embodiment, when the screw 2, the spring washer 1b, the first flat washer 1c, the second flat washer 1d, and the plastic washer 1a are separated after the screw structure 3 is separated from the fastened portion 5, if there is a step on the outer periphery of the plastic washer 1a when refastening, there is a risk that the second flat washer 1d will ride up on the plastic washer 1a when reused. If such riding up occurs, there is a risk that the fastening becomes unstable and the sealing property is affected. However, in the screw structure 3 of the present embodiment, the spring washer 1b, the first flat washer 1c, and the second flat washer 1d are integrated with the screw 2 by the plastic washer 1a, so that the above-mentioned riding-up is unlikely to occur.

[0030] The fastening member 1 may be configured such that the coefficient of friction between the first flat washer 1c and the second flat washer 1d is lower than the coefficient of friction between the second flat washer 1d and the plastic washer 1a. According to the above configuration, as the screw 2 rotates during fastening, the second flat washer 1d slides against the spring washer 1b and the first flat washer 1c which rotate relative to the screw 2, thereby suppressing the rotation of the second flat washer 1d relative to the plastic washer 1a, and the plastic washer 1a can be stably plastically deformed.

[0031] In one embodiment of the present disclosure, a screw structure 3 includes the fastening member 1 of the above-described embodiment and a screw 2. According to the above configuration, it is possible to realize a screw structure 3 that is easy to refasten and has excellent sealing properties. The screw 2 can be made of, for example, metal or resin.

[0032] In one embodiment of the present disclosure, a screw fastening structure 4 includes the screw structure 3 of the above-described embodiment and a fastened portion 5 to which the screw 2 is fastened. According to the above configuration, it is possible to realize a screw fastening structure 4 that is easy to refasten and has excellent sealing properties. The fastened portion 5 can be formed of, for example, metal or resin.

[0033] The fastened portion 5 has a contact surface 5a that contacts the fastening member 1, an insertion hole 5b into which the shaft portion 2b is inserted, and a chamfered portion 5c between the contact surface 5a and the inner peripheral surface 5b1 of the insertion hole 5b, and the plastic washer 1a can or has been plastically deformed by the fastening of the screw 2 until it is pressed against the chamfered portion 5c. According to the above configuration, by inserting the shaft portion 2b into the insertion hole 5b and fastening the screw 2 to the fastened portion 5, the fastening member 1 can be sandwiched between the head 2a and the contact surface 5a, compressed, and plastically deformed. In addition, by plastically deforming the plastic washer 1a until it is pressed against the chamfered portion 5c, the deformation of the plastic washer 1a in the radial outward direction is suppressed by the anchor effect of the chamfered portion 5c. Therefore, it is possible to suppress the generation of a step on the outer peripheral portion of the plastic washer 1a due to the bite of the outer peripheral portion of the second flat washer 1d, and as a result, it is possible to suppress the deterioration of the sealing property due to the generation of the step. In addition, since the plastic washer 1a can be shaped to fit into the chamfered portion 5c, when the screw 2 is removed after being fastened and then fastened again into the fastened portion 5, positioning relative to the fastened portion 5 can be easily performed.

[0034] The fastened portion 5 has a contact surface 5a that contacts the fastening member 1, an insertion hole 5b into which the shaft portion 2b is inserted, and a chamfered portion 5c between the contact surface 5a and the inner peripheral surface 5b1 of the insertion hole 5b. The plastic washer 1a can be or has been plastically deformed until the inner peripheral surface 1a1 of the plastic washer 1a is pressed against the shaft portion 2b by being pressed against the chamfered portion 5c by the fastening of the screw 2. According to the above configuration, the fastening member 1 can be sandwiched between the head 2a and the contact surface 5a and compressed to be plastically deformed by inserting the shaft portion 2b into the insertion hole 5b and fastening the screw 2 to the fastened portion 5. In addition, the chamfered portion 5c can provide a pressing effect that induces the direction of plastic deformation toward the shaft portion 2b, so that the plastic washer 1a can be easily plastically deformed until it is pressed against the shaft portion 2b, and the plastic washer 1a can be well held on the shaft portion 2b.

[0035] The chamfered portion 5c is a C-chamfered portion. According to the above-mentioned configuration, the chamfered portion 5c can be realized with a simple configuration. Note that the chamfered portion 5c is not limited to a C-chamfered portion, and may be, for example, an R-chamfered portion or the like.

[0036] The inclination angle θ of the C-chamfered portion as the chamfered portion 5c with respect to a plane perpendicular to the shaft portion 2b is equal to or greater than 30° and equal to or less than 60°. According to the above configuration, the pressing effect can be efficiently obtained, so that the plastic washer 1a can be more effectively held by the shaft portion 2b.

[0037] 3 to 5 are explanatory diagrams for explaining the pressing effect of the chamfered portion 5c against the shaft portion 2b when the inclination angle θ of the chamfered portion 5c is 45°, 30°, or 60°. FIG. 6 is a simulation result showing the relationship between the inclination angle θ of the chamfered portion 5c and the pressing effect of the chamfered portion 5c against the shaft portion 2b. The load F1 shown in FIG. 3 is the load acting on the chamfered portion 5c from the fastening member 1 that is plastically deformed and pressed against the chamfered portion 5c by fastening the screw 2, and the load F2 corresponds to the radial component when the perpendicular component of the load F1 to the chamfered portion 5c is further divided into the axial component and radial component of the shaft portion 2b. Therefore, the radial component of the normal force by the chamfered portion 5c, i.e., the pressing force toward the radial inside of the shaft portion 2b, is opposite to and has the same magnitude as the load F2. In other words, it can be said that the pressing effect can be obtained more efficiently as the load F2 / load F1 is larger. From the results in FIG. 6, it is understood that the inclination angle θ of the chamfered portion 5c is preferably 30° or more and 60° or less, and most preferably 45°.

[0038] In one embodiment of the present disclosure, a medical device 6 has the screw fastening structure 4 of the above-described embodiment. According to the above configuration, it is possible to realize a medical device 6 having a screw fastening structure 4 that is easy to refasten and has excellent sealing properties. The medical device 6 is, for example, various pumps such as an infusion pump, a syringe pump, and an insulin pump, a blood oxygen meter, a blood glucose meter, an activity meter, an artificial ventilator, a defibrillator, etc.

[0039] In one embodiment of the present disclosure, the electronic device 7 has the screw fastening structure 4 of the embodiment described above. According to the above configuration, the electronic device 7 having the screw fastening structure 4 that is easy to refasten and has excellent sealing properties can be realized. The electronic device 7 of this embodiment is the medical device 6 of the embodiment described above. However, the type of the electronic device 7 is not particularly limited as long as it has the screw fastening structure 4 of the embodiment described above, and may be, for example, a PC-related device, various remote controls, a game machine, a cutting device, an assembly device, or the like, and may be either a home or industrial device. Note that since the electronic device 7 has the screw fastening structure 4 that is easy to refasten and has excellent sealing properties, it is particularly suitable for cases where the electronic device 7 may be flooded.

[0040] In one embodiment of the present disclosure, a fastening method for a screw fastening structure 4 includes a screw fastening structure 4 having a screw structure 3 and a fastened portion 5 to which a screw 2 is fastened, the screw structure 3 having a fastening member 1 and a screw 2, the fastening member 1 is for fastening a screw 2 having a head 2a and a shank 2b and has a plastic washer 1a, and includes a fastening step of plastically deforming the plastic washer 1a by fastening the screw 2 until an inner surface 1a1 of the plastic washer 1a is pressed against the shank 2b.

[0041] According to the above configuration, in the fastening step, the plastic washer 1a is held on the shaft portion 2b by plastic deformation until the inner peripheral surface 1a1 of the plastic washer 1a is pressed against the shaft portion 2b by fastening the screw 2. Therefore, a fastening method of the screw fastening structure 4 that is easy to refasten and has excellent sealing properties can be realized.

[0042] In the fastening method of the screw fastening structure 4, the fastened portion 5 has a contact surface 5a that contacts the fastening member 1, an insertion hole 5b into which the shaft portion 2b is inserted, and a chamfered portion 5c between the contact surface 5a and the inner peripheral surface 5b1 of the insertion hole 5b, and in the fastening step, the plastic washer 1a is plastically deformed until it is pressed against the chamfered portion 5c by the fastening of the screw 2. According to the above configuration, in the fastening step, the plastic washer 1a is plastically deformed until it is pressed against the chamfered portion 5c, so that the deformation of the plastic washer 1a radially outward is suppressed by the anchor effect of the chamfered portion 5c. In addition, the fastening step allows the plastic washer 1a to be shaped so that it can be fitted into the chamfered portion 5c, so that the screw structure 3 formed after the fastening of the screw 2 can be removed from the fastened portion 5 and then easily positioned in the fastened portion 5 when the screw structure 3 is fastened to the fastened portion 5 again.

[0043] In the fastening method of the screw fastening structure 4, the fastened portion 5 has a contact surface 5a that contacts the fastening member 1, an insertion hole 5b into which the shaft portion 2b is inserted, and a chamfered portion 5c between the contact surface 5a and an inner peripheral surface 5b1 of the insertion hole 5b, and in the fastening step, the plastic washer 1a is pressed against the chamfered portion 5c by fastening the screw 2, thereby plastically deforming the plastic washer 1a until the inner peripheral surface 1a1 of the plastic washer 1a is pressed against the shaft portion 2b. According to the above configuration, in the fastening step, a pressing effect can be obtained by the chamfered portion 5c that guides the direction of plastic deformation toward the shaft portion 2b, so that the plastic washer 1a can be easily plastically deformed until it is pressed against the shaft portion 2b, and the plastic washer 1a can be well held on the shaft portion 2b.

[0044] More specifically, the fastening method of the screw fastening structure 4 uses the screw fastening structure 4 of the above-described embodiment.

[0045] The present disclosure is not limited to the above-described embodiment, and various modifications are possible without departing from the spirit and scope of the present disclosure. [Explanation of symbols]

[0046] 1 Fastening members 1a Plastic washer 1a1 Inner surface 1b Spring washer 1c First flat washer 1d Second flat washer 2 screws 2a head 2b Shaft 2b1 thread 2b2 Non-threaded part 3 Screw structure 4. Screw fastening structure 5 Part to be fastened 5a Contact surface 5b Insertion hole 5b1 Inner surface 5c Chamfered part 6. Medical Devices 7 Electronic equipment D1 Outside diameter of plastic washer D2 Outside diameter of second flat washer F1, F2 load θ Tilt angle

Claims

1. A fastening member for fastening a screw having a head and a shaft, A plastic washer is provided. The plastic washer is a fastening member that can be or has been plastically deformed until the inner circumferential surface of the plastic washer is pressed against the shaft portion when the screw is fastened.

2. The fastening member according to claim 1 , wherein the plastic washer is capable of or has been plastically deformed until an inner circumferential surface of the plastic washer is pressed against the threads of the shaft portion by tightening the screw.

3. The fastening member according to claim 1 , comprising, from the head side toward the shaft side, a spring washer, a first flat washer, a second flat washer, and the plastic washer, in this order.

4. The second flat washer and the plastic washer are adjacent to each other, The fastener of claim 3 , wherein the plastic washer has an outer diameter smaller than an outer diameter of the second flat washer.

5. 4. The fastening member according to claim 3, wherein a coefficient of friction between the first flat washer and the second flat washer is lower than a coefficient of friction between the second flat washer and the plastic washer.

6. A screw structure comprising the fastening member according to claim 1 and the screw.

7. A screw fastening structure comprising: the screw structure according to claim 6; and a fastened portion into which the screw is fastened.

8. the fastened portion has a contact surface that contacts the fastening member, an insertion hole into which the shaft portion is inserted, and a chamfered portion between the contact surface and an inner circumferential surface of the insertion hole, The screw fastening structure according to claim 7 , wherein the plastic washer can be or has been plastically deformed by fastening the screw until it is pressed against the chamfered portion.

9. the fastened portion has a contact surface that contacts the fastening member, an insertion hole into which the shaft portion is inserted, and a chamfered portion between the contact surface and an inner circumferential surface of the insertion hole, The screw fastening structure according to claim 7, wherein the plastic washer can be or has been plastically deformed by being pressed against the chamfered portion when the screw is fastened, until the inner circumferential surface of the plastic washer is pressed against the shaft portion.

10. The screw fastening structure according to claim 8 , wherein the chamfered portion is a C-chamfered portion.

11. The screw fastening structure according to claim 10 , wherein an inclination angle of the C-chamfered portion as the chamfered portion with respect to a plane perpendicular to the shaft portion is equal to or greater than 30° and equal to or less than 60°.

12. A medical device having the screw fastening structure according to claim 7.

13. A method for fastening a screw fastening structure, comprising the steps of: The screw fastening structure includes a screw structure and a fastened portion to which the screw is fastened, The screw structure includes a fastening member and the screw, The fastening member is for fastening a screw having a head and a shaft and has a plastic washer; A fastening method for a screw fastening structure, comprising a fastening step of plastically deforming the plastic washer by fastening the screw until the inner circumferential surface of the plastic washer is pressed against the shaft portion.

14. the fastened portion has a contact surface that contacts the fastening member, an insertion hole into which the shaft portion is inserted, and a chamfered portion between the contact surface and an inner circumferential surface of the insertion hole, The fastening method for a screw fastening structure according to claim 13, wherein in the fastening step, the plastic washer is plastically deformed by fastening the screw until the plastic washer is pressed against the chamfered portion.

15. the fastened portion has a contact surface that contacts the fastening member, an insertion hole into which the shaft portion is inserted, and a chamfered portion between the contact surface and an inner circumferential surface of the insertion hole, 14. The fastening method for a screw fastening structure according to claim 13, wherein in the fastening step, the plastic washer is plastically deformed until the inner circumferential surface of the plastic washer is pressed against the shaft portion by fastening the screw to press the plastic washer against the chamfered portion.

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