Fastening structure

Abstract

 The present invention makes it possible to avoid a catching phenomenon in which smooth rotation is prevented by a small screw being caught on an inclined surface of a washer due to screwing, and to also reduce processing costs. In this fastening structure, a threaded shaft (1a) of a bolt (1) for mounting a mounting object is passed through a mount hole of a mount member (2) until the tip portion protrudes, and a small screw (5) attached to a nut (4) on the tip portion via a washer (3) is screwed, thereby fixing the mounting object to the mount member (2). The washer (3) has: an inclined surface (3b) formed on a first surface (3A) facing the nut (4); and a second surface (3B) which is a flat surface on the opposite side to the first surface (3A). The inclined surface (3b) is formed such that its height gradually decreases in a tightening direction of the nut (4). The small screw (5) with a spherical tip (5b) is screwed into the nut (4) such that the screw axis is parallel to the axis of the nut (4), and the tip (5b) of the small screw (5) protrudes and is in contact with the inclined surface (3b). The inclined surface (3b) has a smaller friction coefficient than the second surface (3B) of the washer (3).

Description

Fastening structure

[0001] The present invention relates to a fastening structure using nuts and bolts.

[0002] Conventionally, many methods have been put into practical use to prevent loosening, such as using spring washers, using double nuts, or passing a split pin through the nut and bolt. However, these methods are not effective enough and loosening can occur due to vibration, or the mechanisms and processing are complex and expensive.

[0003] To address this issue, various locking pin mechanisms have been proposed, including one in which a lock pin is inserted into a hole on the inner surface of the nut, expanding the nut's inner surface and generating anti-loosening pressure; and another in which a locking pin insertion hole is drilled diagonally through the nut at a position slightly away from the inner surface of the nut, and a locking pin that is slightly longer than the length of the locking pin insertion hole and has a pointed tip is inserted.The nut is then manually turned to fit and tightened onto the male threads of the bolt until it reaches the washer, at which point it is tightened firmly with a commercially available tool such as a wrench, thereby securing the object sandwiched between the two washers.Further tightening presses the locking pin against the washer, bringing the tip of the locking pin into firm contact with the male threads, thereby pressing against the male threads and creating an anti-loosening effect.Tightening even more strongly causes the tip of the locking pin to bite into the male threads like a stake, further enhancing the anti-loosening effect (see, for example, Patent Documents 1 and 2).

[0004] The nuts described in Patent Documents 1 and 2 are fixed by expanding the inner surface of the nut or by having the tip of the lock pin bite into the threaded rod of the bolt, which damages the threaded rod (threaded portion) and makes it impossible to reuse the nut even if it is loosened for maintenance, etc.

[0005] Therefore, a fastening structure has been proposed that does not loosen when fastened, can be easily loosened during maintenance, and allows the reuse of bolts and nuts.The fastening structure involves passing the threaded rod of a bolt used to attach an object to a workpiece through a mounting hole in the workpiece so that its tip protrudes, and then screwing a machine screw attached to a nut via a washer into the tip with an appropriate torque to secure the object to the workpiece.The washer has a first surface facing the nut that is inclined, and a second surface opposite the first surface that is flat, and the inclined surface is formed so that its height gradually decreases in the direction in which the nut is tightened.The machine screw is screwed into the nut so that its axis is perpendicular to the inclined surface, and the tip of the machine screw protrudes so that it comes into contact with the inclined surface, and the inclined surface has a smaller coefficient of friction than the second surface of the washer (see, for example, Patent Document 3).

[0006] In this way, the machine screw is threaded into the nut so that its axis is perpendicular to the inclined surface on the first surface of the washer, so the threaded part of the bolt (threaded portion) is not damaged and the nut can be reused. Also, even if the nut tries to turn in the loosening direction due to vibration or other reasons, the inclined surface has a smaller coefficient of friction than the second surface, so the machine screw tries to move up the inclined surface, applying a force that presses the washer against the attached workpiece, making the nut less likely to loosen.

[0007] JP 2009-293793 A JP 2011-179679 A JP 2018-138814 A

[0008] However, with the structure of Patent Document 3, (i) the washer has as many inclined surfaces as there are machine screws (i.e., a large number), which increases the cost of machining these surfaces. (ii) Because the machine screws are screwed in at an angle, the screw holes for the machine screws must be machined at an angle relative to the nut axis, which also increases the cost. (iii) Although the tip of the machine screw is designed to strike the washer's inclined surface at a right angle, if the machine screw is screwed in too tightly, it will slightly sink into the inclined surface. If the nut tries to turn in the loosening direction (counterclockwise) due to vibration, the machine screw (with its tip slightly recessed) may get caught on the washer's inclined surface, preventing smooth rotation. In other words, there are issues such as the jamming phenomenon, in which the washer rotates along with the nut.

[0009] The function of this structure is that, provided the nut and washer do not rotate together, the small screw slides up the inclined surface of the washer, preventing the bolt from loosening even when the axial force of the bolt increases and it is subjected to vibration. However, this function is often lost due to the aforementioned catching phenomenon.

[0010] An object of the present invention is to provide a fastening structure that can avoid the aforementioned catching phenomenon and reduce processing costs.

[0011] The present invention provides a fastening structure in which the threaded rod of a bolt for attaching an object to be attached is passed through a mounting hole in an attached member so that its tip protrudes, and a machine screw attached to a nut via a washer is screwed into the tip, wherein the washer has a first surface facing the nut that is inclined, and a second surface opposite the first surface that is flat, and the inclined surface is formed so that its height gradually decreases in the direction in which the nut is tightened, and a machine screw having a curved tip is screwed into the nut so that its screw axis is parallel to the axis of the nut, and the tip of the machine screw protrudes from the nut and is in contact with the inclined surface, and the inclined surface has a smaller coefficient of friction than the second surface of the washer.

[0012] This simplifies drilling the threaded holes for the machine screws, prevents damage to the threaded portion of the bolt, and allows for reuse. Furthermore, even if the nut or bolt attempts to rotate in the loosening direction due to vibration or other factors, the friction coefficients between the inclined surface and the second surface of the washer are different, and the movement of the machine screw presses the washer against the attached member, making it less likely to loosen. Additionally, when the nut attempts to rotate in the loosening direction (counterclockwise) due to vibration, the curved surface shape of the tip of the machine screw allows it to slide without getting caught. This prevents the machine screw (which tends to be slightly threaded) from getting caught on the inclined surface of the washer and preventing it from rotating smoothly (i.e., the washer getting caught and rotating along with the nut).

[0013] In this case, the curved surface shape of the tip of the machine screw can be spherical. The spherical shape is preferably such that, as the tip is pressed against the inclined surface of the washer and tightened, the shape of the tip changes and gradually becomes flat, so that when the machine screw is tightened with a specified maximum force, it fits snugly against the inclined surface of the washer. In this way, the average surface pressure on the tip end face of the machine screw is reduced when the machine screw is fully tightened.

[0014] The inclined surface may have a smoother surface roughness than the second surface, and the inclined surface may have a regular finish, while the second surface may have a rough finish. The inclined surface may be one or more inclined surfaces formed in the circumferential direction around the central hole of the washer.

[0015] Furthermore, the present invention can be applied not only to a fastening structure in which an object to be attached is attached using a nut via a washer, but also to a fastening structure in which an object to be attached is fixed to a workpiece using a headed bolt. That is, the present invention includes a fastening structure in which an object to be attached to a workpiece is fixed using a headed bolt, wherein a washer is placed on the threaded stem of the bolt adjacent to the head of the bolt, the washer has a first surface facing the head of the bolt that is inclined and a second surface opposite the first surface that is flat, the inclined surface is formed so that its height gradually decreases in the direction of tightening of the bolt, a machine screw having a curved tip is screwed into the head of the bolt so that its thread axis is parallel to the axis of the nut, the tip of the machine screw protrudes from the nut and is in contact with the inclined surface, and the inclined surface has a smaller coefficient of friction than the second surface of the washer.

[0016] The present invention simplifies the drilling of threaded holes for machine screws, does not damage the threaded portion of the bolt, and is reusable. Furthermore, even if the nut or bolt attempts to rotate in the loosening direction due to vibration or other factors, the friction coefficients between the inclined surface and the second surface of the washer are different, so the movement of the machine screw presses the washer against the attached member, making it difficult for the machine screw to loosen. In addition, this prevents the machine screw from getting caught on the inclined surface of the washer and preventing it from rotating smoothly when the nut attempts to rotate in the loosening direction (counterclockwise) due to vibration.

[0017] Fig. 1 is a cross-sectional view showing a fastening structure according to the present invention. Fig. 2 is an enlarged view of part A in Fig. 1. Fig. 3 is a perspective view of a washer used in the fastening structure according to the present invention, seen from above. (a) is an explanatory diagram of the surface pressure distribution at the initial point (at contact) when the tightening load is small, and (b) is an explanatory diagram of the surface pressure distribution at the point when the tightening load is fully tightened (at tightening). Fig. 4 is a cross-sectional view showing another embodiment.

[0018] Hereinafter, an embodiment of the present invention will be described.

[0019] FIG. 1 is a cross-sectional view showing a fastening structure according to the present invention, and FIG. 2 is an enlarged view of part A in FIG.

[0020] As shown in Figures 1 and 2, the fastening structure of the present invention is a fastening structure in which the threaded rod 1a of a bolt 1 for attaching an object to be attached (not shown) to a workpiece 2 is passed through a mounting hole in the workpiece 2 so that the tip portion protrudes, and a nut 4 is applied to the tip portion via a washer 3 to fix the object to the workpiece 2.

[0021] As shown in Figure 3, the washer 3 has a central hole 3a through which the threaded rod 1a passes. An inclined surface 3b is formed on the first surface 3A facing the nut 4 in the circumferential direction around the central hole 3a. On the other hand, the second surface 3B is flat. The inclined surface 3b may be single or multiple, and the number of inclined surfaces is unrelated to the number of machine screws 5 described below. Figure 3 shows an example with two inclined surfaces 3b.

[0022] The inclined surface 3b is continuous with a wall surface 3c rising from the end of the nut 4 in the tightening direction. These surfaces 3b, 3c are perpendicular to each other. The inclined surface 3b is inclined with respect to the second surface 3B at an angle θ of approximately 3 to 20°, which is at least equal to or greater than the lead angle of the nut (or bolt) thread.

[0023] The first surface 3A (inclined surface 3b) and the second surface 3B of the washer 3 have different surface roughnesses, which results in different coefficients of friction. The inclined surface 3b is smoother than the second surface 3B, making its coefficient of friction smaller. For example, the inclined surface 3b is finished to a medium finish, while the second surface 3B is finished to a rough finish.

[0024] The nut 4 has a threaded hole 4a, into which a machine screw 5 is screwed, formed around the central hole of the nut 4 in correspondence with the inclined surface 3b so that the screw axis is parallel to the nut axis. The machine screw 5 is screwed into the threaded hole 4a of the nut 4, and the tip 5b protrudes from the nut 4 and is in contact with the inclined surface 3b. The tip 5b of the machine screw 5 has a curved surface shape (for example, a spherical shape) with the center part protruding the most and is rounded.

[0025] If the tip of the machine screw is flat, the maximum outer diameter portion (edge) contacts the inclined surface 3b at a substantially right angle, and the edge of the machine screw 5 that has been driven in will easily get caught on the inclined surface 3b. However, if the tip is curved and rounded as in the present invention, the tip portion (rounded portion) of the machine screw 5 will be slightly recessed, and at the same time, the rounded portion on the inclined surface side will also sink slightly into the inclined surface 3b. However, by adjusting the size of the roundness, even if a slight recess is created in the inclined surface 3b, the roundness makes it easier for the machine screw 5 to slide without getting caught.

[0026] In this way, by rounding the tip of the machine screw 5 into a curved surface, it becomes unnecessary for the machine screw 5 to contact the inclined surface 3b of the washer 3 at a right angle, and the machine screw 5 does not need to be tilted relative to the nut 4, so the threaded hole 4a into which the machine screw 5 is screwed can be formed so that its axis is parallel to the axis of the nut 4. This has the great advantage of allowing for a significant reduction in processing costs while maintaining the function of preventing loosening.

[0027] In other words, with this structure, when the nut 4 attempts to rotate in the loosening direction (i.e., counterclockwise) due to vibration or other factors, the machine screw 5 moves up the inclined surface 3b of the washer 3, which results in an increase in the distance between the top surface of the nut 4 and the second surface 3B of the washer 3. At this time, because the friction coefficients of the first and second surfaces 3A, 3B of the washer 3 are different, the slippage between the inclined surface 3b of the washer 3, which has a smaller friction coefficient, and the tip of the machine screw 5 will occur first, meaning that they will slide against each other before the second surface 3B and the workpiece 2 do. Even a slight slippage increases the distance, and the second surface 3B of the washer 3 is pressed more strongly against the seating surface (workpiece 2), making it increasingly difficult to slip.

[0028] The greater the inclination angle of the inclined surface 3b, the stronger the second surface 3B of the washer 3 is pressed against the seating surface, further increasing the frictional force and making it increasingly difficult to rotate. In other words, the nut 4 will not loosen.

[0029] As an example, if the bolt 1 has a diameter of 36 mm (M36), a length five times the diameter (180 mm), a bolt axial force (= tightening force) of 350 kN, and the inclination depth of the inclined surface 3b of the washer 3 is 0.6 mm, the force (= axial force, or rotational torque) required to (forcefully) loosen the nut 4 will be 196% (approximately twice) of the force required when tightening, which is 100%.

[0030] If the diameter of the bolt 1 is 64 mm and the axial force is 1150 kN (other conditions being the same), the force required to loosen the nut 4 will be 153% (1.5 times). The force required to loosen the nut 4 mentioned above will vary depending on the inclination depth of the inclined surface 3b on the washer 3 (height of the wall surface 3c), and the deeper the depth, the greater the force (loosening rotation torque or axial force) required.

[0031] The fact that 153 to 196% of the force is required to loosen it by force is the same as saying that nut 4 will not loosen. This is because bolts are normally fastened at 60 to 90% of their yield point, so if nearly twice the loosening torque (or axial force) is applied, the bolt will break (before it loosens).

[0032] Nevertheless, the nut 4 usually needs to be able to be easily loosened for maintenance etc. In other words, there are contradictory requirements for the nuts: they should never loosen, but they should be able to loosen easily when they need to be loosened.

[0033] This fastening structure perfectly meets the above-mentioned requirements, and the relationship between the nut 4 and the washer 3 means that the nut 4 will never loosen naturally, even if subjected to vibrations. On the other hand, if you need to loosen the nut 4 for maintenance, you can do so easily with just a hand wrench, for example. In other words, by loosening the machine screw 5 with just a hand wrench, you can easily loosen the nut 4, even if it is a little tighter than when it was originally fastened.

[0034] In other words, with this fastening structure, the relationship between the nut 4 (machine screw 5) and the washer 3 (inclined surface 3b) means that the nut 4 will not loosen on its own even if subjected to vibrations, but if you want to loosen the nut 4 for maintenance purposes, you can easily do so with just a hand wrench, for example.

[0035] Incidentally, paragraph 0050 of the aforementioned Patent Document 3 discloses that, as a measure for making the axis of the machine screw parallel to the axis of the thread of the nut, the tip of the machine screw is made hemispherical, and the sloped side of the washer is also made hemispherical so that the tip of the machine screw is aligned with the hemispherical part of the machine screw, while a concave groove with a semicircular cross section that follows the sloped surface is carved into the sloped surface, and the machine screw is not tilted at an angle but is made vertical, achieving the same effect. This is because, by carving a concave groove with a semicircular cross section that follows the sloped surface into the sloped surface, the contact area between the tip of the machine screw and the concave groove of the sloped surface is increased, thereby reducing (averaging) the contact surface pressure. Otherwise, if the sloped surface were flat, the surface pressure at the tip of the machine screw would increase dramatically, causing it to collapse.

[0036] However, carving a semicircular groove in the washer like this is costly, and because there is naturally a gap (play) between the inside diameter of the washer and the diameter of the machine screw, there is a risk that the washer will shift from the axis of the nut by the amount of play when the machine screw is installed. Because of this misalignment, there is always a possibility that the center of the concave groove on the washer will shift from the center of the machine screw. If this happens, the hemispherical tip of the machine screw may interfere with the side of the concave groove, causing malfunction.

[0037] In contrast, in the present invention, the washer's inclined surface is flat. Therefore, even if the washer's inner diameter is misaligned with the central axis of the nut, the tip of the machine screw simply moves on a flat surface, eliminating the interference problem with the concave groove seen in Patent Document 3. Another important difference is that the roundness of the tip of the machine screw is so large (i.e., slightly spherical) that it is virtually unrecognizable as a hemisphere. This radius is determined by stress and strain precisely calculated using the Hertz equation. Specifically, as the spherical tip of the machine screw is pressed against the washer surface and tightened, the shape of the spherical tip gradually changes (because the machine screw is an elastic body), gradually flattening until it fits snugly into the flat inclined surface when the screw is tightened to the specified maximum torque. As a result, the average surface pressure on the end face of the screw's tip is reduced (i.e., evened out) when fully tightened.

[0038] In other words, while the structure described in paragraph 0050 of Patent Document 3 increases the contact area of ​​the tip of the machine screw from the beginning to reduce the surface pressure, in the present invention, the tip of the machine screw makes "point contact" with the washer surface at the initial stage (when it makes contact) when the tightening load is small (see Figure 4(a)), but when it is fully tightened (when fastened), the tip becomes nearly flat, reducing the surface pressure against the washer (see Figure 4(b)). This physical difference between the two is significant. In this case, the flat surface of the washer also becomes slightly concave, but the amount of this concave is also calculated using Hertz's equation, and the curvature R dimension of the tip of the machine screw is finally determined through repeated actual experiments so that the machine screw can slide up the inclined surface of the washer without hindrance when the nut moves to loosen.

[0039] As described above, the embodiments of the present invention have been described with reference to the drawings. However, various additions and modifications are possible without departing from the spirit and scope of the present invention. For example, while the above-described embodiments employ a fastening structure utilizing the relationship between a nut and a washer, the present invention can also be applied to a fastening structure utilizing the relationship between the head 1b of a headed bolt 1 and a washer 3, as shown in FIG. 5. In this case, a machine screw 5 having a spherical tip 5b is threaded into the head 1b of the bolt 1 with its screw axis parallel to the axis of the bolt 1, and the tip 5b of the machine screw 5 contacts the inclined surface 3b. In this case, the head 1b of the headed bolt 1 is not limited to a hexagonal cross section, but may also be a circular cross section.

[0040] REFERENCE SIGNS LIST 1 Bolt 1a Threaded rod 1b Head 1c Screw hole 2 Mounted member 3 Washer 3A First surface 3a Center hole 3B Second surface 3b Inclined surface 3c Wall surface 4 Nut 4a Screw hole 5 Machine screw 5a Threaded portion 5b Tip

Claims

1. A fastening structure for fixing an object to be attached to a member to be attached by inserting a threaded rod of a bolt for attaching the object to be attached through a mounting hole of the member to be attached and protruding a tip portion, and screwing a small screw attached to a nut through a washer to the tip portion, wherein the washer has an inclined surface formed on a first surface facing the nut, while a second surface opposite to the first surface is a flat surface, the inclined surface is formed such that its height gradually decreases in the tightening direction of the nut, a small screw having a curved tip surface is screwed into the nut so that the screw axis is parallel to the axis of the nut, and the tip of the small screw protrudes from the nut and contacts the inclined surface, and the inclined surface has a smaller coefficient of friction than the second surface of the washer.

2. The fastening structure according to claim 1, wherein the curved surface shape of the tip of the small screw is a spherical shape.

3. The fastening structure according to claim 2, wherein the spherical shape is such that when the tip is pressed against the inclined surface of the washer and tightened, the shape of the tip gradually becomes flat as it changes, and at the time when the small screw is tightened with a specified maximum force, it fits exactly to the inclined surface of the washer.

4. The fastening structure according to claim 1, wherein the inclined surface is smoother in surface roughness than the second surface, the inclined surface is finish-machined, and the second surface is rough-machined.

5. The fastening structure according to claim 1, wherein the inclined surface is formed singly or plurally in the circumferential direction around the central hole of the washer.

6. A fastening structure for fixing an object to be attached to a member to be attached by a bolt with a head, wherein a washer is disposed on the threaded rod of the bolt adjacent to the head of the bolt, the washer has an inclined surface formed on a first surface facing the head of the bolt, while a second surface opposite to the first surface is a flat surface, the inclined surface is formed such that its height gradually decreases in the tightening direction of the bolt, a small screw having a curved tip shape is screwed into the head of the bolt so that the screw axis is parallel to the axis of the nut, and the tip of the small screw protrudes from the nut and contacts the inclined surface, and the inclined surface has a smaller coefficient of friction than the second surface of the washer.

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