Double screw double nut fastener

Abstract

To provide a fastening method capable of eliminating looseness even when exposed to vibration in double nut fastening with a double screw structure.SOLUTION: When a double nut of an inside nut 7 and an outside nut 9 is screwed into a screw part 10 formed with a double screw, a spring washer 8 is interposed between both nuts. By only tightening the outside nut 9, the inside nut 7 is also tightened and the spring washer 8 is compressed. The spring washer 8 can ensure both axial force for clamping an object to be fastened 2 and axial force between the nuts, so that it is difficult to loosen.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a double-nut fastener for a double screw. More specifically, the present invention relates to a double-nut fastener for optimally double-nut fastening a double screw.

Background Art

[0002] The inventors of the present invention have proposed a double-screw structure that is difficult to loosen by double-nut fastening (Patent Documents 1 and 2). The double-screw structure in which two types of screws are formed on the screw shaft is, for example, a first screw (S1) that is a metric pitch screw, and a multi-start screw having the same pitch direction as the first screw (S1) and a lead that is a multiple of the pitch of the first screw (S1). It has a second screw (S2) formed by overlapping one or more extracted screws on the first screw (S1). This double-screw structure is usually used as a fastener by tightening with two nuts: a metric pitch nut screwed onto the first screw (S1) and a high-lead nut (nut for multi-start screw) screwed onto the second screw (S2). This fastening has the advantage that it is only necessary to tighten only the metric pitch nut, and there is no need for fastening operations such as retightening required for double-nut fastening.

[0003] On the other hand, in double-nut fastening using a general screw, a spring washer is proposed to be interposed between the nuts to prevent loosening of the nut (Patent Document 3). Also, although it is not double-nut fastening, in bolt-nut fastening using a normal spring washer, a research report has been made on the anti-rotation prevention function (loosening prevention function) of the spring washer by analysis using the finite element method (Non-Patent Document 1). According to this report, it is reported that the axial force decreases significantly when a spring washer is used. In other words, it is reported that the nut loosens even when a spring washer is interposed when a vibration load is applied to the fastened body with an excitation force in the direction perpendicular to the axis of the bolt.

Prior Art Documents

Patent Documents

[0004] [Patent Document 1] WO2016 / 194842 [Patent Document 2] WO2019 / 230167 [Patent Document 3] Publicly Published Utility Model No. 61-180462 [Non-patent literature]

[0005] [Non-Patent Document 1] "Analysis of Washer Loosening Behavior using Three-Dimensional Finite Element Method" by Narutake Kimura and two others (https: / / www.fml.tu-tokyo.ac.jp / ~izumi / papers / Spring_washer070326.pdf) [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] The double-nut fastening system using a double-screw configuration proposed by the inventors of this invention poses no problems in ordinary structures, but in structures such as railway bridges that are constantly subjected to severe vibrations, the double nuts may loosen. While loosening can be prevented by periodic inspections, it should be avoided as much as possible. The method proposed in Patent Document 3 is for suppressing loosening when fixing terminals of electrical equipment wiring with standardized screws, and is not intended to prevent loosening in robust double-nut fastenings that can be used in large structures. The loosening prevention method using spring washers reported in Non-Patent Document 1 was not used in double-nut fastenings and is based on the premise of using general screws such as metric coarse threads.

[0007] Based on the above background, the present invention aims to achieve the following objectives. The object of the present invention is to provide a double-nut fastening system using a double-screw structure that does not loosen even when exposed to external forces such as vibration, by simply tightening the outer nut. Another object of the present invention is to provide a double-nut fastening system with a double screw configuration that, in a double-nut fastening system using a double screw configuration, can maintain the clamping force (axial force) between the double nuts without reducing the clamping force of the fastened object by tightening only the outer nut. [Means for solving the problem]

[0008] The double-screw double-nut fastening body of the present invention 1 is On the screw shaft, metric Coarse grain It's a screw. ru A first thread, and formed in the same twist direction as the first thread Two threads with a pitch three times that of the aforementioned metric coarse thread A double screw structure in which two types of threads are formed, The first screw is screwed into outside Nut and, The bolt heads of the double screw structure are sandwiched between Parts to be fastened In contact with, The aforementioned outside The nut is screwed into the second thread at an internal position. inside Nut and In a double-nut fastening body with a double screw, The aforementioned outside The nut and the aforementioned inside It is characterized by consisting of a spring-type washer that is elastically deformable and fitted between nuts and into the double screw structure.

[0009] The double-screw double-nut fastening body of the present invention 2 is characterized in that, in the double-screw double-nut fastening body of the present invention 1, the spring-type washer is a spring washer. The double-screw double-nut fastening body of the present invention 3 is characterized in that, in the double-screw double-nut fastening body of the present invention 1, the spring-type washer is a disc spring washer.

[0010] The double-screw double-nut fastening body of the present invention 4 is a double-screw double-nut fastening body of the present invention 1, wherein the fastening is performed in the initial stage of vibration load. component The axial force increases, making it less likely to loosen. [Effects of the Invention]

[0011] The double-nut fastening body of the double-thread of the present invention only tightens the outer nut, and since it can ensure both the securing of the axial force on the fastened body side and the securing of the axial force between the nuts even under a vibration load, it is difficult to loosen.

Brief Description of the Drawings

[0012] [Figure 1] FIG. 1 is a diagram showing an example of a fastening structure in which a fastened body is fixed with bolts and nuts. [Figure 2] FIG. 2 is a partial enlarged cross-sectional view of the double-nut portion of FIG. 1. [Figure 3] FIG. 3 shows the time change of the axial force when a spring washer is sandwiched between the nuts in the double-nut fastening body of the double-thread. FIG. 3(a) shows the time change of the axial force of the screw shaft between the nuts, and FIG. 3(b) is a diagram showing the time change of the axial force of the screw shaft of the fastened body portion. [Figure 4] FIG. 4 shows the time change of the axial force when a disc spring washer is sandwiched between the nuts in the double-nut fastening body of the double-thread. FIG. 4(a) shows the time change of the axial force between the nuts, and FIG. 4(b) is a diagram showing the time change of the axial force of the fastened body portion. [Figure 5] FIG. 5 is a comparative example, and shows the time change of the axial force when there is no spring washer or the like between the nuts in a conventional double-nut fastening body of the double-thread. FIG. 5(a) shows the time change of the axial force between the nuts, and FIG. 5(b) is a diagram showing the time change of the axial force of the fastened body portion.

Embodiments for Carrying Out the Invention

[0013] The first embodiment of the present invention will be described below with reference to the drawings. Figure 1 shows an example of a fastening structure in which a fastened object is fixed with a bolt and nut. This fastening structure 1 is a double nut consisting of two nuts, with a spring washer 8 interposed between the double nuts. In the splice joint of a steel bridge, a through hole 3 is made in the fastened member 2, which is made of two overlapping plates. The cylindrical portion 6 of the bolt 4 is inserted into this through hole 3. The fastened member 2 is sandwiched between the bolt head 5 of the bolt 4 and the inner nut 7, and the spring washer 8 is inserted on the outside of the inner nut 7, and further, an outer nut 9 is screwed on the outside of this to fix it. In this example, the bolt 4 is a special screw with a double thread formed thereon. The threaded portion 10 of the bolt 4 is formed so that two types of threads, a first thread and a second thread, overlap.

[0014] In this example, the first thread is a single-thread metric coarse thread as defined by industrial standards. The second thread is formed in the same twist direction as the first thread, as if removing material from the metric coarse thread. The lead (Ln) is three times the pitch (P) of the metric coarse thread, and it is a two-thread thread with a changed phase (a two-thread thread, obtained by removing one thread from a three-thread thread, arranged at equal angular intervals). The details of this double thread are described in detail in Patent Documents 1 and 2 and are publicly known technology, and are not the gist of the present invention, so a detailed explanation is omitted. In this example, the fastening structure 1 is constructed by inserting a bolt 4 into a through hole 3 of the member to be fastened 2, manually screwing an inner nut 7 onto the threaded portion 10 of the bolt 4, then inserting a spring washer 8 onto the threaded portion 10, and finally screwing an outer nut 9 onto the threaded portion 10.

[0015] This fastening structure 1 is fastened by turning only the outer nut 9 with a set torque using a wrench (not shown). This means that the tightening and loosening of the inner nut 7, which are required in general double-nut fastening operations, are not necessary. When the outer nut 9 is screwed into the threaded portion 10 with torque T, an axial force F is generated in the cylindrical portion 6 of the bolt 4. The fastening structure 1 shown in Figure 1 fastens the fastened member 2 by turning the outer nut 9, and no other fastening operations are performed. Therefore, it is essential that the outer nut 9 does not loosen (rotate) due to external forces such as vibration after being fastened to the predetermined torque.

[0016] For this reason, after turning the outer nut 9 to a set torque T, it is necessary to maintain a constant axial force of the bolt 4 for tightening the fastened member 2, and also to ensure an axial force between the inner nut 7 and the outer nut 9, which act as a locking mechanism, so that the outer nut 9 does not loosen. In a typical double-nut fastening, the process of tightening the inner nut 7 can be said to be an operation to maintain the axial force between the double nuts. In other words, an ideal fastening structure 1 requires that by simply tightening the outer nut 9 to a set torque, two axial forces—the bolt axial force F for tightening the fastened member 2 and the axial force between the outer nut 9 and the inner nut 7—be properly maintained.

[0017] This results in an optimal fastening structure that provides the fastening function of fastening structure 1 while also preventing screw loosening. When the outer nut 9 is tightened with torque T, the axial force F of the screw shaft is uniquely determined once the friction coefficients of the seating surface and screw surface of the outer nut 9 are determined, and therefore cannot be adjusted unless the friction coefficients are changed with lubricants, etc. Note that in a conventional double nut fastening structure without a spring washer 8, it is possible to increase the axial force between the nuts between the outer nut 9 and the inner nut 7 by tightening, but when tightened, the axial force F of the screw shaft decreases. In the case of a normal double nut fastening structure, in order to secure this axial force F, it is necessary to initially increase the tightening torque of the inner nut 7 and outer nut 9 above the design torque, and then tighten the inner nut 7.

[0018] Figure 2 is a partially enlarged view of the fastening structure 1 shown in Figure 1, showing the state before the outer nut 9 is tightened. When tightening is started with the outer nut 9, the outer nut 9 comes into contact with the spring washer 8. At this time, the spring washer 8 maintains a distance Δy in the axial direction (see Figure 2). As the outer nut 9 is further rotated, the spring washer 8 is sandwiched between the inner nut 7 and the outer nut 9, narrowing the distance, until finally the distance t is approximately the same as the thickness of the spring washer 8 (see Figure 1). The elastic deformation of the spring washer 8 at this time is conserved as energy. As can be understood from the above explanation, a disc spring with a similar function may be used instead of the spring washer 8.

[0019] Tightening torque for inner nut 7 When the outer nut 9 is turned, the spring washer 8 is rotated via its seating surface, and the rotation of the spring washer 8 rotates the inner nut 7. In other words, the rotational torque from the outer nut 9 to the inner nut 7 is transmitted solely by frictional force to the seating surface of the outer nut 9 on the spring washer 8 side and to the spring washer 8 in contact with this seating surface. Furthermore, the rotational torque of the spring washer 8 is transmitted to the inner nut 7 via the inner seating surface of the spring washer 8 and the outer seating surface of the inner nut 7.

[0020] When the outer nut 9 is tightened with torque T, the spring washer 8 is compressed (conserved as elastic energy), and this spring force generates a load W in the axial direction of the outer nut 9. Here, in this embodiment, the double screw configuration satisfies the condition "θ1 (angle of inclination of the thread surface of the outer nut 9 (angle of inclination of the first thread)) < θ2 (angle of inclination of the thread surface of the inner nut 7 (angle of inclination of the second thread))". Since the inner nut 7 is rotationally driven only by the frictional force due to the rotation of the outer nut 9, it does not rotate even if the outer nut 9 is rotated once a predetermined torque is reached. Therefore, when the outer nut 9 is tightened with rotational torque T, the inner nut 7 rotates by a predetermined angle, then stops rotating, and the spring washer 8 between the inner nuts 7 is compressed (as shown in Figure 1). In this way, simply tightening the outer nut 9 tightens the inner nut 7 as well, and compresses it with the compressive force W of the spring washer 8.

[0021] Axial force between the inner nut 7 and the outer nut 9 due to the outer nut 9 When the outer nut 9 is rotated against the axial load W, the spring washer 8 is pressed, and the spring washer 8 is compressed from Δy to a thickness t. The load W on the spring washer 8 becomes the axial force between the inner nut 7 and the outer nut 9 and is stored as elastic energy. The load W is reduced simply by tightening the outer nut 9, and the inner nut 7 and Outer nut 9 Since this becomes the axial force between the nuts, it becomes the force that locks both nuts, so there is no need to tighten the inner nut 7 as in a normal double nut fastening. Furthermore, unlike bolt fastenings using spring washers in reported non-double nut fastenings, the fastening in this embodiment will not loosen (see Non-Patent Literature 1).

[0022] As understood from the above description, in the fastening structure 1 shown in FIG. 1, with respect to the bolt 4, as the force in its axial direction, by simply tightening the outer nut 9, the above axial force F and load W are applied. Specifically, the shaft portion 6 of the bolt 4 becomes the axial force F, and the axial force between the outer nut 9 and the inner nut 7 theoretically becomes "F + W". The elastic deformation of the spring washer 8 becomes the elastic energy accumulated in the spring washer 8 as long as the inner nut 7 or the outer nut 9 does not move or rotate. At this time, the load W of the spring washer 8 does not decrease, so the probability of loosening between the outer nut 9 and the inner nut 7 decreases.

[0023] Therefore, the relationship of the axial force applied to the bolt 4 of the fastening structure 1 of the present invention is as follows. F (axial force of the cylindrical portion 6) < F + W (axial force between the inner nut 7 and the outer nut 9) As understood from the above description, the fastening structure 1 of the present invention maintains the axial force "F" required for design necessary for fixing the fastened member 2, and as long as the axial force "F + W" between the inner nut 7 and the outer nut 9 is maintained, it will not loosen unless it is a load beyond expectation. That is, it is the sum of the axial force F of the cylindrical portion 6 generated by tightening the outer nut 9 and the axial force W between the inner nut 7 and the outer nut 9. Also, the magnitude of the load W is determined by the spring constant k of the spring washer 8.

Example

[0024] Based on the above premise, a test piece similar to the above fastening structure 1 was vibrated to test the loosening of the screw. Hereinafter, the example of the present invention will be described as a loosening experiment. [Experiment 1] [Loosening experiment conditions] (Specifications of the test piece in Experiment 1) For the test piece in Experiment 1, a fastening body composed of a bolt, a nut, and a spring washer with the specifications shown in Table 1 below was subjected to a loosening test with a loosening testing machine.

Table 1

[0025] (Test machine) As shown in Table 2 below, the loosening test was performed using a Junker testing machine (J121, manufactured by Vibrationmaster GmbH (located in Luxembourg, France)) compliant with ISO 16130, DIN 16130, DIN 25201-4, and DIN 65151 standards. The test conditions are as shown in Table 2 below. [Table 2]

[0026] (Test Procedure) The general procedure for the test using the above-mentioned test machine is as follows (1) to (5). (1) Apply lubricant (molybdenum disulfide) to the bolt to be tested and set it in the testing machine. (2) A multi-start nut, spring washer (SW), or disc spring attached to the bolt. Washer Install the bolts in the order of a single nut. The bolt axial force is controlled by tightening the outer nut with a standard torque wrench to achieve an axial force of approximately 15.00 kN (the tightening torque was approximately 43-47 Nm). (3) Start the vibration test (load in the X-axis direction (Figure 1)) and repeatedly apply a lateral displacement of a specific amplitude and frequency to the specimen mounting fixture to apply a dynamic lateral load to the bolted fasteners. (4) Monitor the change in axial force during the test (until 2,000 cycles are completed). (5) If the axial force is completely lost or the bolt breaks before reaching the specified 2,000 cycles, the test shall be terminated at that point.

[0027] [Results of Experiment 1] Table 3 below shows the loosening test results from Experiment 1. [Table 3]

[0028] [Experimental results of the loosening test] The numerical results in Table 3 above show that the axial force was maintained even after 2,000 cycles. (1) Residual axial force (%) when a "disc spring washer" is incorporated between nuts 42.6-96.8% (2) Residual axial force (%) when a spring washer is incorporated between nuts 88.3-93.6% Based on the above results, the double-nut fastening body of the double screw in the embodiment of the present invention has a bolt axial force that stays within a predetermined value and is resistant to loosening.

[0029] [Results of Experiment 1] Table 4 below shows the axial force test results for Comparative Example 1 (screw shaft and nut are standard products), Comparative Example 2, and the Example (experimental example). The bolt used in Comparative Example 1 was a metric coarse thread bolt (M12) of the Japanese Industrial Standards, and two identical nuts of the same type, standardized (type 2), were used. The bolt and nut used in Comparative Example 2 were those with the specifications shown in Table 1. The bolt and nut in the Example of the present invention are the same specifications as those in Comparative Example 2, and the only difference from Comparative Example 2 (without SW) is that a spring washer ((SW) Japanese Industrial Standard No. 3) is interposed between the two nuts. The fastening conditions before the test were as follows: Comparative Example 1: both nuts were tightened to 42 Nm each, Comparative Example 2 and Examples Only the outer nut 9 was tightened to 42 Nm. Note that the "10-time average" in Table 4 means the average value obtained from 10 tests.

[0030] [Table 4]

[0031] [Experimental results of axial force in Experiment 1] From the numerical results in Table 4 above, the tightening torque of the nuts in Comparative Example 1, Comparative Example 2, and the Example was 42 Nm. The axial force was almost the same before the experiment, but the axial force after the experiment differed in each case as follows. (1) Axial force between nuts (kN) Comparative Example 1 (37.3) > Example (29.2) > Comparative Example 2 (23.4) (2) Bolt axial force (kN) Comparative Example 2 (18.3) > Example (17.5) > Comparative Example 1 (8.2) From the above results, Comparative Example 1 (existing technology) has a high axial force between nuts (kN) but a low bolt axial force. Comparative Example 2 (without SW) has a high bolt axial force but a low axial force between nuts. In the embodiment of the present invention, the axial force between nuts and the bolt axial force are within predetermined values, and the fastener is well-balanced, making it less likely to loosen.

[0032] [Experiment 2] [Experiment on the time evolution of axial force due to load] Next, in Experiment 2, when a vibration load was applied to a double-nut fastening of a double-screwed structure using a loosening tester, the time change of the axial force was measured. (Specifications of the test specimen for Experiment 2) The test specimens for Experiment 2 were fasteners consisting of bolts, nuts, and spring washers with the specifications shown in Table 5 below, and were subjected to loosening tests using the same loosening tester as in Experiment 1. Except for the difference in bolt thread length (L), nearly identical fasteners were used. [Table 5]

[0033] The test equipment and test conditions are as shown in Table 6. [Table 6]

[0034] The general procedure for the load test is the same as described above. Table 7 below shows the loosening test results, illustrating the time change of axial force when a vibration load of 200 (cycles / 16 sec) was applied to a disc spring washer, spring washer (SW, for right-hand threads), and spring washer (SW, for left-hand threads). This experimental data was obtained by repeating the same test three times with the same test piece. There were no significant differences in the axial force in any of the tests. [Table 7]

[0035] Figures 3-5 show the time change of axial force when a vibration load of 200 (cycles / 16 sec) is applied. Figure 3 shows data for a spring washer (for right-hand threads), Figure 4 shows data for a disc spring washer, and Figure 5 shows data without a washer. In detail, Figure 3 shows the time change of axial force when a spring washer (SW) is placed between the nuts in a double-nut fastening of a double-threaded bolt. Figure 3(a) shows the time change of axial force on the screw shaft between the nuts, and Figure 3(b) shows the time change of axial force on the screw shaft of the fastened part. As can be seen from the data in Figure 3(a), the axial force between the nuts decreases over time, but as shown in Figure 3(b), the axial force on the screw shaft of the fastened part is large at the beginning of the vibration load. This means that, due to the spring washer, the axial force between the nuts is greater than the axial force on the fastened part at the time of fastening. The torque coefficient of the inner nut (second nut) 7 is smaller than that of the outer nut (first nut) 9 because the lead angle is larger. Therefore, when a vibration load is applied, the inner nut 7 (second nut) rotates or moves until it reaches a stable position, which reduces the axial force between the two nuts, but increases the axial force at the fastened part.

[0036] These data demonstrate that the double-nut fastening of the double-screwed structure is resistant to loosening even under vibration load. Note that the data in Figures 3 and 4 are obtained when only the outer nut (first nut) 9 is turned, tightening the fastened portion to an axial force of 20 kN. Figure 4 shows the time change of the axial force when a disc spring washer is placed between the nuts in the double-nut fastening of the double-screwed structure. Figure 4(a) shows the time change of the axial force between the nuts, and Figure 4(b) shows the time change of the axial force in the fastened portion. Similar to the data for the spring washer in Figure 3, as can be seen from the data in Figure 4(a), the axial force between the nuts decreases, but as shown in the data in Figure 4(b), the axial force in the fastened portion increases in the initial stages of vibration load.

[0037] Figure 5 is a comparative example, showing the time change of axial force in a conventional double-screw double-nut fastening assembly of the standard specifications when there is no spring washer or the like between the nuts. Figure 5(a) shows the time change of axial force between the nuts, and Figure 5(b) shows the time change of axial force in the fastened portion. However, the conventional double-screw double-nut fastening assembly in Figure 5 differs from the data shown in Figures 3 and 4 in that both the inner and outer nuts are tightened with the same torque. In the case of a conventional double-screw double-nut fastening assembly of the standard specifications, the axial force in the fastened portion decreases even at the initial stage of vibration loading. [Industrial applicability]

[0038] The double-screw double-nut fastening system of the present invention is less prone to loosening in bridges for railways and roads, various mobile machinery, industrial machinery, etc., and is particularly ideal for fastening structures in parts exposed to vibration. [Explanation of Symbols]

[0039] 1…Fastening structure 2…Parts to be fastened 3…Through hole 4... Bolts 6…Cylindrical section 7...Inner nut 8...Pulling Washer 8 9...Outer nut 10... Threaded part 13… F,W…axial force T...Tightening Torque

Claims

1. A double screw structure having two types of threads formed on the screw shaft: a first thread with one groove, which is a metric coarse thread, and a second thread with two grooves, which is formed in the same twist direction as the first thread and has a pitch three times that of the metric coarse thread. The outer nut screwed onto the first screw, The inner nut, which is screwed into the second thread, is in contact with the fastened member sandwiched between the bolt heads of the double thread structure and is located inside the outer nut. In a double-nut fastening body with a double screw, Between the outer nut and the inner nut, and fitted into the double screw structure, and A double-screw double-nut fastening body characterized by being made of the same material.

2. In the double-screw double-nut fastening body described in claim 1, The aforementioned spring-type washer is a spring washer. A double-screw, double-nut fastening body characterized by these features.

3. In the double-screw double-nut fastening body described in claim 1, The aforementioned spring-type washer is a disc spring washer. A double-screw, double-nut fastening body characterized by these features.

4. In the double-screw double-nut fastening body described in claim 1, In the initial stages of vibration loading, the axial force of the fastened member increases, making it less likely to loosen. A double-screw, double-nut fastening body characterized by these features.

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