JPH01224509A - Privative screw excellent in fatigue property - Google Patents

Abstract

PURPOSE:To prolong fatigue lifetime in a screw used in such a part where a load remarkably fluctuates as an accumulator by eliminating all contact surfaces on thread crests on such a side as to receive a tensile load so as to form all the thread contact surfaces at the same height. CONSTITUTION:Truncation of all threads of a female screw 20 at the same height eliminates contact surfaces on the thread crests, resulting in identity of all screw contact height L. This equalizes portions Fw where the threads Fs of the female screw come into contact with the threads Ms of a male screw so as to average charge loads bearing on the threads Fs and Ms irrespective of change in screwing or fastening positions, thereby resulting in almost the same bending moment generated in the bottoms of the threads Fs and Ms. Therefore, smaller peak bending moment generated in the thread bottoms in comparison with that in a conventional screw can enhance fatigue property as well as prolong fatigue lifetime.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application This invention relates to screws that receive tensile loads and are used in places such as accumulators and cylinders where loads fluctuate rapidly. This invention relates to a missing screw with excellent fatigue properties.

BACKGROUND ART For example, an accumulator as a fluid device divides the inside of a container body into a gas chamber and a liquid chamber by a plug, and closes both ends of the chamber with a side plate, and expands and contracts a bladder according to fluctuations in fluid pressure in a liquid circuit. Although it functions as a pulsation absorber and a shock absorber, parallel screws are used as means for fixing the container body and the side plate.

Problem to be Solved by the Invention By the way, when the pressure inside the accumulator increases and the side plate is pressed outward, loads in the axial and circumferential directions, so-called variable loads, are repeatedly applied to the screw within the range from 0 to the maximum load. However, this load is not uniformly shared by each thread, but is largely biased in the direction of the tensile force.

As a result, stress concentration occurs at the root of the female screw tip, which is subjected to a large tensile load, leading to breakage from there.

Therefore, in order to solve this problem, we developed a patented ``threaded joint with excellent fatigue characteristics using a tapered external thread'' (US Pat. No. 4,189,975, Japanese Patent Publication No. 56
-53651) may be used.

As shown in FIG.
The external thread 4 is formed with a thread m7 according to the above-mentioned patent.
A test accumulator was manufactured in which the peak height h was gradually reduced by ~ml, and a conventional accumulator using the triangular screw in the standard state was manufactured, and the seal diameter d"1 was manufactured.
The load sharing ratio and fatigue life of each screw thread of each accumulator were investigated under the conditions of 0.04 mm, internal pressure P = 0 to 318 kg/c+', and frequency of 2.5 H2.

As a result, the load sharing ratio was more averaged in the test accumulator than in the conventional accumulator, but the fatigue life of the test accumulator was shorter than that of the conventional accumulator.

Incidentally, in the test accumulator, the thread with the highest load sharing ratio is the second thread m2 from the tip 2m,
The rate is 18.5%, and in the conventional accumulator, the rate is 21% for the first screw out from the tip.
In addition, the fatigue life of the conventional accumulator is 560.0.
00 times, and in the test accumulator, it was 380.00θ times.

Normally, when the load sharing ratio of the screw thread decreases, the fatigue life of the screw increases.However, in the test accumulator, the fatigue life of the test accumulator was shortened.

When we investigated the cause, we found that during the bending moment applied to each female thread root f, the maximum bending moment was applied to the second female thread root f2 of the female thread tip 2e, and the fracture occurred from there. . That is, when the male thread 4 is pressed in the direction of arrow Y, each thread fm of the female thread becomes a cantilever beam that receives a shared load on the lower surface, and the female thread fm
The peak height fh can be seen as a span that affects the magnitude of the bending moment.

Therefore, when the thread height fh is made uniform, if the shared load of each screw thread is not made equal, the larger the shared load is, the larger the bending moment becomes, making it easier to break.

Therefore, the bending moment per unit that occurs at the root of the female thread of the conventional accumulator and the test accumulator is
The results were calculated from the load sharing ratio and the thread height as shown in Figure 4.

In this figure, A is a conventional accumulator, B is
The test accumulator is shown.
A peak bending moment PB of 3.5 kg*/m occurs, and that moment PB is equal to the peak bending moment P A = 11.4 k of the conventional accumulator A.
g am larger than 1 van, and therefore it became clear that the screw was susceptible to fatigue failure.

In view of the above circumstances, it is an object of the present invention to improve the fatigue characteristics of screws and extend their fatigue life.

Means for Solving the Problems This invention, in screwed screws that receive fluctuating loads, eliminates the contact surface of the top of the entire thread on the side that receives the tensile load so that the height of the entire contact becomes the same height. By forming
This is an attempt to solve the above problems.

When a fluctuating load is applied to the threaded screws due to changes in the applied load, a shared load is applied to each thread of the screw on the side receiving the tensile load, but the contact heights of all threads remain at the same height. Since the screw contact area is the same, the load shared by the screw threads will be approximately the same no matter how the screw tightening position changes.

Therefore, the bending moment generated at each thread root is averaged, and the peak bending moment is greatly reduced.

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described with reference to the accompanying drawings, in which the same reference numerals have the same names and functions.

FIG. 2 is a longitudinal cross-sectional view of the accumulator ACC, in which the interior of the container body 10 is partitioned into a gas chamber 12 and a liquid chamber 13 by a bladder 11, and both ends 14.
15 is closed by side plates 16.17.

Both ends 14.15 and side plates 16.17 of this main body 10 are
The threaded portion S is formed as shown in FIG. 1.

That is, a female thread 20 is formed inside the end portion 14 of the container body 10 to be screwed into the male thread 19 of the side plate 16 .

This female thread 20 has 10 female threads FS, but the tops of all the threads are cut to the same height to eliminate the contact surface at the top of the threads, so that the full thread contact height L is the same height. The portion where the thread FS of the female thread contacts the thread MS of the male thread, the so-called contact area FW of the female thread, is made the same.

The groove bottoms f, ~flo are formed in an arc shape, and the radius fr thereof is 0.1 to 0.18 times larger than the pitch.

Each thread height mh of the male thread 19 is formed to a standard height.

In addition, 25 is a stopper for preventing screwing in more than necessary, and C is a center line.

Next, the operation of this embodiment will be explained.

When the hydraulic pressure in the hydraulic circuit 30 fluctuates and liquid is pressurized into the accumulator ACC from the supply/discharge port 23, the bladder 11 is compressed and the pressure in the gas chamber 12 increases, causing the side plate 16 to move as shown by arrow A2.
Press in the direction.

Therefore, since the male thread 19 is also pushed in the same direction, a load is also applied to the female thread 20 screwed therewith, so that a shared load W, -, is applied to each thread FS+ +a.

However, if this shared load is too large, the thread F
S undergoes deformation within the elastic limit and plastic deformation, and as the pitch of the female thread changes, the load that each thread/thread FS actually receives is only the load corresponding to the crest of the deformation within the elastic limit, and the rest The load is transferred to the next thread.

Further, if the screw contact area is large relative to the transmitted load to be shared, the screw will bear the load corresponding to the deformation within the elastic limit, and the other load will be transferred to the next screw thread.

This shared load is regulated by the size of the contact surface fifFW of the female thread, but since the contact height L of all threads FS + o-1 is equal, the contact area FW is equal, so the screw tightening No matter how the mounting position changes, the internal diameter of the female thread that receives the tensile load is determined so that the load actually shared by each thread FSI 10 is approximately equal. The inner diameter of the female thread is determined so that the load on the total number of threads is equal to the product of the contact area of the thread and the material stress.

In other words, since the contact height and shared load of each thread are equal,
The bending moment generated at each screw root f+10 is also approximately equal.

Therefore, the peak bending moment PC per unit generated at the thread root of the female thread 20 is reduced to a greater extent than that of the conventional accumulator, so that the fatigue life can be extended.

Incidentally, when an accumulator using the screw according to the present invention was manufactured and the bending moment generated at the root f+-10 of the female thread and the fatigue life of the screw were investigated under the same conditions as in the above experiment, the bending moment was as shown in Fig. 4. As shown in straight line C, the peak bending moment pc is 3.482Kg, +
nm/n+m, and the fatigue life is 10,000
,000 times or more, which is more than 20 times that of conventional accumulators.

It goes without saying that the present invention can be used not only for triangular threads but also for square threads, round threads, trapezoidal threads, etc.

In addition, instead of trimming the tops of all the threads of the female thread to the same height, as shown in Figure 3, the top contact surface FSL of the female thread FS is shaved to make the female contact height L the same. It may be formed to a certain height.

Effects of the Invention In this invention, the contact surface of all the thread crests of the threads on the side receiving the tensile load is eliminated and all the thread contact heights are made the same height, so that the contact area of each thread is equal and becomes smaller.

Therefore, the actual shared loads of the threads are averaged regardless of changes in the screwing position and the tightening position, and the bending moments generated at the root of the threads of the threads are also approximately equal.

Therefore, the peak bending moment generated at the thread root is reduced compared to the conventional example, improving fatigue characteristics and extending fatigue life.

[Brief explanation of the drawing]

1 to 4 are diagrams showing an embodiment of the present invention. FIG. 1 is an enlarged longitudinal cross-sectional view, FIG. 2 is an enlarged view of the screw portion, and FIG.
The figure is a longitudinal sectional view of the accumulator, Figure 3 is an enlarged view showing another embodiment and corresponds to Figure 1, Figure 4 is a diagram showing the relationship between each thread root and bending moment, and Figure 5 19 is a diagram showing a part of an enlarged sectional view of a conventional example. 19 Male thread 20 Female thread FS Female thread thread L Contact height MS Male thread

Claims (1)

[Claims] In screws that are screwed together and are subject to fluctuating loads, the contact surface on the top of all the threads on the side that receives the tensile load is eliminated so that the contact height of all the threads is the same height.It has excellent fatigue properties. Missing screw.