JPH0786366B2 - Stepped screw with excellent fatigue characteristics - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a screw subjected to repeated tensile force used in a place where load fluctuations such as an accumulator or a cylinder is severe, and a stepped screw having a long fatigue life. It is about.

2. Description of the Related 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 bladder, and closes both ends of the chamber with side plates and changes in hydraulic pressure of a liquid circuit. According to the above, the bladder is expanded and contracted to perform a pulsation absorbing action, a shock absorber action and the like, and a parallel screw is used as a fixing means for the container body and the side plate.

By the way, when the pressure in the accumulator rises and the side plate is pushed outward, axial and circumferential loads, so-called fluctuating loads, are repeatedly applied to the screw over a range from 0 to the maximum load. The threads are not evenly distributed, but are largely biased in the tensile force direction.

Therefore, stress concentration occurs at the root of the female screw tip, which receives a large tensile load, and fracture occurs from there.

Therefore, in order to solve this problem, an already patented "screw joint with a fatigue characteristic using a tapered male screw" (see U.S. Pat. No. 4189975, Japanese Patent Publication No. 56-53651) is used. Can be used.

As shown in FIG. 8, the present inventor forms the female screw 2 of the container body 1 and the male screw 4 of the side plate 3 with a 60-degree triangular screw M106.8 × 2, and according to the patent, A test accumulator in which the thread height h of the threads m _{7 to} m ₁ is gradually reduced is manufactured, and a conventional accumulator in which the triangular screw is used in a standard state is manufactured, and a seal diameter d = 104 mm and an internal pressure p = The load sharing ratio and fatigue life of each thread of each accumulator were investigated under the conditions of 0 to 318 kg / cm ² and frequency of 2.5 HZ.

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

Attributable to the largest thread load distribution rate, in the test accumulator on the second thread m ₂ from the tip 2m, that rate is 18.5%, in the conventional accumulators, the first thread from the distal end In the mountain, the rate was 21%, and the fatigue life was 560.000 times in the conventional accumulator and 380.000 times in the test accumulator.

When the load sharing ratio of the screw thread decreases, the screw fatigue life is usually extended, but in the above test accumulator,
Contrary to this, the fatigue life became shorter.

Then, when the cause was investigated, each female thread root f ₁ to ₁₀
It was found that the peak bending moment was added to the second female thread root f ₂ of the female thread tip portion 2e in the maximum bending moment applied to, and the bending moment amplitude became the maximum, and the fracture occurred from there. That is, when the male screw 4 is pressed in the direction of the arrow Y, each thread fm of the female thread becomes a state of a cantilever that receives a shared load on the lower surface, and the thread height fh of the female thread fm is the bending moment. It can be seen as the span that affects the size.

Therefore, when the peak heights fh are made uniform, if the shared load of each thread is not uniform, the larger the shared load, the larger the peak bending moment, and also the maximum bending moment amplitude, It is easy to destroy.

Therefore, the maximum bending moment per unit generated at the root of the female thread of the conventional accumulator and the test accumulator was calculated from the load sharing ratio and the average contact height.
It was as shown in the figure. In this figure, A is a conventional accumulator, and B is a test accumulator, but the peak bending moment PB per unit of 13.5 kgmm / mm from the tip screw 2e of the test accumulator B to the second female thread root f2. And the moment PB is the peak bending moment PA per unit of the conventional accumulator A.
＝ 11.4kgmm / mm, therefore, the bending moment amplitude is also large, and it has been clarified that the screw is easily fractured by fatigue.

The present invention has been made in view of the above circumstances, and an object thereof is to improve the fatigue characteristics of a screw to prolong the fatigue life of the screw.

Means and Actions for Solving the Problems The present invention is characterized in that the internal threads are formed so that the line connecting the crests of the internal threads becomes stepwise, thereby averaging the maximum bending moment per unit of the thread root. The hit peak bending moment is greatly reduced and the bending moment amplitude is reduced to prevent fatigue fracture.

Embodiment An embodiment of the present invention will be described with reference to the accompanying drawings. FIG. 2 is a vertical sectional view of an accumulator (ACC), and the inside of a container body 10 is partitioned by a bladder 11 into a gas chamber 12 and a liquid chamber 13. Further, both end portions 14 and 15 of the main body 10 have side plates 16 and 17
It has been closed by.

Both ends 14 and 15 of the main body 10 and the side plates 16 and 17 are screwed to each other, and the screw portion S thereof is formed as shown in FIG.

That is, inside the end 14 of the container body 10, the male screw of the side plate 16
A female screw 20 that is screwed with the screw 19 is formed.

The female screw 20 has ten female threads FS. The contact height L of each female thread is from the female thread FS10 to the female thread FS.
In the standard thread portion 50 up to 7, the portion where the thread FS of the female thread contacts the thread MS of the male thread, that is, the so-called female thread contact area FW, which is formed at the standard height, is made the same. in the low mountain portion FL from mountain FS5 to thread FS _1, wherein the contact height L and summit cut molding threads at the same height thread FS ₁₀ - lower than ₇ contact area of internal thread FW To reduce.

Crest of thread FS6 intermediate mountain portion 51, thread FS ₁₀ standard crests 50 - ₇ lower than the summit of and thread F of Teiyama portion FL
S ₅ - ₁ of higher forms from the summit portion, the maximum bending moment per unit applied to said thread FS ₆ are thread FS standard ridges 50
Larger than the maximum bending moment per unit applied to, and threads FS ₅ of Teiyama portion FL - to be smaller than the maximum bending moment per unit applied to _1.

Internal thread 20, since it is formed as described above, a line connecting each screw crest becomes a thread FS ₈ and stepped with declining the peak height between the thread FS _6, a so-called stepped screw Naru but this low mountain area FL
The number of screw threads FS formed in is determined as follows. When the number of screw threads is N, the load Wx that acts on the x-th thread FSx on the tensile force side of the screw is Wx = W × wNx (1) W: Total load WNx: Load sharing ratio for thread FSx And the maximum bending moment Mx per unit applied to the x-th thread root fx at this time is Mx = [(DL−D ₀ ) + (D ₀ −D) / 2)] / 2 × W × wNx / (Π
× DL) (2) DL: Female thread root diameter D0: Male thread outer diameter D: Female thread inner diameter Di or low mountain portion inner diameter Dx.

At this time, if D = Dx is determined so that Mx = maximum bending moment M0 per unit of thread root within fatigue limit, the screw contact area
FWx is calculated from FWx = (D ₀ −Dx) × π / 4 (3), and when the contact surface pressure Px per unit area is calculated from this contact area FW and load Wx, Px = Wx / FWx (4) Becomes

Therefore, first, the load W ₁ = W × wN ₁ acting on the first screw thread FS ₁ is obtained using the equation (1), and the bending moment M ₁ = [(DL− D ₀ ) + (D ₀ −D)
/ 2)] / 2 × W × wN ₁ / (π × DL) is calculated.

At this time, D = Dx is calculated so that the maximum bending moment per unit M ₁ = the maximum bending moment M per unit, and the screw contact area FW ₁ = (D ₀ −Dx) × π / 4 using the formula (3).
Then, using equation (4), the contact surface pressure P ₁ = W ₁ / FW ₁
Ask for.

As a result, when P ₁ <material tensile strength σB, the load W ₂ = W × wN ₂ acting on the second screw thread FS ₂ is calculated using the formula (1) and is substituted into the formula (2). D female thread standard inner diameter
The maximum bending moment M ₂ i per unit when Di is
The maximum bending moment M ₂ x per unit when the inner diameter Dx of the low mountain portion is obtained, respectively.

Then, the one having the maximum bending moment M ₂ per unit <the maximum bending moment bending moment M ₀ per unit, for example, the inner diameter Dx of the maximum bending moment M ₂ x per unit is adopted.

Hereinafter, the same work is repeated until the adopted inner diameter becomes the female screw standard inner diameter Di. When P ₁ > σ _B , the screw thread undergoes plastic deformation, so the first screw thread FS ₁ receives only the load of FW ₁ × σ _B , even though the load W ₁ = W × wN ₁ acts. I can't.

Thus, the load W ₂ applied to the next thread FS ₂ is, (W-FW ₁ ×
σ _B ) × w (N-1) ₁ .

Substituting the load W ₂ at this time into the formula (2) and the maximum bending moment M ₂ i per unit when D is the female thread standard inner diameter Di, and the maximum per unit when D is the inner diameter Dx of the low mountain portion. The bending moments M ₂ x are obtained respectively, and then the same processing as described above is performed to determine the number of threads in the low mountain portion.

Female thread roots f _{1 to} f ₁₀ are formed in an arc shape and have a radius fr
Is 0.1 to 0.18 times the pitch.

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

In addition, 25 is a stopper for preventing excessive screwing, and C is a center line.

Next, the operation of this embodiment will be described. Hydraulic circuit 30
Fluid pressure fluctuates and the accumulator (ACC)
When the liquid is pressed into the bladder 11, the bladder 11 is compressed and the pressure in the gas chamber 12 rises to press the side plate 16 in the arrow A2 direction.

As a result, the male screw 19 is also pushed in the same direction, and the load is also applied to the female screw 20 that is screwed together with this.
₁ - to _10, shared load W ₁ - ₁₀ is applied.

However, if this shared load is too large, the thread FS
Causes the plastic deformation within the elastic limit, and the pitch of the screw that has undergone the plastic deformation changes, but the load actually received by each thread FS is only the range of the force within the elastic limit.

The amount of this force has been restricted by the size of the contact area FW of the internal thread, the contact area FW is thread FS ₁₀ since the contact height L of the thread FS increases higher - than the ₈ Low mountain area F
Thread FS ₆ of L - contact area FW of ₁ becomes smaller.

Further, the internal thread FS ₁₀ - ₁ contact height L, each thread root f ₁₀
The maximum bending moment per unit of _{1 is set} to be almost even.

Therefore, the peak bending moment PC and the bending moment amplitude per unit generated at the thread root of the female screw tip portion 20E are greatly reduced as compared with the female screw of the conventional accumulator, so that the fatigue life can be extended.

In this connection, produced an accumulator using a screw according to the present invention, the experiments female thread valleys under the same conditions as the bottom f ₁ - When checking up per unit occurring ₁₀ bending moments and fatigue life of the screw, a bending up per unit The moment is as shown by the curve C in Fig. 4, the peak bending moment PC per unit is 4.7kg.mm/mm, and the fatigue life is 1
It was over 0,000,000 times, more than 20 times that of conventional accumulators.

It goes without saying that the present invention can be applied not only to the triangular screw but also to a square screw, a round screw, a trapezoidal screw and the like, and to the same place of a container such as a cylinder.

Also, when a tensile load is applied to both ends of the screw,
As shown, the threads of the end portion 20E of the internal thread 20 FS ₁ - ₃
To form a low mountain portion FL by shaping the crest into the same height to make the contact height of the female screw 20 stepwise and to face the end of the male screw 19, that is, the other end of the female screw 20. Side end thread MS ₁₀
- to shape the summit cut to form a low mountain portion ML of ₈ at the same height,
The male screw 19 may be formed in a step shape.

Further, instead of trimming and shaping the screw to reduce the contact height, the contact surface L of the screw thread FS may be ground to reduce the contact height L of the screw as shown in FIG. .

As shown in FIG. 6, an internal thread FS ₁₀ of the internal thread 20 - the standard ridges 50 and the up ₇ to the standard height, an internal thread FS ₃ - and summit law shaping ₁ up at the same height The low ridge portion FL may be used, and the intermediate thread portion FS _{6 to 4} may be used as the intermediate ridge portion 51.

Each female thread crest of the intermediate crest portion 52 is located on the slant line h1 connecting the crest of the female thread FS _{7 and} the crest of the female thread FS ₃ .

Also, as shown in FIG. 7, each female thread FS ₁₀ of the standard thread 50 is
~ FS ₈ has an intersection point X ₁ with a line 50a connecting the female screw crests and each female screw thread FS _{7 to} FS ₄ connecting the female screw crests 51a of the intermediate mountain portion 51, and has a center OP on the female screw 20 side. It is located on the tangent t ₁ of the circle of radius RO, and also the line FLa connecting the female thread crests FS _{3 to} FS ₁ of the low ridge FL and the female thread FS ₇ 〜 of the intermediate ridge 51. The intersection X ₂ with the line 51a connecting the crests of the female thread of FS ₄ may be located on the tangent t ₂ of the circle of radius R ₁ having the center 0 m on the male thread 20 side.

At this time, the line connecting the line 50a, the line 51a, and the line FLa becomes a curve that gradually separates from the extension line 50b of the line 50a toward the female screw tip portion 20E.

In each of the above-described embodiments, the case where the tensile load is applied to the female screw has been described. However, when the tensile load is applied to the screw in the opposite case, it is needless to say that the male screw may be formed as described above. Is.

EFFECTS OF THE INVENTION Since the stepped screw according to the present invention is configured as described above, since the shared load related to each screw thread is averaged, the peak bending moment per unit generated at the root of the screw is also substantially equal.

Therefore, the stepped screw has excellent fatigue characteristics.

[Brief description of drawings]

1 to 7 are views showing an embodiment of the present invention, FIG. 1 is an enlarged view of a main part of FIG. 2, and FIG. 2 is a longitudinal sectional view of an accumulator, FIG. 3 and FIG. FIG. 7 is an enlarged cross-sectional view showing another embodiment and corresponds to FIG. 1, and FIG. 4 is a view showing the relationship between each thread root and the maximum bending moment per unit,
FIG. 8 is a vertical sectional view showing a conventional example. 19 …… Male thread 20 …… Male thread FW …… Contact area L …… Contact height

Claims (5)

[Claims] 1. A screw between a container main body and a side plate, wherein the container main body receives a fluctuating internal pressure to apply a tensile load to the screw, and the screw consists of a female screw of the container main body and a male screw of the side plate. In the screw, each of which has a crest and a valley bottom, and which can be screwed to connect the container main body and the side plate, a line in which the female screw connects each female screw crest is stepped. It is characterized in that the diameter of each root of the female screw is formed to be substantially uniform, and the maximum bending moment per unit of each root becomes uniform when the female screw receives a fluctuating load. Stepped screw with excellent fatigue characteristics. 2. A screw between a container main body and a side plate, wherein the container main body receives a variable internal pressure to apply a tensile load to the screw, and the screw consists of an internal thread of the container main body and an external thread of the side plate. In the screw, each of which has a crest and a valley bottom, and which can be screwed to connect the container main body and the side plate, the crest of the female thread is the same as the male thread. It has a contact surface for connecting so that it can be screwed, and the contact surface is gradually reduced so that the line connecting the crests of the threads becomes stepwise, and the diameters of the roots of the female threads are substantially uniform. A stepped screw with excellent fatigue characteristics, characterized in that the maximum bending moment per unit of each root becomes uniform when the internal thread is subjected to fluctuating load. 3. A screw between the container main body and the side plate, wherein the container main body receives a variable internal pressure to apply a tensile load to the screw, and the screw is composed of an internal thread of the container main body and an external thread of the side plate. In the screw, each of which has a crest and a valley bottom, and which can be screwed to connect the container main body and the side plate, the female screw is gradually lowered along its longitudinal direction. The female thread has a mountain portion, an intermediate mountain portion, and a standard mountain portion, and the female thread has its peak contact surface reduced so that the line connecting the mountain peak ends of the female thread has a stepped shape. A stepped screw with excellent fatigue characteristics, characterized in that the root diameters of the threads are formed to be approximately uniform, and the maximum bending moment per unit of each root becomes uniform when the female thread is subjected to fluctuating load. 4. A stepped screw having excellent fatigue characteristics according to claim 3, wherein the contact heights of the low ridge portion and the intermediate ridge portion are adjusted by cutting the ridges. 5. A stepped screw having excellent fatigue characteristics according to claim 3, wherein the contact heights of the low peak portion and the intermediate peak portion are adjusted by cutting the peak contact surfaces.