JPH0221012A - Stepped screw superior in fatigue characteristics - Google Patents

Abstract

PURPOSE:To improve fatigue characteristics by eliminating the contact face of the respective ridge tops of the screw end of a screw at a side for receiving tension load. CONSTITUTION:A female screw 20 is formed inside the end 14 of a vessel body 10, at a low side portion from the ridge FS5 of the female screw 20 to the ridge FS1, the tops of the ridges are cut to be formed at an equal height, its contact height L is formed lower than ridges FS10-FS7 to reduce the contact area Fn of the female screw. Accordingly, sharing load with respect to the respective ridges is averaged so that peak bending moment per unit generated at the bottom of thread becomes nearly equal. Accordingly, a stepped screw becomes superior to fatigue characteristics.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application This invention relates to screws that are subjected to repeated tensile forces and are used in places such as accumulators and cylinders where the load fluctuates rapidly. It is related to.

Problems to be Solved by Prior Art and Inventions 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 a side plate, and also prevents fluid pressure fluctuations in a liquid circuit. The bladder is expanded and contracted in response to the pulsation absorption and shock absorber functions, and parallel screws are used as means for fixing the container body and the side plate.

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 screws over a range from 0 to the maximum load. The threads do not share the load evenly, but are largely biased in the direction of the tensile force.

Therefore, stress concentration occurs at the bottom of the valley at the tip of the ring, which receives a large tensile load, and the ring breaks 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 Patent No. 41119975, Japanese Patent Publication No. 5).
6-53651) 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.
The load sharing ratio and fatigue life of each screw thread of each accumulator were investigated under the following conditions: 0.04 mm, internal pressure p = 0 to 318 kg/cm21, and frequency 2.5H2.

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 thread with the largest load sharing ratio is the second thread m2 from the tip 2 m,
The skewer is 18.5%, and in the conventional accumulator, the first thread from the tip is 21%.
In addition, the fatigue life of the conventional accumulator is 560.0.
00 times, and in the test accumulator it was 380.000 times.

Normally, the fatigue life of a screw increases when the load sharing ratio of the screw threads decreases, but in the above test accumulator,
On the other hand, the fatigue life was shortened.

So, when we investigated the cause, we found that each female thread root fl~
During the maximum bending moment applied to ◎, the female thread tip 2e
A peak bending moment is applied to the second female thread root f2, and the bending moment f? @ also reached its maximum value, and it was found that it was destroyed from there. That is, the male thread 4 is indicated by the arrow Y.
When pressed in the direction, each thread fm of the female thread becomes a cantilever beam that receives a shared load on the lower surface, and the height fh of the female thread fm is considered to be the span that affects the magnitude of the bending moment. be able to.

Therefore, when the thread height fh is equalized, if the shared load of each thread is not equalized, the larger the shared load, the larger the peak bending moment will be.
The bending moment amplitude also reaches its maximum, making it more likely to break.

Therefore, the maximum bending moment per unit that occurs 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. A peak bending moment per unit of 13.5 kgmm /1111 is generated from the tip screw 2e of the test accumulator B to the second female thread root f2. PB occurs and the moment PB is larger than the peak bending moment per unit PA=11.4 kgm/- of the conventional accumulator A, so that the bending moment (the demoment amplitude is also large and
It has become clear that screws are 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 the fatigue life of screws.

Means for Solving the Problems and Effects This invention provides that in screws that are screwed together and that are subjected to fluctuating loads, the screw on the side that receives the tensile load lacks the contact surface of each thread crest at the end of the screw, and the contact surface is reduced. By making the height lower than that of other parts, the maximum bending moment per unit at the thread root is averaged, the peak bending moment per unit is greatly reduced, and the amplitude of the bending moment is reduced to prevent fatigue failure. It is something.

Embodiment An embodiment of the present invention will be described with reference to the accompanying drawings. FIG. 2 is a longitudinal cross-sectional view of an accumulator (ACC). , and both ends 14.1 of this main body 10
5 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, and the contact height of each female thread is 10 female threads.
In the standard ridge portion 50 from FS7 to female thread FS7, a portion formed at a standard height where the female thread FS contacts the male thread MS, the so-called contact surface f? tFW, is the same, and in the low ridge portion PL from thread FS5 to thread FSI, the threads are top-cut to the same height and the contact height is determined from the thread FS,, -7. Lower the contact area FW of the female thread.

The top of the thread FS6 of the intermediate thread 51 is the standard thread 50.
lower than the top of the thread FSI07 and the low crest F
The thread FS of L is formed higher than the top of the thread FS, -, and the maximum bending moment per unit applied to the thread FS6 is larger and lower than the maximum bending moment per unit applied to the thread FS of the standard thread part 50. Thread FS of crest FL,
−, the maximum bending moment per unit applied to

Since the female thread 20 is formed as described above, the line connecting the tops of each thread has a stepped shape with the thread height gradually decreasing between the thread FS and the thread FS6, resulting in a so-called stepped thread. The number of threads FS formed in this low ridge portion FL is determined as follows. When the number of screw threads is N, the load Wx acting on the Xth thread FSx on the tension side of the screw is Wx
=WXwNx (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 Do) + ( D
o-D)/2))/2 XWxwNx/ (rxDL)
(2) DL: Female thread root diameter DO: Male thread outer diameter D: Female thread inner diameter Di, or can be expressed as the inner diameter x of the low crest.

At this time, if D=Dx is determined so that Mx=maximum bending moment MO per thread root unit within the fatigue limit, the thread contact surface flF W x is F W X = (D o −
D
W x (4).

Therefore, first, use equation (1) to find the load W1 = WXwN1 acting on the first thread FS, and use equation (2) to calculate the bending moment Ml = ((D L
Do ) + (Do −D )/2))/2 X
Find WXwNt/(πXDL).

At this time, the maximum bending moment per unit is M.

=D=Dx so that the maximum bending moment per unit is M
While determining the screw contact area FW1 using equation (3),
= (D,) - Dx)

Find /FW.

As a result, when Ps<material tensile strength σB, (1)
The load acting on the second thread FS2 using the formula W2 = WX
Determine wN2 and substitute it into equation (2), and calculate the maximum bending moment M21 per unit when D is the standard inner diameter Di of the internal thread, and the maximum bending moment M2x per unit when D is the inner diameter Dx of the low crest, respectively. demand.

Then, the one that gives the maximum bending moment M2 per unit (maximum bending moment M per unit), for example, the inner diameter Dx of the maximum bending moment Mix per unit, is adopted.

Thereafter, similar operations are repeated until the adopted inner diameter becomes the female thread standard inner diameter D1.

When PR>σS, the screw thread undergoes plastic deformation, so the first screw thread FS can only receive a load of FW1×σB even though the load W1=WXWN1 is applied.

Therefore, the load W2 applied to the next thread FS is (W-
FW, Xσ, )XW(N-1)1.

The load at this time! ! Substituting W2 into equation (2), D becomes
The maximum bending moment M2i per unit when the standard internal diameter of the female thread is Di and the maximum bending moment per unit M2x when the internal diameter of the low crest part is Dx are determined, respectively, and then the same process as above is performed to Determine the number of threads on the crest.

The female thread root bottoms f1 to flO are formed in an arc shape, and the radius fr thereof is 0.1 to 0.16 times as large as 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 prada 11 is compressed and the pressure in the gas chamber 12 increases, causing the side plate 16 to move in the direction of arrow A2. Press.

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, -, o is applied to each thread FSI 10.

However, if this shared load is too large, the thread F
S causes deformation within the elastic limit and plastic deformation, and the pitch of the thread that caused the plastic deformation changes, but the load that each thread FS can actually bear is only the range of the force of the deformation within the elastic limit. It is.

The amount of this force is regulated by the size of the contact surface 1'iFW of the female thread, but this contact surface IFW is
The higher the contact height, the larger the screw thread F'5to.
- Thread FS of low thread part FL compared to -s, contact area F of -t
W becomes smaller.

Further, the contact height of each female thread FSIOI is set to such a height that the maximum bending moment per unit generated at each thread root f10-1 is also approximately equal.

Therefore, the peak bending moment PC and the bending moment amplitude per unit generated at the thread root of the female thread tip 20E are greatly reduced compared to the female thread 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 maximum bending moment per unit generated at the root of the female thread fl -1° and the fatigue life of the screw were investigated under the same conditions as in the above experiment, it was found that the maximum bending moment per unit was The moment is
The curve C in Figure 4 shows that the peak bending moment pc per unit is 4.7 Kg, m+a/mm, and the fatigue life is more than 10,000,000 cycles, which is 20 times longer than that of the conventional accumulator. It has more than doubled.

It goes without saying that the present invention is applicable not only to triangular threads but also to square threads, round threads, trapezoidal threads, etc., and also to the same part of a container such as a cylinder.

Also, when a tensile load is applied to both ends of the screw, the third
As shown in the figure, the thread F of one end 20E of the female thread 20
Cut the peaks of S, -, to the same height and shape the low mountain PL
The contact height of the female thread 20 is stepped, and the thread M S , o-8 at the end of the male thread 19, that is, the end opposite to the other end of the female thread 20 is formed. The male thread 19 may be formed into a stepped shape by cutting the top of the crest to the same height to form the low ridge portion ML.

Furthermore, instead of trimming the top of the thread to lower the contact height, as shown in Figure 5, the contact surface of the top of the female thread FS may be shaved to lower the contact height of the female thread. .

As shown in FIG. 6, the female threads F's, o of the female thread 20 are
-, the standard height is set to 50, and the female thread F
! Shape the peaks of i and +t to the same height and make low mountain 1.
gFL, and the portion up to the female thread FS6-4 may be used as the intermediate thread portion 5I.

Each female thread crest of this intermediate ridge portion 51 is located on the diagonal line hI connecting the crest of the female thread FS7 and the female thread FS3.

Further, as shown in FIG. 7, each female thread F of the standard thread part 50 is
The intersection point XI of the line 50a connecting the female thread crests of S, o to FS, and the line 51a connecting the female thread crests of each of the female threads FS7 to FS4 of the intermediate ridge portion 51 has a center OP on the female thread 20 side. Tangent to a circle with radius RO 1. position on top, and
A line FLa connecting the tops of the female threads FS, ~FS, of the low mountain portion FL and each female thread FS, ~F of the intermediate mountain portion 51
The intersection point X2 with the line 51a connecting the female thread crests of S4 is the tangent t2 of a circle with radius R1 having the center Om on the male thread 20 side.
It may be positioned above.

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

In addition, in each of the above embodiments, the case where a tensile load is applied to the female thread has been explained, but in contrast, when a tensile load is applied to the thread, it goes without saying that the male thread may be formed as described above. It is.

Effects of the Invention Since the stepped screw according to the present invention is configured as described above, how can each thread be divided? Since the f-force is averaged, the peak bending moment per unit generated at the thread root also becomes approximately equal.

This results in stepped screws with excellent fatigue properties.

[Brief explanation of the drawing]

1 to 7 are diagrams showing an embodiment of the present invention, FIG. 1 is an enlarged view of the main part of FIG. 2, FIG. 2 is a longitudinal sectional view of the accumulator, and FIGS. 3 and 5 to Figure 7 is an enlarged cross-sectional view showing an example of each of the five ponds, and corresponds to Figure 1. Figure 4 is a diagram showing the relationship between each thread root and the maximum bending moment per unit.
FIG. 8 is a longitudinal sectional view showing a conventional example. 19 ... Male thread 20 ... Male thread FW ... Contact area L ... Contact height Fig. 1

Claims (5)

[Claims] (1) In screws that are screwed together and are subjected to fluctuating loads, the screw on the side that receives the tensile load has a missing contact surface at the top of each thread at the end of the screw in order to reduce the peak bending moment per unit. A stepped screw with excellent fatigue properties, characterized by a contact height lower than that of other parts. (2) In screws that are screwed together and are subjected to fluctuating loads, the screw on the side that receives the tensile load has a low ridge portion and a middle ridge portion that are continuous from one end to the other in order to reduce the peak bending moment per unit. A stepped screw with excellent fatigue properties, characterized by having a standard crest and a standard ridge. (3) The stepped screw with excellent fatigue properties according to claim 2, wherein the contact height of the low ridge portion and the intermediate ridge portion is adjusted by cutting the top of the thread. (4) Fatigue characteristics according to claim 2, characterized in that the contact height of the threads of the low thread portion and the middle thread portion is adjusted by cutting the contact surface of the top portion of each thread. Excellent stepped screw. (5) Excellent fatigue properties according to claim 2, characterized in that the contact height of the screw thread in the middle thread part is lower than the contact height of the standard thread part and higher than the contact height of the low thread part. Stepped screw.