JP2001153107A - Hydraulic cylinder - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the strength of a piston fastening portion in simple structure without using special high-strength material for or apply heat treatment to a piston rod in a hydraulic cylinder. SOLUTION: A recess 21 is formed as an axial directional insertion part on an end surface 20 on the piston rod 3 side of a piston 4, and the tip of the rod 3 is inserted into the recess 21 of the piston 4 to ensure a coaxial property. Coaxial bolt screw and bolt through holes 24 and 25 are opened on the end surface 23 of the tip the rod 3 and the piston 4 respectively, to fix the piston 4 by inserting a bolt 22 into the hole 24 from the hole 25, in a condition where the surface 20 of the piston 4 is oppositely contacted to the surface 23 of the tip of the rod 3. A taper 30a expanding toward the surface 23 is formed in the entrance portion in the lower hole 30 of the hole 24.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a hydraulic cylinder, and more particularly to a hydraulic cylinder used for a hydraulic working machine such as a hydraulic construction machine.

[0002]

2. Description of the Related Art In a hydraulic working machine such as a hydraulic shovel, which is a typical example of a hydraulic construction machine, a hydraulic cylinder is used as an actuator for driving a working member. As shown in FIG. 14, the hydraulic cylinder is provided at a cylinder main body 102, a piston rod 103 moving within the cylinder main body 102, and a distal end of the piston rod 103, and the inside of the cylinder main body 102 has a rod side chamber 1.
07a and a piston 104 partitioned into a bottom chamber 107b. As such a piston fastening structure of a hydraulic cylinder, a piston rod 103 is generally used.
A small-diameter piston insertion portion 103j is provided at the front end portion of the piston through a step portion 103m, a male screw portion 103g is formed at the front end portion of the piston insertion portion 103j, and the piston 104 is inserted into the piston insertion portion 103j. Nut 112 into the male screw portion 103 so as to abut against the portion 103m.
g, the piston 104 is fixed and fastened to the piston rod 103.

[0003] Such a piston fastening structure is disclosed, for example, in Japanese Utility Model Publication No. Hei 7-16888. FIG. 15 shows another example of the conventional piston fastening structure. This example is disclosed in Japanese Utility Model Laid-Open No. 57-203103, in which a piston insertion portion 103 for inserting a piston 104 is provided.
j is further provided with an annular groove 103k, and the piston 104 is fitted to the step 103m until the annular groove 103k is brought into contact with the step 103m.
3k is fitted with a half-ring-shaped flange 160 obtained by dividing a circular ring into two in the radial direction, and this flange 160 is fixed to the piston 104 with bolts 170, so that the piston 1
04 is fixed and fastened to the piston rod 103.
In this case, an O-ring 180 is provided between the piston insertion portion 103j and the piston 104 to seal the space between the chambers 107a and 107b in the cylinder body 102 separated by the piston 104.

[0004]

In the hydraulic cylinder shown in FIG. 14, when pressure oil is supplied to the bottom chamber 107b, the piston rod 103 moves to the left in the drawing to extend the hydraulic cylinder. Side chamber 107
When the pressure oil is supplied to a, the piston rod 103 moves to the right in the figure and the hydraulic cylinder contracts. In a hydraulic working machine such as a hydraulic excavator, such a hydraulic cylinder frequently expands and contracts.
The pressure of a or 107b acts.

By the way, in the conventional piston fastening structure shown in FIG. 14, when the pressure of the chamber 107a or 107b acts on the piston 104, the piston 104 closes the nut 112 with the male screw portion 103g provided on the piston insertion portion 103j. The pressure in the chamber 107a or 107b is
The piston rod 103 was more likely to be damaged than the male screw portion 103g.

FIG. 16 shows the relationship between the maximum principal stress acting on the male screw portion 103g of the piston insertion portion 103j and the number of screw threads. Here, the number of threads is counted from the load point when the nut 112 is tightened, that is, from the contact surface between the piston 104 and the nut 112. As can be seen from this figure, the maximum tensile stress is applied to the first thread portion on the external thread portion 103g,
In addition, the tensile stress repeatedly increases and decreases between the time of pressing on the rod side and the time of pressing on the bottom side, and breakage of the male screw portion 103g occurs in the first thread portion. Therefore, it is necessary to increase the strength of the first thread portion of the male screw portion 103g by making the material of the piston rod 103 a high-strength material or performing heat treatment on the male screw portion 103g.

Further, in the conventional piston fastening structure shown in FIG. 15, the pressure applied to the piston 104 is received at the cross section of the annular groove 103k of the piston insertion portion 103j, and the portion is easily damaged from this portion, as shown in FIG. Similarly, the strength of the piston rod 103 needs to be increased. Also,
In this structure, two half ring-shaped flanges 160 are required, and there is a problem that the number of parts is increased.

An object of the present invention is to provide a hydraulic cylinder having a piston fastening structure which can improve the strength of a piston fastening portion with a simple structure without using a special high-strength material for the piston rod or performing heat treatment. It is.

[0009]

(1) In order to achieve the above object, according to the present invention, a piston is fastened to the tip of a piston rod, and the inside of a cylinder body is divided into a rod-side chamber and a bottom-side chamber. In the hydraulic cylinder, the end surface of the piston rod is connected to the end surface of the piston rod at least partially in contact with the end surface of the end of the piston rod through a bolt through hole formed in the end surface of the piston rod. A bolt is inserted into and tightened in a bolt screw hole provided on the piston rod, and the piston is directly fixed to the piston rod with the bolt, and an entrance portion of a pilot hole of a bolt screw hole formed in the end face of the end of the piston rod. It is assumed that a taper extending toward the end face is formed.

[0010] By directly fixing the piston to the piston rod with a bolt as described above, the force applied to the piston during the operation of the hydraulic cylinder is received by the bolt,
Tensile stress acts on the bolt, but it is basically possible to use ordinary high-strength bolts and it is easy to make the material stronger, so that sufficient strength against tensile stress is obtained. can get. On the bolt screw hole side of the piston rod, the screw portion is basically a female screw and a compressive stress acts on this screw portion, so that sufficient strength can be obtained with ordinary piston rod material. For this reason, it is not necessary to use a high-strength material for the piston rod, and it is not necessary to increase the strength by heat treatment, so that the piston rod can be made inexpensively with a low-cost material. Further, since the bolt is directly fixed with bolts, the fatigue strength against external force is also improved, the strength of the piston fastening portion is improved with a simple structure, and the life of the piston rod can be improved.

Further, since it is directly fixed with bolts, it can be constructed with a minimum number of parts.

Further, since the conventional small-diameter piston insertion portion is not required, there is no need to provide an extra step on the piston rod, and the problem of breakage at the step is reduced.

The above is the operation based on the configuration of the invention of the prior application proposed by Japanese Patent Application No. 10-149467. The present invention
The invention of the prior application is improved to further improve the strength of the piston fastening structure.

That is, when the piston is directly fixed to the piston rod with a bolt, a tensile stress acts on the bolt and a compressive stress acts on the bolt screw hole as described above.
These stresses are maximum at the bolt thread at the opening of the bolt screw hole, that is, at the first thread portion of the bolt counted from the end face of the tip of the piston rod, and stress concentration occurs at this portion. In response to this concentrated stress, a taper is formed at the entrance of the pilot hole of the bolt screw hole formed in the end face of the tip end of the piston rod as described above to form a taper that spreads toward the end face, thereby opening the bolt and the bolt screw hole of the bolt screw hole. Stress concentration at the portion is reduced, and the strength of the piston fastening portion can be further improved.

(2) In the above (1), preferably,
The angle of the taper is in the range of 8-10 °.

Thus, the stress concentration at the opening of the bolt screw hole can be alleviated while securing a certain height of the screw thread at the entrance of the pilot hole of the bolt screw hole.

(3) In the above (1), the taper is formed in the range of the first to second to fourth ridges of the female screw of the bolt screw hole counted from the end face.

This makes it possible to alleviate the stress concentration at the opening of the bolt screw hole, while keeping the number of screw threads of the bolt screw hole as complete as possible.

[0019]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows a piston fastening structure of a hydraulic cylinder according to a first embodiment of the present invention. The hydraulic cylinder 1 has a cylinder body 2, a piston rod 3, and a piston 4. It comprises a cylindrical tube 5 having one end 5a closed and the other end 5b open, and a rod-side cylinder head 6 fixed to the open end 5b of the tube 5. The piston rod 3 extends through the rod-side cylinder head 6 into and out of the tube 5. The piston rod 3 is slidable in the tube 5 at the tip of the piston rod 3 located in the cylinder body 2 and inside the cylinder body 2. Is divided into a chamber 7a on the rod side and a chamber 7b on the bottom side.
A seal ring 13, wear rings 14a and 14b, and contamination seals 15a and 15b are provided on the outer periphery of the piston 4. The rod side cylinder head 6 has a chamber 7
A supply / discharge port 8 for hydraulic oil to / from a is provided, and a closed end 5a of the tube 5 is provided with a supply / discharge port 9 for hydraulic oil to / from the chamber 7b.

The chamber 7b on the bottom side of the supply / discharge port 9
When the supply / discharge port 8 communicating with the rod-side chamber 7a is connected to the tank, the piston 4 slides toward the rod-side cylinder head 6 on the left side in the drawing to displace the piston rod 3 into the cylinder body 3 And the hydraulic cylinder 1 is extended. When pressure oil is supplied to the chamber 7a on the rod side via the supply / discharge port 8 and the supply / discharge port 9 communicating with the chamber 7b on the bottom side is connected to the tank, the piston 4 closes the closed end of the tube 5 on the right side in the figure. Part 5a
The piston cylinder 3 is slid and displaced to move the piston rod 3 into the cylinder body 2, and the hydraulic cylinder 1 contracts. Attachment part 1 provided at tube closed end 5a of cylinder body 2
0 and a mounting portion 11 provided at the outer end of the piston rod 3
By pivotally connecting one to the fixed member and the other to the movable member, the movable member can be driven.

Next, the piston fastening structure which is a feature of the present invention will be described.

End surface 2 of piston 4 on piston rod 3 side
0 has a recess 21 as an axial fitting portion,
The piston end face 20 is divided into a bottom face 21a of the recess 21 and an end face portion 20a facing the chamber 7a on the rod side, and the piston 4 is mounted by fitting the tip of the piston rod 3 into the recess 21 so that the piston rod is mounted. 3 and coaxiality. Further, an axial bolt screw hole 24 is formed in the end face 23 of the tip of the piston rod 3, an axial bolt through hole 25 is formed in the recess bottom surface 21 a of the piston 4, and the end face 23 of the tip of the piston rod 3 is formed.
The piston 4 is fixed by inserting and tightening the bolt 22 from the bolt through hole 25 into the bolt screw hole 24 in a state where the concave bottom surface 21 a of the end surface 20 of the piston 4 faces the piston rod 3 and fastened to the piston rod 3. . The number of bolts 22 may be an appropriate number, but in the present embodiment, six bolts 22 are used as shown in FIG. Further, the bolt 22 has been described as an example of a round head bolt, but may be a hexagonal head bolt.

FIG. 3 shows an end surface 23 of the distal end of the piston rod 3.
The enlarged view of the bolt screw hole 24 opened in FIG. In FIG. 3, reference numeral 30 denotes a prepared hole of the bolt screw hole 24, and reference numeral 31 denotes a screw portion of the bolt screw hole 24, that is, a female screw thread. The pilot hole 30 of the bolt screw hole 24 is formed with a taper 30a that extends toward the end face 23 at the entrance thereof, and accordingly, the tip of the thread 31 of the female screw is cut and lowered. The angle of the taper 30a is preferably in the range of 8 to 10 °, and is preferably formed in the range from the first peak to the second to fourth peaks counted from the end face 23. In the illustrated example, the taper 30a has Angle 8
゜, formed from the first peak to the third peak.

In the piston fastening structure of the present embodiment, first, the piston 4 is directly fastened to the piston rod 3 with the bolt 22 as described above, so that a sufficient strength can be obtained with a normal piston rod material. Becomes

That is, by directly fastening the piston 4 to the piston rod 3 with the bolts 22, the force applied to the piston 4 is received by the plurality of bolts 22. At this time, a tensile stress acts on the bolt 22, but basically, a normal high-strength bolt can be used for the bolt 22, and it is easy to make the material stronger,
Sufficient strength against tensile stress is obtained. On the bolt screw hole 24 side of the piston rod 3, the screw portion is basically a female screw and a compressive stress acts on this screw portion, so that sufficient strength can be obtained with ordinary piston rod material.

Further, a taper 30a is formed at the entrance of the pilot hole 30 of the bolt screw hole 24, whereby the strength of the piston fastening structure is further improved.

FIG. 4 shows the relationship between the maximum principal stress acting on the bolt 22 and the number of threads when the taper 30a is not formed at the entrance of the pilot hole 30 of the bolt screw hole 24. here,
The number of threads is a load point when the bolt 22 is tightened, that is, is counted from the end face 23 of the piston rod 3. As can be seen from this figure, the maximum tensile stress is applied to the first thread portion of the bolt 22, that is, the portion indicated by reference numeral 32 in FIG. 3, and stress concentration occurs in this portion.

FIG. 5 shows the relationship between the maximum principal stress acting on the bolt 22 and the number of threads when a taper 30a is formed at the entrance of the pilot hole 30 of the bolt screw hole 24. From this figure, it can be seen that when the taper 30a is formed, the maximum tensile stress of the first thread portion of the bolt 22 is reduced, and the concentrated stress is reduced.

That is, when the piston 4 is directly fixed to the piston rod 3 with the bolt 22, the bolt 2
2, a tensile stress acts on the screw portion (female screw) of the bolt screw hole 24, and a compressive stress acts on the screw portion (female screw) of the bolt screw hole 24. Counting from the end surface 23 of the tip of the bolt 22, the maximum is at the first peak portion 32 of the bolt 22, and stress concentration occurs at this portion. By forming the taper 30a which spreads toward the end face 23 at the entrance portion of the pilot hole 30 of the bolt screw hole 24 as described above, the stress at the bolt screw hole opening of the bolt 23 and the bolt screw hole 24 against this concentrated stress. Concentration is reduced, and the strength of the piston fastening structure can be further improved.

Here, as a result of the examination, if the angle of the taper 30a is in the range of 8 to 10 °, the height of the screw thread at the entrance portion of the pilot hole of the bolt screw hole is secured to some extent while the bolt screw hole opening is formed. If the taper 30a is formed in the range from the first peak to the second to fourth peaks counted from the end face 23, if the taper 30a is formed from the end face 23, the thread height of the bolt screw hole can be reduced. It was found that the stress concentration at the opening of the screw hole at the opening of the bolt screw hole can be alleviated while the maximum number of complete threads remains.

According to the present embodiment, the following effects can be obtained.

1) In the conventional general piston fastening structure shown in FIG.
(See FIG. 14).
Since a tensile stress acts on 3 g, a high-strength material must be used for the piston rod. On the other hand, in the piston fastening structure of the present invention, since it is not necessary to provide a male thread portion on the piston rod 3, there is no need to use a high-strength material for the piston rod 3, and it is not necessary to increase the strength by heat treatment, so that the cost is reduced. The piston rod 3 can be made inexpensively with a suitable material.

2) Conventionally, the tensile stress acts on the male thread portion of the piston rod, but in the present invention, it acts on the bolt 22.
Normal high-strength bolts can be used for the bolts 22 and it is easy to make the material stronger,
The strength of the piston fastening portion can be improved with a simple structure, and the life of the piston rod can be significantly improved.

3) By forming the taper 30a at the entrance of the pilot hole 30 of the bolt screw hole 24, concentrated stress is reduced, and the strength of the piston fastening structure can be further improved.

4) By fixing using bolts 22,
Fatigue strength against external force is also improved.

5) In the conventional piston fastening structure shown in FIG. 15, two flanges 160 were required, and the number of parts was large. However, in the present invention, such parts are unnecessary, and the number of parts is small. it can.

6) Since the conventional small-diameter piston insertion portion is not required, the piston rod may be a round bar having no step, and there is no problem of breakage at the step.

7) The entire length of the rod is determined by the length up to the head of the bolt 22, so that the effective stroke of the hydraulic cylinder 1 can be made longer than when a large nut 112 is used as in the conventional case.

8) Since a plurality of bolts 22 are used, the tightening torque of each bolt can be reduced, and assembly and disassembly are easy.

9) Conventionally, the piston rod
In many cases, maintenance is performed by disassembling the piston assembly. For example, when replacing the rod-side cylinder head or replacing the rod seal attached to the cylinder head, remove the nut 112 (see FIG. 14). Each time, the nut had to be put on a special machine to loosen the nut, and the nut had to be loosened using this special machine. The tightening torque of the nut 112 is about 1000
kgfm. In the present invention, the tightening torque of one bolt 22 is as small as about 70 kgfm. For this reason,
The bolt 22 can be loosened by manpower, and the bolt can be loosened without the need for a large-scale dedicated machine, and the serviceability is high.

As described above, according to this embodiment, the strength of the piston fastening portion can be improved with a simple structure without using a special high-strength material for the piston rod 3 or performing heat treatment (1) to 5 above. )). Further, the effect of securing the effective stroke length and improving disassembly / assembly and serviceability can be obtained (the above 6) to 8)).

Another embodiment of the present invention will be described with reference to FIGS. In the figure, the same parts as those shown in FIG. 1 are denoted by the same reference numerals.

FIG. 6 shows a second embodiment of the present invention, in which a piston 4A is fastened to a piston rod 3A with one bolt 22A. In this case, bolt 2
2A requires a single bolt to apply a large tightening force required for fastening, so it is preferable to use a hexagonal head bolt as shown in the figure.

Further, similarly to the first embodiment, the pilot hole 30 of the bolt screw hole 24A is formed with a taper 30a (the inclination is not shown) which spreads toward the end face 23 at the tip end of the piston rod 3 at the entrance. Have been.

FIG. 7 shows a third embodiment of the present invention, in which a hole 30 for an axial fitting portion is formed in the center of the end face 23 of the piston rod 3B, and the end face 2 of the piston 4B is formed.
A projection 31 of a fitting portion in the axial direction is provided at a center portion on the 0 side, and the projection 31 is fitted into the hole 30 to ensure coaxiality between the piston rod 3B and the piston 4B. A counterbore hole 32 for accommodating the head of the bolt 22 is formed in the opening of the bolt through hole 25 of the piston 4B on the side of the chamber 7b.
Is formed so that the head of the bolt 22 does not protrude from the end face of the piston 4B.

Further, similarly to the first embodiment, the pilot hole 30 of the bolt screw hole 24 is formed with a taper 30a (inclination not shown) extending toward the end surface 23 at the tip end of the piston rod 3 at the entrance. Have been.

FIG. 8 shows a fourth embodiment of the present invention, in which a hole 30 and a protrusion 3 of the fitting portion shown in FIG.
1 is enlarged to form a hole 30C and a projection 31C, respectively.
The bolt screw hole 24 and the through hole 25 of the bolt 22 are formed in the hole 30C and the protrusion 31C.

Further, similarly to the first embodiment, the pilot hole 30 of the bolt screw hole 24 is formed with a taper 30a (inclination not shown) extending toward the end face 23 at the tip end of the piston rod 3 at the entrance. Have been.

FIG. 9 shows a fifth embodiment of the present invention, in which the positioning for ensuring the coaxiality between the piston rod 3D and the piston 4D is replaced by a fitting portion and a pin 35 is used.
This is done by: Here, two pins 35 are used symmetrically on the diameter line as shown in FIG. When assembling, a pin hole 3 is formed in each of the end faces 20 and 23 while maintaining the coaxiality of the piston rod 3D and the piston 4D.
After the pins 6 and 37 are opened in advance and the pin 35 is press-fitted into one of the pin holes 36 and 37, the other pin hole is press-fitted into the pin 35 and positioned, and fixed with the bolt 22 in this state.

Further, similarly to the first embodiment, the pilot hole 30 of the bolt screw hole 24 is formed with a taper 30a (inclination not shown) extending toward the end surface 23 of the tip end of the piston rod 3 at the entrance. Have been.

The above-described effects 1) to 7) can be obtained also by the second embodiment shown in FIG.

According to the embodiments shown in FIGS. 7 to 9, the effects of 1) to 9) are obtained, and the effect of the above 7) is such that the entire length of the rod is up to the piston end face. Therefore, the effective stroke of the hydraulic cylinder can be further lengthened.

It should be noted that some hydraulic cylinders have a cushion device for alleviating the impact at the stroke end of the piston rod. In the above embodiment, the present invention is applied to a piston fastening structure of a hydraulic cylinder without such a cushion device. Although the invention is applied, Japanese Patent Application No. 10-14 / 1998
FIGS. 8 to 12 of No. 9467 show an embodiment in which a piston of a hydraulic cylinder provided with a cushion ring (cushion device on the rod side) is directly fixed to a piston rod with bolts. A hydraulic cylinder having a cushion plunger (bottom side cushion device) provided on a piston rod directly fixed to a piston with a bolt is shown, and the present invention may be applied to a device provided with such a cushion device. Of course, also in this case, the same effect as the above embodiment can be obtained.

[0055]

According to the present invention, the strength of the piston fastening portion can be improved with a simple structure without using a special high-strength material for the piston rod or performing heat treatment. The effect of improvement in assemblability and serviceability is obtained.

Further, since the tapered portion is formed at the entrance of the pilot hole of the bolt screw hole, the strength of the piston fastening structure can be further improved.

[Brief description of the drawings]

FIG. 1 is a sectional view of a hydraulic cylinder including a piston fastening structure according to a first embodiment of the present invention.

FIG. 2 is a front view of a piston fastening portion shown in FIG.

FIG. 3 is an enlarged view of a bolt screw hole formed in an end face of a tip end of a piston rod.

FIG. 4 is a diagram showing the relationship between the maximum principal stress acting on the bolt and the number of threads when no taper is formed at the entrance of the pilot hole of the bolt screw hole.

FIG. 5 is a diagram showing the relationship between the maximum principal stress acting on the bolt and the number of threads when a taper is formed at the entrance of the pilot hole of the bolt screw hole.

FIG. 6 is a sectional view of a piston fastening structure of a hydraulic cylinder according to a second embodiment of the present invention.

FIG. 7 is a sectional view of a piston fastening structure of a hydraulic cylinder according to a third embodiment of the present invention.

FIG. 8 is a sectional view of a piston fastening structure of a hydraulic cylinder according to a fourth embodiment of the present invention.

FIG. 9 is a sectional view of a piston fastening structure of a hydraulic cylinder according to a fifth embodiment of the present invention.

FIG. 10 is a front view of a piston fastening portion shown in FIG. 9;

FIG. 11 is a cross-sectional view of a conventional piston fastening structure of a hydraulic cylinder.

FIG. 12 is a sectional view of a piston fastening structure of a hydraulic cylinder according to another related art.

FIG. 13 is a view showing a stress state acting on a male screw portion of a piston rod.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Hydraulic cylinder 2 Cylinder main body 3 Piston rod 4 Piston 5 Tube 6 Rod side cylinder head 7a, 7b Chamber 8, 9 Supply / discharge port 20 Piston rod side end surface 20a Piston end surface facing rod side chamber 21 Recess 21a Bottom surface 21b Inner peripheral surface of piston recess 22 Bolt 23 End face of piston rod 24 Bolt screw hole 25 Bolt through hole 30 Bolt screw hole pilot hole 30a Taper 31 Bolt screw hole thread 32 First bolt portion of bolt

Continued on the front page (72) Inventor Mitsuhiro Yoshimoto 650, Kandamachi, Tsuchiura-shi, Ibaraki F-term in the Tsuchiura Plant of Hitachi Construction Machinery Co., Ltd. (Reference) 3H081 AA03 BB02 CC24 DD02 EE27 HH01

Claims (3)

[Claims] In a hydraulic cylinder in which a piston is fastened to a tip of a piston rod and the inside of a cylinder body is divided into a chamber on a rod side and a chamber on a bottom side, a rod-side end face of the piston is attached to an end face of the tip of the piston rod. In a state of at least partial face-to-face contact, a bolt is inserted into a bolt screw hole opened in the end face of the tip of the piston rod from a bolt through hole opened in the end face on the rod side of the piston and tightened, and the piston is tightened with this bolt. A hydraulic cylinder which is directly fixed to a piston rod, and has a taper which extends toward the end face at an entrance portion of a pilot hole of a bolt screw hole formed in an end face of a tip end of the piston rod. 2. The hydraulic cylinder according to claim 1, wherein the angle of the taper is in the range of 8 to 10 °. 3. The hydraulic cylinder according to claim 1, wherein the taper is formed in a range from the first crest to the second to fourth crests of the female screw of the bolt screw hole counted from the end face. Hydraulic cylinder.