Method for manufacturing piston rod for buffer

Abstract

To provide a cylinder device and a method for manufacturing a piston rod which have an excellent abrasion resistance, less aging in a slide property, and an improved smoothness from a dry state to a stationary state.SOLUTION: A cylinder device of the present invention has: a bottomed cylindrical cylinder; seal means provided at an opening part of the cylinder; and a piston rod provided projecting from the opening part of the cylinder and slidable with the seal means. The piston rod is chromium-plated, and a slide surface of the piston rod in an axial direction has a projection valley depth Rvk:0.06 μm or more and less than a plating thickness, a projection peak height Rpk:0 μm or more and 0.04 μm or less, and a core part level difference Rk:0.08 μm or more and 0.16 μm or less, as characteristic evaluation parameters of a plateau structure surface described in JIS B 0671-2 and ISO 13565-2.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing a piston rod for a shock absorber. This application claims priority based on Japanese Patent Application No. 2020-057547, filed on March 27, 2020, the contents of which are incorporated herein by reference. [Background technology]

[0002] Conventionally, techniques have been proposed for improving wear resistance and sliding characteristics by specifying the surface properties of automotive sliding parts. For example, Patent Document 1 below discloses a technique for polishing grooves on the contact surface with an element of a pulley for a belt-type continuously variable transmission with a wrap film, thereby processing the surface roughness to a peak height Rpk of 0.09 μm or less and a valley depth Rvk of 0.4 to 1.3 μm. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2011-137492 Summary of the Invention [Problem to be solved by the invention]

[0004] Regarding the surface properties of automotive sliding parts, in the case of piston rods, from the viewpoint of sliding characteristics and corrosion resistance, chrome plating is applied to the sliding surface with the oil seal, and then polishing processes such as buffing and super-finishing are carried out. If the protruding peaks on the sliding surface of the piston rod that comes into contact with the oil seal are high, the attacking force on the oil seal will be strong, which may cause wear of the oil seal. Also, if the protruding valleys on the sliding surface of the piston rod are shallow, the oil film will be insufficient, and if the protruding valleys are deep, there is a risk of strong interference with the oil seal.

[0005] Furthermore, research by the present inventors has revealed that if the level difference of the core portion on the sliding surface of the piston rod is small, it affects the ease of conformability from the dry state to the steady state. Here, when the piston rod is a piston rod for a suspension device of an automobile, the level difference of the core portion is thought to have an effect on the ride comfort of the automobile. That is, if the surface quality of the sliding surface of the piston rod is inappropriate, it may result in a deterioration in the ride comfort of the automobile and the sliding characteristics may change over time. Also, if the surface quality of the sliding surface of the piston rod is inappropriate, the sliding characteristics may change over time.

[0006] The problem to be solved by the present invention is to provide a cylinder device and a method for manufacturing a piston rod that can improve the wear resistance of the sliding surface of the piston rod, thereby reducing changes in the sliding characteristics of the piston rod over time and improving the ease of conformance from a dry state to a steady state. [Means for solving the problem]

[0007] The present invention employs the following aspects. (1) A method for manufacturing a shock absorber piston rod, comprising: a cylinder in which a working fluid is sealed; an oil seal provided at an opening of the cylinder; a piston rod provided so as to protrude from the opening of the cylinder and slide in an axial direction while contacting the oil seal, the piston rod having a sliding surface that can repeatedly move in and out between the inside and outside of the cylinder; a plating step of forming a chrome plating layer on the sliding surface of the shock absorber piston rod; and a polishing step of forming a plurality of oil reservoir grooves on the sliding surface, in which the working fluid can be retained, using a first polishing film while rotating the shock absorber piston rod around its axis after the plating step. and a second polishing process step, after the first polishing process step, using a second polishing film while rotating the shock absorber piston rod around its axis, to form the oil reservoir grooves into a plateau shape, wherein in the first polishing process step, the first polishing film is fed out while the shock absorber piston rod is being rotated, and the first polishing film is brought into contact with the shock absorber piston rod for polishing, and the pressing force with which the first polishing film is pressed against the shock absorber piston rod is 0.15 to 0.3 MPa. (2) The manufacturing method of a shock absorber piston rod described in (1) above may be as follows: the first polishing process and the second polishing process include a first roller that can rotate about its axis, a second roller that is arranged at a distance from the first roller and can rotate about its axis, and a third roller to which the first abrasive film or the second abrasive film is supplied, the shock absorber piston rod is placed on the boundary between the first roller and the second roller, the first roller and the second roller rotate about their axes to rotate the shock absorber piston rod about their axes, the third roller is placed on the boundary between the first roller and the second roller, the third roller rotates about its axis while the first abrasive film or the second abrasive film is supplied to the third roller, and the shock absorber piston rod is pressed against the first roller and the second roller to perform the polishing process. (3) The manufacturing method of a shock absorber piston rod according to (2) above may be performed as follows: the third roller is pressed against the shock absorber piston rod by a pressure head that presses the third roller toward the shock absorber piston rod. (4) The method for manufacturing a shock absorber piston rod described in (3) above may be performed as follows: the pressure head is supported so as to be freely movable in the axial direction of the third roller, and is supported so as to be freely movable back and forth along the axial direction of the third roller while pressing the third roller toward the shock absorber piston rod. [Effects of the Invention]

[0008] According to the above aspects of the present invention, the sliding surface of the piston rod has a suitable depth of the protruding valley portion, a suitable height of the protruding peak portion, and a suitable level difference between the core portion. This provides excellent wear resistance, is less aggressive to the oil seal, reduces wear of the oil seal, and minimizes changes in sliding characteristics over time. Furthermore, the piston rod has good conformability from a dry state to a steady state, and when applied to a piston rod for an automobile suspension device, it can provide a suspension device that does not degrade the ride comfort of the automobile. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a graph showing an example of a surface roughness curve of a sliding surface of a piston rod obtained by a method for manufacturing a piston rod according to an embodiment of the present invention. [Figure 2] The piston rod has a sliding surface that has the surface roughness curve shown in FIG. [Figure 3] FIG. 4 is a front view showing the outer shape of the piston rod and the position of the sliding surface. [Figure 4] FIG. 3 is a diagram showing the upper structure of the shock absorber, and is an enlarged cross-sectional view of part A in FIG. 2. [Figure 5] 3 is a diagram showing the tip of the piston rod, and is an enlarged cross-sectional view of part B in FIG. 2. FIG. [Figure 6] FIG. 4 is a flow chart showing an example of a manufacturing process of the piston rod. [Figure 7] 7 is a flowchart showing an example of polishing the outer diameter of the piston rod (polishing treatment step S6 in FIG. 6). FIG. [Figure 8] FIG. 2 is a perspective view showing an example of a polishing device used to polish the outer diameter of the piston rod, illustrating a main part of the polishing device. [Figure 9] This is a partially enlarged cross-sectional view of the portion including the sliding surface of the piston rod, as viewed in a cross section including the central axis CL, in which the outer diameter of the piston rod is polished with a #400 polishing film. [Figure 10] FIG. 1 is a partially enlarged cross-sectional view of the piston rod including the sliding surface, taken along a cross section including the central axis CL, showing the piston rod when its outer diameter is polished with a #320 polishing film. [Figure 11] FIG. 1 is a partially enlarged cross-sectional view of the piston rod including the sliding surface, taken along a cross section including the central axis CL, showing the piston rod with its outer diameter polished with a #1000 polishing film. [Figure 12] FIG. 1 is a partially enlarged cross-sectional view of the piston rod including the sliding surface, taken along a cross section including the central axis CL, showing the piston rod when its outer diameter is polished with a #2000 polishing film. [Figure 13] FIG. 2 is an explanatory diagram showing the relationship between the height of the peaks (Rpk), the level difference between the core (Rk), and the depth of the valleys (Rvk) in a smoothed roughness curve. [Figure 14] FIG. 10 is an explanatory diagram for determining the height of the protruding peak (Rpk), the level difference between the core (Rk), and the depth of the protruding valley (Rvk), and shows the position of the straight line that separates the protruding portions on the surface roughness curve. [Figure 15] FIG. 10 is an explanatory diagram showing the ratio of the load length of a profile curve element to the evaluation length obtained from the surface roughness curve. [Figure 16]FIG. 1 is an explanatory diagram showing the position of the straight line where the secant line of the load curve drawn when ΔMr, which is the difference in load length ratio, is set to 40%, has the gentlest slope. [Figure 17] FIG. 10 is an explanatory diagram showing a state in which the straight line is extended to ΔMr=0% and 100% and divided into a protruding peak portion and a protruding valley portion. [Figure 18] FIG. 10 is an explanatory diagram showing how to find the height of a right triangle that is equal to the cross-sectional area A1 of the protruding peak. [Figure 19] FIG. 1 is an explanatory diagram of a reproduction test method performed on an example sample. [Figure 20] 10 is a graph showing the evaluation test results of the examples obtained by changing the test conditions in outer diameter polishing (polishing process step S6 in FIG. 6). [Figure 21] 1 is a graph showing the relationship between the protrusion valley depth (Rvk) and the amount of change in friction force (ΔN) in a plurality of piston rods manufactured in the examples. [Figure 22] 1 is a graph showing the relationship between the level difference (Rk) of the core portion and the amount of change in frictional force (ΔN) in a plurality of piston rods manufactured in the examples. [Figure 23] 1 is a graph showing the relationship between the protruding valley depth (Rvk) and the maximum friction force (PP) for a plurality of piston rods manufactured in the examples. [Figure 24] 1 is a graph showing the relationship between the level difference (Rk) of the core portion and the maximum friction force (PP) for a plurality of piston rods manufactured in the examples. [Figure 25] 1 is a graph showing the relationship between the protruding peak height (Rpk) and the amount of oil seal adhesion in a plurality of piston rods manufactured in the examples. [Figure 26] 1 is a graph showing the relationship between the level difference (Rk) of the core portion and the amount of oil seal adhesion in a plurality of piston rods manufactured in the examples. [Figure 27] 1 is a table showing the polishing films used in the first to fourth polishing treatment steps. [Figure 28] 1 is a table showing the measurement results of the protruding valley depth, the protruding peak height, and the level difference of the core portion for each of Samples 1 to 10. [Figure 29] 10 is a table showing vibration conditions. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, a cylinder device and a method for manufacturing a piston rod according to one embodiment of the present invention will be described. The embodiments described below are specifically described to provide a better understanding of the gist of the present invention, and unless otherwise specified, do not limit the present invention. The drawings used to describe the embodiments below are shown at appropriately altered scales to make each part easier to see. The symbol CL in Figures 2 to 5 and the following description indicates the center line of the cylinder device and the piston rod.

[0011] Fig. 1 is a graph showing an example of the surface roughness curve of the sliding surface of a piston rod whose surface has been polished according to a manufacturing method described below. Fig. 2 is a cross-sectional view showing the overall structure of a shock absorber, which is a cylinder device equipped with the same piston rod, taken along a cross section including the central axis CL. The sliding surface of this piston rod has the characteristics of the surface roughness curve shown in Fig. 1. Before describing the surface roughness of the sliding surface of the piston rod, the overall configuration of a cylinder device equipped with a piston rod will be described.

[0012] "Cylinder device" The cylinder device 1 shown in Fig. 2 is a shock absorber used in suspension devices for vehicles such as automobiles and railcars, and specifically in strut-type suspension devices for automobiles. This cylinder device 1 has a cylindrical inner tube (cylinder) 2 in which a working fluid is sealed, and a cylindrical outer tube 3 with a bottom that is larger in diameter than the inner tube 2 and is provided on the outer periphery of the inner tube 2, forming a reservoir chamber R between the inner tube 2 and the outer tube 3 in which a working fluid and a working gas are sealed. In other words, the cylinder device 1 is a double-tube shock absorber in which the inner tube 2 is provided inside the outer tube 3.

[0013] The outer cylinder 3 is composed of a cylindrical side wall 7 and a bottom 8 that closes one axial end of the side wall 7. The inner cylinder 2 is cylindrical. The inner cylinder 2 is engaged with the bottom 8 of the outer cylinder 3 via an annular base valve 13 attached to one axial end of the inner cylinder 2. The inner cylinder 2 is also engaged with an opening 9 on the side wall 7 of the outer cylinder 3 on the opposite side from the bottom 8 via an annular metal rod guide 11 attached to the other axial end of the inner cylinder 2.

[0014] The base valve 13 is positioned in the radial direction by being placed on the bottom 8 of the outer cylinder 3 while being fitted and fixed to the inner cylinder 2. As a result, the base valve 13 is arranged coaxially with the outer cylinder 3. The rod guide 11 is fitted into the inner cylinder 2 and the side wall portion 7 of the outer cylinder 3, thereby arranging the other axial end of the inner cylinder 2 coaxially with the outer cylinder 3. An annular oil seal (sealing member) 15 is disposed on the side of this rod guide 11 opposite the bottom 8. This oil seal 15 is also fitted into the inner periphery of the opening 9 side of the side wall 7. The opening 9 of the outer cylinder 3 is located axially outward of the inner cylinder 2, and therefore also serves as the opening of the cylinder. On the side of the outer cylinder 3 opposite the bottom 8, an engagement portion 16 is formed by bending the outer cylinder 3 radially inward. The oil seal 15 is supported on one end of the cylinder by having its outer axial outer side engaged by the engagement portion 16.

[0015] A piston 25 is slidably fitted within the inner cylinder 2. This piston 25 divides the interior of the inner cylinder 2 into a first chamber 22 and a second chamber 23. The first chamber 22 is provided between the piston 25 and the rod guide 11 within the inner cylinder 2. The second chamber 23 is provided between the piston 25 and the base valve 13 within the inner cylinder 2. The second chamber 23 within the inner cylinder 2 is divided from a reservoir chamber R by the base valve 13 provided on one end side of the inner cylinder 2.

[0016] A metal piston rod 21 is connected to the piston 25 by a nut 26. The piston rod 21 has a cylindrical large-diameter portion 21a with a constant outer diameter, passes through the rod guide 11 and the oil seal 15, and extends from the inner cylinder 2 and the outer cylinder 3, i.e., from one end side of the cylinder, to the outside. The large-diameter portion 21a of the piston rod 21 is slidably inserted into the rod guide 11 and the oil seal 15, respectively. One end of the piston rod 21 is located inside the outer cylinder 3 and the inner cylinder 2, and the other end is located outside the outer cylinder 3 and the inner cylinder 2.

[0017] A small diameter portion 21b is formed on one end side of the large diameter portion 21a of the piston rod 21. A piston 25 is inserted into this small diameter portion 21b, and a nut 26 is screwed onto a thread formed on the tip side of the small diameter portion 21b, thereby attaching the piston 25 to the piston rod 21. An annular groove 21c is formed in the large diameter portion 21a of the piston rod 21 at a position close to the base end of the small diameter portion 21b. A ring-shaped internal stopper 24 is attached so as to engage with this annular groove 21c. A ring-shaped rebound rubber 19 is arranged on the upper side of the internal stopper 24.

[0018] As shown in Figure 3, in the large diameter portion 21a of the piston rod 21, a sliding range A for the rod guide 11 and the oil seal 15 is defined between a position P1 slightly spaced away from the annular groove 21c toward the opposite side from the piston mounting side and a connection portion P2 of the bolt portion 21d, which serves as the mounting portion to the vehicle body. Although not shown in the drawings, the outer surface of the piston rod 21 is formed with a chrome plating layer. Furthermore, the surface of sliding range A of large diameter portion 21a of piston rod 21 (the sliding surface of the chrome plating layer; hereinafter, sometimes simply referred to as "sliding surface A") is a polished surface. This polished surface has the following characteristics evaluation parameters for plateau structure surfaces described in JIS B 0671-2 and ISO 13565-2: a peak height Rpk of 0 μm or more and 0.04 μm or less, a core level difference Rk of 0.08 μm or more and 0.16 μm or less, and a valley depth Rvk of 0.06 μm or more and less than the plating thickness. The polished surface will be described in detail later.

[0019] The piston rod 21 moves axially integrally with the piston 25. The oil seal 15, through which the large diameter portion 21a of the piston rod 21 is inserted, seals the gap between the outer cylinder 3 and the piston rod 21, preventing the working liquid in the inner cylinder 2 and the working gas and working liquid in the reservoir chamber R from leaking to the outside.

[0020] As shown in Fig. 5, piston 25 is formed with passages 27 and 28 penetrating therethrough in the axial direction. Passages 27 and 28 allow communication between first chamber 22 and second chamber 23. Piston 25 is provided with an annular disc valve 28a on the axial side opposite bottom 8, which is capable of closing passage 28 when it abuts against piston 25. Piston 25 is also provided with an annular disc valve 27a on the axial side facing bottom 8, which is capable of closing passage 27 when it abuts against piston 25.

[0021] When the piston rod 21 moves toward the compression side, increasing the amount of penetration into the inner cylinder 2 and the outer cylinder 3, the disc valve 28a causes the piston 25 to move in a direction that narrows the second chamber 23. As a result, when the pressure in the second chamber 23 becomes higher than the pressure in the first chamber 22 by a predetermined value or more, the disc valve 28a opens the passage 28, generating a damping force. When the piston rod 21 moves toward the extension side, increasing the amount of protrusion from the inner cylinder 2 and the outer cylinder 3, the disc valve 27a causes the piston 25 to move in a direction that narrows the first chamber 22. As a result, when the pressure in the first chamber 22 becomes higher than the pressure in the second chamber 23 by a predetermined value or more, the disc valve 27a opens the passage 27, generating a damping force.

[0022] 2 is formed with a passage 28 and a passage 29 that penetrate in the axial direction. The passages 28, 29 allow communication between the second chamber 23 and the reservoir chamber R. An annular disc valve 30 is disposed on the axial side of the base valve 13 facing the bottom 8, and is capable of closing the passage 28 by abutting against the base valve 13. In addition, an annular disc valve 31 is disposed on the axial side of the base valve 13 opposite the bottom 8, and is capable of closing the passage 29 by abutting against the base valve 13.

[0023] The disc valve 30 is a check valve that allows the flow of hydraulic fluid from the second chamber 23 toward the reservoir chamber R via the passage 28, while restricting the flow of hydraulic fluid in the opposite direction via the passage 28. The disc valve 30 functions as a damping valve that opens the passage 28 when the piston rod 21 moves toward the compression side and the pressure in the second chamber 23 becomes higher than the pressure in the reservoir chamber R by a predetermined value or more, thereby generating a damping force.

[0024] The disc valve 31 is a check valve that allows the flow of hydraulic fluid from the reservoir chamber R toward the second chamber 23 through the passage 29, and restricts the flow of hydraulic fluid in the opposite direction through the passage 29. When the piston rod 21 moves in the extension direction and the piston 25 moves toward the first chamber 22, the pressure in the second chamber 23 drops below the pressure in the reservoir chamber R, and the disc valve 31 opens the passage 29. At that time, the disc valve 31 is a suction valve that allows the hydraulic fluid to flow from the reservoir chamber R into the second chamber 23 without generating any substantial damping force.

[0025] As shown in FIG. 2, a cylindrical mounting eye 33 is fixed to the outside of the bottom 8 of the outer cylinder 3. The cylinder device 1 is mounted between the mounting eye 33 and a portion of the piston rod 21 that is outside the oil seal 15 and a relative moving portion that is the mounting target. In the cylinder device 1, the piston 25 slides axially within the inner cylinder 2 together with the piston rod 21 due to relative movement that occurs in the mounting target portion, changing the volumes of the first chamber 22 and the second chamber 23. At that time, a damping force is generated by the flow resistance of oil fluid that occurs in the piston 25 and the base valve 13. For example, the piston rod 21 of the cylinder device 1 is connected to the vehicle body, and the mounting eye 33 is connected to the vehicle wheel, generating a damping force against the relative movement of the wheel with respect to the vehicle body.

[0026] As shown in Figure 4, the rod guide 11 has a generally stepped annular shape. The rod guide 11 has a large-diameter portion 11a on one axial side, the outer circumferential surface of which is cylindrical, and a small-diameter portion 11b on the other axial side, the outer circumferential surface of which is cylindrical and has a smaller diameter than the outer circumferential surface of the large-diameter portion 11a. The large-diameter portion 11a and the small-diameter portion 11b are formed coaxially. The large-diameter portion 11a of the rod guide 11 fits into the inner circumferential surface of the outer cylinder 3, and the small-diameter portion 11b fits into the inner circumferential surface of the inner cylinder 2.

[0027] An annular protrusion 11c having a circular ring shape and protruding in the axial direction is formed on the end of the rod guide 11 on the side of the large diameter portion 11a in the axial direction. A communication hole 11d is formed in the annular protrusion 11c at a radially inner position thereof, penetrating the rod guide 11 along the axial direction. The communication hole 11d opens to a reservoir chamber R between the outer cylinder 3 and the inner cylinder 2 on the side opposite to the annular protrusion 11c in the axial direction of the rod guide 11.

[0028] The oil seal 15 has a seal member main body 37, which is an integrally molded product in which a metal annular member 36 is embedded in a synthetic rubber sealing material 35, and an annular metal spring 38. The annular member 36 is used to maintain the shape of the sealing material 35 and provides the seal member main body 37 with the strength necessary to fix it to the target portion. The seal member main body 37 is supported on one end of the cylinder by the radial position of the annular member 36 being sandwiched between the annular protrusion 11c of the rod guide 11 and the locking portion 16 of the outer cylinder 3.

[0029] The seal member 35 has a dust lip portion 35a, an oil lip portion 35b, a seal ring portion 35c, and a check lip portion 35d.

[0030] The dust lip portion 35a extends in a cylindrical shape from the inner circumferential surface of the annular member 36 to one side in the axial direction. The oil lip portion 35b extends in a cylindrical shape from the inner circumferential surface of the annular member 36 to the other side in the axial direction. The seal member main body 37 has the large diameter portion 21a of the piston rod 21 inserted inside the oil lip portion 35b and the dust lip portion 35a so as to be able to slide therethrough. The dust lip portion 35a and the oil lip portion 35b seal the gap between themselves and the large diameter portion 21a of the piston rod 21.

[0031] The seal ring portion 35c protrudes in an annular shape from the outer peripheral surface of the annular member 36 on the same side as the oil lip portion 35b in the axial direction. The seal ring portion 35c simultaneously contacts the annular protrusion 11c of the rod guide 11 and the outer cylinder 3, thereby sealing the gap between the outer cylinder 3 and the rod guide 11.

[0032] The check lip portion 35d has a circular cylindrical shape and protrudes from a radially intermediate position of the annular member 36 on the same side as the oil lip portion 35b in the axial direction. The check lip portion 35d abuts radially inward of the annular protrusion 11c of the rod guide 11. The check lip portion 35d functions as a check valve that allows working fluid leaking from a gap between the rod guide 11 and the large diameter portion 21a of the piston rod 21 to flow into the reservoir chamber R via the communication hole 11d, while regulating the flow of working fluid in the reverse direction. The spring 38 is attached to the outer periphery of the oil lip portion 35b. The spring 38 presses the oil lip portion 35b against the outer periphery of the large diameter portion 21a of the piston rod 21 to seal the gap.

[0033] When the vehicle is running, the piston rod 21 or the outer cylinder 3 of the cylinder device 1 is repeatedly subjected to external impacts. Each time an impact force is received, the piston rod 21 moves to the contraction side or the extension side, and a damping force is applied at that time. In this way, the cylinder device 1 functions as a shock absorber used in a strut-type suspension of an automobile. Furthermore, since a chrome-plated layer is provided on the outer peripheral surface of the large diameter portion 21a of the piston rod 21 to form a sliding surface, even if the large diameter portion 21a repeatedly slides against the sealing member, the chrome-plated layer exhibits excellent sliding characteristics and excellent wear resistance.

[0034] In the piston rod 21 of this embodiment, the sliding surface A has the following characteristics evaluation parameters for a plateau structure surface described in JIS B 0671-2 and ISO 13565-2: a protruding peak height Rpk of 0 μm or more and 0.04 μm or less, a core level difference Rk of 0.08 μm or more and 0.16 μm or less, and a protruding valley depth Rvk of 0.06 μm or more and less than the plating thickness. FIG. 1 shows an example of a roughness curve of the sliding surface A of the piston rod 21 polished so that the protruding valley depth Rvk, the protruding peak height Rpk, and the core level difference Rk fall within these ranges.

[0035] Since the protruding valley depth Rvk is 0.06 μm or more and less than the plating thickness of the chrome plating layer, a sufficient amount of oil can be retained in the valley. This provides excellent oil retention, allowing the piston rod 21 to slide smoothly. In particular, when the piston rod 21 starts reciprocating and sliding repeatedly from a stopped state, the sliding surface A of the piston rod 21 conforms well from a dry state to a steady state. Therefore, when this piston rod 21 is used in an automobile suspension device, a suspension device (cylinder device) 1 can be provided that does not degrade the ride comfort of the automobile. In addition, the sliding surface A has a suitable depth of the protruding valleys, height of the protruding peaks, and level difference between the core portion, so that the piston rod 21 has excellent wear resistance and is less aggressive to the oil seal 15, thereby reducing wear of the oil seal 15 and minimizing changes in sliding characteristics over time.

[0036] "Piston rod manufacturing method" FIG. 6 is a flow chart showing an outline of a method for manufacturing the piston rod 21. In the introduction step S1, a rod material such as a steel rod made of a type of steel suitable for a piston rod is prepared. This rod material is subjected to heat treatment such as induction hardening and tempering in the heat treatment step S2 to perform the surface hardening treatment required for a piston rod. Next, a cutting process S3 and a grinding process S4 are performed to obtain the outer shape of piston rod 21 shown in Fig. 3. This results in the outer shape of piston rod 21 having small diameter portion 21b, annular groove 21c, bolt portion 21d, etc. Subsequently, in the plating step S5, the piston rod is subjected to a chrome plating process to form a chrome plating layer with a film thickness of about 20 μm, thereby producing a rod material. Subsequently, in the polishing process step S6, the rod material is subjected to outer diameter polishing as described below, to obtain the desired piston rod 21.

[0037] FIG. 7 shows details of the polishing process S6. The polishing process S6 includes a first polishing process S61, a second polishing process S62, a third polishing process S63, and a fourth polishing process S64. The first polishing process S61 and the second polishing process S62 can be collectively referred to as a polishing process S6-1 for forming an oil reservoir. The third polishing process S63 and the fourth polishing process S64 can be collectively referred to as a polishing process S6-2 for forming a plateau surface. Although the polishing process step S6-1 for forming oil reservoirs is made up of two processes, the first polishing process step S61 and the second polishing process step S62, the present invention is not limited to this and polishing may be made up of one process or three or more processes. Although the plateau surface forming polishing process step S6-2 is made up of two processes, the third polishing process step S63 and the fourth polishing process step S64, the present invention is not limited to this and polishing may be made up of one process or three or more processes.

[0038] FIG. 8 shows the main components of a film polishing device suitable for use in carrying out the above-mentioned polishing steps. The polishing processing device 40 of this embodiment has drive rollers 41, 42 that are arranged close to each other with parts of their circumferential surfaces close to each other, so that they are horizontally aligned and rotatable about their respective axes. The device is configured so that the rod material 21A for a piston rod to be machined can be placed on the boundary between the drive rollers 41, 42 that are arranged close to each other. When the rod material 21A is placed on the boundary between the drive rollers 41 and 42 that are arranged close to each other, the supported rod material 21A can be rotated around its axis as the drive rollers 41 and 42 rotate.

[0039] A backup roller 43 that is rotatable about its axis is horizontally disposed above the rod material 21A on the boundary between the drive rollers 41 and 42. A polishing film 44 can be supplied to the bottom side of this backup roller 43 from a film supply device (not shown). The polishing film 44 is a long, rectangular film, and is supplied, as indicated by arrow a, from a film supply device (not shown) provided on one side of the backup roller 43 in a direction perpendicular to the central axis of the backup roller 43 toward the bottom side of the backup roller 43. The polishing film 44 then moves, as indicated by arrow b, to a film winding device (not shown) provided on the other side of the backup roller 43 in a direction perpendicular to the central axis of the backup roller 43, where it is taken up. With this polishing processing device 40, the required length of polishing film 44 can be wound around the film supply device and continuously supplied to the bottom side of the backup roller 43.

[0040] The backup roller 43 is supported by a vertical movement mechanism (not shown) so that it can rotate freely while maintaining a horizontal position. The backup roller 43 is supported by the vertical movement mechanism so that its vertical position can be finely adjusted. A pressure head 45 supported by a vertical / front-rear movement mechanism (not shown) is provided above the backup roller 43. This pressure head 45 can press the backup roller 43 downward with a predetermined pressure while descending from a position slightly above the backup roller 43, as indicated by arrow c. The pressure head 45 is also supported by the vertical / front-rear movement mechanism (not shown) so that it can move axially of the backup roller 43, as indicated by arrow d. Therefore, the pressure head 45 is supported so that it can move back and forth along the axial direction of the backup roller 43 while pressing the backup roller 43 downward with a predetermined force.

[0041] In the following description, an example will be described in which the outer diameter of the piston rod 21 is polished using a polishing device equipped with four polishing processing devices 40 shown in FIG. As an example, of four polishing machines, a polishing film with a grit size (#600) is loaded as the polishing film for the first polishing machine. A polishing film with a grit size (#320) is loaded as the polishing film for the second polishing machine. A polishing film with a grit size (#1000) is loaded as the polishing film for the third polishing machine. A polishing film with a grit size (#2000) is loaded as the polishing film for the fourth polishing machine. Abrasive film is a lapping film made of synthetic resin such as PET (polyethylene resin), with abrasive grains fixed to one side with an adhesive layer, and the abrasive grains have a specific grain size depending on the grit size fixed onto the film.

[0042] Of the four polishing apparatuses described above, the first polishing process step S61 can be performed in the first polishing apparatus, the second polishing process step S62 can be performed in the second polishing apparatus, the third polishing process step S63 can be performed in the third polishing apparatus, and the fourth polishing process step S64 can be performed in the fourth polishing apparatus. When a low-grit abrasive film (first abrasive film) is used in the first abrasive processing step S61 and the second abrasive processing step S62, grooves with large valley depths and widths can be formed on the surface of the piston rod. Figure 9 shows the outline of the grooves formed when a #400 abrasive film is used for abrasive processing. Figure 10 shows the outline of the grooves formed when a #320 abrasive film is used for abrasive processing. As is clear from comparing the grooves shown in Figure 9 with those shown in Figure 10, when a polishing film with a lower (smaller) grit size is used, the valley width becomes wider and the valley depth becomes deeper. At the same time, the width of the peaks of the peaks between the valleys becomes smaller, so the real contact area e when the piston rod comes into contact with the oil seal becomes smaller.

[0043] A polishing process is performed using a polishing film (second polishing film) with a higher grit size in a third polishing process step S63 and a fourth polishing process step S64. In these polishing processes, the grooves with large valley widths and large valley depths formed in the first polishing process step and the grooves with large valley depths formed between those grooves are polished to reduce the height of the grooves without changing the depth of the grooves. FIG. 11 shows the general shapes of the valleys and peaks that are generated when the valleys and peaks formed in the first polishing process step S61 and the second polishing process step S62 are polished using a #1000 polishing film. FIG. 12 shows the general shapes of the valleys and peaks that are generated when the valleys and peaks formed in the first polishing process step S61 and the second polishing process step S62 are polished using a #2000 polishing film. In FIG. 11, the peaks of the portion marked "polished" are removed, whereas in FIG. 12, the peaks of the portion marked "polished" are removed.

[0044] As can be seen from a comparison of the shapes of the valleys and peaks shown in Figure 11 with the shapes of the valleys and peaks shown in Figure 12, even in the third and fourth polishing processes in which the valleys and peaks formed by the first polishing process step S61 and the second polishing process step S62 were polished using a polishing film with a higher (larger) grit number than those used in the first and second polishing process steps, the shapes of the valleys and peaks formed when the grit number is low (#1000) are different from the shapes of the valleys and peaks formed when the grit number is high (#2000).

[0045] Comparing the valleys and peaks after polishing with a #1000 polishing film as shown in Figure 11 and the valleys and peaks after polishing with a #2000 polishing film as shown in Figure 12, the #1000 polishing film polished a larger amount of the peaks, while the #2000 polishing film polished a smaller amount of the peaks. Therefore, the width of the top of one peak, which affects the true contact area when the piston rod comes into contact with the oil seal, is narrower in width e2 of the peak in Figure 12 than in width e1 of the peak in Figure 11.

[0046] Therefore, the real contact area of ​​the piston rod outer surface after polishing with a #1000 polishing film is larger than the real contact area of ​​the piston rod outer surface after polishing with a #2000 polishing film. Also, as is clear from a comparison of Figures 11 and 12, the depth of the valleys that form oil reservoirs on the piston rod outer surface after polishing with a #2000 polishing film is deeper than the depth of the valleys that form oil reservoirs on the piston rod outer surface after polishing with a #1000 polishing film.

[0047] Based on the above relationship, it is possible to adjust the true contact area of ​​the piston rod and the depth of the valleys that become oil reservoirs by appropriately adjusting the grit size of the abrasive film used in the first polishing process S61 and the second polishing process S62, after which abrasive film with a lower grit size is used in the third polishing process S63 and the fourth polishing process S64, which are performed using abrasive films with a higher grit size. As a result, it is thought that it is possible to reduce changes in the sliding characteristics of the piston rod over time and improve the ease of conformance when the piston rod slides from a dry state to a steady state.

[0048] Taking into consideration the various conditions of the peaks and valleys after polishing shown in Figures 9 to 12, and from the test results described later, it has been found that the surface of the sliding range A of the large diameter portion 21a of the piston rod 21 (sliding surface A, which is the surface of the chrome plating layer) is desirably a polished surface in which the protruding peak height Rpk is 0 μm or more and 0.04 μm or less (0 to 0.04 μm), the core level difference Rk is 0.08 μm or more and 0.16 μm or less (0.08 to 0.16 μm), and the protruding valley depth Rvk is 0.06 μm or more and less than the plating thickness, which are the characteristic evaluation parameters of plateau structure surfaces described in JIS B 0671-2 and ISO 13565-2. As described above, when the chromium plating layer thickness is 20 μm, the protruding valley depth Rvk is 0.06 μm or more and less than 20 μm.

[0049] "(Rvk), (Rpk), (Rk)" The protruding valley depth (Rvk), protruding peak height (Rpk), and core level difference (Rk) used as evaluation parameters in this embodiment will be described below. In this embodiment, a smoothed roughness curve of the sliding surface A is obtained from the actually measured profile curve. The smoothed roughness curve is expressed by removing the wavelength of the waviness curve from the measured raw data (profile curve) using a filter. The cutoff value λc used in this embodiment is λc=0.8 mm.

[0050] Regarding the smoothed roughness curve, the above-mentioned axial profile curve data is filtered with a phase compensation filter to calculate a first average line, and the lower part is removed from this first average line. After removing the lower part, the data is further filtered with the same phase compensation filter to calculate a second average line, and the second average line is subtracted from the profile curve to obtain the smoothed roughness curve.

[0051] Figure 13 shows the relationship between the protruding peaks, protruding valleys, cores, and evaluation length ln for the smoothed roughness curve. Furthermore, Figure 13 shows the relationship between the load length ratio, the load curve corresponding to the smoothed roughness curve, the equivalent line, the protruding peak height (Rpk), the level difference of the core (Rk), the protruding valley depth (Rvk), and the most gently sloping line. In Figure 13, Mr1 indicates the load length ratio (unit: percentage) at the point where the line separating the protruding peaks and the core of the roughness curve intersects with the load curve. Furthermore, Mr2 indicates the load length ratio (unit: percentage) at the point where the line separating the protruding valleys and the core of the roughness curve intersects with the load curve.

[0052] These relationships can be determined by the procedures shown in FIGS. FIG. 14 shows the smoothed roughness curve obtained as described above. FIG. 15 shows the state in which the ratio of the load length of the profile curve element at the cutting level c to the evaluation length ln is calculated. As shown in FIG. 16, the position where the secant line of the load curve drawn with the difference in load length ratio ΔMr set to 40% has the gentlest slope is found. As shown in FIG. 17, the secant lines are extended to Mr=0% and Mr=100% for the original smoothed roughness curve, dividing it into protruding peaks and valleys. As shown in FIG. 18, the height of a right triangle that is equal to the cross-sectional area A1' of the protruding peak is determined, and Mr1 is used as the base.

[0053] Here, a load curve is created by summing the widths of the protruding peaks while moving the cutting line for the obtained smoothed roughness curve corresponding to the evaluation length from the top to the bottom. A 40%-long equivalent line is drawn on this load curve, and the intersection of the equivalent line with the 0% vertical axis is taken as the upper level of the core part, and the intersection of the equivalent line with the 100% vertical line is taken as the lower level of the core part. The difference between this upper limit level of the core part and the lower limit level of the core part is taken as the level difference Rk of the core part.

[0054] Then, the area enclosed by the vertex of the load curve and the upper level of the core is equivalent to a triangle (equivalent triangular area A1'), and the height of this equivalent triangle is taken as the protruding peak height Rpk.Furthermore, the area enclosed by the load curve between the 100% vertical line and the lower level of the core is equivalent to a triangle (equivalent triangular area A2'), and the height of this equivalent triangle is taken as the protruding valley depth Rvk.

[0055] In the sliding surface A of the piston rod 21 of this embodiment, the protruding peak height Rpk defined as above is 0 μm or more and 0.04 μm or less (0 to 0.04 μm), the core level difference Rk is 0.08 μm or more and 0.16 μm or less (0.08 to 0.16 μm), and the protruding valley depth Rvk is 0.06 μm or more and less than the plating thickness, making it a polished surface. The test results described below show that if the protruding valley depth Rvk is less than 0.06 μm, the amount of change in frictional force and the frictional force increase, so it is desirable that the protruding valley depth Rvk be 0.06 μm or more.

[0056] The test results described below show that if the level difference Rk of the core portion is less than 0.08 μm, the amount of change in frictional force and the frictional force increase, so it is desirable that the level difference Rk of the core portion be 0.08 μm or more. Test results described below show that if the protruding peak height Rpk exceeds 0.04 μm, the amount of peeling of the oil seal increases, so it is desirable that the protruding peak height Rpk be 0.04 μm or less. Test results described later show that the amount of peeling of the oil seal increases when the level difference Rk of the core portion exceeds 0.16 μm, so it is desirable that the level difference Rk of the core portion be 0.16 μm or less. [Example]

[0057] Steel bars (diameter 12.5 mm, length 200 mm) made of JIS S25C were surface hardened (high-frequency quenching and tempering), machined, outer diameter machined and chrome-plated to obtain piston rod samples 1 to 10. Next, the polishing process step S6 was performed in accordance with the first to fourth polishing process steps S61 to S64 described above with reference to Fig. 7. To perform the polishing process step S6, a polishing processing device 40 having drive rollers 41 and 42, a backup roller 43, a polishing film 44, and a pressure head 45 shown in Fig. 8 was used.

[0058] During polishing, the rotation speed of the drive roller 41 was 1400 rpm, and the pressing force of the pressure head 45 was 0.15 to 0.3 MPa. In the first polishing process step S61 and the second polishing process step S62, polishing films of any one of grit sizes 1, 2, and 3 shown in Fig. 27 were used. In the third polishing process step S63 and the fourth polishing process step S64, polishing was performed using polishing films of any one of grit sizes 4, 5, and 6 shown in Fig. 27. In the first polishing process S61 and the second polishing process S62, grooves effective for retaining lubricant were formed, and in the third polishing process S63 and the fourth polishing process S64, polishing was performed to form a plateau surface.

[0059] The protruding valley depth Rvk, protruding peak height Rpk, and core level difference Rk, which are the characteristic evaluation parameters of the plateau structure surface specified in JIS B 0671-2 (2002) and ISO 13565-2 described above, were measured for Samples 1 to 10. The average values ​​of these measurement results are shown in Figure 28. The roughness was measured using a stylus surface roughness tester at five locations in the axial direction of sliding surface A of samples 1 to 10 under the conditions of an evaluation length of 4 mm and a cutoff (λc) of 0.8 mm.

[0060] For each sample, a micro-amplitude vibrator was used to measure the maximum friction force at each frequency and evaluate the change in friction force over time. Friction force measurements were performed at frequencies of 0.2, 0.5, 1, 2, 5, 10, 15, 20, 30, and 40 Hz, with the measurement range being ±1 mm from the piston rod mid-length, and the piston speed being 0.002 to 0.290 m / s. In the test examples described below, the maximum friction force difference (PP) was defined as the difference (peak to peak) between the maximum friction force on the extension side and the maximum friction force on the compression side of the suspension at each frequency. The amount of change in friction force was defined as the difference between the initial value and the maximum friction force (PP) after three cycles.

[0061] As detailed conditions for the frequency dependency test, as shown in Fig. 19, a smoothing vibration (amplitude: 30 mm, frequency: 0.53 Hz, vibration time: 10 minutes) was performed, followed by vibration (10 conditions) at the frequencies shown in Fig. 29. Similar tests were performed three times at 1-minute intervals.

[0062] After that, after leaving it to stand for 1 hour, vibration was applied again under the conditions shown in Figure 29, and a reproduction test was carried out by repeating the test three times at 1-minute intervals.

[0063] FIG. 20 shows Lissajous waveforms of samples 1, 6, and 7 among samples 1 to 10 shown in FIG. 28, when the test conditions are 1 mm-10 Hz among the test conditions shown in FIG.

[0064] As can be seen from the measurement results shown in FIG. 20, when comparing the Lissajous waveforms before and after one hour, the friction value shown on the vertical axis for sample 1 fluctuates greatly over the three measurements. In contrast, Samples 6 and 7 exhibited small fluctuations in friction shown on the vertical axis, and provided favorable results. The value of the level difference Rk of the core portion in Sample 6 was 0.06 μm. The value of the level difference Rk of the core portion in Sample 7 was 0.15 μm, which was larger than 0.06 μm. Therefore, it was found that the value of the protruding valley depth Rvk is important for each sample.

[0065] Therefore, the relationship between the protruding valley depth Rvk and the amount of change in frictional force ΔN was determined for samples 1 to 7 out of samples 1 to 10 shown in FIG. 28, and the results are shown in FIG. The change in frictional force ΔN was defined as the difference (PP) between the maximum frictional force at the initial test time and that after three cycles. Next, for Samples 1 to 7, which show the results shown in FIG. 21, the relationship between the level difference Rk of the core portion and the amount of change in frictional force ΔN was determined, and the results are shown in FIG.

[0066] From the results shown in FIG. 21, it was found that in order to reduce the amount of change in friction, the protruding valley depth Rvk needs to be 0.06 μm or more. From the results shown in FIG. 22, it was found that in order to reduce the amount of change in friction, the level difference Rk of the core portion needs to be 0.08 μm or more.

[0067] Next, the relationship between the protruding valley depth Rvk and the maximum frictional force difference (PP) was determined for samples produced under the same conditions as samples 1 to 7 whose results are shown in Fig. 21, and the results are shown in Fig. 23. The maximum frictional force difference (PP) is the difference (Peak to Peak) between the maximum frictional force on the extension side and the maximum frictional force on the compression side of the suspension at each frequency. Furthermore, for the same samples as above, the relationship between the level difference Rk of the core portion and the maximum frictional force difference (PP) was determined and the results are shown in FIG.

[0068] From the results shown in FIG. 23, it was found that in order to reduce the maximum frictional force difference (PP), the protruding valley depth Rvk needs to be 0.06 μm or more. From the results shown in FIG. 24, it was found that in order to reduce the maximum frictional force difference (PP), the level difference Rk of the core portion needs to be 0.08 μm or more. From the above results, it was found that an appropriate oil groove can be formed on the sliding surface of the piston rod by setting the core level difference Rk to 0.08 μm or more and the protruding valley depth Rvk to 0.06 μm or more. Therefore, it was found that the sliding surface of the piston rod outside the oil seal can quickly recover from a dry state to a wet (steady) state, improving the compatibility between the oil and the piston rod.

[0069] Next, for accelerated testing, samples created under the same conditions as samples 1 to 10 whose results are shown in Figure 21 were assembled in an oil-free state into cylinder devices (shock absorbers) having the structures shown in Figures 2 to 5. Then, using a micro-amplitude vibration device, the amount of seal wear was evaluated with an amplitude of ±1 mm, a frequency of 15 Hz, a measurement area of ​​the rod mid-length of ±1 mm, and 2,700 test cycles. The amount of seal wear was evaluated by observing the amount of oil seal adhered to the surface of the piston rod after the test using a laser microscope. Fig. 25 shows the relationship between the amount of oil seal adhesion and the protruding peak height Rpk, and Fig. 26 shows the relationship between the amount of oil seal adhesion and the level difference Rk of the core portion.

[0070] From the relationship shown in Figure 25, it was found that the value of the protruding peak height Rpk makes a big difference in the amount of oil seal adhesion when it is set at a boundary of 0.04 μm. Also, from the relationship shown in Figure 26, it was found that the value of the core level difference Rk makes a big difference in the amount of oil seal adhesion when it is set at a boundary of 0.16 μm. Therefore, it was found that by setting the peak height Rpk to 0.04 μm or less and the core level difference Rk to 0.16 μm or less, it was possible to reduce the attack on the oil seal and improve its wear resistance. Furthermore, by summarizing the results shown in Figures 25 and 26, it is clear that by setting the peak height Rpk to 0.02 μm or more and 0.04 μm or less, it is possible to reliably obtain a piston rod with excellent sliding characteristics.

[0071] The gist of one embodiment of the present invention based on the above description will be summarized below. (1) The cylinder device of this aspect comprises a cylindrical cylinder with a bottom, a sealing member provided at an opening of the cylinder, and a piston rod that protrudes from the opening of the cylinder and slides relative to the sealing member. The sliding surface of the piston rod has a chrome-plated layer, and the sliding surface has a protruding valley depth Rvk of 0.06 μm or more and less than the plating thickness, a protruding peak height Rpk of 0 μm or more and 0.04 μm or less, and a core level difference Rk of 0.08 μm or more and 0.16 μm or less, which are characteristic evaluation parameters for plateau structure surfaces described in JIS B 0671-2 and ISO 13565-2.

[0072] (2) The method for manufacturing a piston rod of the same aspect is a method for manufacturing a piston rod having a chrome plating layer formed on the sliding surface, and includes a polishing treatment step performed after the formation of the chrome plating layer. The polishing process includes an initial polishing process in which a first polishing film is used to polish the surface to obtain an initial polished surface having a protruding valley depth Rvk of 0.06 μm or more and less than the plating thickness of the chrome plating, which is a characteristic evaluation parameter for plateau structure surfaces specified in JIS B 0671-2 and ISO 13565-2; and a final polishing process in which a second polishing film having a finer mesh than the first polishing film is used to polish the initial polished surface to form a plateau surface having a protruding peak height Rpk of 0 μm or more and 0.04 μm or less, and a core level difference Rk of 0.08 μm or more and 0.16 μm or less, which are lubricity parameters for the plateau structure surface.

[0073] (3) In the method for manufacturing a piston rod described in (2) above, the following method may be employed: in the initial polishing step, the first polishing film is formed with an abrasive having a coarse grit size of #600 or less to form the initial polished surface; and in the final polishing step, the second polishing film is formed with an abrasive having a fine grit size of #800 or more to form the plateau surface. The first polishing film may be formed with an abrasive having a grit size of #120 or more and #600 or less. The second polishing film may be formed with an abrasive having a grit size of #800 or more and #3000 or less. [Industrial Applicability]

[0074] According to each of the above aspects of the present invention, the sliding surface of the piston rod has a suitable depth of the protruding valley portion, a suitable height of the protruding peak portion, and a suitable level difference between the core portion. This provides excellent wear resistance, is less aggressive to the oil seal, reduces wear of the oil seal, and minimizes changes in sliding characteristics over time. Furthermore, the piston rod has good conformability from a dry state to a steady state, and when applied to a piston rod for an automobile suspension device, it can provide a suspension device that does not degrade the ride comfort of the automobile. Therefore, the present invention has great industrial applicability. [Explanation of symbols]

[0075] 1...cylinder device (shock absorber), 2...inner cylinder, 3...outer cylinder, 11...rod guide, 15...oil seal (sealing means), 21...piston rod, 21a...large diameter portion, 25...piston, A...sliding surface

Claims

1. a cylinder in which a hydraulic fluid is sealed; an oil seal provided at an opening of the cylinder; a manufacturing method of a piston rod for a shock absorber, the piston rod being provided to protrude from the opening of the cylinder, sliding in an axial direction while contacting the oil seal, and having a sliding surface that can repeatedly move in and out between the inside and outside of the cylinder, a plating step of forming a chrome plating layer on the sliding surface of the shock absorber piston rod; a first polishing process step of forming a plurality of oil reservoir grooves in which the working fluid can be retained on the sliding surface by using a first polishing film while rotating the shock absorber piston rod about its axis after the plating process; a second polishing process step of forming the oil reservoir groove into a plateau shape using a second polishing film while rotating the shock absorber piston rod around its axis after the first polishing process step; and In the first polishing process, the first polishing film is fed out while the shock absorber piston rod is rotated, and the first polishing film is brought into contact with the shock absorber piston rod for polishing, and a pressing force for pressing the first polishing film against the shock absorber piston rod is 0.15 to 0.3 MPa.

2. A method for manufacturing a shock absorber piston rod according to claim 1, The first polishing process and the second polishing process include: a first roller that is rotatable about an axis; a second roller disposed apart from the first roller and rotatable about an axis; a third roller to which the first abrasive film or the second abrasive film is supplied; and a third roller is placed on the boundary between the first roller and the second roller, and the third roller rotates around its axis while the first abrasive film or the second abrasive film is supplied to the third roller, and the third roller rotates around its axis, and the shock absorber piston rod is pressed against the first roller and the second roller to perform a polishing process.

3. A method for manufacturing a shock absorber piston rod according to claim 2, The third roller is pressed against the shock absorber piston rod by a pressure head that presses the third roller toward the shock absorber piston rod.

4. A method for manufacturing a shock absorber piston rod according to claim 3, a pressure head supported so as to be freely movable in the axial direction of the third roller, and supported so as to be freely movable back and forth along the axial direction of the third roller while pressing the third roller toward the shock absorber piston rod;

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