Hydraulic fluid for hydraulic cylinder device and hydraulic cylinder device

Abstract

To provide a hydraulic fluid for a hydraulic cylinder device and a hydraulic cylinder device using the hydraulic fluid, which can improve durability of sliding parts compared to conventional ones while ensuring wear resistance equal to or higher than that of conventional hydraulic fluids.SOLUTION: A hydraulic fluid for a hydraulic cylinder device according to the present invention comprises a base oil and an additive added to the base oil, wherein the hydraulic fluid has an aniline point in a range of 85°C or higher and 110°C or lower, and the hydraulic fluid does not contain phosphate esters in excess of 0.1 pt.mass as additives for 100 pts.mass of the base oil.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a hydraulic oil for a hydraulic cylinder device and a hydraulic cylinder device using the hydraulic oil. [Background technology]

[0002] Generally speaking, a hydraulic cylinder device is a device that has a piston, a piston rod fixed to the piston, and hydraulic oil housed in a cylinder tube, and causes the piston and piston rod to move back and forth by injecting and expelling hydraulic oil, and damps the reciprocating motion of the piston rod by the flow resistance of the hydraulic oil.

[0003] To prevent undesired loss of hydraulic oil pressure or leakage of hydraulic oil to the outside during the reciprocating motion of the piston and piston rod, a piston seal (also called piston packing or piston band) is provided between the cylinder tube and the piston, and a rod seal (also called rod packing or oil seal) is provided between the piston rod and the rod cover (also called rod guide). That is, the piston seal slides relative to the cylinder tube, and the piston rod reciprocates while sliding relative to the rod seal.

[0004] Piston seals and rod seals are components that prevent hydraulic oil from losing pressure and leaking to the outside, so it is important that they adhere closely to the object they slide against. Synthetic rubber and synthetic resin are usually used as materials for piston seals and rod seals. For these reasons, a large frictional force is generated between the seal material and the object it slides against. On the other hand, from the viewpoint of smooth reciprocating motion of the piston and piston rod and durability of the sliding parts, it is necessary to reduce the frictional force and ensure wear resistance in the sliding parts.

[0005] In response to such conflicting demands, for example, Patent Document 1 (JP Patent Publication 5-255683) teaches a hydraulic oil composition for shock absorbers, which is characterized by containing (a) a base oil, (b) at least one compound selected from the group consisting of phosphate esters, phosphites, and amine salts of phosphate esters, and (c) an alkanolamine. According to Patent Document 1, it is said that a hydraulic oil for shock absorbers is provided which exhibits excellent friction characteristics even under conditions in which minute vibrations and minute amplitudes are applied to the shock absorber, and which also exhibits good wear resistance and friction characteristics even when used in shock absorbers that use sliding members made of polytetrafluoroethylene resin. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 5-255683 Summary of the Invention [Problem to be solved by the invention]

[0007] The present inventors conducted research aimed at further improving the smoothness of reciprocating motion of a piston and a piston rod in a hydraulic cylinder device and the durability of the sliding parts. In the course of the research, they found that the durability of the sliding parts between the piston rod and the rod seal may unexpectedly decrease due to dust and dirt attached to the piston rod. Then, they conducted further research to clarify the mechanism of the decrease in durability and found that certain components (especially phosphate esters) contained in conventional hydraulic oils may be strongly related to this.

[0008] However, in hydraulic oils for hydraulic cylinder devices, phosphate esters have traditionally been considered to be important additive components that contribute to improving wear resistance, and if phosphate esters were simply eliminated from hydraulic oils, there is a possibility that the old problem of insufficient wear resistance being obtained would resurface.

[0009] The present invention has been made to solve the above problems, and a first object of the present invention is to provide a hydraulic oil for a hydraulic cylinder device that can improve the durability of sliding parts compared to conventional devices while ensuring wear resistance equal to or higher than that of conventional hydraulic oils. A second object of the present invention is to provide a hydraulic cylinder device in which the durability of sliding parts is improved compared to conventional devices by using the hydraulic oil. [Means for solving the problem]

[0010] (I) One aspect of the present invention is a hydraulic oil for a hydraulic cylinder device, A base oil and an additive added to the base oil, The hydraulic oil has an aniline point in the range of 85°C to 110°C, The additive does not contain more than 0.1 parts by mass of phosphate esters per 100 parts by mass of the base oil. The present invention provides a hydraulic oil for a hydraulic cylinder device, characterized in that

[0011] In the present invention, the following improvements and modifications can be freely combined in the above-mentioned hydraulic oil for hydraulic cylinder devices (I). (i) The additives include 0.1 parts by mass or more and 3 parts by mass or less of zinc dialkyldithiophosphate and 0.1 parts by mass or more and 3 parts by mass or less of boron-containing alkenyl succinimide relative to 100 parts by mass of the base oil. (ii) The base oil is a mineral oil and / or a synthetic oil. (iii) The kinetic viscosity of the hydraulic oil at 40°C is 8 mm 2 / s or more 17 mm 2 / s. (iv) The additive does not contain more than 0.01 parts by mass of phosphate esters per 100 parts by mass of the base oil. (v) The additives further include one or more selected from the group consisting of oiliness improvers, antioxidants, ashless dispersants, metal-based detergent-dispersants, viscosity index improvers, pour point depressants, metal deactivators, rust inhibitors, and antifoaming agents.

[0012] (II) Another aspect of the present invention is a hydraulic cylinder device using hydraulic oil, The hydraulic cylinder device includes a cylinder tube, a piston capable of reciprocating inside the cylinder tube, a piston rod fixed to the piston, and a rod seal in which the piston rod slides, The rod seal is made of acrylonitrile-butadiene rubber having an acrylonitrile content of 24% by mass or less, The hydraulic oil is any one of the hydraulic oils for hydraulic cylinder devices described above. The present invention provides a hydraulic cylinder device characterized by the above-mentioned.

[0013] According to the present invention, the above-mentioned hydraulic cylinder device (II) can be modified in any combination of the following improvements and modifications. (vi) The piston rod has a chrome plating layer on its surface having an arithmetic mean roughness of 0.1 μm or less. (vii) The rod seal has a Shore A hardness at 20° C. of 75 or more and 90 or less. (viii) The hydraulic cylinder device is a hydraulic shock absorber for an automobile. Effect of the Invention

[0014] According to the present invention, it is possible to provide a hydraulic oil for a hydraulic cylinder device that can improve the durability of sliding parts compared to conventional hydraulic oils while ensuring wear resistance equal to or higher than that of conventional hydraulic oils. Furthermore, by using the hydraulic oil, it is possible to provide a hydraulic cylinder device in which the durability of sliding parts is improved compared to conventional hydraulic oils. Problems, configurations, and effects other than those described above will become clear from the description of the embodiments below. [Brief description of the drawings]

[0015] [Figure 1] 1 is a schematic cross-sectional view of an automobile hydraulic shock absorber, which is an example of a hydraulic cylinder device according to the present invention. FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. The drawings are exaggerated to facilitate understanding of the description. The present invention is not limited to the specific embodiments described, and can be appropriately combined with known technologies or improved based on known technologies without departing from the technical idea of ​​the invention.

[0017] (Basic concept of the present invention) As described above, the inventors of the present invention have found that the durability of the sliding part between the piston rod and the rod seal may be unexpectedly reduced due to dust and dirt adhering to the piston rod while conducting research aimed at further improving the smoothness of the reciprocating motion of the piston and the piston rod in a hydraulic cylinder device and the durability of the sliding part. Then, after further research to clarify the mechanism of the durability reduction, they found that there is a possibility that certain components (especially phosphate esters) contained in conventional hydraulic oils are strongly related to this.

[0018] On the other hand, phosphate esters have been considered important additives that contribute to improving wear resistance. The effect of phosphate esters in improving wear resistance is said to be based on the following mechanism.

[0019] Phosphate esters added to hydraulic oils are catalytically active on the surfaces of the cylinder tube and piston rod, releasing protons. The release of protons triggers the stable formation of an adsorption film on the surfaces of the cylinder tube and piston rod. The anti-wear effect of this adsorption film can reduce wear at the sliding parts.

[0020] However, according to the research and studies of the present inventors, it was found that hydraulic oils with phosphate esters added have a negative effect of swelling the synthetic rubber (e.g., acrylonitrile butadiene rubber, NBR) of the rod seal and softening the surface hardness. It was also thought that the softening of the rod seal causes dust and dirt adhering to the piston rod to scratch the surface of the rod seal during sliding, and that this causes hydraulic oil to leak from the scratched area.

[0021] Therefore, the present inventors have conducted extensive research into hydraulic oils that can reduce the amount of phosphate esters added to the hydraulic oil, while maintaining wear resistance equal to or greater than that of conventional hydraulic oils, and can improve the durability of sliding parts. As a result, they have found that the desired effect can be obtained by adding a zinc dialkyldithiophosphate and a boron-containing alkenyl succinimide in combination. The present invention has been completed based on this finding.

[0022] [Hydraulic cylinder oil] The hydraulic oil for hydraulic cylinder devices according to the present invention contains a base oil and an additive added to the base oil, the aniline point of the hydraulic oil is in the range of 85°C to 110°C, and does not contain more than 0.1 parts by mass of phosphoric acid esters as additives per 100 parts by mass of base oil. Moreover, it is preferable that the hydraulic oil of the present invention contains a combination of zinc dialkyldithiophosphate and boron-containing alkenyl succinimide instead of reducing the amount of phosphoric acid esters added.

[0023] First, in the present invention, the term "phosphate esters" refers to orthophosphate compounds or phosphite compounds. Examples of orthophosphate compounds include tricresyl phosphate, triphenyl phosphate and its derivatives, trixylenyl phosphate, cresyl diphenyl phosphate, 2-ethylhexyl diphenyl phosphate, and tris(2-ethylhexyl) phosphate. Examples of phosphite compounds include lauryl acid phosphate, oleyl acid phosphate, dilauryl hydrogen phosphite, distearyl hydrogen phosphite, and dioleyl hydrogen phosphite.

[0024] As mentioned above, it has been found that hydraulic oils containing orthophosphate compounds or phosphite compounds have a negative effect of swelling synthetic rubber (e.g., NBR) of rod seals and softening the surface hardness. In the present invention, in order to suppress this negative effect, the amount of phosphate esters added is suppressed to 0.1 parts by mass or less per 100 parts by mass of base oil. The amount of phosphate esters added is more preferably 0.01 parts by mass or less, and even more preferably not intentionally added.

[0025] Each component constituting the hydraulic oil will be described in detail below.

[0026] The base oil used in the present invention may be a mineral oil and / or a synthetic oil that has been conventionally used as a base oil for hydraulic cylinders. The base oil may be a single mineral oil or a synthetic oil, or a combination of multiple mineral oils and synthetic oils.

[0027] Examples of mineral oils include paraffin-based mineral oils and naphthene-based mineral oils, etc. Examples of synthetic oils include polybutene-based synthetic oils, polypropylene-based synthetic oils, poly-α-olefin-based synthetic oils, ester-based synthetic oils, polyalkylene glycol-based synthetic oils, polyphenyl ether-based synthetic oils, polyglycol-based synthetic oils, polyphenyl ether-based synthetic oils, alkylbenzene-based synthetic oils, synthetic naphthene-based synthetic oils, ionic liquids, etc.

[0028] The aniline point means the lowest temperature at which the same volume of aniline and the measurement sample exist as a homogeneous solution, and is one of the indices for estimating the swelling property of the contacting sealing material. In the present invention, the aniline point of the hydraulic oil is preferably in the range of 85°C to 110°C. If the aniline point of the hydraulic oil is less than 85°C, the sealing material will swell and soften more than necessary, and will be easily scratched by sliding (scratch resistance will decrease). On the other hand, if the aniline point of the hydraulic oil is more than 110°C, the sealing material will shrink and harden more than necessary, and the sealing property of the hydraulic oil will be easily reduced. The aniline point of the hydraulic oil is more preferably in the range of 85°C to 105°C, and even more preferably in the range of 90°C to 105°C.

[0029] From the viewpoint of the seal material, when the surface hardness (e.g., durometer hardness / Shore A hardness) of the seal material before contact with the hydraulic oil is used as a standard, softening / hardening due to contact with the hydraulic oil within a range of ±10% is considered to be within the acceptable range in terms of the scratch resistance / adhesion of the seal material. It is more preferable to limit the variation in the surface hardness of the seal material due to contact with the hydraulic oil to a softening / hardening within a range of ±5%.

[0030] In other words, by controlling the aniline point of the hydraulic oil within the range of 85°C or higher and 110°C or lower, the variation in surface hardness of the sealing material due to contact with the hydraulic oil can be limited to within ±10%, resulting in a balance between the scratch resistance of the sealing material and the sealing ability of the hydraulic oil.

[0031] As described above, it is preferable to add a zinc dialkyldithiophosphate and a boron-containing alkenyl succinimide in combination to the hydraulic oil of the present invention, instead of reducing the amount of phosphate esters added.

[0032] In the present invention, zinc dialkyldithiophosphate is an additive that contributes to wear resistance and oxidation resistance. The alkyl group of zinc dialkyldithiophosphate may be a primary alkyl group, a secondary alkyl group, or a tertiary alkyl group, and a primary alkyl group or a secondary alkyl group is preferred. The number of carbon atoms in the alkyl group is preferably 1 to 24, and more preferably 4 to 12.

[0033] In order to achieve both oxidation resistance and wear resistance, the amount of zinc dialkyldithiophosphate added is preferably 0.1 to 3 parts by mass per 100 parts by mass of base oil. If the amount added is less than 0.1 parts by mass, the positive effects (oxidation resistance and wear resistance) cannot be sufficiently obtained, and if it exceeds 3 parts by mass, it becomes difficult to dissolve in the base oil, which may cause sludge formation.

[0034] In the present invention, the boron-containing alkenyl succinimide is an additive that contributes to wear resistance and sludge dispersibility, and examples thereof include boron-containing polybutenyl succinimide, boron-containing polybutenyl benzylamine, etc. They may be used alone or in combination of two or more kinds.

[0035] In order to achieve both anti-wear properties and sludge dispersibility, the amount of boron-containing alkenyl succinimide added is preferably 0.1 to 3 parts by mass per 100 parts by mass of base oil. If the amount added is less than 0.1 parts by mass, the positive effects (anti-wear properties and sludge dispersibility) cannot be sufficiently obtained, and if it exceeds 3 parts by mass, it becomes difficult to dissolve in the base oil, which may cause sludge formation.

[0036] The hydraulic oil of the present invention may further contain, as an additive, one or more selected from the group consisting of an oiliness improver, an antioxidant, an ashless dispersant, a metal-based detergent-dispersant, a viscosity index improver, a pour point depressant, a metal deactivator, a rust inhibitor, and an antifoaming agent, depending on the application of the hydraulic cylinder device. Among these additives, it is preferable to contain one or more selected from the group consisting of an antioxidant, a metal-based detergent-dispersant, a viscosity index improver, a rust inhibitor, and an antifoaming agent.

[0037] The total amount of additives added is preferably 0.2 parts by mass or more and 20 parts by mass or less, and more preferably 5 parts by mass or more and 10 parts by mass or less, per 100 parts by mass of the base oil.

[0038] As the oiliness improver, for example, aliphatic alcohols and derivatives thereof, fatty acid esters and derivatives thereof, fatty acids and derivatives thereof, amine compounds and derivatives thereof, amide compounds and derivatives thereof, etc. can be suitably used.

[0039] As the antioxidant, for example, amine-based antioxidants and derivatives thereof, phenol-based antioxidants and derivatives thereof, sulfur-based antioxidants and derivatives thereof, carbamate-based antioxidants and derivatives thereof, etc. can be suitably used.

[0040] Suitable examples of ashless dispersants that can be used include succinimide and its derivatives, benzylamine and its derivatives, and esters and its derivatives.

[0041] As the metal-based detergent-dispersant, for example, calcium salicylate and its derivatives, calcium phenate and its derivatives, calcium sulfonate and its derivatives, potassium borate and its derivatives, etc. can be suitably used.

[0042] As the viscosity index improver, for example, polymethacrylate and its derivatives, polyisobutylene and its derivatives, olefin copolymers and its derivatives, etc. can be suitably used.

[0043] As the pour point depressant, for example, polymethacrylate and its derivatives, polybutene and its derivatives, polyalkylstyrene and its derivatives, etc. can be suitably used.

[0044] As the metal deactivator, for example, benzotriazole and its derivatives, thiadiazole and its derivatives, etc. can be suitably used.

[0045] As the rust inhibitor, for example, imidazole and its derivatives, benzotriazole and its derivatives, thiadiazole and its derivatives, etc. can be suitably used.

[0046] As the defoaming agent, for example, a silicone-based defoaming agent and its derivatives, a polyacrylate-based defoaming agent and its derivatives, etc. can be suitably used.

[0047] The preferred kinetic viscosity of the hydraulic oil varies depending on the application of the hydraulic cylinder device. For example, when the hydraulic cylinder device is a hydraulic shock absorber for an automobile, the kinetic viscosity at 40° C. is preferably 8 mm 2 / s or more 17 mm 2 / s is preferred.

[0048] [Hydraulic cylinder device] Next, an embodiment of the hydraulic cylinder device according to the present invention will be described. Here, a hydraulic shock absorber for an automobile will be taken as an example. Naturally, the hydraulic cylinder device according to the present invention is not limited to the hydraulic shock absorber for an automobile.

[0049] Fig. 1 is a schematic cross-sectional view of an automobile hydraulic shock absorber, which is an example of a hydraulic cylinder device according to the present invention. The automobile hydraulic shock absorber 100 shown in Fig. 1 is a telescopic rod, twin-cylinder type hydraulic shock absorber, and the cylinder portion 10 has a double structure of an outer cylinder 20 and an inner cylinder 30 (also called a cylinder tube). A piston 40 is disposed inside the inner cylinder 30, and one end of a piston rod 50 is fixed to the piston 40 and the other end is disposed so as to protrude from the cylinder portion 10 (outer cylinder 20, inner cylinder 30), and hydraulic oil 60 is sealed in the inner cylinder 30 so as to fill it.

[0050] On the side of the cylindrical portion 10 from which the piston rod 50 protrudes, a rod guide 70 and a guide bush 72 are arranged to suppress radial movement of the piston rod 50, and a rod seal 74 is arranged to prevent undesired external leakage of the hydraulic oil 60.

[0051] The piston 40 is provided with a piston valve 42 that generates a damping force by utilizing the flow resistance of the hydraulic oil 60, and a piston seal 44 that prevents the hydraulic oil 60 from undesirably losing pressure.

[0052] As the piston rod 50 extends and retracts (resulting in the reciprocating motion of the piston 40), hydraulic oil 60 passes through a piston valve 42 provided on the piston 40 to flow in and out of the upper piston chamber A and the lower piston chamber B in the inner cylinder 30, and also passes through a base valve 32 provided at the bottom of the inner cylinder 30 to flow in and out of the reservoir chamber C between the inner cylinder 30 and the outer cylinder 20.

[0053] The telescopic rod twin-cylinder hydraulic shock absorber generates a damping force by utilizing the flow resistance when the hydraulic oil 60 passes through the piston valve 42 and the base valve 32. In general, in the telescopic rod twin-cylinder hydraulic shock absorber, the damping force in the compression direction is controlled by the base valve 32, and the damping force in the extension direction is controlled by the piston valve 42.

[0054] Due to the above-mentioned mechanism, the automobile hydraulic shock absorber 100 has three main sliding parts (the inner cylinder 30 / piston seal 44, the piston rod 50 / guide bush 72, and the piston rod 50 / rod seal 74). It is important to ensure wear resistance for each sliding part, but if the rod seal 74 is damaged at the sliding part of the piston rod 50 / rod seal 74, the hydraulic oil 60 is likely to leak to the outside from the damaged part.

[0055] There are no particular limitations on the materials of the guide bush 72, rod seal 74, and piston seal 44, and conventional synthetic rubber or resin can be used. For example, in the case of the above-mentioned automotive hydraulic shock absorber 100, from the viewpoints of oil resistance, sealing properties, and cold resistance, the material of the guide bush 72 and the rod seal 74 is preferably acrylonitrile-butadiene rubber (NBR) having a Shore A hardness of 75 to 90 at 20°C and an acrylonitrile content of 30 mass% or less. NBR having an acrylonitrile content of 24 mass% or less is more preferable.

[0056] The piston rod 50 is chrome-plated on its surface from the viewpoint of rust prevention and wear resistance. The chrome-plated layer preferably has a surface roughness (arithmetic mean roughness Ra) of 0.1 μm or less. If the Ra of the chrome-plated layer exceeds 0.1 μm, the frictional force increases at the sliding portion of the piston rod 50 / guide bush 72 and the sliding portion of the piston rod 50 / rod seal 74, which causes a decrease in the smoothness of the reciprocating motion. The average thickness of the chrome-plated layer is preferably in the range of 10 μm to 30 μm. If the average thickness is out of this range, it becomes difficult to control Ra or excessive time is required for film formation.

[0057] The hydraulic cylinder device according to the present invention uses the hydraulic oil according to the present invention as the hydraulic oil 60, and therefore can suppress softening of synthetic rubber seal materials (e.g., rod seal 74), thereby suppressing damage to the rod seal 74 caused by dust and dirt adhering to the piston rod 50, and suppressing external leakage of the hydraulic oil 60. Furthermore, the hydraulic oil according to the present invention can ensure wear resistance equal to or greater than that of conventional hydraulic oils, so the hydraulic cylinder device according to the present invention improves durability at each sliding portion. EXAMPLES

[0058] The present invention will be explained in more detail below with reference to various experiments, however, the present invention is not limited to the configurations and structures described in these experiments.

[0059] [Experiment 1] (hydraulic oil immersion test for seal materials) In this experiment, an accelerated test was conducted to investigate the change in the surface hardness of the seal material by immersing it in hydraulic oils with different aniline points. The seal material used was an NBR plate (thickness 2 mm, A437, manufactured by NOK Corporation) with a low nitrile composition (acrylonitrile content 24% by mass or less) used in hydraulic shock absorbers for automobiles.

[0060] Six types of base oil with different oil types and refinement degrees were prepared. Polymethyl methacrylate was added to each base oil as a viscosity index improver to improve the kinematic viscosity at 40°C to 12±1 mm. 2 The kinematic viscosity was measured in accordance with JIS K 2255, and the aniline point was measured in accordance with JIS K 2256.

[0061] The above NBR plates were immersed in these hydraulic oils A1 to F1, heated to 100°C, and held for 504 hours, after which the surface hardness of the NBR plates was measured. The surface hardness of the NBR plates was measured using a type A durometer in accordance with JIS K 6253. The durometer hardness (Shore A hardness) of the surface of the NBR plates before the hydraulic oil immersion test was 84. The test results of Experiment 1 are summarized in Table 1.

[0062] [Table 1]

[0063] As shown in Table 1, it can be seen that the surface hardness of the NBR plate after the immersion test changes depending on the aniline point of the hydraulic oil. Specifically, if the aniline point of the hydraulic oil is relatively low, the surface of the NBR plate softens after the immersion test, and if the aniline point of the hydraulic oil is relatively high, the surface of the NBR plate hardens after the immersion test. This is thought to be because if the aniline point is low, the hydraulic oil easily penetrates into the seal material, causing it to swell and soften, and if the aniline point is high, the components in the seal material easily dissolve into the hydraulic oil, causing it to shrink and harden.

[0064] [Experiment 2] (Sliding test using dust-mixed hydraulic oil) In this experiment, a sliding test was conducted using hydraulic oil mixed with dust to investigate whether the seal material would be scratched (scratch resistance). Seven types of JIS Z 8901 test powder 1 (Kanto loam) were used as the test dust. Hydraulic oils A1 to F1 prepared in experiment 1 were mixed and dispersed with test dust at a ratio of 1 part by mass per 100 parts by mass of base oil to prepare hydraulic oils A2 to F2.

[0065] The sliding tests were carried out using a ball-on-disk friction and wear tester (MTM traction tester, PCS Instruments, UK). SUJ2 (high carbon chromium bearing steel) with a diameter of 3 / 4 inch was used as the ball test piece, and the same type of NBR plate as in Experiment 1 was used as the disk test piece. Other test conditions were a load of 5 N, a peripheral speed of 0.5 m / s, and a sliding time of 5 hours, and dust-mixed hydraulic oil was appropriately dropped onto the sliding surface. The oil temperatures of the dust-mixed hydraulic oil to be dropped were two types: 10°C and 80°C.

[0066] After the sliding test, the surface of the NBR plate was observed under an optical microscope to determine whether there were scratches in the sliding direction and, if there were scratches, their length was measured. Those where no scratches were observed or confirmed in the sliding direction were judged as "no scratches," those where scratches less than 10 mm in length were observed or confirmed in the sliding direction were judged as "small scratches," and those where scratches 10 mm or more in length were observed or confirmed in the sliding direction were judged as "large scratches." The test results of Experiment 2 are summarized in Table 2. Table 2 also lists some of the results from Table 1.

[0067] [Table 2]

[0068] As shown in Table 2, in the sliding test using hydraulic oil A2, scratches of various sizes were observed and confirmed on the surface of the NBR plate after the test, which shows that the scratch resistance of the sealing material was reduced. In other words, hydraulic oil A2, which has an aniline point of less than 85°C and softens the surface hardness of the sealing material by more than 10% upon contact with the sealing material, is confirmed to reduce the scratch resistance of the sealing material. The reason why the scratch resistance in the sliding test at an oil temperature of 80°C was relatively better than that at an oil temperature of 10°C is thought to be because the high temperature caused a large deformation of the NBR plate, which was the disk test piece, which increased the contact area with the ball test piece and reduced the contact surface pressure with the ball test piece.

[0069] On the other hand, hydraulic oils B2 to F2, which have an aniline point of 85°C or higher and do not soften the surface of the seal material much due to contact, did not have any particular adverse effect on the scratch resistance of the seal material. Note that the sliding test using the ball-on-disk friction and wear tester is a test that forcibly applies a load, so it is not possible to judge or evaluate the deterioration of the hydraulic oil sealing performance caused by the shrinkage and hardening of the seal material due to the use of hydraulic oil F2.

[0070] [Experiment 3] (Investigation of the effects of phosphate esters and additives of the present invention on the scratch resistance of sealing materials) In this experiment, the effects of phosphate esters and the additives of the present invention (zinc dialkyldithiophosphate, boron-containing alkenyl succinimide) on the scratch resistance of sealing materials were investigated.

[0071] Using hydraulic oil C2 (dust-containing hydraulic oil) prepared in experiment 2 as a base, hydraulic oils C3a to C3g were prepared by adding 0 to 2 parts by mass of phosphate esters, 0.1 to 0.5 parts by mass of zinc dialkyldithiophosphate, and 0.1 to 0.5 parts by mass of boron-containing alkenyl succinimide to 100 parts by mass of the base oil. Tricresyl phosphate was used as the phosphate esters. Zinc di(2-ethylhexyl)dithiophosphate was used as the zinc dialkyldithiophosphate. Boron-containing polybutenyl succinimide was used as the boron-containing alkenyl succinimide. The specifications of hydraulic oils C3a to C3g are shown in Table 3 below.

[0072] First, the 40°C kinematic viscosity and aniline point of hydraulic oils C3a to C3g were investigated in the same manner as in Experiment 1. In addition, a sliding test was conducted in the same manner as in Experiment 2 to investigate the effect on the scratch resistance of the sealing material. The sliding test was conducted under conditions of an oil temperature of 10°C, which was the oil temperature at which scratches were likely to occur in Experiment 2. The results are shown in Table 3.

[0073] [Table 3]

[0074] As shown in Table 3, it can be seen that the addition of 0.5 parts by mass or more of phosphate esters tends to slightly lower the aniline point of the hydraulic oil. Also, in hydraulic oils C3a to C3d, in which 0.5 parts by mass or more of phosphate esters were added, scratches of various sizes were observed and confirmed on the surface of the NBR plate after the sliding test, indicating that the scratch resistance of the sealing material was reduced.

[0075] On the other hand, the addition of zinc dialkyldithiophosphate and boron-containing alkenyl succinimide does not appear to affect the aniline point of the hydraulic oil. Also, hydraulic oils C3e to C3g, in which the amount of phosphate esters added is kept to 0.1 parts by mass or less, are confirmed to have no significant adverse effect on the scratch resistance of the sealing material.

[0076] [Experiment 4] (Investigation of the effects of phosphate esters and additives of the present invention on wear resistance) As mentioned above, phosphate esters have been added to conventional hydraulic oils as important components that contribute to improving wear resistance. In this experiment, we investigated the effects of phosphate esters and the additives of the present invention (zinc dialkyldithiophosphate, boron-containing alkenyl succinimide) on the wear resistance of sliding parts.

[0077] Using the hydraulic oil C1 (dust-free hydraulic oil) prepared in experiment 1 as a base, hydraulic oils C4a to C4j were prepared by adding 0 to 0.5 parts by mass of phosphate esters, 0 to 5 parts by mass of zinc dialkyldithiophosphate, and 0 to 5 parts by mass of boron-containing alkenyl succinimide to 100 parts by mass of the base oil. The phosphate esters, zinc dialkyldithiophosphate, and boron-containing alkenyl succinimide were the same as those used in experiment 3. The specifications of hydraulic oils C4a to C4j are shown in Tables 4 to 6 described below. Hydraulic oil C1 alone was also used separately as a reference sample.

[0078] First, before the sliding test, the appearance of each hydraulic oil was visually checked (transparent or opaque). The sliding test was performed using transparent hydraulic oil, but the sliding test was not performed for opaque hydraulic oil because it was determined that the additives were not sufficiently dissolved.

[0079] The sliding test to investigate the wear resistance was performed using a reciprocating friction tester (UMT TriboLab, Bruker Japan Co., Ltd.) assuming sliding between a cylinder tube and a piston seal. A polytetrafluoroethylene piece (thickness 8 mm, silica filler content 15 mass%, type A durometer hardness 90) was used as the fixed test piece, and an S45C piece (carbon steel for mechanical construction, arithmetic mean roughness Ra of the surface is about 0.2 μm) was used as the movable test piece. The reason for setting the arithmetic mean roughness Ra of the S45C piece surface to about 0.2 μm is for the accelerated test. Other test conditions were: movable test piece temperature 60°C, surface pressure 4 MPa, stroke 10 mm, frequency 5 Hz, sliding time 5 hours, and 20 μL of hydraulic oil was applied to the sliding surface.

[0080] After the sliding test, the thickness of the polytetrafluoroethylene piece was measured with a micrometer, and the amount of displacement was taken as the amount of wear (unit: μm). The mass of the S45C piece was measured with a precision electronic balance, and the amount of change was taken as the amount of wear (unit: mg). The results are shown in Tables 4 to 6.

[0081] [Table 4]

[0082] [Table 5]

[0083] [Table 6]

[0084] As shown in Table 4, in the sliding test using hydraulic oil C1, which does not contain any anti-wear agent, the abrasion amounts of both the PTFE and S45C pieces were very large. In contrast, in the sliding test using hydraulic oil C4a, which contains a conventional anti-wear agent such as a phosphate ester, the abrasion amounts of both the PTFE and S45C pieces were significantly smaller than those of hydraulic oil C1, reaffirming the conventional effectiveness of the anti-wear agent.

[0085] Hydraulic oil C4b and hydraulic oil C4c are made by adding only the additive of the present invention, zinc dialkyldithiophosphate or boron-containing alkenyl succinimide. In the sliding test using these, the abrasion amount of the PTFE piece and the abrasion amount of the S45C piece increased compared to the hydraulic oil C4a, and it is understood that the addition of the additive alone is not a substitute for phosphate esters.

[0086] On the other hand, as shown in Tables 5 and 6, in hydraulic oils C4d to C4g and C4i to C4k in which zinc dialkyldithiophosphate and boron-containing alkenyl succinimide were co-added, the PTFE piece wear amount and S45C piece wear amount were reduced compared to hydraulic oil C4a. It was confirmed that the co-addition of these additives is effective in improving wear resistance.

[0087] However, hydraulic oils C4h and C4l, which contained more than 3 parts by mass of these additives, were found to be opaque during visual inspection, and it can be said that excessive addition of these additives is also not desirable.

[0088] From the above experimental results, it was confirmed that the hydraulic oil according to the present invention can improve the durability (especially scratch resistance) of the seal material while ensuring wear resistance equal to or greater than that of conventional hydraulic oils in the sliding parts of the hydraulic cylinder device. Furthermore, by using the hydraulic oil according to the present invention, it is possible to provide a hydraulic cylinder device in which the wear resistance and durability of the sliding parts are improved compared to conventional ones.

[0089] The above-mentioned embodiments and experiments are described to aid in understanding of the present invention, and the present invention is not limited to the specific configurations described. For example, it is possible to replace a part of the configuration of the embodiments with a configuration according to the technical common sense of a person skilled in the art, and it is also possible to add a configuration according to the technical common sense of a person skilled in the art to the configuration of the embodiments. In other words, it is possible to delete, replace, or add a part of the configuration of the embodiments or experiments in this specification without departing from the technical idea of ​​the invention. [Explanation of symbols]

[0090] 100...Hydraulic shock absorbers for automobiles, 10: Cylinder portion, 20: Outer cylinder, 30: Inner cylinder, 32: Base valve, 40...piston, 42...piston valve, 44...piston seal, 50: Piston rod, 60: Hydraulic oil, 70...rod guide, 72...guide bush, 74...rod seal.

Claims

1. A hydraulic oil for a hydraulic cylinder device, A base oil and an additive added to the base oil, The hydraulic oil has an aniline point in the range of 85°C to 110°C, The additive does not contain more than 0.1 parts by mass of phosphate esters per 100 parts by mass of the base oil.

2. A hydraulic oil for a hydraulic cylinder device comprising:

2. 2. The hydraulic oil for hydraulic cylinder devices according to claim 1, 1. A hydraulic oil for a hydraulic cylinder device, comprising, as the additives, 0.1 parts by mass or more and 3 parts by mass or less of zinc dialkyldithiophosphate and 0.1 parts by mass or more and 3 parts by mass or less of boron-containing alkenyl succinimide per 100 parts by mass of the base oil.

3. 3. The hydraulic oil for hydraulic cylinder devices according to claim 2, 2. A hydraulic oil for a hydraulic cylinder device, wherein the base oil is a mineral oil and / or a synthetic oil.

4. The hydraulic oil for a hydraulic cylinder device according to any one of claims 1 to 3, The kinetic viscosity of the hydraulic oil at 40°C is 8 mm 2 / s or more 17 mm 2 4. A hydraulic oil for a hydraulic cylinder device, characterized in that:

5. The hydraulic oil for a hydraulic cylinder device according to any one of claims 1 to 3, 1. A hydraulic oil for hydraulic cylinder devices, comprising: an additive that does not contain more than 0.01 parts by mass of phosphate esters per 100 parts by mass of the base oil.

6. The hydraulic oil for a hydraulic cylinder device according to any one of claims 1 to 3, 1. The hydraulic oil for a hydraulic cylinder device, further comprising, as the additive, one or more selected from the group consisting of an oiliness improver, an antioxidant, an ashless dispersant, a metal-based detergent-dispersant, a viscosity index improver, a pour point depressant, a metal deactivator, a rust inhibitor, and an antifoaming agent.

7. A hydraulic cylinder device using hydraulic oil, The hydraulic cylinder device includes a cylinder tube, a piston capable of reciprocating inside the cylinder tube, a piston rod fixed to the piston, and a rod seal in which the piston rod slides, The rod seal is made of acrylonitrile-butadiene rubber having an acrylonitrile content of 24% by mass or less, The hydraulic oil is a hydraulic oil for a hydraulic cylinder device according to any one of claims 1 to 3. A hydraulic cylinder device characterized by:

8. The hydraulic cylinder device according to claim 7, The hydraulic cylinder device is characterized in that the piston rod has a chrome plating layer formed on its surface having an arithmetic mean roughness of 0.1 μm or less.

9. The hydraulic cylinder device according to claim 8, 1. A hydraulic cylinder device, wherein the rod seal has a Shore A hardness of 75 or more and 90 or less at 20°C.

10. The hydraulic cylinder device according to claim 9, 13. A hydraulic cylinder device comprising: a hydraulic shock absorber for an automobile.

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