Hydraulic oil for hydraulic cylinder device and hydraulic cylinder device

Abstract

Provided are: a hydraulic oil for a hydraulic cylinder device, capable of improving durability of a sliding part more than before while securing abrasion resistance equal to or higher than when a conventional hydraulic oil is used; and a hydraulic cylinder device using the hydraulic oil. This hydraulic oil for a hydraulic cylinder device according to the present invention comprises a base oil and an additive added to the base oil, the hydraulic oil having an aniline point in a range of 85°C-110°C, and not including more than 0.1 parts by mass of phosphoric acid esters as the additive based on 100 parts by mass of the base oil.

Description

Hydraulic oil for hydraulic cylinder device and hydraulic cylinder device

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

[0002] A hydraulic cylinder device is generally a device in which a piston, a piston rod fixed to the piston, and hydraulic oil are housed in a cylinder tube, and the piston and piston rod are caused to reciprocate by the inflow and outflow of hydraulic oil, and the reciprocating motion of the piston rod is damped by the flow resistance of the hydraulic oil.

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

[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 typically used as materials for piston seals and rod seals. This creates a large frictional force between the sealing material and the object it slides against. However, from the perspective of smooth reciprocating motion of the piston and piston rod and the durability of the sliding parts, it is necessary to reduce frictional force and ensure wear resistance in the sliding parts.

[0005] In response to these conflicting demands, for example, Patent Document 1 (JP 5-255683 A) 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 phosphate ester amine salts, and (c) an alkanolamine. Patent Document 1 claims to provide a hydraulic oil for shock absorbers that exhibits excellent friction characteristics even under conditions in which the shock absorber is subjected to minute vibrations and minute amplitudes, and that also exhibits good wear resistance and friction characteristics when used in shock absorbers that use sliding members made of polytetrafluoroethylene resin.

[0006] Japanese Patent Application Publication No. 5-255683

[0007] The present inventors conducted research aimed at further improving the smoothness of the reciprocating motion of the piston and piston rod in a hydraulic cylinder device and the durability of the sliding parts. During this research, they discovered that dust and dirt adhering to the piston rod can sometimes cause an unexpected decrease in durability of the sliding part between the piston rod and the rod seal. Further research was conducted to clarify the mechanism behind this decrease in durability, and they found that certain components (particularly phosphate esters) contained in conventional hydraulic oils may be strongly related to this decrease.

[0008] However, in hydraulic oils for hydraulic cylinder devices, phosphate esters have traditionally been considered important additive components that contribute to improving wear resistance, and simply excluding phosphate esters from hydraulic oils may resurface the old problem of insufficient wear resistance being obtained.

[0009] The present invention has been made to solve the above-mentioned problems, and a primary 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 hydraulic oils while ensuring wear resistance equal to or greater than that of conventional hydraulic oils. A secondary object of the present invention is to provide a hydraulic cylinder device that uses the hydraulic oil and thereby improves the durability of sliding parts compared to conventional hydraulic oils.

[0010] (I) One aspect of the present invention provides a hydraulic oil for a hydraulic cylinder device, comprising a base oil and an additive added to the base oil, the hydraulic oil having an aniline point in the range of 85°C or higher and 110°C or lower, and the additive does not contain more than 0.1 parts by mass of a phosphate ester per 100 parts by mass of the base oil.

[0011] The present invention allows the following improvements and modifications to be freely combined in the hydraulic oil (I) for hydraulic cylinder devices. (i) The additives include 0.1 to 3 parts by mass of zinc dialkyldithiophosphate and 0.1 to 3 parts by mass of boron-containing alkenyl succinimide per 100 parts by mass of the base oil. (ii) The base oil is a mineral oil and / or a synthetic oil. (iii) The kinematic viscosity of the hydraulic oil at 40°C is 8 mm 2 / s or more 17 mm 2 (iv) The additives do 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 contain 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.

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

[0013] The present invention allows the following improvements and modifications to be freely combined in the above hydraulic cylinder device (II): (vi) The piston rod has a chrome plating layer formed on its surface with an arithmetic mean roughness of 0.1 μm or less; (vii) The rod seal has a Shore A hardness of 75 or more and 90 or less at 20°C; (viii) The hydraulic cylinder device is a hydraulic shock absorber for an automobile.

[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 greater than that of conventional hydraulic oils. Furthermore, by using this 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.

[0015] 1 is a cross-sectional schematic view of an automobile hydraulic shock absorber, which is an example of a hydraulic cylinder device according to the present invention.

[0016] Hereinafter, embodiments of the present invention will be described with reference to the drawings. The drawings are exaggerated to facilitate understanding of the description. Furthermore, 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 within the scope of the technical idea of ​​the invention.

[0017] (Basic Concept of the Present Invention) As described above, the present inventors have found, in the course of conducting research aimed at further improving the smoothness of the reciprocating motion of a piston and a piston rod in a hydraulic cylinder device and the durability of the sliding parts, that dust and dirt adhering to the piston rod can cause an unexpected decrease in durability of the sliding part between the piston rod and the rod seal. Further research has been conducted to clarify the mechanism behind this decrease in durability, and it has been found that certain components (particularly phosphate esters) contained in conventional hydraulic oils may be strongly involved.

[0018] On the other hand, phosphate esters have long 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 proton release 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 reduces wear at the sliding parts.

[0020] However, according to the research and studies of the present inventors, it was found that hydraulic oils containing phosphate esters have the negative effect of swelling the synthetic rubber (e.g., acrylonitrile butadiene rubber, NBR) of the rod seal, softening its surface hardness. It was 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, resulting in external leakage of hydraulic oil from the scratched areas.

[0021] Therefore, the present inventors 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 better than conventional hydraulic oils and improving the durability of sliding parts. As a result, they found that the desired effects can be achieved by adding a zinc dialkyldithiophosphate and a boron-containing alkenyl succinimide in combination. The present invention was completed based on this finding.

[0022] [Hydraulic Cylinder Device Hydraulic Oil] The hydraulic oil for hydraulic cylinder devices according to the present invention comprises a base oil and an additive to be added to the base oil, the aniline point of the hydraulic oil being in the range of 85°C or higher and 110°C or lower, and not containing more than 0.1 parts by mass of phosphate esters as additives per 100 parts by mass of base oil. Furthermore, it is preferable that the hydraulic oil of the present invention contains a combination of a zinc dialkyldithiophosphate and a boron-containing alkenyl succinimide, instead of reducing the amount of phosphate esters added.

[0023] First, in the present invention, phosphate esters refer 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 ester compounds or phosphite ester compounds have the negative effect of swelling synthetic rubber (e.g., NBR) in rod seals and softening their surface hardness. In the present invention, to suppress this negative effect, the amount of phosphate esters added is limited 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 added intentionally.

[0025] Each component of 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 two or more mineral oils or synthetic oils.

[0027] Examples of mineral oils include paraffin-based mineral oils and naphthenic 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 naphthenic synthetic oils, and ionic liquids.

[0028] The aniline point refers to the lowest temperature at which the same volume of aniline and a measurement sample exist as a homogeneous solution, and is one index 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 below 85°C, the sealing material will swell and soften more than necessary, making it more susceptible to scratches due to sliding (reduced scratch resistance). On the other hand, if the aniline point of the hydraulic oil is above 110°C, the sealing material will shrink and harden more than necessary, making it more likely to lose its sealing ability. 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 perspective 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 the reference, a softening / hardening variation of ±10% due to contact with the hydraulic oil is considered to be within the acceptable range from the viewpoint 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 variation 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 fluctuation in the surface hardness of the sealing material due to contact with the hydraulic oil can be limited to within ±10%, thereby achieving both 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 can be a primary alkyl group, a secondary alkyl group, or a tertiary alkyl group, with primary alkyl groups and secondary alkyl groups being preferred. The alkyl group preferably has 1 to 24 carbon atoms, more preferably 4 to 12 carbon atoms.

[0033] 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 part 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] 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 part by mass, the positive effects (anti-wear properties and sludge dispersibility) cannot be sufficiently obtained, while 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, depending on the application of the hydraulic cylinder device, one or more additives 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. Among these additives, it is preferable to contain one or more additives 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 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 ashless dispersants include succinimide and its derivatives, benzylamine and its derivatives, and esters and its derivatives.

[0041] Suitable metal-based detergent-dispersants include, for example, calcium salicylate and its derivatives, calcium phenate and its derivatives, calcium sulfonate and its derivatives, and potassium borate and its derivatives.

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

[0043] Suitable pour point depressants include, for example, polymethacrylate and its derivatives, polybutene and its derivatives, and polyalkylstyrene and its derivatives.

[0044] Suitable metal deactivators include, for example, benzotriazole and its derivatives, and thiadiazole and its derivatives.

[0045] Suitable examples of the rust inhibitor include imidazole and its derivatives, benzotriazole and its derivatives, and thiadiazole and its derivatives.

[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 kinematic viscosity of the hydraulic oil varies depending on the application of the hydraulic cylinder device. For example, if the hydraulic cylinder device is a hydraulic shock absorber for an automobile, from the viewpoint of low-temperature fluidity, the preferred kinematic viscosity at 40°C is 8 mm 2 / s or more 17 mm 2 / s is preferred.

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

[0049] Figure 1 is a cross-sectional schematic diagram of an automotive hydraulic shock absorber, which is an example of a hydraulic cylinder device according to the present invention. The automotive hydraulic shock absorber 100 shown in Figure 1 is a telescopic rod, twin-cylinder hydraulic shock absorber, and its cylindrical portion 10 has a double structure consisting 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 cylindrical portion 10 (outer cylinder 20, inner cylinder 30). 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 unwanted 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 the piston valve 42 provided on the piston 40 to move in and out of the upper piston chamber A and the lower piston chamber B within the inner cylinder 30, and also passes through the base valve 32 provided at the bottom of the inner cylinder 30 to move in and out of the reservoir chamber C between the inner cylinder 30 and the outer cylinder 20.

[0053] The twin-tube hydraulic shock absorber with a telescopic rod generates a damping force by utilizing the flow resistance of the hydraulic oil 60 when it passes through the piston valve 42 and the base valve 32. In a twin-tube hydraulic shock absorber with a telescopic rod, the damping force in the compression direction is generally 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-described mechanism, the automotive hydraulic shock absorber 100 has three main sliding parts (inner cylinder 30 / piston seal 44, piston rod 50 / guide bush 72, and piston rod 50 / rod seal 74). It is important to ensure wear resistance at each sliding part, but if the rod seal 74 is damaged at the sliding part between the piston rod 50 and rod seal 74, external leakage of the hydraulic oil 60 from the damaged part becomes more likely.

[0055] There are no particular limitations on the materials for the guide bushing 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-described automotive hydraulic shock absorber 100, from the viewpoints of oil resistance, sealing performance, and cold resistance, the material for the guide bushing 72 and 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% by mass or less. NBR with an acrylonitrile content of 24% by mass or less is more preferred.

[0056] The surface of the piston rod 50 is chrome-plated for 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 chrome-plated layer has an Ra greater than 0.1 μm, friction increases at the sliding portions of the piston rod 50 / guide bush 72 and the piston rod 50 / rod seal 74, reducing the smoothness of 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 outside this range, it may be difficult to control Ra or excessive time may be 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 sealing 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, and therefore the hydraulic cylinder device according to the present invention improves durability at each sliding part.

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

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

[0060] Six types of base oils with different oil types and refinement degrees were prepared. Polymethyl methacrylate was added to each base oil as a viscosity index improver to achieve a kinematic viscosity of 12±1 mm at 40°C. 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]

[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 during the immersion test, while if the aniline point of the hydraulic oil is relatively high, the surface of the NBR plate hardens during the immersion test. This is thought to be because a low aniline point makes it easier for the hydraulic oil to penetrate into the sealing material, causing it to swell and soften, while a high aniline point makes it easier for components in the sealing material to 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 or not the sealing 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 to 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 the dust-mixed hydraulic oil was appropriately dropped onto the sliding surface. The oil temperature of the dust-mixed hydraulic oil to be dropped was two types: 10°C and 80°C.

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

[0067]

[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, indicating a reduction in the scratch resistance of the sealing material. In other words, hydraulic oil A2, which has an aniline point below 85°C and softens the surface hardness of the sealing material by more than 10% upon contact with the sealing material, was 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 higher temperature increased the deformation of the NBR plate disk specimen, increasing the contact area with the ball specimen and reducing the contact surface pressure with the ball specimen.

[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.However, because the sliding test using the ball-on-disk friction and wear tester is a test in which a load is forcibly applied, it is not possible to determine or evaluate the deterioration of the hydraulic oil sealing performance due to shrinkage and hardening of the seal material caused by 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 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 base oil. Tricresyl phosphate was used as the phosphate ester. 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 kinematic viscosity at 40°C and aniline point of hydraulic oils C3a to C3g were investigated in the same manner as in Experiment 1. Then, a sliding test similar to that in Experiment 2 was conducted to investigate the effect on the scratch resistance of the sealing material. The sliding test was performed under the condition of an oil temperature of 10°C, which was the temperature at which scratches were most likely to occur in Experiment 2. The results are also shown in Table 3.

[0073]

[0074] As shown in Table 3, adding 0.5 parts by mass or more of phosphate esters tends to slightly lower the aniline point of the hydraulic oil. Furthermore, for hydraulic oils C3a to C3d, which contained 0.5 parts by mass or more of phosphate esters, 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.Furthermore, it has been confirmed that hydraulic oils C3e to C3g, in which the amount of phosphate esters added is kept to 0.1 parts by mass or less, do not have any particular adverse effect on the scratch resistance of the sealing material.

[0076] [Experiment 4] (Investigation of the effects of phosphate esters and the 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, the effects of phosphate esters and the additives of the present invention (zinc dialkyldithiophosphate and boron-containing alkenyl succinimide) on the wear resistance of sliding parts were investigated.

[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 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 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 (checked for transparency or opacity). The sliding test was performed using transparent hydraulic oils, but the sliding test was not performed on opaque hydraulic oils because it was determined that the additives had not been sufficiently dissolved.

[0079] The sliding test to investigate wear resistance was performed using a reciprocating friction tester (UMT TriboLab, Bruker Japan Co., Ltd.) simulating the sliding between a cylinder tube and a piston seal. A polytetrafluoroethylene specimen (8 mm thick, containing 15% silica filler by mass, type A durometer hardness 90) was used as the fixed specimen, and an S45C specimen (carbon steel for mechanical construction, arithmetic mean roughness Ra approximately 0.2 μm) was used as the moving specimen. The arithmetic mean roughness Ra of the S45C specimen surface was set to approximately 0.2 μm for accelerated testing. Other test conditions were: moving specimen temperature 60°C, surface pressure 4 MPa, stroke 10 mm, frequency 5 Hz, sliding time 5 hours, and 20 μL of hydraulic oil 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 recorded as the amount of wear (unit: μm). The mass of the S45C piece was also measured with a precision electronic balance, and the amount of change was recorded as the amount of wear (unit: mg). The results are shown in Tables 4 to 6.

[0081]

[0082]

[0083]

[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 specimens were very large. In contrast, in the sliding test using hydraulic oil C4a, which contains a conventional anti-wear agent, phosphate esters, the abrasion amounts of both the PTFE and S45C specimens were significantly reduced compared to those of hydraulic oil C1, reaffirming its effectiveness as an anti-wear agent.

[0085] Hydraulic oils C4b and C4c contain the additives of the present invention, zinc dialkyldithiophosphate or boron-containing alkenyl succinimide, added alone. In sliding tests using these fluids, the abrasion loss of the PTFE and S45C specimens increased compared to the abrasion loss of the hydraulic oil C4a, demonstrating that adding these additives 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, which were co-added with zinc dialkyldithiophosphate and boron-containing alkenyl succinimide, the PTFE and S45C piece wear volumes were reduced compared to hydraulic oil C4a. This confirms that the co-addition of these additives is effective in improving wear resistance.

[0087] However, hydraulic oils C4h and C4l, to which more than 3 parts by mass of these additives were added, 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 (particularly scratch resistance) of the sealing material at the sliding parts of a hydraulic cylinder device while ensuring wear resistance equal to or greater than that of conventional hydraulic oils. 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-described embodiments and experiments have been described to aid in understanding the present invention, and the present invention is not limited to the specific configurations described. For example, it is possible to replace part of the configuration of the embodiments with configurations that are within the technical common sense of those skilled in the art, and it is also possible to add configurations that are within the technical common sense of those skilled in the art to the configuration of the embodiments. In other words, it is possible to delete, replace, or add part of the configurations of the embodiments and experiments in this specification, as long as they do not deviate from the technical concept of the invention.

[0090] 100...Automotive hydraulic shock absorber, 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, comprising: a base oil; and an additive added to the base oil, the hydraulic oil having an aniline point in the range of 85°C or higher and 110°C or lower, and not containing more than 0.1 parts by mass of phosphate esters as the additive per 100 parts by mass of the base oil.

2. The hydraulic oil for hydraulic cylinder devices according to claim 1, characterized in that the additives include from 0.1 to 3 parts by mass of zinc dialkyldithiophosphate and from 0.1 to 3 parts by mass of boron-containing alkenyl succinimide per 100 parts by mass of the base oil.

3. A hydraulic oil for a hydraulic cylinder device according to claim 1 or 2, characterized in that the base oil is a mineral oil and / or a synthetic oil.

4. The hydraulic oil for hydraulic cylinder devices according to any one of claims 1 to 3, wherein 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 hydraulic cylinder devices according to any one of claims 1 to 4, characterized in that the additive 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 hydraulic cylinder devices according to any one of claims 1 to 5, 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 having a cylinder tube, a piston capable of reciprocating inside the cylinder tube, a piston rod fixed to the piston, and a rod seal against which the piston rod slides, the rod seal being made of acrylonitrile-butadiene rubber having an acrylonitrile content of 24 mass% or less, and the hydraulic oil being a hydraulic oil for hydraulic cylinder devices as defined in any one of claims 1 to 6.

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

9. A hydraulic cylinder device according to claim 7 or 8, characterized in that the rod seal has a Shore A hardness at 20°C of 75 or more and 90 or less.

10. The hydraulic cylinder device according to any one of claims 7 to 9, characterized in that the hydraulic cylinder device is a hydraulic shock absorber for an automobile.

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