Hydraulic oil for hydraulic cylinder device, and hydraulic cylinder device

Abstract

Provided are a hydraulic oil for a hydraulic cylinder device and a hydraulic cylinder device using the hydraulic oil, the hydraulic oil being capable of improving the durability of a sliding portion compared to conventional hydraulic oil while ensuring wear resistance equal to or higher than that when conventional hydraulic oil is used. A hydraulic oil for a cylinder device according to the present invention is characterized by containing a base oil and an additive added to the base oil, the aniline point of the hydraulic oil being in the range of 85-110 DEG C inclusive, and containing no more than 0.1 parts by mass of a phosphoric acid ester as the additive per 100 parts by mass of the base oil.

Description

TECHNICAL FIELD

[0001] The present application relates to a hydraulic cylinder device working oil and a hydraulic cylinder device using the same. BACKGROUND

[0002] A hydraulic cylinder device is roughly a device in which a piston, a piston rod fixed to the piston, and a working oil are built in a cylinder, and the piston and the piston rod are reciprocated by the in-and-out of the working oil, or the reciprocation of the piston rod is attenuated by the flow resistance of the working oil.

[0003] In order to prevent the unwanted pressure leakage of the working oil, the external leakage of the working oil, during the reciprocation of the piston and the piston rod, a piston seal (also called a piston gasket, a piston band) is provided between the cylinder and the piston, and a rod seal (also called a rod gasket, an oil seal) is provided between the piston rod and a rod cover (also called a rod guide). That is, the piston seal slides with respect to the cylinder, and the piston rod reciprocates while sliding with respect to the rod seal.

[0004] The piston seal and the rod seal are components for preventing the pressure leakage / external leakage of the working oil, and therefore it is important that they are tightly fitted to the sliding objects. As the material of the piston seal and the rod seal, synthetic rubber and synthetic resin are generally used. Thus, a large frictional force is generated between the seal and the sliding object. On the other hand, from the viewpoint of the smoothness of the reciprocation of the piston and the piston rod and the durability of the sliding portion, reduction of the frictional force of the sliding portion and securing of the wear resistance are required.

[0005] In response to such opposite requirements, for example, Patent Literature 1 (Japanese Patent Application Laid-Open No. 5-255683) teaches a hydraulic working oil composition for a damper, characterized by containing (a) a base oil, (b) at least one compound selected from the group consisting of a phosphate ester, a phosphite ester, and a phosphate amine salt, and (c) an alkanol amine. According to Patent Literature 1, it is possible to provide a hydraulic working oil for a damper that exhibits excellent frictional properties even under conditions in which a damper is subjected to a micro-vibration and a micro-amplitude, and also exhibits good wear resistance and frictional properties even when used for a damper using a sliding member composed of a polytetrafluoroethylene resin.

[0006] PRIOR ART DOCUMENTS

[0007] PATENT LITERATURE

[0008] Patent Literature 1: Japanese Patent Application Laid-Open No. 5-255683 SUMMARY

[0009] PROBLEMS TO BE SOLVED BY THE INVENTION

[0010] The present inventors have conducted research with the aim of further improving the smoothness of reciprocating motion of a piston and a piston rod and the durability of a sliding portion in a hydraulic cylinder device. In this research, it was found that the durability of the sliding portion between the piston rod and a rod seal was sometimes unexpectedly reduced due to dust and dirt adhering to the piston rod. Furthermore, further repeated research to elucidate the mechanism of this reduction in durability revealed that a prescribed component (particularly, a phosphoric acid ester) contained in a conventional working oil was likely to be closely related thereto.

[0011] However, in a working oil for a hydraulic cylinder device, it has been conventionally considered that a phosphoric acid ester is an important additive component that contributes to improvement in wear resistance, and in the case of simply excluding the phosphoric acid ester from the working oil, it is likely that the conventional problem that insufficient wear resistance is not obtained is restored.

[0012] The present application was completed in order to solve the above-described problems, and a first object is to provide a working oil for a hydraulic cylinder device that is capable of ensuring wear resistance that is equal to or higher than that in the case of using a conventional working oil while improving the durability of a sliding portion as compared to the past. In addition, a second object is to provide a hydraulic cylinder device in which the durability of a sliding portion is improved as compared to the past by using the working oil.

[0013] Approach for solving the problem

[0014] (I) One embodiment of the present application provides a working oil for a hydraulic cylinder device, characterized by comprising a base oil and an additive added to the base oil, the aniline point of the working oil being in a range of 85°C or higher and 110°C or lower, and, with respect to 100 parts by mass of the base oil, as the additive, a phosphoric acid ester is not contained in an amount exceeding 0.1 part by mass.

[0015] The present application can freely combine and apply the following modifications and changes in the working oil (I) for a hydraulic cylinder device.

[0016] (i) With respect to 100 parts by mass of the base oil, as the additive, 0.1 part by mass or more and 3 parts by mass or less of zinc dialkyldithiophosphate and 0.1 part by mass or more and 3 parts by mass or less of a boron-containing alkenyl succinimide are contained.

[0017] (ii) The base oil is a mineral oil and / or a synthetic oil.

[0018] (iii) The kinematic viscosity at 40°C of the working oil is 8 mm 2 / s or more and 17 mm 2 / s or less.

[0019] (iv) With respect to 100 parts by mass of the base oil, as the additive, a phosphoric acid ester is not contained in an amount exceeding 0.01 part by mass.

[0020] (v) As the above-mentioned additive, one or more selected from the group consisting of oiliness improver, antioxidant, ashless dispersant, metal-based cleaning dispersant, viscosity index improver, pour point depressant, metal deactivator, antirust agent, and antifoaming agent is further contained.

[0021] (II) Another aspect of the present application provides a hydraulic cylinder device using a working oil, characterized by comprising a cylinder, a piston capable of reciprocating inside the cylinder, a piston rod fixed to the piston, and a rod seal for the piston rod to slide, wherein the rod seal is composed of acrylonitrile-butadiene rubber having an acrylonitrile content of 24 mass% or less, and the working oil is the above-mentioned working oil for any of the hydraulic cylinder devices.

[0022] The present application can freely combine and apply the following improvements and modifications in the above-mentioned hydraulic cylinder device (II).

[0023] (vi) The above-mentioned piston rod is formed with a chromium plating layer having an arithmetic average roughness of 0.1 μm or less on the surface thereof.

[0024] (vii) The above-mentioned rod seal has a Shore A hardness of 75 or more and 90 or less at 20°C.

[0025] (viii) The above-mentioned hydraulic cylinder device is a hydraulic damper for an automobile.

[0026] Effects of the Invention

[0027] According to the present application, it is possible to provide a working oil for a hydraulic cylinder device which can improve the durability of a sliding portion compared to the past while ensuring wear resistance equivalent to or higher than that in the case of using a conventional working oil. In addition, it is possible to provide a hydraulic cylinder device in which the durability of a sliding portion is improved compared to the past by using the working oil. The problems, structures, and effects other than the above will be clear from the following description of the embodiments. BRIEF DESCRIPTION OF DRAWINGS

[0028] Figure 1 is an example of the hydraulic cylinder device according to the present application, and is a cross-sectional view of a hydraulic damper for an automobile. DETAILED DESCRIPTION

[0029] Hereinafter, the embodiments according to the present application will be described with reference to the drawings. The drawings are exaggerated for the sake of understanding. In addition, the present application is not limited to the specific embodiments described below, and can be appropriately combined with known techniques or modified based on known techniques without departing from the technical idea of the present application.

[0030] (Basic Idea of the Present Application)

[0031] As mentioned above, during our 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, we discovered a situation where dust and dirt adhering to the piston rod caused an unexpected decrease in the durability of the sliding parts between the piston rod and the rod seal. Furthermore, after repeated studies to elucidate the mechanism of this durability reduction, we found that certain components (especially phosphate esters) contained in conventional working oils may be closely related to it.

[0032] On the other hand, phosphate esters have previously been considered important additives that help improve wear resistance. The wear resistance improvement effect of phosphate esters is believed to be based on the following mechanism.

[0033] Phosphate esters added to the working oil are catalyzed to release protons on the surfaces of the cylinder and piston rod. This proton release triggers the stable formation of an adsorbed film on the cylinder and piston rod surfaces. Furthermore, the wear-resistant properties of this adsorbed film reduce wear on sliding parts.

[0034] However, according to the investigation and research of the inventors, it has been clarified that working oils containing phosphate esters have a negative effect, causing the synthetic rubber (e.g., acrylonitrile-butadiene rubber, NBR) of the rod seal to swell and soften the surface hardness. Furthermore, it is believed that due to the softening of the rod seal, dust and dirt adhering to the piston rod can damage the surface of the rod seal during sliding, leading to leakage of working oil from the damaged area to the outside.

[0035] Therefore, the inventors conducted in-depth research on working oils that, while reducing the amount of phosphate esters added, can improve the durability of sliding parts compared to previous methods while maintaining at least the same level of wear resistance. Their results showed that the desired effect could be achieved by combining zinc dialkyl dithiophosphate and boron-containing alkenyl succinimide. This invention is based on this insight.

[0036] Hydraulic cylinder assembly working fluid

[0037] The hydraulic cylinder device working oil of the present invention comprises a base oil and additives added to the base oil. The aniline point of the working oil is in the range of 85°C to 110°C, and the additives do not contain more than 0.1 parts by weight of phosphate esters relative to 100 parts by weight of the base oil. Furthermore, the working oil of the present invention preferably incorporates zinc dialkyl dithiophosphate and boron-containing alkenyl succinimide instead of reducing the amount of phosphate esters added.

[0038] First, in this invention, phosphate esters refer to orthophosphate ester compounds or phosphite ester compounds. Examples of orthophosphate ester compounds include tricresyl phosphate, triphenyl phosphate and its derivatives, tri(xyl) phosphate, toluene diphenyl phosphate, 2-ethylhexyl diphenyl phosphate, and tri(2-ethylhexyl) phosphate. Examples of phosphite ester compounds include lauryl acid phosphate, oleyl acid phosphate, dilauryl hydrogen phosphite, distearyl hydrogen phosphite, and dioleyl hydrogen phosphite.

[0039] As previously stated, working oils containing orthophosphate or phosphite compounds have a negative effect of causing the synthetic rubber (e.g., NBR) of rod seals to swell and soften the surface hardness. In this invention, to suppress this negative effect, the amount of phosphate esters added is controlled to 0.1 parts by weight or less relative to 100 parts by weight of base oil. More preferably, the amount of phosphate esters added is 0.01 parts by weight or less, and even more preferably, it is added unintentionally.

[0040] The following is a detailed description of the components that make up the working oil.

[0041] The base oil used in this invention can appropriately utilize mineral oils and / or synthetic oils that have been conventionally used as base oils for hydraulic cylinders. This base oil can be a single mineral oil or a synthetic oil, or a combination of multiple mineral oils and synthetic oils.

[0042] Examples of mineral oils include paraffinic mineral oils and cycloalkane-based mineral oils. Examples of synthetic oils include polybutene-based synthetic oils, polypropylene-based synthetic oils, polyalphaolefin-based synthetic oils, ester-based synthetic oils, polyalkylene glycol-based synthetic oils, polyphenylene ether-based synthetic oils, polyethylene glycol-based synthetic oils, polyphenylene ether-based synthetic oils, alkylbenzene-based synthetic oils, synthetic cycloalkane-based synthetic oils, and ionic liquids.

[0043] The aniline point refers to the lowest temperature at which equal volumes of aniline and the test sample exist as a homogeneous solution, and it is one of the indicators for estimating the swelling property of the sealing material in contact. In this invention, the aniline point of the working oil is preferably in the range of 85°C or higher and 110°C or lower. When the aniline point of the working oil is lower than 85°C, the sealing material swells and softens excessively, thus easily becoming damaged due to sliding (reduced damage resistance). On the other hand, when the aniline point of the working oil is higher than 110°C, the sealing material shrinks and solidifies excessively, thus easily reducing the sealing performance of the working oil. More preferably, the aniline point of the working oil is in the range of 85°C or higher and 105°C or lower, and even more preferably, it is in the range of 90°C or higher and 105°C or lower.

[0044] It should be noted that, from the perspective of sealing materials, when using the surface hardness of the sealing material before contact with the working oil (e.g., hardness tester hardness / Shore A hardness) as a reference, softening / hardening within ±10% of the variation caused by contact with the working oil is considered acceptable from the viewpoint of the sealing material's damage resistance / sealing performance. More preferably, the variation in surface hardness of the sealing material caused by contact with the working oil should be suppressed to a softening / hardening range of ±5%.

[0045] In other words, by controlling the aniline point of the working oil within the range of 85°C above and 110°C below, the variation in surface hardness of the sealing material caused by contact with the working oil can be suppressed within ±10%. As a result, the damage resistance of the sealing material and the sealing performance of the working oil can be balanced.

[0046] As mentioned above, the working oil of the present invention preferably incorporates zinc dialkyl dithiophosphate and boron-containing alkenyl succinimide in combination to replace the reduction of the amount of phosphate esters added.

[0047] In this invention, zinc dialkyl dithiophosphate is an additive that contributes to wear resistance and oxidation resistance. The alkyl group in zinc dialkyl dithiophosphate can be primary, secondary, or tertiary alkyl, preferably primary or secondary alkyl. The alkyl group preferably has 1 to 24 carbon atoms, more preferably 4 to 12.

[0048] To balance antioxidant and wear-resistant properties, the amount of zinc dialkyl dithiophosphate added is preferably 0.1 parts by weight or more and 3 parts by weight or less per 100 parts by weight of base oil. If the addition amount is less than 0.1 parts by weight, the positive effects (antioxidant and wear-resistant properties) cannot be fully achieved; if it is greater than 3 parts by weight, it is difficult to dissolve in the base oil and may become a major cause of sludge formation.

[0049] Boron-containing alkenyl succinimide is an additive in this invention that helps with wear resistance and sludge dispersibility; examples include boron-containing polybutenyl succinimide and boron-containing polybutenyl benzylamine. It can be used alone or in combination.

[0050] To balance wear resistance and sludge dispersibility, the amount of boron-containing alkenyl succinimide added is preferably 0.1 parts by weight or more and 3 parts by weight or less per 100 parts by weight of base oil. If the addition amount is less than 0.1 parts by weight, the positive effects (wear resistance and sludge dispersibility) cannot be fully achieved; if it is greater than 3 parts by weight, it is difficult to dissolve in the base oil and may become a major cause of sludge formation.

[0051] The working oil of the present invention may further include, depending on the application of the hydraulic cylinder device, one or more additives selected from oiliness improvers, antioxidants, ashless dispersants, metal-based cleaning dispersants, viscosity index improvers, pour point depressants, metal passivators, rust inhibitors, and defoamers. Among these additives, it is preferable to include one or more additives selected from antioxidants, metal-based cleaning dispersants, viscosity index improvers, rust inhibitors, and defoamers.

[0052] The total amount of additives added is preferably 0.2 parts by weight or more and 20 parts by weight or less relative to 100 parts by weight of base oil, more preferably 5 parts by weight or more and 10 parts by weight or less.

[0053] As an oiliness improver, it can appropriately utilize aliphatic alcohols and their derivatives, fatty acid esters and their derivatives, fatty acids and their derivatives, amine compounds and their derivatives, amide compounds and their derivatives, etc.

[0054] As antioxidants, for example, amine antioxidants and their derivatives, phenolic antioxidants and their derivatives, sulfur antioxidants and their derivatives, carbamate antioxidants and their derivatives, etc. can be appropriately utilized.

[0055] As an ashless dispersant, succinimide and its derivatives, benzylamine and its derivatives, esters and their derivatives, etc., can be appropriately utilized.

[0056] As a metal-based cleaning dispersant, calcium salicylate and its derivatives, calcium phenolate and its derivatives, calcium sulfonate and its derivatives, potassium borate and its derivatives, etc., can be appropriately utilized.

[0057] As viscosity index improvers, polymethacrylates and their derivatives, polyisobutylene and their derivatives, olefin copolymers and their derivatives, etc., can be appropriately utilized.

[0058] As pour point depressants, polymethacrylates and their derivatives, polybutenes and their derivatives, polyalkylstyrene and their derivatives, etc., can be appropriately utilized.

[0059] As metal passivating agents, benzotriazole and its derivatives, thiadiazole and its derivatives, etc. can be appropriately utilized.

[0060] As rust inhibitors, imidazole and its derivatives, benzotriazole and its derivatives, thiadiazole and its derivatives, etc., can be appropriately utilized.

[0061] As defoamers, for example, organosilicon defoamers and their derivatives, polyacrylate defoamers and their derivatives can be appropriately utilized.

[0062] The preferred kinematic viscosity of the working oil varies depending on the application of the hydraulic cylinder device. For example, in the case of a hydraulic cylinder device used as a hydraulic damper in an automobile, from the viewpoint of low-temperature fluidity, the preferred kinematic viscosity at 40°C is 8 mm. 2 / s or higher and 17mm 2 / s or less.

[0063] [Hydraulic cylinder assembly]

[0064] Next, one embodiment of the hydraulic cylinder device according to the present invention will be described. Here, a hydraulic shock absorber for automobiles will be used as an example. Of course, the hydraulic cylinder device of the present invention is not limited to a hydraulic shock absorber for automobiles.

[0065] Figure 1 This is an example of the hydraulic cylinder device involved in the present invention, and is a cross-sectional schematic diagram of a hydraulic shock absorber for automobiles. Figure 1 The illustrated automotive hydraulic damper 100 is a rod-telescopic / multi-cylinder type hydraulic damper, with the cylinder portion 10 having a dual structure of an outer cylinder 20 and an inner cylinder 30 (also referred to as a cylinder). The piston 40 is disposed inside the inner cylinder 30, and the piston rod 50 is configured such that one end is fixed to the piston 40 and the other end protrudes from the cylinder portion 10 (outer cylinder 20, inner cylinder 30), and the working oil 60 is sealed in such a way that the inner cylinder 30 is filled.

[0066] A rod guide 70 and a guide bushing 72 are provided on the protruding side of the piston rod 50 of the cylinder 10 to suppress the radial movement of the piston rod 50, and a rod seal 74 is provided to prevent unwanted external leakage of the working oil 60.

[0067] The piston 40 is equipped with a piston valve 42 that generates a damping force by utilizing the flow resistance of the working oil 60, and a piston seal 44 for preventing unwanted pressure leakage of the working oil 60.

[0068] Through the extension and retraction of the piston rod 50 (which causes the reciprocating motion of the piston 40), the working oil 60 enters and exits the upper piston chamber A and lower piston chamber B inside the inner cylinder 30 through the piston valve 42 provided on the piston 40, and enters and exits the storage chamber C between the inner cylinder 30 and the outer cylinder 20 through the bottom valve 32 provided at the bottom of the inner cylinder 30.

[0069] The rod-telescopic / multi-cylinder type hydraulic damper utilizes the flow resistance of the working oil 60 as it passes through the piston valve 42 and the bottom valve 32 to generate damping force. It should be noted that in the rod-telescopic / multi-cylinder type hydraulic damper, the bottom valve 32 is generally used to control the damping force in the retraction direction, and the piston valve 42 is used to control the damping force in the extension direction.

[0070] According to the aforementioned mechanism, the automotive hydraulic damper 100 mainly has three sliding parts (inner cylinder 30 / piston seal 44, piston rod 50 / guide bushing 72, piston rod 50 / rod seal 74). Ensuring wear resistance is important for any sliding part, but if the rod seal 74 is damaged at the sliding part of the piston rod 50 / rod seal 74, it can easily lead to leakage of working oil 60 from the damaged portion to the outside.

[0071] The materials for the guide bushing 72, rod seal 74, and piston seal 44 are not particularly limited, and conventional synthetic rubbers and resins can be used. For example, in the case of the automotive hydraulic damper 100 described above, from the viewpoints of oil resistance, sealing, and cold resistance, the materials for the guide bushing 72 and rod seal 74 are preferably acrylonitrile-butadiene rubber (NBR) with a Shore A hardness of 75 or higher and 90 or lower at 20°C and an acrylonitrile content of 30% by mass or lower. More preferably, NBR with an acrylonitrile content of 24% by mass or lower is used.

[0072] From the viewpoint of rust prevention and wear resistance, the piston rod 50 can be chrome-plated on its surface. The surface roughness (arithmetic mean roughness Ra) of this chrome plating layer is preferably 0.1 μm or less. If the Ra of the chrome plating layer exceeds 0.1 μm, the friction at the sliding parts of the piston rod 50 / guide bushing 72 and the piston rod 50 / rod seal 74 increases, becoming a major cause of reduced smoothness of reciprocating motion. Furthermore, the average thickness of this chrome plating layer is preferably in the range of 10 μm or more and 30 μm or less. If the average thickness deviates from this range, it becomes difficult to control Ra, or the film formation time becomes excessively long.

[0073] Because the hydraulic cylinder device of the present invention uses the working oil 60 described herein, softening of the synthetic rubber sealing material (e.g., rod seal 74) can be suppressed. Therefore, damage to the rod seal 74 caused by dust and dirt adhering to the piston rod 50 can be suppressed, thereby preventing leakage of the working oil 60 to the outside. Furthermore, the working oil described herein ensures wear resistance equal to or better than conventional working oils, thus improving the durability of the sliding parts of the hydraulic cylinder device of the present invention.

[0074] Example

[0075] The present invention will be further illustrated below through various experiments. However, the present invention is not limited to the composition and structure described in these experiments.

[0076] [Experiment 1]

[0077] (Working oil immersion test of sealing materials)

[0078] In this experiment, the sealing material was immersed in working oils with different aniline points to conduct accelerated tests to investigate the changes in the surface hardness of the sealing material. As the sealing material, an NBR plate (2 mm thick, manufactured by NOK Corporation, A437) with a low acrylonitrile composition (acrylonitrile content of 24% by mass or less) used in automotive hydraulic shock absorbers was used.

[0079] Six base oils of different types and purity were prepared. For each base oil, polymethyl methacrylate (PMMA) was added as a viscosity index improver to adjust the kinematic viscosity at 40°C to 12 ± 1 mm. 2 The working oils A1 to F1 were prepared for testing, with a viscosity index of approximately 200 and a kinematic viscosity of approximately 200. The kinematic viscosity was determined according to JIS K 2255, and the aniline point was determined according to JIS K 2256.

[0080] The NBR boards were immersed in working oils A1 to F1, heated to 100°C and held for 504 hours, and then the surface hardness of the NBR boards was measured. The surface hardness of the NBR boards was measured using a type A hardness tester according to JIS K 6253. The surface hardness (Shore A hardness) of the NBR boards before the working oil immersion test was 84. The test results of Experiment 1 are summarized in Table 1.

[0081] [Table 1]

[0082] Table 1 Specifications of working oils A1 to F1 and results of working oil immersion tests on NRB plates

[0083]

[0084] PAO: Polyalphaolefin

[0085] As shown in Table 1, the surface hardness of the NBR board after the immersion test varies depending on the aniline point of the working oil. Specifically, when the aniline point of the working oil is low, the immersion test softens the surface of the NBR board; when the aniline point of the working oil is high, the immersion test cures the surface of the NBR board. This is because if the aniline point is low, the working oil easily penetrates into the sealing material, causing the sealing material to swell and soften; if the aniline point is high, the components within the sealing material easily dissolve into the working oil, causing the sealing material to shrink and cure.

[0086] [Experiment 2]

[0087] (Sliding test using dust-mixed working oil)

[0088] In this experiment, a sliding test was conducted using working oil mixed with dust to investigate whether the sealing material suffered damage (damage resistance). The seven types of test powder 1 (Kanto loam) according to JIS Z 8901 were used as the test dust. Working oils A1 to F1 were prepared by mixing and dispersing the test dust in 1 part by mass of 100 parts by mass of base oil to prepare working oils A2 to F2, relative to working oils A1 to F1 prepared in Experiment 1.

[0089] The sliding test was conducted using a ball-disc friction and wear testing machine (PCS Instruments, UK; MTM traction testing machine). SUJ2 (high carbon chromium bearing steel) with a diameter of 3 / 4 inch was used as the ball test piece, and NBR plates of the same type as in Experiment 1 were used as the disc test piece. Other test conditions were set as follows: load 5 N, circumferential speed 0.5 m / s, and sliding time 5 hours. A suitable amount of dust-mixed working oil was added dropwise to the sliding surface. The temperature of the added dust-mixed working oil was set at both 10°C and 80°C.

[0090] After the sliding test, the surface of the NBR board was observed using an optical microscope to determine the presence and length of damage in the sliding direction. Damage that could not be observed or confirmed in the sliding direction was classified as "no damage," damage less than 10 mm in length observed and confirmed in the sliding direction was classified as "minor damage," and damage longer than 10 mm observed and confirmed in the sliding direction was classified as "major damage." The results of Experiment 2 are summarized in Table 2. Table 2 also includes a portion of the results from Table 1.

[0091] [Table 2]

[0092] Table 2 Specifications and sliding test results of working oils A2 to F2

[0093]

[0094] As shown in Table 2, in the sliding test using working oil A2, damage of varying sizes was observed and confirmed on the surface of the NBR plate after the test, indicating a reduction in the damage resistance of the sealing material. In other words, it was confirmed that working oil A2, with an aniline point below 85℃ and softened by contacting the sealing material and increasing its surface hardness by more than 10%, reduced the damage resistance of the sealing material. It should be noted that the relatively better damage resistance in the sliding test at 80℃ compared to 10℃ is attributed to the increased deformation of the NBR plate (disc test piece) at higher temperatures, leading to a larger contact area with the ball test piece and a reduced contact pressure between them.

[0095] On the other hand, working oils B2 to F2, which have an aniline point above 85°C and cause minimal softening of the sealing material surface due to contact, do not have a particularly adverse effect on the damage resistance of the sealing material. It should be noted that the sliding test conducted using a ball-and-disc friction and wear tester is a test that applies a forced load; therefore, it is impossible to determine or evaluate the reduction in the sealing performance of the working oil caused by shrinkage and hardening of the sealing material due to the use of working oil F2.

[0096] [Experiment 3]

[0097] (Investigation of the effects of phosphate esters related to the damage resistance of sealing materials and the additives in this invention)

[0098] In this experiment, the effects of phosphate esters and the additives in this invention (zinc dialkyl dithiophosphate and boron-containing alkenyl succinimide) on the damage resistance of the sealing material were investigated.

[0099] Based on the working oil C2 (working oil containing dust) prepared in Experiment 2, working 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 dialkyl dithiophosphate, and 0.1 to 0.5 parts by mass of boron-containing alkenyl succinimide to 100 parts by mass of the base oil. Trimethylbenzene phosphate was used as the phosphate ester. Zinc di(2-ethylhexyl) dithiophosphate was used as the zinc dialkyl dithiophosphate. Boron-containing polybutenyl succinimide was used as the boron-containing alkenyl succinimide. The specifications of working oils C3a to C3g are shown in Table 3 below.

[0100] First, the kinematic viscosity and aniline point of the working oils C3a to C3g at 40°C were investigated in the same manner as in Experiment 1. Additionally, the same sliding test as in Experiment 2 was conducted to investigate the effect on the damage resistance of the sealing material. The sliding test conditions were the oil temperature of 10°C, which is prone to damage in Experiment 2. The results are recorded in Table 3.

[0101] [Table 3]

[0102] Table 3 Specifications and sliding test results of working oils C3a to C3g

[0103]

[0104] PEs: Phosphate esters

[0105] ZnDTP: Zinc dialkyl dithiophosphate

[0106] BASI: Boron-containing alkenyl succinimide

[0107] As shown in Table 3, it can be seen that adding more than 0.5 parts by mass of phosphate esters tends to slightly lower the aniline point of the working oil. Furthermore, after sliding tests, working oils C3a to C3d containing more than 0.5 parts by mass of phosphate esters showed visible damage of varying sizes on the surface of the NBR plates, indicating a reduction in the damage resistance of the sealing material.

[0108] On the other hand, it was found that the addition of zinc dialkyl dithiophosphate and boron-containing alkenyl succinimide had no effect on the aniline point of the working oil. Furthermore, it was confirmed that keeping the amount of phosphate esters added to below 0.1 parts by weight in working oils C3e to C3g did not have a particularly adverse effect on the damage resistance of the sealing material.

[0109] [Experiment 4]

[0110] (Investigation of the effects of phosphate esters related to wear resistance and additives in this invention)

[0111] As mentioned earlier, phosphate esters have been added to conventional working fluids as important components that help improve wear resistance. In this experiment, the effects of phosphate esters and the additives of this invention (zinc dialkyl dithiophosphate and boron-containing alkenyl succinimide) on the wear resistance of sliding parts were investigated.

[0112] Based on the working oil C1 (dust-free working oil) prepared in Experiment 1, working 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 dialkyl dithiophosphate, and 0 to 5 parts by mass of boron-containing alkenyl succinimide to 100 parts by mass of the base oil. The same substances as in Experiment 3 were used for the phosphate esters, zinc dialkyl dithiophosphate, and boron-containing alkenyl succinimide. The specifications of working oils C4a to C4j are shown in Tables 4 to 6 below. Working oil C1 monomer was also used separately as a reference sample.

[0113] First, before the sliding test, visually inspect each working oil (checking for transparency or opacity). Use transparent working oil for the sliding test; use opaque working oil, as the additives cannot be fully dissolved, and do not perform the sliding test.

[0114] The wear resistance test was conducted using a reciprocating friction testing machine (Bruker Japan Co., Ltd., UMT TriboLab) simulating the sliding between the cylinder and piston seals. A polytetrafluoroethylene sheet (8 mm thick, containing 15% by mass silica filler, with a type A hardness of 90) was used as the stationary test piece, and an S45C sheet (carbon steel for mechanical structures, with an arithmetic mean surface roughness Ra of approximately 0.2 μm) was used as the movable test piece. The arithmetic mean surface roughness Ra of the S45C sheet was set to approximately 0.2 μm to accelerate the test. Other test conditions included a movable test piece temperature of 60°C, a surface pressure of 4 MPa, a stroke of 10 mm, a frequency of 5 Hz, a sliding time of 5 hours, and the application of 20 μL of working oil to the sliding surface.

[0115] After the sliding test, the thickness of the PTFE sheet was measured using a micrometer, and the displacement was recorded as the wear amount (in μm). Additionally, the mass of the S45C sheet was measured using a precision electronic balance, and the change in mass was recorded as the wear amount (in mg). The results are recorded in Tables 4-6.

[0116] [Table 4]

[0117] Table 4 Specifications and sliding test results of working oils C1, C4a to C4c

[0118]

[0119] PTFE: Polytetrafluoroethylene

[0120] [Table 5]

[0121] Table 5 Specifications and sliding test results of working oils C4d to C4h

[0122]

[0123] [Table 6]

[0124] Table 6 Specifications and sliding test results of working oils C4i to C4l

[0125]

[0126] As shown in Table 4, in the sliding test using working oil C1 without wear-resistant agents, the wear amounts of both PTFE and S45C sheets were very high. In contrast, in previous sliding tests using working oil C4a, which contains phosphate esters as wear-resistant agents, the wear amounts of both PTFE and S45C sheets were significantly reduced compared to working oil C1, further confirming the effectiveness of the previous wear-resistant agents.

[0127] Working oils C4b and C4c are working oils in which zinc dialkyl dithiophosphate or boron-containing alkenyl succinimide, as additives of the present invention, are added individually. In sliding tests using them, the wear of PTFE flakes and S45C flakes increased compared to working oil C4a, indicating that the addition of this additive alone cannot replace phosphate esters.

[0128] On the other hand, as shown in Tables 5 and 6, it can be seen that in working oils C4d to C4g and C4i to C4k, which simultaneously contain zinc dialkyl dithiophosphate and boron-containing alkenyl succinimide, the wear of PTFE sheets and S45C sheets is reduced compared to that in working oil C4a. This confirms that the co-addition of these additives is effective in improving wear resistance.

[0129] However, working oils C4h and C4l, which contained more than 3 parts by weight of these additives, were found to become opaque during visual inspection, indicating that it is not advisable to add these additives in excess.

[0130] Based on the above experimental results, it has been confirmed that the working oil involved in this invention can ensure wear resistance of sliding parts in hydraulic cylinder devices at a level equal to or higher than that of conventional working oils, while improving the durability (especially damage resistance) of sealing materials. Furthermore, by using the working oil involved in this invention, hydraulic cylinder devices with improved wear resistance and durability of sliding parts compared to the past can be provided.

[0131] The above-described embodiments and experiments are provided to aid in understanding the present invention, and the present invention is not limited to the specific configurations described. For example, a portion of the configuration of the embodiments can be replaced with a configuration based on common technical knowledge of those skilled in the art, and conversely, a configuration based on common technical knowledge of those skilled in the art can be added to the configuration of the embodiments. That is, the present invention can delete, replace, or add other configurations to a portion of the configurations of the embodiments and experiments described in this specification without departing from the inventive concept.

[0132] Symbol Explanation

[0133] 100…Hydraulic shock absorbers for automobiles

[0134] 10…cylinder, 20…outer cylinder, 30…inner cylinder, 32…bottom valve,

[0135] 40…piston, 42…piston valve, 44…piston seal,

[0136] 50…piston rod, 60…working oil,

[0137] 70… rod guide, 72… guide bushing, 74… rod seal.

Claims

1. A working oil for a hydraulic cylinder device, characterized in that, It contains base oil and additives added to that base oil. The aniline point of the working oil is in the range of 85°C above and 110°C below. As an additive, relative to 100 parts by weight of the base oil, it does not contain more than 0.1 parts by weight of phosphate esters.

2. The working oil for the hydraulic cylinder device according to claim 1, characterized in that, As the additive, relative to 100 parts by weight of the base oil, it contains 0.1 parts by weight and 3 parts by weight of zinc dialkyl dithiophosphate and 0.1 parts by weight and 3 parts by weight of boron-containing alkenyl succinimide.

3. The working oil for the 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 working oil for the hydraulic cylinder device according to any one of claims 1 to 3, characterized in that, The working oil has a kinematic viscosity of 8 mm at 40°C. 2 / s or higher and 17mm 2 / s or less.

5. The working oil for the hydraulic cylinder device according to any one of claims 1 to 4, characterized in that, As the additive, it does not contain more than 0.01 parts by weight of phosphate esters relative to 100 parts by weight of the base oil.

6. The working oil for the hydraulic cylinder device according to any one of claims 1 to 5, characterized in that, As an additive, it also includes one or more selected from oiliness improvers, antioxidants, ashless dispersants, metal cleaning dispersants, viscosity index improvers, pour point depressants, metal passivators, rust inhibitors, and defoamers.

7. A hydraulic cylinder device that uses working oil, characterized in that, The hydraulic cylinder assembly includes a cylinder, a piston capable of reciprocating inside the cylinder, a piston rod fixed to the piston, and a rod seal for sliding the piston rod. The rod seal is made of acrylonitrile-butadiene rubber with an acrylonitrile content of less than 24% by mass. The working oil is the working oil for the hydraulic cylinder device according to any one of claims 1 to 6.

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

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

10. The hydraulic cylinder device according to any one of claims 7 to 9, characterized in that, The hydraulic cylinder device is a hydraulic damper for automobiles.

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