Lubricating oil for a rolling bearing in a high-speed rotating equipment

Abstract

To provide a lubricating oil which is used for a high-speed rotating equipment such as a high-speed dental air turbine hand piece and is excellent in biosafety (reduced harm to human body), environmental conservation (safety to the environment), heat resistance (autoclaving resistance) and bearing durability.

Description

FIELD OF INVENTION

[0001] The present invention relates to a lubricating oil for a rolling bearing used in a high-speed rotating equipment.

[0002] More specifically, the present invention relates to a novel and high-performance lubricating oil used for a rolling bearing that is a major component of a high-speed rotating equipment (an air turbine hand piece) used in, among others, the medical and food-processing fields.

[0003] Further more specifically, the present invention relates to a novel and high-performance lubricating oil that is excellent in biosafety (reduced harm to human body), environmental conservation (safety to the environment) and bearing durability and is to be applied to a rolling bearing used in a high-speed cutter (an air turbine hand piece).

BACKGROUND ART

[0004] A high-speed cutter is generally constructed of a rotary shaft for fixedly holding various cutting tools thereon, a drive unit for rotating the rotary shaft, and a bearing unit rotatably supporting the rotary shaft.

[0005] As a high-speed cutter of this type, an odontotherapeutic high-speed cutter (i.e., an air turbine hand piece) can be mentioned by way of example.

[0006] Bearing units for the above-described odontotherapeutic high-speed cutters (i.e., an air turbine hand pieces) are known to include those having a ball bearing system making use of balls (rolling elements) and those equipped with a (non-contact) air bearing system making use of an air bearing.

[0007] Paying attention, for example, to the bearing mechanisms of dental air turbine hand pieces, two types of air turbine hand pieces are known, one being of the ball bearing turbine type and the other of the air bearing turbine type.

[0008] The former type, i.e., the ball bearing turbine type can be considered to be a high-speed rotation type for approximately 200,000 to 400,000 rpm, while the latter type, that is, the air bearing turbine type can be considered to be a super high-speed rotation type for approximately 300,000 to 500,000 rpm.

[0009] It is, however, to be noted that the above-described revolution speeds of the ball bearing turbine type and the air bearing turbine type are general values. For example, though the dental air turbine hand piece already proposed by the inventors of the present application (Japanese Patent No. 3,122,302 and U.S. Pat. No. 5,562,446) is of the ball bearing turbine type, it has high performance so that super high-speed rotation can be achieved.

[0010] To facilitate the understanding of the conventional art and the present invention, a description will now be made about the construction of an equipment to which the lubricating oil according to the present invention is applied, namely, of a dental high-speed cutter (i.e., a dental air turbine hand piece).

[0011] FIG. 1 to FIG. 2 show the construction of a dental air turbine hand piece. FIG. 1 is a perspective view illustrating the overall construction, and FIG. 2 is a cross-sectional view illustrating the internal construction of the head portion and the neck portion in particular.

[0012] As is depicted in FIG. 1, the dental air turbine hand piece designated generally by letter A is composed of a head portion H, which carries a cutting tool B(5) fixedly held on a rotor shaft (drive shaft) of an air turbine, and a grip portion G.

[0013] The neck portion N of the grip portion G is connected to the head H and is internally provided with means for supplying compressed air to the air turbine arranged within the head portion H and also for discharging compressed air from the air turbine.

[0014] FIG. 2 illustrates the internal construction of the head portion H and the neck portion N of the dental air turbine hand piece A.

[0015] As is illustrated in FIGS. 1 and 2, in the head portion H, a turbine rotor shaft 3 with turbine blades 2 disposed at a peripheral edge portion thereof is arranged within a chamber 11 of a head 1, and the turbine rotor shaft 3 is rotatably supported within the head 1 by way of a bearing unit 4.

[0016] The head 1 is composed of a head main part 12 and a cap part 13. Within the head main part 12, the bearing unit 4 is arranged to rotatably support the turbine rotor shaft 3. The cutting tool 5 that performs a dental treatment is fixedly held in a bore which is formed through the turbine rotor shaft 3 along its central axis. Incidentally, the cutting tool 5 is provided on its peripheral side wall with a chuck 51 for holding the cutting tool 5 in place within the bore.

[0017] The bearing unit 4 is of a ball bearing type and is constructed of an inner ring 41, an outer ring 42, rolling elements 43 and a retainer 44. The bearing unit 4 can be provided on an outer periphery or side wall thereof with O-rings for providing the bearing unit with self-centering function and/or with known wave washers for enhancing the rigidity of the shaft.

[0018] A main part 6 of the neck portion N is provided with an air supply passage 7 and an air inlet 71 for supplying compressed air to the turbine blades 2 arranged within the chamber 11 and also with air discharge passages 8,9 and air outlets 81,91 for discharging compressed air from the chamber 11.

[0019] In the above-described internal construction of the dental air turbine hand piece A as illustrated in FIG. 2, the means for supplying and discharging compressed air is the one proposed by the inventors of the present application (Japanese Patent No. 3,122,302 and U.S. Pat. No. 5,562,446) and is of a new construction absolutely unseen in the conventional art.

[0020] Accordingly, FIG. 2 contains other reference symbols in addition to those referred to in the above to describe the individual elements (members). Although a description of these additional symbols is omitted herein, the construction of the conventional dental air turbine hand piece can be easily understood on the basis of FIG. 2.

[0021] As described above, the dental air turbine hand piece A—which is equipped with the air supply and discharge means shown in FIG. 2 and already proposed by the inventors of the present application—belongs to the category of conventional hand pieces with a rolling bearing built therein, but it makes it possible to obtain rotation of an extremely high speed and hence a large torque.

[0022] In the above-described dental air turbine hand piece of the ball bearing type, its bearing unit is in the form of a miniature bearing unit. Since the turbine rotor shaft rotates at a high speed of approximately 200,000 to 400,000 revolutions per minute, the temperature inside the bearing unit becomes high, and the bearing unit is exposed to a large stress. For a lubricating oil to be applied to a bearing which is used under the above-described severe conditions, it is, therefore, extremely important to control its quality and properties.

[0023] Further, the above-described dental air turbine hand piece of the ball bearing type is used in the oral cavity. Accordingly, the dental air turbine hand piece is used by spraying or dropping a lubricating oil to the bearing unit; in other words, the turbine is used under an environment of minimum lubrication, and is subjected to high-pressure and high-temperature treatment for sterilization and disinfection (which is also called “autoclaving”, which is applied under the following conditions: steam pressure, 2.4 kgf/cm2; temperature, 135° C.; time, five minutes).

[0024] The lubricating oil for use in the above bearing unit is, therefore, required to have properties sufficient to withstand such severe use conditions, such as oxidation resistance.

[0025] As lubricating oils for rolling bearing units in high-speed cutters such as a dental air turbine hand piece, a variety of lubricating oils have been used or proposed to date.

[0026] For example, it is widely practiced to supply a lubricating oil by spraying it with flon or LPG. As lubricating oils, those making use of refined mineral oils, such as paraffin, as base oils are known.

[0027] The above-described lubricating oils are typically of the petroleum base and are prepared by fractionating and refining petroleum into various fractions and incorporating additives, such as an oxidation inhibitor, therein as needed.

[0028] As base oil components of the above-described lubricating oils, there are known synthetic oils such as glycol esters and low molecular weight polyolefins in addition to natural mineral oils.

[0029] Furthermore, edible oils such as animal oils and vegetable oils are also known to be in use as lubrication oils for precision machines, machine tools, ship engines and the like. In general, however, these edible oils are used by being added in proportions of from 10 to 20 wt. % to lubricating oil of the mineral oil base.

[0030] Incidentally, the edible oils have a problem in oxidation resistance. It is, therefore, a common practice to use such edible oils in combination with one or more of various oxidation stabilizers (oxidation inhibitors).

[0031] In view of the above-described severe use conditions for dental air turbine hand pieces, dental air turbine hand pieces equipped with retainers impregnated with fluorinated oils, which have excellent heat resistance, permit sterilization and disinfection (autoclaving) and have superb lubricity, have been proposed recently in Japanese Patent Application Publication (Kokoku) No. HEI 5-43884 and Japanese Utility Model Application Laid-Open (Kokai) No. HEI 7-10553.

[0032] Incidentally, the retainers are in the form of porous members obtained by sintering powder of a polyimide resin. The fluorinated oils have properties such that they are inactive, are excellent in heat resistance, chemical resistance and solvent resistance and, even when exposed to high temperatures, do not form a solid deterioration material. It is, therefore, possible to consider that the above-described proposed dental air turbine hand pieces use these properties for the retainers.

[0033] Further, Japanese Patent Application Laid-Open (Kokai) No. HEI 6-165790 discloses an embodiment of impregnating with a lubricating oil a snap retainer of a ball bearing in a dental air turbine hand piece to be described briefly hereinafter, although its direct subject matter is the snap retainer itself.

[0034] More specifically, in the dental air turbine hand piece disclosed in Japanese Patent Application Laid-Open (Kokai) No. HEI 6-165790 referred to in the above, the snap retainer of the ball bearing is characterized in that:

[0035] (i) the retainer is a snap retainer, in which a ball-holding pocket is formed on one side of a synthetic resin cylinder having a fabric-made fiber layer therein, and chamfered portions are formed on opening-side end edges of the pocket; and

[0036] (ii) the fiber layer of the retainer is impregnated with a lubricating oil.

[0037] The snap retainer of the ball bearing disclosed in Japanese Patent Application Laid-Open (Kokai) No. HEI 6-165790 is to improve the balance of rotation by the feature (i) above, thereby making it possible to protect the retainer from wearing and also to avoid an increase in rotating torque, both of which would otherwise occur due to contact between the retainer and the outer ring.

[0038] However, the specific composition of the lubricating oil impregnated in the retainer is not clear in Japanese Patent Application Laid-Open (Kokai) No. HEI 6-165790. As a matter of fact, Japanese Patent Application Laid-Open (Kokai) No. HEI 6-165790 does not disclose whatsoever the specific composition of the edible oil, although it describes an embodiment of charging an edible oil into a housing in the description of the conventional art. It is the belief of the inventors of the present application that the lubricating oil or edible oil disclosed in Japanese Patent Application Laid-Open (Kokai) No. HEI 6-165790 is still no better than the level of the conventionally-proposed lubrication oils in view of the technical level in the present field of art.

[0039] Further, Japanese Patent Application Laid-Open (Kokai) No. HEI 6-212179 discloses mixing of ceramic powder in a lubricating oil to reduce the replenishing frequency of a lubricating oil to a bearing of a dental air turbine. This is a proposal based on a finding that advance mixing of ceramic powder in a lubricating oil can improve the lifetime of the lubricating oil.

[0040] The above-described various lubricating oils, which have been proposed to date, are still accompanied by various drawbacks which should be improved as described below in order to permit their application in high-speed rotating equipments such as an odontotherapeutic air turbine hand piece having a rolling bearing.

[0041] For example, the above-described lubricating oils of the mineral oil base, such as of the liquid paraffin base, or of the synthetic oil base or the lubricating oils obtained by adding edible oils to them still involve matters to be improved from the viewpoint of biosafety and environmental conservation.

[0042] In addition, the fluorinated oils proposed in the above-described Japanese Patent Application Publication (Kokoku) No. HEI 5-43884 and Japanese Utility Model Application Laid-Open (Kokai) No. HEI 7-10553, such as perfluoropolyether (PFPE) and perfluoropolyalkyl ether (PFAE), are excellent in heat resistance, chemical resistance and solvent resistance; and even when exposed to high temperatures, they form no solid deterioration material. They are, accordingly, preferred as lubricating oils for a high-speed rotating equipments such as an air turbine hand piece. However, the fluorinated oils still involve matters to be improved from the viewpoint of environmental destruction and biosafety.

[0043] Furthermore, the above-described Japanese Patent Application Laid-Open (Kokai) No. HEI 6-165790 discloses impregnation of a retainer of a rolling bearing, which is formed of a shaped member made of a phenol resin and having a fiber layer, with a lubricating oil; and it suggests use of an edible oil as a lubricating oil. As will be descried in detail subsequently herein, a great majority of general edible oils are drying oils; and when they become dry, they are formed into resinous solids. They are, accordingly, not suited as durable lubricating oils for bearings.

[0044] As described above, most of general edible oils are dry oils and are susceptible to oxidation. It is, therefore, a common practice to use them by adding a synthetic oxidation inhibitor therein. In an edible-oil-base lubricating oil composed primarily of drying oils and added with an oxidation inhibitor, it is necessary to keep in mind the potential risk that a reaction product to be formed between the oxidation inhibitor and a metal dissolved out from a bearing system may be a material harmful for the body.

[0045] According to the above-described Japanese Patent Application Laid-Open (Kokai) No. HEI 6-212179, ceramic powder is mixed in a lubricating oil to improve the lifetime of the lubricating oil so that the replenishing frequency of the lubricating oil to a lubrication system can be reduced. In an odontotherapeutic cutter of the high-speed rotation type (an air turbine hand piece), however, the race (retainer) and balls of the bearing are ground by the ceramic powder so that a metal harmful for the body may be dissolved out, or a serious damage can be given to the bearing mechanism when the lubricating oil becomes scarce under the environment of minimum lubrication.

[0046] As described above, the conventional lubricating oils for rolling bearings, said oils being for use in a high-speed rotating equipments, for example, an odontotherapeutic high-speed cutter (an air turbine hand piece), still involve drawbacks when evaluated in view of the following properties:

[0047] (i) biosafety (reduced harm to the body),

[0048] (ii) environmental conservation (safety),

[0049] (iii) heat resistance (the possibility of sterilization and disinfection by autoclaving), and

[0050] (iv) durable lubrication system.

DISCLOSURE OF INVENTION

[0051] The present invention is completed in view of the above-described problems of the conventional art.

[0052] Incidentally, the direct motive for the present invention resided in the lack of a lubricating oil of excellent properties for a high-performance and high-speed rotating dental air turbine hand piece of a ball bearing turbine type previously proposed by the inventors of the present application (Japanese Patent No. 3,122,302 and U.S. Pat. No. 5,562,446).

[0053] The present invention provides a novel lubricating oil that has excellent properties in connection with the above-described evaluation characteristics such as biosafety.

[0054] More specifically, the present invention provides a novel lubricating oil capable of meeting the requirements that in a high-speed rotating equipment such as an odontotherapeutic high-speed cutter (a ball bearing type air turbine hand piece), it can be used under large torques produced by high-speed rotation, it has an excellent heat resistance, in other words, it can permit sterilization and disinfection by autoclaving under high temperature and high pressure, and it can be used stably over a long period of time.

[0055] In other words, the present invention provides a novel lubricating oil that is for a rolling bearing used as a principal element in a high-speed rotating equipment of a ball bearing type operated at 200,000 rpm or higher or even at 300,000 rpm or higher such as an odontotherapeutic high-speed cutter and is capable of establishing a durable lubrication system without needing a reduction in revolution speed in a high-speed rotating equipment.

[0056] The inventors of the present application conducted diligent researches in an attempt to provide a lubricating oil that has the above-described high performance. As a result of the researches, the inventors previously proposed a lubricating oil composed of a specific nondrying vegetable oil for use in the ball bearing of a high-speed rotating equipment (Japanese Patent No. 3,195,545 and U.S. Pat. No. 5,981,448).

[0057] The lubricating oil of the above-described nondrying vegetable oil previously proposed by the inventors is comprised of:

[0058] (i) 60 wt. % or greater of at least one type of monovalent unsaturated fatty acid that contains one unsaturated bond per molecule, and

[0059] (ii) 30 wt. % or less of a polyvalent unsaturated fatty acid that contains at least two unsaturated bonds per molecule.

[0060] More specifically, the nondrying vegetable oil previously proposed by the inventors of the present application is constructed from at least one component selected from the group consisting of:

[0061] olive oil,

[0062] arachis oil, and

[0063] oleysol oil.

[0064] The inventor of the present application subsequently conducted further diligent researches on vegetable oil type lubricating oils that have a higher performance surpassing that of the previously proposed lubricating oil.

[0065] As a result, the inventor of the present application discovered that a nondrying vegetable oil obtained from rice bran has a high performance as a lubricating oil used in the bearing of a high-speed rotating equipment (an air turbine hand piece). Such a nondrying vegetable oil obtained from rice bran is a similar nondrying vegetable oil type lubricating oil to the previously proposed oil but differs therefrom; and when focusing on the constituent components of glycerol esters, in this oil from rice bran:

[0066] the content of monovalent unsaturated fatty acids (in concrete terms, oleic acid) that are relatively stable against oxidation is lower than that of the previously proposed oil, and

[0067] the content of divalent unsaturated fatty acids (in concrete terms, linolic acid) that are relatively unstable with respect to oxidation is higher than that of the previously proposed oil.

[0068] The invention of the present application was made based upon the above-described findings.

[0069] The present invention provides a rice bran type lubricating oil for a ball bearing of a high-speed rotating equipment which is superior in terms of biosafety and environmental protection, which is superior in terms of heat resistance and durability, and which originates in a natural vegetable that is superior in terms of economy.

[0070] In summary, the present invention relates is a lubricating oil for a rolling bearing of a high-speed rotating equipment, and the present invention is characterized in that the lubricating oil comprises a nondrying vegetable oil extracted from rice bran.

[0071] More specifically, the present invention relates to a lubricating oil for a rolling bearing of a high-speed rotating equipment which is characterized in that the oil is a nondrying vegetable oil extracted from rice bran and is a rice bran type nondrying vegetable oil that contains:

[0072] oleic acid (monovalent unsaturated fatty acid)

[0073] 40-45 wt. %

[0074] linolic acid (divalent unsaturated fatty acid)

[0075] 35-40 wt. %

[0076] palmitic acid (C1-6 saturated fatty acid)

[0077] 15-20 wt. %

[0078] other

[0079] balance (100 wt. %)

BRIEF DESCRIPTION OF DRAWINGS

[0080] FIG. 1 is a perspective view of a dental air turbine hand piece of a high-speed rotation type to which a lubricating oil according to the present invention is applied; and

[0081] FIG. 2 is a cross-sectional view of a head portion (H) and a neck portion (N) of the dental air turbine hand piece of FIG. 1, FIG. 2 showing the structures of the head portion (H) and neck portion (N).

BEST MODE FOR CARRYING OUT THE INVENTION

[0082] As described above, the characteristic feature of the present invention resides in the novel constitution of a lubricating oil applied to a rolling bearing of a high-speed rotating equipment, for example, an odontotherapeutic high-speed cutter (an air turbine hand piece), having as an essential element a rolling bearing rotatably supporting a rotary shaft thereon.

[0083] More specifically, the characteristic feature of the present invention resides in that, in a high-speed rotating equipment, for example, a high-speed cutter (an air turbine hand piece), having a rolling bearing element composed of an outer ring, an inner ring, rolling elements (ball bearings) and a retainer made of a metal or a heat-resistant synthetic resin and rotatably supporting a rotary shaft with air turbine blades fixed thereon, a lubricating oil applied to the rolling bearing element is composed of a nondrying oil made of vegetable oils excellent in biosafety and environmental conservation and superb in heat resistance (so that sterilization and disinfection by autoclaving is feasible) and durability in place of a conventional lubricating oil of the mineral oil base or of the synthetic oil base.

[0084] The technical structure and embodiments of the present invention will be described below in detail.

[0085] First, a description will be made in detail about the characteristic feature of the present invention in which the lubricating oil is composed of a vegetable oil, especially, a specific nondrying vegetable oil.

[0086] To faciliate the understanding of the present invention, vegetable oils will be described below in general.

[0087] Roughly dividing, vegetable oils can be classified into the following three types:

[0088] (i) Nondrying Oils:

[0089] The term “nondrying oil” means an oil which does not form any film-like matter (resinous solid) even when dried (oxidized) in the form of a thin layer in air.

[0090] The nondrying oil of this type contains unsaturated fatty acids, each of which contains two or more double bonds per molecule (hereinafter called “poly-unsaturated fatty acids), only in small amounts and is primarily composed of the glyceride (glycerol ester) of oleic acid (which contains one double bond per molecule), and its iodine value (a scale indicating the degree of unsaturation of an oil) is 100 or smaller.

[0091] Representative examples of nondrying oils of this type include, among others, olive oil, arachis oil and oleysol oil.

[0092] As described above, the present invention relates to a rice bran type nondrying oil. Such oil has not attracted much attention in the past, and use only as edible oils has been recognized.

[0093] As described above, the inventor of the present application has secured Japanese Patent No. 3,195,545 and U.S. Pat. No. 5,981,448 in which functions and advantages far superior to those of conventional lubricating oils are obtained when nondrying vegetable oils such as olive oil, arachis oil and oleysol oil, etc. as described above are used as lubricating oils in a bearing of a high-speed rotating equipment.

[0094] The rice bran type nondrying oil that is the principal component of the lubricating oil of the present invention, as will be described in detail later, belongs to the category of a nondrying vegetable oil, and it differs from the oil previously proposed by the inventor.

[0095] (ii) Semidrying Oils:

[0096] The “semidrying oil” is an oil which shows properties intermediate between the nondrying oil described above and the drying oil described below.

[0097] The iodine value of the semidrying oil is from 100 to 130.

[0098] Representative examples of the semidrying oils of this type include, among others, rapeseed oil, sesame oil and cotton seed oil.

[0099] (iii) Drying Oils:

[0100] The term “drying oil” means an oil which forms a film (resinous solid) when dried (oxidized) in the form of a thin layer in air.

[0101] The drying oil of this type is composed of glycerides of fatty acids having high degrees of unsaturation (for example, linoleic acid contains two double bonds and linolenic contains three double bonds). These glycerides absorb oxygen in the air and undergo oxidative polymerization, thus easily forming a film-like matter. The iodine value of such a drying oil is 130 or greater.

[0102] Representative examples of drying oils of this type include linseed oil and tung oil.

[0103] Among the above-described various types of vegetable oils, the nondrying oils are oils and fats (glycerol esters of fatty acids) each of which does not make a film-like matter (resinous solid) even when it is formed into a thin layer and dried (oxidized). Since they are excellent in heat resistance (so that sterilization and disinfection by autoclaving is feasible) and durability, they are suitable as lubricating oils for a bearing used in a high-speed rotating equipment (an air turbine hand piece).

[0104] The present invention adopts, as a lubricating oil for a bearing in a high-speed rotating equipment (an air turbine hand piece), a rice bran oil that is extracted, as a nondrying vegetable oil, from rice bran.

[0105] The rice bran type nondrying vegetable oil according to the present invention will be described below in detail in comparison with olive oil, which is a typical example of the nondrying vegetable oil the inventor of the present application has previously proposed (see Japanese Patent No. 3,195,545 and U.S. Pat. No. 5,981,448).

[0106] Olive oil will be described first, and then the rice bran oil of the present invention will be described in comparison with the olive oil.

[0107] Olive oil is an oil (glycerol esters) available from drupes of Olea Europaea. Roughly driving, its components can be classified into the following three types:

[0108] (i) unsaturated resin acids;

[0109] (ii) saturated resin acids; and

[0110] (iii) various trace components.

[0111] The unsaturated resin acids in olive oil are generally composed of mono-unsaturated and di- and higher-unsaturated (poly-unsaturated) resin acids.

[0112] The kinds and contents of the unsaturated resin acids in olive oil will be shown below:

1) Oleic acid (mono-unsaturated) 56.0-83.0%
CH3(CH2)7CH═CH(CH2)7COOH
2) Linoleic acid (poly-unsaturated) 3.5-20.0%
CH3(CH2)4CH═CHCH2CH═CH(CH2)7COOH
3) Palmitoleic acid (mono-unsaturated) 0.3-3.5%
CH3(CH2)5CH═CH(CH2)7COOH
4) Linolenic acid (poly-unsaturated) 0.0-1.5%
CH3CH2CH═CHCH2CH═CHCH2CH═
CH(CH2)7COOH
5) Gadoleic acid (mono-unsaturated) 0.0-0.05%
CH3(CH2)9CH═CH(CH2)7COOH

[0113] As described above, olive oil abundantly contains oleic acid which is a mono-unsaturated fatty acid. Olive oil also contains poly-unsaturated fatty acids, such as linoleic acid, in small amounts.

[0114] As has also been described above, a poly-unsaturated fatty acid is susceptible to oxidation. Nonetheless, olive oil has excellent oxidation resistance as a whole because, as will be described subsequently herein, olive oil contains tocopherols (vitamin E) as trace components and poly-unsaturated fatty acids such as linolenic acid are protected from oxidative deterioration owing to the anti-oxidation action of the tocopherols (vitamin E).

[0115] A description will next be made about saturated fatty acid components in olive oil.

[0116] The kinds and contents of saturated fatty acids in olive oil will be shown below:

1) Palmitic acid CH3(CH2)14COOH  7.5-20.0%
2) Stearic acid CH3(CH2)16COOH   0.5-3.5%
3) Myristic acid CH3(CH2)12COOH  0.0-0.05%
4) Arachidic acid CH3(CH2)18COOH 0.0-0.05%
5) Behenic acid CH3(CH2)20COOH   0.0-0.05%
6) Lignoceric acid CH3(CH2)22COOH 0.0-0.05%

[0117] As is recognized from the foregoing, olive oil can be considered to have low contents of saturated fatty acids which cause hypercholesterolemia.

[0118] Next, a description will be made on various trace components in olive oil.

[0119] The kinds of various trace components in olive oil will hereinafter be described along with their properties and functions.

[0120] (1) Unsaponifiable Materials:

[0121] (a) Sterols

[0122] (b) Hydrocarbons

[0123] Squalene

[0124] Aromatic hydrocarbons (which impart inherent sensory characteristics, namely, aroma and flavor)

[0125] (c) Tocopherols (oxidation-preventing function)

[0126] α-tocopherol (vitamin E) (prevention of blackening and polymerization)

[0127] β-tocopherols (prevention of rancidity which would otherwise be caused by the existence of one or more heavy metals)

[0128] Incidentally, olive oil and other nondrying oils such as arachis oil and oleysol oil contain α-tocophenal (vitamin E) as mach as 150 wt ppm (150 mg/kg).

[0129] (d) Triterpene alcohols

[0130] Cycloalterenol

[0131] Erythrodiol

[0132] (e) Fat-soluble vitamins

[0133] Vitamins A, D (anti-oxidation effects)

[0134] (2) Phospholipids, Chlorophyll and Derivatives:

[0135] (a) Phospholipids

[0136] (b) Chlorophyll (anti-oxidation effect)

[0137] (3) Phenolic Compounds:

[0138] (a) Phenolic compounds (anti-oxidation effects)

[0139] (b) Polyphenols (anti-oxidation effects)

[0140] As described above, olive oil has higher contents of various trace components, which act against oxidation of oils and fats, than other nondrying oils and drying oils; and it can, therefore, provide a lubricating oil excellent in heat resistance (so that sterilization and disinfection by autoclaving is feasible) and durability.

[0141] The inventor of the present application discovered that the lubricating characteristics improve as the total content of free fatty acids (saturated or unsaturated) contained in nondrying vegetable oils decreases.

[0142] The free fatty acids contained in nondrying vegetable oils (occasionally called “free fatty acids” in the following description) will be described.

[0143] Generally, oils and fats (fats such as beef tallow, lard and butter, etc. and fatty oils such as rapeseed oil, tung oil and linseed oil, etc) are composed of glycerol esters of higher fatty acids.

[0144] More specifically, various types of fatty acids (saturated or unsaturated) are present in nondrying vegetable oils as esters shown by the following formula (1)

3 molecules of fatty acid+1 molecule of glycerol→1 molecule of triglyceride (ester)  (1)

[0145] However, the above-described nondrying vegetable oils include various types of fatty acids (free fatty acids) that do not bond with glycerol (CH2OH—CHOH—CH2OH). If the total content of the above-described free fatty acids is indicated by free acidity, then the acidity drops as the free acidity value drops, and the viscosity also shifts toward a higher viscosity level. Accordingly, nondrying vegetable oils with a low free acidity are superior in terms of durability as lubricating oils for use in a rolling bearing.

[0146] If the quality of “Olive Oil” (trade name of olive oil manufactured by Golden Eagle Olive Products, U.S.A.) is classified based upon the free acidity, the following results are obtained:

1). Free acidity of refined oil  0.1 wt. %
2). Free acidity of extra-virgin olive oil 1.1 wt. %
3). Free acidity of virgin olive oil 3.2 wt. %

[0147] The free acidity of nondrying vegetable oils such as olive oil, etc. can be lowered by a method in which, for instance, a 5 to 10% aqueous solution of sodium hydroxide is added to olive oil and heated, the olive oil is saponified so as to produce glycerol and sodium salts of fatty acids, and the glycerol thus produced forms ester bonds with free fatty acids, and then the oil and fat components are separated and removed by centrifugal separation, etc. The resulting olive oil is low in free acidity.

[0148] Next, the rice bran type nondrying vegetable oil (called “rice bran oil”) of the present invention will be described. The oil has the properties and characteristics described below.

[0149] In the present invention, various types of rice bran oils can be used. The “Komeabura” (pure rice oil) rice bran oil manufactured by Tsuno Food Industries Co., Ltd. is one of them.

[0150] In regards to rice bran oils, it can be remembered that a major oil-related syndrome incident occurred in 1968 in Japan when a heat transfer medium (PCB) was mixed with rice bran oil in the final deodorization process.

[0151] Subsequently, however, the deodorization process was changed to a steam distillation system in which a high-pressure boiler is used, and no heat transfer medium is employed. Safety is thus now ensured.

[0152] The most common method form manufacturing rice bran oil includes the following steps: original rice oil→degumming step→dewaxing step→deacidifying step→decoloring step→deodorization step.

[0153] 1). Qualitative and Quantitative Analysis Results for Fatty Acids in Rice Bran Oil:

[0154] A qualitative analysis of rice bran oil was performed by hydrolyzing the rice bran oil, converting the oil into a methyl ester with trimethylsilyldiazomethan, and then performing an analysis using a gas chromatography/mass spectrometry (GC/MS).

[0155] A quantative analysis of rice bran oil was performed by gas chromatography (GC). The results obtained (contents, shown by wt. %) are shown below:

Oleic acid (monovalent unsaturated) 41.0
Linolic acid (divalent unsaturated) 36.6
Eicosenic acid (monovalent unsaturated) 0.7
Myristic acid (C13 saturated)    0.3
Palmitic acid (C15 saturated)    15.9
Stearic acid (C17 saturated)     1.8
Arachidonic acid (C19 saturated) 0.7
Other                            3.0
(Note: The carbon number for saturated fatty acids are values that leave in carboxyl groups)

[0156] Compared to olive oil, rice bran oil has the following special features:

[0157] The content of oleic acid which is a monovalent unsaturated acid that is relatively stable against oxidation is small; and

[0158] The content of linolic acid which is a divalent unsaturated acid that is relatively unsatable with respect to oxidation is large.

[0159] When rice bran oil is viewed only based upon these points, rice bran oil might appear to have poor lubricating characteristics compared to olive oil. However, as will be seen below, rice bran oil shows superior lubricating characteristics because of other components contained in rice bran oil

[0160] 2) Mass Analysis of Free Oleic Acid/Linolic Acid:

[0161] After being dissolved in THF (tetrahydrofuran), rice bran oil was analyzed by high-performance liquid chromatography (HPLC). The results obtained (contents following liberation, shown by wt. %)

Free oleic acid   less than 0.01
Free linolic acid less than 0.01

[0162] Thus, it is recognized that the free fatty acid content of rice bran oil is less than {fraction (1/10)} that of the highest grade of olive oil (refined oil). It is inferred that this characteristic contributes greatly to the oxidation resistance, etc.

[0163] 3). Quantative Analysis of α-Tocophenol (Vitamin E)

[0164] Following dissolution in an eluent solution (n-Hx/2POH=99/1), analysis was performed by high-performance liquid chromatography (HPLC). A quantative analysis was performed also for δ-tocophenol. The results obtained (contents, shown by wt.ppm) are shown below.

α-tocophenol 600
δ-tocophenol less than 10

[0165] It is recognized that the α-tocophenol (vitamin E) content of rice bran oil is far higher than that of olive oil (the α-tocophenol content of olive oil is approximately 150 wt.ppm). It is inferred that this characteristic contributes greatly to the oxidation resistance, etc.

[0166] Differences between rice bran oil, which belongs to nondrying vegetable oils and makes up the lubricating oil of the present invention for a rolling bearing for a high-speed rotating equipment (a dental air turbine hand piece), and other edible oils, which include nondrying vegetable oils such as olive oil, are shown below in Table 1.

[0167] In Table 1, the notes have the following meanings:

[0168] (1) composed primarily of oleic acid, and containing palmitoleic acid,

[0169] (2) linoleic acid,

[0170] (3) linolenic acid, and

[0171] (4) composed of palmitic acid, stearic acid, lauric acid and myristic acid.

[0172] In Table 1, asterisk (*) indicates vegetable oils for comparison.

TABLE 1
Comparison between Nondrying
Vegetable Oils and other Edible Oils
	Average constituent fatty acids (%)
	Unsaturated fatty
	acids	Saturated fatty
	Mono-	Di-	Tri-	acids	Vitamin E
	(1)	(2)	(3)	(4)	(mg/kg)
Rice bran oil	40˜45	35˜40	—	15˜20	600
Olive oil*	70	12	0.5-1.5	15	150
Arachis oil*	60	25	—	15	150
Oleysol oil*	80	10	—	10	—
Rapeseed oil*	60	22	10	8	150
Sunfrower oil*	20	70	10	10	250
Soybean oil*	23	55	7	15	175
Corn oil*	26	60	2	12	200
Coconut oil*	8	2	<1	90	—
(solid fat)

[0173] In the lubricating oil for use in the rolling bearing of a high-speed rotating equipment (an air turbine hand piece) provided by the present invention, the rice bran oil, which constitutes the base oil and belongs to the category of nondrying vegetable oil, can be used as an independent (sole) base oil, and it can be blended with other nondrying vegetable oils (such as olive oil, arachis oil, oleysol oil, etc.) besides rice bran oil so as to form a blended type lubricating oil.

[0174] Furthermore, oil-absorbing synthetic resin particles can be added to the rice bran oil as an additive component.

[0175] The inventor of the present invention have found that particles of an oil-absorbing synthetic resin (oil-absorbing crosslinked polymer) such as a crosslinked polymer of the acrylate ester type are extremely effective for improving the retention of the lubricating oil for a rolling bearing without impairing the properties of the rice bran oil.

[0176] A description will be made below on the oil-absorbing crosslinked polymer which is preferred for improving the properties of rice bran oil useful in the practice of the present invention.

[0177] Incidentally, the oil-absorbing crosslinked polymer itself is known in the present field of art from Japanese Patent Application Laid-Open (Kokai) No. HEI 5-337367 and Japanese Patent Application Publication (Kokoku) No. HEI 3-143996.

[0178] However, needless to say, it is found for the first time by the inventor of the present application that oil-absorbing crosslinked polymer imparts excellent lubrication characteristics for the rice bran type lubricating oil used in a rolling bearing of a high-speed rotation system.

[0179] In the present invention, when oil-absorbing crosslinked polymer is co-used, several times more of excellent lubrication characteristics can be obtained compared to cases in which such oil-absorbing crosslinked polymer is not co-used.

[0180] The solubility parameter (SP value) of rice bran oil used as a principal component of the lubricating oil of the present invention is between 6 and 9. It is, therefore, preferable that the oil-absorbing crosslinked polymer used in the present invention have a similar SP value from the viewpoint of compatibility. In other words, it is preferable that an oil-absorbing crosslinked polymer of 9 or smaller in SP value be added to the lubricating oil of the present invention which contains rice bran oil as its principal component.

[0181] The above-described oil-absorbing crosslinked polymer can be produced generally by copolymerization of:

[0182] (i) 90 to 99.9 wt. % of a monomer (A) containing one polymerizable unsaturated group per molecule and capable of producing a polymer having an SP value of 9 or smaller, and

[0183] (ii) 0.1 to 10 wt. % of a crosslinking monomer (B) containing at least two polymerizable unsaturated group per molecule.

[0184] The monomer (A) above comprises at least one polymerizable unsaturated monomer containing:

[0185] (1) at least one C2-C30 aliphatic hydrocarbon group; and selected from the group consisting of:

[0186] (2) alkyl(meth)acrylate, alkylaryl(meth)acrylate, alkyl(meth)acrylamide, alkylaryl(meth)acrylamide, alkylaryl(meth)acrylamdie, fatty acid vinyl esters, alkylstyrenes, and α-olefins.

[0187] The crosslinking monomer (B) above comprises, for instance:

[0188] ethylene glycol di(meth)acrylate, diethylene glycol di(meth)arylagte, polyethylene glycol di(meth)acrylate, propyleneglycol di(meth)acrylate, polypropylene glycol di(meth)acrylate, 1,3-butylene glycol di(meth)acrylate, neopentyl glycol di(meth)acrylate, N,N′-methylene bisacrylamide, N,N′-propylenebisacrylamide, glycerol tri(meth)acrylate, trimethylolpropane tri(meth)-acrylate, and divinylbenzene.

[0189] The above-described oil-absorbing crosslinking polymer can also be produced by using a monomer containing two polymerizable unsaturated groups per molecule, specifically a diene monomer. Illustrative examples of oil-absorbing crosslinked polymers making use of diene monomers of this type can include polymers of butadiene, isoprene, cyclopentadiene and 1,3-pentadiene, and their hydrogenation products; and copolymers of the above-described dienes with other polymerizable monomers such as styrene and α-olefins such as butylene, and their hydrogenation products.

[0190] Incidentally, the above-described crosslinking monomers can be used as the polymerizable monomers.

[0191] Further, the oil-absorbing crosslinked polymer may be composed of a crosslinked copolymer of ethylene with another olefin. Examples of the another olefin copolymerize with ethylene can include propylene, butylene and pentene. Further, as crosslinking monomers, those mentioned above can be used.

[0192] The above-described oil-absorbing crosslinked polymer is in the form of particles with an average particle size from 0.5 to 2,000 μm, and it can be added in a desired amount to the rice bran oil. As an oil-absorbing crosslinked polymer of this type, it is possible to use “Oleosorb PW-190” (trade name) or “Oleosorb PW-170” (trade name), both acrylate ester polymers produced by Nippon Shokubai Co., Ltd.

[0193] Next, a description will be made about a bearing, particularly a retainer (a retaining element used in a bearing), to which the above-described lubricating oil according to the present invention that is composed primarily of the rice bran oil, is applied.

[0194] In the present invention, the retainer can be those made of phenol resin or polyimide resin. A polyimide resin retainer will be described below.

[0195] The polyimide resin (hereinafter abbreviated as “PI resin”) used in the present invention is a resin obtained by subjecting an aromatic carboxylic acid and an aromatic amine to condensation polymerization and containing imide bonds in the backbone thereof (which may be either thermoplastic or thermosetting), and it is excellent in heat resistance, chemical resistance, mechanical properties and electrical characteristics.

[0196] In the present invention, the term “PI resin” should be interpreted to also include polyamideimide resins (hereinafter abbreviated as “PAI resins”) containing imide bonds and amide bonds in their backbones.

[0197] In the present invention, a commercially-available PI resin or PAI resin can be conveniently used as the PI resin or PAI resin for molding the retainer. As commercially-available PI resins and PAI resins, the following resins can be listed including their chemical structural formulas.

[0198] (i) PI Resins:

[0199] (1) “P84-HT” (trade name; product of Lenzing AG, Austria; represented by the below-described chemical formula (1) in which R represents an alkylene group);

[0200] (2) “TI-3000” (trade name; product of Toray Industries, Inc.; represented by the below-described chemical formula (2));

[0201] (3) “UIP-S” (trade name; product of Ube Industries, Ltd.; represented by the below-described chemical formula (3));

[0202] (4) “Vespel” (trade name; product of E.I. du Pont de Nemours & Co., Ltd.; represented by the below-described chemical formula (2));

[0203] (5) “Aurum” (trade name; product of Mitsui-Toatsu Chemicals Inc.; represented by the below-described chemical formula (4)); and

[0204] (6) Others, including “Meldin 8100” and “Meldin 900”, products of Furon, U.S.A.

[0205] (ii) PAI Resins:

[0206] (1) “Torlon 4000 TF” (trade name; product of Amoco Chemical Corp.; represented by the below-described chemical formula (5) in which Ph represents a phenylene group) 1

EXAMPLES

[0207] The present invention will hereinafter be described in further detail by the following examples.

[0208] The lubricating oil according to the present invention, which is composed primarily of rice bran oil and is applicable to a rolling bearing of a dental air hand piece, was compared with various known lubricating oils so as to substantiate the superiority of the lubricating oils of the present invention.

[0209] The rolling bearing of a dental air hand piece (see FIG. 1 and FIG. 2) furnished for use in the test of the example is a miniature rolling bearing of the open type which is provided with a snap retainer and has the dimensions:

[0210] (i) Inner diameter of an outer ring: 6.350 mm

[0211] (ii) Inner diameter of an inner ring: 3.175 mm

[0212] (iii) Width: 2.380 mm

[0213] The above bearing unit was installed in a dental air turbine hand piece, and the test was conducted under the conditions of air supply pressure, 2.5 kgf/cm2; air supply rate, 26 l/min; and revolution speed, about 400,000 rpm.

[0214] Namely, using the dental air turbine hand piece (see FIG. 1 and FIG. 2), the lubricating oils according to the present invention primarily composed of rice bran oil and the various known lubricating oils were tested under the above-described conditions. The test results are shown in Table 2.

[0215] In Table 2, the notes have the following meanings:

[0216] (1) <Heat Resistance Test (Autoclaving Resistance; Cycles)>

[0217] An autoclaving apparatus (“ALPHI”, trade name; manufactured by J. MORITA MFG. CORP.) was used. Autoclaving resistance is expressed in terms of cycles until the rotation of the dental air turbine hand piece became unstable and the efficiency of rotation dropped to 10% (about 40,000 rpm).

[0218] Conditions for the treatment in the autoclaving apparatus were: steam pressure, 2.4 kgf/cm2; temperature, 135° C.; time, 5 minutes.

[0219] (2) <Bearing Resistance (Continuous Operation; Hrs)>

[0220] Each lubricating oil was applied to a lubrication system of the dental air hand piece at the beginning, and a continuous operation was then performed at about 400,000 rpm without additional feeding of the lubricating oil. The bearing resistance is expressed in term of hours until the rotation become unstable and the revolution speed dropped to 10% (about 40,000 rpm).

[0221] The specific constructions of various retainers shown in Table 2 are as follows:

[0222] (i) Non-Porous PI/PAI•R:

[0223] This means a non-porous retainer of the polyimide (PI) base or the polyamideimide (PAI) base.

[0224] The non-porous PI•R was obtained by forming “Vespel SP-1” (trade name, product of E.I. du Pont de Nemours & Co., Ltd.) into a retainer-like shape.

[0225] On the other hand, the non-porous PAI•R was obtained by forming “Torlon 4203” (trade name, product of Teijin-Amoco Engineering Plastics, Ltd.) into a retainer-like shape.

[0226] (ii) Porous P•R:

[0227] This means a retainer of the phenol resin base.

[0228] Incidentally, the porous P•R was obtained by impregnating inter-thread spaces of a woven fabric, which had been rolled plural times into a pipe-like shape, with a phenol resin in a vacuum state, shaping the thus-impregnated fabric under heat and then machining the shaped fabric into a retainer-like shape.

[0229] (iii) Porous PI/PAI•R:

[0230] This means a porous retainer composed of a sintered body of polyimide (PI) base or polyamideimide (PAI) base powder.

[0231] The porous PI•R was obtained by compression-forming “UIP-S” (trade name, product of Ube Industries, Ltd.) under a forming pressure of 4000 kgf/cm2, sintering the green compact at 400° C. in a nitrogen gas atmosphere and then machining the sintered compact into a retainer-like shape (porosity: about 13% by volume).

[0232] The porous PAI-R was obtained by classifying and sifting “Torlon 4000TF” (trade name, product of Amoco Chemical Corp., U.S.A.) into an average particle size of 20 μm, compression-forming the thus-shifted powder under a preforming pressure of 2,800 kgf/cm2, sintering the green compact at 300° C., and then machining the sintered compact into a retainer-like shape (porosity: about 14% by volume).

[0233] In Table 2,

[0234] (i) As the paraffin oil (liquid paraffin), a conventional spray-type paraffin oil produced by an odontotherapy-related maker was used.

[0235] (ii) As the fluorinated oil, “FOMBLIN” (trade name, product of Ausimont S.P.A., Italy) was used.

TABLE 2
Evaluation of Characteristics of Various Lubricating Oils
	Heat resistance (1)	Bearing resistance (2)
	(autoclaving resistance, cycles)	(continuous rotation, hrs)
			(i)	(ii)	(iii)	(i)	(ii)	(iii)
	Bio-	Environmental	Non-porous	Porous	Porous	Non-porous	Porous	Porous
Lubricating Oil	safery	Conservation	PI/PAI * R	P * R	PI/PAI * R	PI/PAI * R	P * R	PI/PAI * R
Vegetable	Nondrying	Rice bran oil	A	B	1300<	1300<	1500<	3.00	28.00	35.00
oil	oil	Refined olive oil	A	B	1000<	1000<	1000<	1.50	25.00	29.75
		Extra virgin oil	A	B	1000<	1000<	1000<	2.00	23.50	25.50
		Virgin olive oil	A	B	1000<	1000<	1000<	1.75	16.50	18.25
		Arachis oil	A	B	1000<	1000<	1000<	1.50	14.75	18.50
	Semidrying	Rapeseed oil	A	B	300	300	300	0.25	2.25	4.50
	oil
Mineral	Paraffin oil		C	C	1000<	200	1000<	0.50	6.75	8.50
oil	Parafin 100 parts by weight +	C	C	1000<	150	1000<	1.00	4.75	17.00
	acrylic resin 5 parts by weight
Synthetic	Fluorinated oil	C	C	1000<	1000<	1000<	1.50	14.50	17.50
oil
A: Excellent,
B: Good,
C: poor

[0236] As seen from Table 2, it is recognized that the lubricating oil according to the present invention which is composed primarily of rice bran oil and used in dental cutters of high-speed rotation (an air turbine hand piece) is excellent in the various characteristics of biosafety, environmental conservation, heat resistance (autoclaving resistance) and bearing durability compared to the conventional lubricating oils including nondrying vegetable oil such as olive oil, semidrying vegetable oil such as rapeseed oil, mineral oil and synthetic oil. Furthermore, the rice bran oil when applied in mist form has an excellent characteristic to blood circulation in periodontal tissues and cleaning of cavity in tooth (or prevention of formation of sumea layer), and it is also excellent in economical efficiency compared to other nondrying vegetable oils.

[0237] Furthermore, the lubricating oil of the present invention is excellent in economy because it costs lower than fluorinated oils.

[0238] Semidrying oils and drying oils, other than nondrying vegetable oils, and paraffin oil (liquid paraffin) show considerable color changes when tested or left over at 135° C. for 175 hours in air. In other words, these oils of different varieties are inferior in oxidation resistance as well. In the tests, nondrying oils such as rice bran oil, olive oil, arachis oil, etc. did not show color changes.

[0239] The lubricating oil according to the present invention which is composed primarily of rice bran oil and is applicable to a high-speed dental air turbine hand piece has a high biodegradation speed and, even when evaluated based on the Interim Water Quality Standards for Effluents (an ordinance of the Office of the Prime Minister), is more preferable than conventional mineral oils. Incidentally, according to the Interim Water Quality Standards for Effluents, the permissible limit for effluents is 30 mg/liter in the case of vegetable oils as opposed to 5 mg/liter in the case of conventional mineral oils.

Claims

1. A lubricating oil for a rolling bearing used in a high-speed rotating equipment, wherein said lubricating oil is comprised of a nondrying vegetable oil extracted from rice bran.

2. The lubricating oil according to claim 1, wherein said nondrying vegetable oil extracted from rice bran comprises, as a component of glycerol ester thereof: 40-45 wt. % of oleic acid (monovalent unsaturated fatty acid), 35-40 wt. % of linolic acid (divalent unsaturated fatty acid), 15-20 wt. % of palmitic acid (C1-6 saturated fatty acid), and other element (balance (100 wt. %)).

3. The lubricating oil according to claim 2, wherein said nondrying vegetable oil extracted from rice bran comprises free fatty acid which, as a component other than glycerol ester of fatty acid, includes: less than 0.01 wt. % of free oleic acid, and less than 0.01 wt. % of free linolic acid.

4. The lubricating oil according to claim 2, wherein said nondrying vegetable oil extracted from rice bran contains at least 500 wt.ppm of α-tocophenol (vitamin E)

5. The lubricating oil according to claim 3, wherein said nondrying vegetable oil extracted from rice bran contains at least 500 wt.ppm of α-tocophenol (vitamin E)

6. A lubricating oil for a rolling bearing used in a high-speed rotating equipment, said lubricating oil comprising: 1) a nondrying vegetable oil extracted from rice bran, as a principal component; and 2) one type of nondrying vegetable oil selected from the group consisting of olive oil, arachis oil and oleysol oil, as another component.

7. The lubricating oil according to any one of claims 1 to 6, wherein said nondrying vegetable oil contains oil-absorbing synthetic resin particles.

8. The lubricating oil according to claim 7, wherein said oil-absorbing synthetic resin particles are made of crosslinked polymer obtained by polymerizing a monomer mixture composed of: (A) 90 to 99.9 wt. % of a monomer composed of a monomer, which has a solubility parameter (sp value) of at most 9, as a principal component, and containing one polymerizable unsaturated group per molecule, and (B) 0.1 to 10 wt. % of a crosslinking monomer having at least two polymerizable unsaturated groups per molecule.

9. The lubricating oil according to claim 8, wherein said monomer (A) contains at least one C3-C30 aliphatic hydrocarbon group and at least polymerizable unsaturated group selected from the group consisting of residual groups of alkyl(meth)acrylates, alkylaryl(meth)acrylates, alkyl(meth)acrylamides, alkylaryl(meth)acrylamides, fatty acid vinyl esters, alkylstyrenes and α-olefins.

10. The lubricating oil according to claim 7, wherein said oil-absorbing synthetic resin particles are made of a diene-type crosslinked polymer obtained by polymerization of a diene monomer.

11. A lubricating oil for a rolling bearing used in a high-speed rotating equipment, wherein said rolling bearing is provided with a rolling bearing element comprising an outer ring, an inner ring, rolling elements and a retainer to rotatably support a rotary shaft thereon to which air turbine blades are fixed, and said lubricating oil used by being dropped to or impregnated in said retainer is comprised of nondrying vegetable oil extracted from rice bran.

12. The lubricating oil according to claim 11, wherein said high-speed rotating equipment is a dental air turbine hand piece of a ball bearing type.

13. The lubricating oil according to claim 11 or 12, wherein said nondrying vegetable oil extracted from rice bran contains oil-absorbing synthetic resin particles.