US20180327688A1

US20180327688A1 - Lubricating oil composition

Info

Publication number: US20180327688A1
Application number: US15/772,643
Authority: US
Inventors: Ayano NAITOU; Mitsuhiro Nagakari
Original assignee: Shell Oil Co
Current assignee: Shell USA Inc
Priority date: 2015-11-04
Filing date: 2016-11-03
Publication date: 2018-11-15
Also published as: WO2017076999A1; BR112018009126A2; JP6666691B2; EP3371289A1; JP2017088651A; RU2018120363A; CN108350386A; BR112018009126A8

Abstract

An object of the present invention is to obtain a lubricating oil for machine tools, which has good lubricating properties and abrasion resistance and also exhibits a high flash point and a low pour point. A lubricating oil composition comprises, as a base oil, a base oil which includes an oil containing 20 mass % to 49 mass % of n-paraffin components and 51 mass % to 80 mass % of i-paraffin components at a quantity of 90 mass % or more of the total quantity of the base oil and which has a kinematic viscosity at 40° C. of 1 to 5 mm2/s. By blending a small quantity of β-dithiophosphorylated propionic acid and/or an acidic phosphoric acid ester in this base oil, a lubricating oil composition suitable for use for a main shaft of a machine tool is obtained. The lubricating oil composition for a main shaft of a machine tool has a flash point of 100° C. or more, a pour point of −10° C. or less, and a Shell four-ball wear mark diameter of 0.7 mm or less.

Description

FIELD OF THE INVENTION

The present invention relates to a lubricating oil composition suitable for use for a main shaft of a machine tool.

BACKGROUND OF THE INVENTION

The main shaft of a machine tool is rotated at a high speed in order to raise the working speed of the machine tool. The functions of a lubricating oil used for the main shaft are cooling and lubrication of the main shaft, and the lubricating oil needs to have a low viscosity in order to attain high cooling efficiency. Wear resistance is also an important property required of the lubricating oil in order to cope with the impact load of the main shaft. In addition, in cold periods during winter, low temperature fluidity is also required in order to obtain good startability of the machine tool.
Further, in some cases, lubricating oils used for machine tools are used not only for lubrication of shaft bearing parts, as mentioned above, but also for lubrication of gear parts, and the like, and in such cases, load-bearing properties are further required as an important property.
The applicant of the present application has previously attained good results in developing a lubricating oil composition with excellent lubricating properties, which is capable of sufficiently exhibiting performance for a longtime even when used under conditions becoming increasingly severe with speeding up, pressure increase, and downsizing of industrial machines and of securing the lifetime of machines, by blending β-dithiophosphorylated propionic acid with a mineral oil or a synthetic oil, see for example Japanese Patent Application Publication No. 2002-265971.
The inventors of the present invention have made various examinations and studies in order to obtain a lubricating oil composition which has even better lubricating properties and wear resistance as well as having a high flash point.
Conventional lubricating oils for main shafts of machine tools have a flash point as low as less than 100° C. and are subject to regulations under the Fire Service Act, but oils having a high flash point, that is, a flash point of 100° C. or higher, are covered by exception provisions under the Fire Service Act, and regulations for storage and preservation of such oils are relaxed, which facilitates handling thereof. It is also necessary for the pour point to be low from the perspective of low temperature startability.

SUMMARY OF THE INVENTION

According to the present invention, a lubricating oil composition suitable for use for a main shaft of a machine tool is obtained by using, as a base oil for the lubricating oil composition, abase oil which comprises an oil containing 20 mass % to 49 mass % of n-paraffin components and 51 mass % to 80 mass % of i-paraffin components at a quantity of 90 mass % or more of the total quantity of the base oil and which has a kinematic viscosity at 40° C. of 1 to 5 mm²/s, and blending a small quantity of β-dithiophosphorylated propionic acid and/or an acidic phosphoric acid ester with the base oil.

DETAILED DESCRIPTION OF THE INVENTION

The lubricating oil composition of the present invention exhibits good lubricating properties with good low temperature fluidity at shaft bearings of machine tools, and the like, and also exhibits excellent wear resistance and a high flash point (COC) of 100° C. or higher, and can therefore be effectively used as a lubricating oil composition for a main shaft of a machine tool.
The base oil of the present invention can be a paraffin-based base oil obtained by, for example, subjecting a kerosene/light oil fraction, which is obtained by atmospheric distillation of crude oil, to an appropriate combination of refining processes, such as hydrocracking.
In this type of base oil, n-paraffin components and i-paraffin components are contained at fixed proportions, and among paraffin components, it is preferable for the content of n-paraffin components to be 20 mass % to 49 mass % and the content of i-paraffin components to be 51 mass % to 80 mass %, and more preferable for the content of n-paraffin components to be 20 mass % to 29 mass % and the content of i-paraffin components to be 71 mass % to 80 mass %.
Base oil components constituted from these n-paraffin components and i-paraffin components account for 90 mass % or more, and preferably 95 mass % or more, of the total quantity of the base oil in the composition. Moreover, the remainder of the base oil may contain naphthene-based components and aromatic components, but if the total content of naphthene-based components and aromatic components exceeds 10 mass %, the flash point and oxidation stability deteriorate.
The kinematic viscosity at 40° C. of this type of base oil is 0.5 to 10 mm²/s, and preferably 1 to 5 mm²/s.
In addition, the total sulfur content in the base oil should be 10 ppm or less, and preferably 1 ppm or less, and the total nitrogen content in the base oil should be less than 10 ppm, and preferably less than 1 ppm.
The number of hydrocarbon carbon atoms in the base oil having a kinematic viscosity at 40° C. of 1 to 5 mm²/s is distributed within the range 10 to 24. In addition, the number of carbon atoms in a base oil having a kinematic viscosity at 40° C. of 1.98 to 2.42 mm²/s is distributed within the range 12 to 16.
Gas chromatography mass spectrometry methods (GC-MS) are known as methods for measuring the content of paraffin components, naphthene-based components and aromatic components in base oils. GC-MS is a method in which hydrocarbons, which have been separated according to retention time by gas chromatography, are subjected to mass spectrometry, in which the molecular weights and content proportions of the separated hydrocarbons are measured.
By denoting the carbon number as n, the molecular weight of a paraffin component is 2n+2, and naphthene-based components and aromatic components having ring structures within the molecule do not have molecular weights of 2n+2. By utilizing this characteristic, it is possible to determine the paraffin component content in a base oil by quantitatively determining the proportion of paraffin components in which the number of carbon atoms falls within the range 10 to 24.
Furthermore, gas chromatography/flame ionization detection (GC-FID) is one method of measuring the content values of straight chain n-paraffins and branched chain i-paraffins among paraffin components. In GC-FID, n-paraffins and i-paraffins are separated according to number of carbon atoms by the difference in retention time between paraffins, and the content proportions thereof can be quantitatively determined according to detection area. Therefore, by measuring the proportions of n-paraffins in a C_10-24fraction by means of GC-FID, it is possible to quantitatively determine the content of n-paraffins.
Moreover, n-paraffins include n-decane, n-undecane, n-dodecane, n-tridecane, n-tetradecane, n-pentadecane, n-hexadecane, n-heptadecane, n-octadecane, n-nonadecane, n-eicosane, n-heneicosane, n-docosane, n-tricosane and n-tetracosane.
As the base oil, a GTL (gas-to-liquid) base oil synthesised by a Fischer Tropsch polymerization method, which is a technique for obtaining liquid fuels from natural gas, can be advantageously used as the base oil component of the present invention due to having an extremely low sulfur content and aromatic content, a high constituent proportion of paraffin components, exhibiting excellent oxidation stability and having extremely low evaporative losses compared to base oils obtained by refining crude oil.
The viscosity characteristics of this GTL base oil should generally be a kinematic viscosity at 40° C. of 1.5 to 5.5 mm²/s, and preferably 1.98 to 2.42 mm²/s. In addition, the total sulfur content is generally less than 1 ppm, and the total nitrogen content is generally less than 1 ppm. One example of this type of GTL base oil is SHELL GTL Solvent GS250TM.
A small quantity of β-dithiophosphorylated propionic acid is blended in this base oil. This β-dithiophosphorylated propionic acid is a compound such as that represented by formula 1 below.
S═P(—OR₁)₂S CH₂CH(R₂)COOH (1)
In formula 1, R₁represents a branched alkyl group having 3 to 8 carbon atoms, and R₂represents a hydrogen atom or an alkyl group having 1 to 4 carbon atoms.
R₁can be a branched alkyl group such as an isopropyl group, a branched butyl group, a branched pentyl group, a branched hexyl group, a branched heptyl group or a branched octyl group. In addition, R₂can be a hydrogen atom, a methyl group, an ethyl group, a propyl group, a butyl group, or the like, but a methyl group is particularly preferred.
Specific examples of this type of compound include 3-(O,O-diisopropyl-dithiophosphoryl)-propionic acid, 3-(O,O-diisopropyl-dithiophosphoryl)-2-methyl-propionic acid, 3-(O,O-diisobutyl-dithiophosphoryl)-propionic acid and 3-(O,O-diisobutyl-dithiophosphoryl)-2-methyl-propionic acid.
As mentioned above, the β-dithiophosphorylated propionic acid should be used at a quantity of 0.01 mass % or more and less than 2 mass % relative to the total quantity of the lubricating oil composition.
It is possible to blend a small quantity of an acidic phosphoric acid ester in the base oil. This acidic phosphoric acid ester is a compound such as that represented by formula 2 below.
In formula 2, m denotes an integer of 1 or 2, and R denotes a straight chain or branched chain saturated or unsaturated hydrocarbon having 6 to 22 carbon atoms.
Specific examples of this acidic phosphoric acid ester include oleyl acid phosphate, stearyl acid phosphate and 2-ethylhexyl acid phosphate.
As mentioned above, the acidic phosphoric acid ester should be used at a quantity of 0.01 mass % or more and less than 2 mass % relative to the total quantity of the lubricating oil composition.
This acidic phosphoric acid ester can be used in combination with the β-dithiophosphorylated propionic acid.
By adding an oxygen-containing organic compound as a solubilizing agent in the present composition, dispersibility of the blended components mentioned above is improved and the performance of these components can be further improved. This type of oxygen-containing organic compound that is a solubilizing agent is at least one type of compound selected from among alcohols, esters, ethers, ketones, aldehydes, carbonates and derivatives thereof.
Among these, polyalkylene glycols (PAG) are particularly preferred. These polyalkylene glycols are compounds in which a plurality of alkylene glycols are polymerised, and are represented by formula 3 and formula 4 below, but are not particularly limited thereto.
HO—(C_nH_(2n+1)O)_a—H (3)
In formula 3, n is an integer between 2 and 4 and a is an integer.
HO—(C_pH_(2p+1)O)_s—(C_qH_(2q+1)O)_t—H (4)
In formula 4, p and q are each an integer between 2 and 4, and s and t are both integers, but cannot both be 0).
This PAG is a material having low oil solubility, and is therefore preferably at least one type of compound selected from among the group consisting of polyethylene glycol, polypropylene glycol and polybutylene glycol. In addition, the weight average molecular weight of the PAG is 200 to 10,000, preferably 200 to 6000, and more preferably 200 to 4000.
Moreover, if the weight average molecular weight is less than 200, solubility in the base oil is improved, but evaporative properties deteriorate, whereas if the weight average molecular weight exceeds 10,000, solubility in the base oil deteriorates.
This PAG is a substance having low oil solubility, as mentioned above, and should therefore be contained at a quantity of 0.01 to 10.0 mass %, preferably 0.1 to 5.0%, and more preferably 0.1 to 3.0% relative to the total quantity of the lubricating oil composition.
If necessary, a variety of publicly known additives, such as amine-based or phenol-based antioxidants, rust inhibitors, steric stabilizers, viscosity modifiers, dispersing agents, pour point depressants and anti-foaming agents, can be blended as appropriate in the lubricating oil composition of the present invention.

EXAMPLES

The lubricating oil composition for a main shaft of a machine tool of the present invention will now be explained in greater detail through the use of working examples, comparative examples and base oil examples, but is in no way limited to these examples.
The following materials were prepared in order to prepare the working examples, comparative examples and base oil examples.

Base Oils

Base Oil 1: GTL (gas-to-liquid) base oil (kinematic viscosity at 40° C.: 2.396 mm²/s, density at 15° C.: 0.7760, n-paraffin component content 23% and i-paraffin component content 77%, as determined by gas chromatography) (SHELL GTL Solvent GS250).
Base Oil 2: i-paraffin-based oil (kinematic viscosity at 40° C.: 2.623 mm²/s, density at 15° C.: 0.7987, n-paraffin component content <1% and i-paraffin component content ≥99%, as determined by gas chromatography) (Shell Paraol 250).
Base Oil 3: Tetradecane (kinematic viscosity at 40° C.: 2.087 mm²/s, density at 15° C.: 0.7664, n-paraffin component content ≥99% and i-paraffin component content <1%, as determined by gas chromatography).
Base Oil 4: Pentadecane (kinematic viscosity at 40° C.: 2.458 mm²/s, density at 15° C.: 0.7723, n-paraffin component content ≥99% and i-paraffin component content <1%, as determined by gas chromatography).
Base Oil 5: Low viscosity naphthene base oil (kinematic viscosity at 40° C.: 2.891 mm²/s, density at 15° C.: 0.8864, Ca=10% and Cn=60%, as determined by n-d-M ring analysis (ASTM D3238)) (SNH-3 manufactured by Sankyo Yuka Kogyo Kabushiki Kaisha).
Base Oil 6: API Group I base oil (kinematic viscosity at 40° C.: 24.54 mm²/s, viscosity at 15° C.: 0.8620, Ca=3.0%, Cn=28.2% and Cp=68.7%, as determined by n-d-M ring analysis (ASTM D3238)), paraffin component content: 68.7 mass %, the majority of which is i-paraffins.

Additives

Additive 1: β-dithiophosphorylated propionic acid (Irgalube 353)
Additive 2: Ethyl β-dithiophosphorylated propionate (Irgalube 63)
Additive 3: 2-ethylhexyl acid phosphate (Phoslex A-8)
Additive 4: Oleyl acid phosphate (Phoslex A-18D)
Additive 5: Tricresyl phosphate
Additive 6: Dioleyl hydrogen phosphate (Chelex H-18D)
Additive 7: Polyalkylene glycol (UCON OSP18)
Base Oil Examples 1 to 5 below were prepared in order to investigate the properties and performance of base oil compositions that constitute lubricating oil compositions.

Base Oil Example

Base Oil Example 1 comprises only Base Oil 2.

Base Oil Examples 2 to 5

Base Oil Examples 2 to 5 are constituted by the compositions shown in Table 3.
The working examples and comparative examples given below were prepared.

Working Example 1

The lubricating oil composition of Working Example 1 was obtained by adding 0.05 mass % of Additive 1 to 99.95 mass % of Base Oil 1, and mixing thoroughly.

Working Examples 2 to 5

The lubricating oil compositions of Working Examples 2 to 5 were obtained in the same way as Working Example 1, except that the compositions shown in Table 1 were used.

Comparative Examples 1 to 7

The lubricating oil compositions of Comparative Examples 1 to 7 were obtained in the same way as Working Example 1, except that the compositions shown in Table 2 were used. Moreover, Comparative Example 1 is the same as Base Oil Example 1.

Tests

The following tests were carried out as appropriate in order to investigate the properties and performance of the working examples, comparative examples and base oil examples.

Kinematic Viscosity at 40° C.

Kinematic viscosity (mm²/s) at 40° C. was measured in accordance with JIS K2283.

Density

Density (g/cm³) at 15° C. was measured using a vibration method in accordance with JIS K2249-1.

Flash Point

Flash point was measured in accordance with JIS K2265-4 using a Cleveland open cup type automatic flash point measurement apparatus.
The thermometer used was a no. 32 thermometer specified in JIS B7410 (COC).
Test evaluations were carried out using the following criteria.
100° C. or higher: ∘ (pass)
Lower than 100° C.: × (fail)

Pour Point

Pour point (° C.) was measured in accordance with JIS K2269. The thermometer used was a no. 10 thermometer specified in JIS B7410 (PP). Test evaluations were carried out using the following criteria.
−10° C. or lower: ∘ (pass)
Higher than −10° C.: × (fail)

Abrasion Resistance Test: Shell Four-Ball Wear Test

The test equipment and test method were as follows: The test was carried out in accordance with ASTM D4172, a load of 15 kgf was applied, the tester was rotated for 30 minutes at a speed of 1800 rpm at an oil temperature of 54° C., and the diameter (mm) of the abrasion mark generated at the point of contact was measured.
Test evaluations were carried out using the following criteria.
Abrasion mark diameter of 0.70 mm or less: ∘ (pass) Abrasion mark diameter exceeding 0.70 mm: × (fail)

Results

Test results for the working examples and comparative examples are shown in Tables 1 and 2.
Test results for the base oil examples are shown in Table 3.
As shown in Table 3, base oils in which a higher quantity of i-paraffin components than n-paraffin components was blended, which are represented by Base Oil Examples 2 and 3, were more preferred in terms of flash point than Base Oil Example 1, which comprised only i-paraffin components. In addition, it was found that Base Oil Examples 2 and 3 exhibited a superior pour point to Base Oil Example 4, which contained 50% of i-paraffin components and 50% of n-paraffin components, and Base Oil Example 5, which comprised only a mixture of n-paraffin components.
With regard to the working examples and comparative examples, Working Example 1 was obtained by blending Additive 1 in Base Oil 1, as shown in Table 1, but achieved excellent results, namely a high flash point of 126° C., a low pour point of −25° C., and an abrasion mark diameter of 0.50 mm in the abrasion resistance test. Working Example 2 was obtained by blending Additive 3 in Base Oil 1, and also achieved excellent results, namely a high flash point of 122° C., a low pour point of −25° C., and an abrasion mark diameter of 0.57 mm in the abrasion resistance test.
Working Example 3 was obtained by blending Additive 4 in Base Oil 1, and achieved even better results, namely a high flash point of 128° C., a low pour point of −25° C. and an abrasion mark diameter of 0.10 mm or less in the abrasion resistance test.
Working Example 4 was obtained by blending Additive 5 in Working Example 1, and achieved excellent results, namely a high flash point of 128° C., a low pour point of −25° C. and an abrasion mark diameter of 0.32 mm in the abrasion resistance test.
Working Example 5 is an example in which Base Oil 1 is used together with 5 mass % of a Group I base oil. Base Oil 6 contains 68.7 mass % of paraffin components, the majority of which is i-paraffin components, and even if a small quantity of Base Oil 6 is contained, the proportions of i-paraffin components and n-paraffin components in the composition is not significantly altered, and Base Oil 6 exhibited tolerably good results, namely a high flash point of 126° C., a low pour point of −25° C. and an abrasion mark diameter of 0.49 mm in the abrasion resistance test. Moreover, the content of paraffin components (the total content of n-paraffin components and i-paraffin components) in the base oil compositions shown in Table 1 is 98.5 mass % relative to the total base oil quantity, and these base oil compositions contain components other than paraffin components.
However, Comparative Example 1 comprised only Base Oil 2, as shown in Table 2, and exhibited a preferred pour point of −50° C. or lower, but had a flash point of 100° C. or lower and caused seizure in the abrasion resistance test, and could therefore not achieve a favourable result. Comparative Example 2 was obtained by blending Additive 1 in Base Oil 2, and achieved a preferred pour point of −50° C. or lower and a preferred abrasion mark diameter of 0.50 mm, but had a flash point of 100° C. or lower, and could not therefore achieve a favourable result.
Comparative Example 3 was obtained by blending Additive 1 in Base Oil 5, and achieved a preferred pour point of −50° C. or lower and a preferred abrasion mark diameter of 0.30 mm, but Base Oil 5 is a naphthene base oil and has a low paraffin component content of approximately 30%, and therefore had an unacceptable flash point of 96° C.
Comparative Example 4 was obtained by blending Additive 2 in Base Oil 1, and achieved preferred results in terms of pour point and flash point, but had a highly unfavourable abrasion mark diameter of 0.93 mm. Comparative Example 5 was obtained by blending Additive 5 in Base Oil 1, and achieved preferred results in terms of pour point and flash point, but had an unacceptable abrasion mark diameter.
Comparative Example 6 was obtained by blending Additive 6 in Base Oil 1, and achieved preferred results in terms of pour point and flash point, but had an unfavourable abrasion mark diameter. In addition, Comparative Example 7 was obtained by adding Additive 7 to Comparative Example 4, and was acceptable in terms of pour point and flash point, but was judged to have an unacceptable abrasion mark diameter.

TABLE 1

Working	Working	Working	Working	Working
Example 1	Example 2	Example 3	Example 4	Example 5

Base Oil 1	99.95	99.95	99.95	98.95	94.95
Base Oil 2	—	—	—	—	—
Base Oil 3	—	—	—	—	—
Base Oil 4	—	—	—	—	—
Base Oil 5	—	—	—	—	—
Base Oil 6	—	—	—	—	5
Additive 1	0.05	—	—	0.05	0.05
Additive 2	—	—	—	—	—
Additive 3	—	0.05	—	—	—
Additive 4	—	—	0.05	—	—
Additive 5	—	—	—	—	—
Additive 6	—	—	—	—	—
Additive 7	—	—	—	1	—
Base oil
composition
n-paraffins	23%	23%	23%	23%	24%
i-paraffins	77%	77%	77%	77%	76%
Kinematic	2.396	2.396	2.396	2.546	2.460
viscosity (40° C.)
Density (15° C.)	0.7760	0.7760	0.7760	0.7780	0.7803
Flash point (COC)	126	122	128	128	126
Shell four-ball	0.50	0.57	<0.10	0.32	0.49
wear test
Pour point (° C.)	−25	−25	−25	−25	−25

TABLE 2

Comparative	Comparative	Comparative	Comparative	Comparative	Comparative	Comparative
Example 1	Example 2	Example 3	Example 4	Example 5	Example 6	Example 7

Base Oil 1	—	—	—	99.95	99.95	99.95	98.95
Base Oil 2	100	99.95	—	—	—	—	—
Base Oil 3	—	—	—	—	—	—	—
Base Oil 4	—	—	—	—	—	—	—
Base Oil 5	—	—	99.95	—	—	—	—
Base Oil 6	—	—	—	—	—	—	—
Additive 1	—	0.05	0.05	—	—	—	—
Additive 2	—	—	—	0.05	—	—	0.05
Additive 3	—	—	—	—	—	—	—
Additive 4	—	—	—	—	—	—	—
Additive 5	—	—	—	—	0.05	—	—
Additive 6	—	—	—	—	—	0.05	—
Additive 7	—	—	—	—	—	—	1
Base oil
composition
n-paraffins	<1%	<1%	—	23%	23%	23%	23%
i-paraffins	≥99%	≥99%	—	77%	77%	77%	77%
Kinematic	2.623	2.623	2.891	2.396	2.396	2.396	2.542
viscosity
(40° C.)
Density (15° C.)	0.7897	0.7897	0.8864	0.7760	0.7760	0.7760	0.7780
Flash point	96	98	96	126	128	132	126
(COC)
Shell four-ball	Seizure	0.50	0.3	0.93	0.87	0.80	0.88
wear test
Pour point (° C.)	<−50.0	<−50.0	<−50.0	−25	−25	−25	−25

TABLE 3

Base Oil	Base Oil	Base Oil	Base Oil	Base Oil
Example 1	Example 2	Example 3	Example 4	Example 5

Base Oil 1	—	—	—	—	—
Base Oil 2	100	80	70	50	—
Base Oil 3	—	10	15	25	50
Base Oil 4	—	10	15	25	50
Base Oil 5	—	—	—	—	—
Base Oil 6	—	—	—	—	—
Base oil
composition
n-paraffins	<1%	20%	30%	50%	≥99%
i-paraffins	≥99%	80%	70%	50%	<1%
Kinematic	2.623	2.488	2.430	2.359	2.267
viscosity (40° C.)
Density (15° C.)	0.7897	0.7924	0.7890	0.7833	0.7694
Flash point (COC)	96	106	110	112	130
Shell four-ball	—	—	—	—	—
wear test
Pour point (° C.)	<−50.0	−20.0	−15.0	−5.0	5.0

Claims

1. A lubricating oil composition for a main shaft of a machine tool, the composition comprising a base oil, which includes an oil containing 20 mass % to 49 mass % of n-paraffin components and 51 mass % to 80 mass % of i-paraffin components at a quantity of 90 mass % or more of the total quantity of the base oil, and which has a kinematic viscosity at 40° C. of 1 to 5 mm²/s, wherein the base oil contains at least one component selected from among β-dithiophosphorylated propionic acid represented by formula 1 below and an acidic phosphoric acid ester represented by formula 2 below:

wherein, in formula 1, R₁represents a branched alkyl group having 3 to 8 carbon atoms, and R₂represents a hydrogen atom or an alkyl group having 1 to 4 carbon atoms and in formula 2, m denotes an integer of 1 or 2, and R represents a straight chain or branched chain hydrocarbon having 6 to 22 carbon atoms.

2. The lubricating oil composition for a main shaft of a machine tool according to claim 1, wherein the β-dithiophosphorylated propionic acid is 3-(O,O-diisobutyl-dithiophosphoryl)-2-methyl-propionic acid and is contained at a quantity of 0.01 mass % or more and less than 2 mass % relative to the total quantity of the composition.

3. The lubricating oil composition for a main shaft of a machine tool according to claim 1, wherein the acidic phosphoric acid ester is a compound in which R in formula 2 is a straight chain or branched chain hydrocarbon having 8 to 18 carbon atoms and is contained at a quantity of 0.01 mass % or more and less than 2 mass % relative to the total quantity of the composition.

4. The lubricating oil composition for a main shaft of a machine tool according to claim 1, wherein the base oil is a base oil derived from Fischer-Tropsch polymerization.

5. The lubricating oil composition for a main shaft of a machine tool according to claim 1, which further comprises an oxygen-containing organic compound as a solubilizing agent at a quantity of 0.01 to 5 mass % relative the total quantity of the composition.

6. The lubricating oil composition for a main shaft of a machine tool according to claim 1, wherein the flash point (COC) of the composition is 100° C. or higher.