JP2005163893A - Rolling bearing for machine tool spindle - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a rolling bearing for a machine tool spindle capable of sufficiently elongating its service life even under high speed rotation of 1,400,000 dmn or more. <P>SOLUTION: In this rolling bering for the machine tool spindle, a plurality of rolling elements are rotatably retained between an inner ring and an outer ring by a cage, and the grease including fine particles composed of an inorganic compound of average particle size of 2 μm or less by 0.1-7 mass% to the total grease, and applying the lubricant having dynamic viscosity of 15 to 40 mm<SP>2</SP>/s at 40°C as base oil is sealed. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention is suitable for a spindle support portion of a machine tool that slides and rotates at high speed, represented by a lathe, drilling machine, boring machine, milling machine, grinding machine, honing machine, super finishing machine, lapping machine, etc. The present invention relates to a rolling bearing for a machine tool spindle to be incorporated.

  A spindle of a machine tool such as that described above usually incorporates a rolling bearing for supporting the spindle, and is generally used in combination with an angular ball bearing or a cylindrical roller bearing. Machining accuracy and productivity of machine tools largely depend on the rotation speed of the spindle, and in order to increase productivity, it is necessary to increase the rotation speed of the spindle. However, if the rolling bearing is used under high speed rotation, the heat generation of the bearing becomes noticeable, and the contact surface pressure between the rolling elements and the inner and outer rings increases due to centrifugal force, so the spindle operating conditions are significantly deteriorated. As a result, the risk of bearing damage represented by wear and seizure increases. In addition, since heat generation increases due to high-speed rotation, there is a risk of thermal deformation of the machine tool, which also affects machining accuracy.

  In order to prevent a fatal situation from occurring in such a bearing system and to avoid a decrease in machining accuracy due to thermal deformation of the entire machine tool, an appropriate lubrication method is selected under high-speed rotation to rotate the spindle support. Heat generation in the bearing must be suppressed as much as possible. Conventionally, the lubrication of rolling bearings for supporting spindles of machine tools that rotate at high speeds has achieved the cooling effect associated with the supply of lubricating oil, so the oil-air lubrication method, nozzle jet lubrication method, and underlace lubrication method have been adopted. . However, in these lubrication methods, since it is indispensable to introduce a lubricating oil supply device, an installation area for that purpose must be ensured, and the overall machine tool is prevented from being made compact. Further, in these lubrication methods, the lubricating oil is continuously consumed, and the operating cost of the lubricating oil supply device is also required, so that the operating cost of the entire machine tool increases. Various measures have been taken to reduce operating costs, but the situation is almost reached.

  Grease is generally used as a lubrication system for bearings, but bearing heat generation due to grease shearing is large, and the reliability of bearing durability is low compared to the above oil-air lubrication methods. This is a lubrication system that is essentially unsuitable for rolling bearings for supporting the spindle of machine tools. However, if grease lubrication can be realized, it is possible to enjoy merits such as downsizing of the machine tool and reduction of operation costs that cannot be supported by the above-described respective lubrication methods that continuously supply the lubricating oil. Grease lubrication also has the advantage of contributing to environmental conservation because it does not consume a large amount of lubricating oil, unlike an oil-air lubrication system.

  From such a background, the present applicant first encapsulates grease containing fine diamond (diamond-like carbon) and fine particles (cluster diamond) composed of amorphous carbon, so that the metal surfaces that are sliding surfaces are bonded to each other. Has been proposed to suppress direct contact and improve the seizure resistance and wear resistance of the bearing (see Patent Document 1).

JP 2003-28174 A

  There is a strong demand for higher performance of machine tools, and bearings are also required to be able to sufficiently withstand high-speed rotation such as dmn exceeding 1.4 million. The grease described in Patent Document 1 has improved seizure resistance and wear resistance by the action of cluster diamond, but may not be able to cope with such high speed rotation sufficiently. In addition, cluster diamond is expensive and disadvantageous in terms of cost.

  The present invention has been made in view of such a situation, and provides a rolling bearing for a machine tool spindle that exhibits a sufficiently long life even under a high-speed rotation condition of, for example, dmn of 1.4 million or more by improving the sealed grease. For the purpose.

The object of the present invention is to hold a plurality of rolling elements between an inner ring and an outer ring so as to be freely rollable by a cage, and to make fine particles of an inorganic compound having an average particle diameter of 2 μm or less into the total amount of grease. Achieved by a rolling bearing for a machine tool spindle characterized in that it contains 0.1 to 7% by mass of grease and a grease based on a lubricating oil having a kinematic viscosity at 40 ° C. of 15 to 40 mm ² / s. Is done.

  The rolling bearing for a machine tool main spindle of the present invention can efficiently suppress direct contact between metal surfaces by enclosing grease containing fine particles made of an inorganic compound having a specific particle diameter and containing a low-viscosity base oil. Further, the seizure preventing effect and the wear preventing effect are further improved. In addition, unlike a method of continuously supplying lubricating oil such as the oil-air lubrication method, the grease can be enclosed and used, so that the operating cost can be reduced and the space can be saved.

  Hereinafter, the present invention will be described in detail.

  The rolling bearing for a machine tool spindle of the present invention is not particularly limited in terms of structure, and for example, an angular ball bearing 10 shown in FIG. 1 can be exemplified. An angular ball bearing 10 shown in the figure is configured by a plurality of balls 13 held between an inner ring 11 and an outer ring 12 by a cage 14 so as to roll freely. The balls 13 can also be made of a ceramic such as silicon nitride or silicon carbide. In the present invention, the following specific grease is enclosed in the bearing space formed by the inner ring 11, the outer ring 12 and the balls 13.

In the encapsulated grease, the type of base oil is not particularly limited as long as the kinematic viscosity at 40 ° C. is 15 to 40 mm ² / s. When a base oil having a kinematic viscosity at 40 ° C. exceeding 40 mm ² / s is used, the temperature rise increases and the problem of seizure occurs. Further, from the viewpoint of increasing the bearing temperature, the lower the kinematic viscosity of the base oil, the better. However, if the kinematic viscosity is too low, the oil film formed on the sliding portion becomes thin and the bearing durability is lowered. Therefore, in the present invention, a base oil having a kinematic viscosity at 40 ° C. of 15 mm ² / s or more is used.

  Examples of the base oil include mineral oils and synthetic oils. As the mineral oil-based lubricating oil, it is possible to use a refined oil obtained by appropriately combining mineral oil under reduced pressure distillation, oil removal, solvent extraction, hydrocracking, solvent dewaxing, sulfuric acid washing, clay refining, hydrorefining, etc. . Examples of the synthetic oil-based lubricating base oil include hydrocarbon-based oils, aromatic base oils, ester-based oils, ether-based oils, and the like. Examples of the hydrocarbon oil include normal paraffin, isoparaffin, polybutene, polyisobutylene, 1-decene oligomer, and poly-α-olefin such as 1-decene and ethylene co-oligomer. Examples of the aromatic oil include alkyl naphthalene oils such as monoalkylnaphthalene, dialkylnaphthalene, and polyalkylnaphthalene. Examples of the ester oil include dibutyl sebacate, di-2-ethylhexyl sebacate, dioctyl adipate, diisodecyl adipate, ditridecyl adipate, ditridecyl tartrate, and diester oil such as methyl acetyl cinolate, trioctyl trimellitate, Aromatic ester oils such as tridecyl trimellitate and tetraoctyl pyromellitate, polyol esters such as trimethylolpropane caprylate, trimethylolpropane verargonate, pentaerythritol-2-ethylhexanoate, pentaerythritol verargonate Oil, carbonate ester oil and the like. Examples of the ether oil include polyglycols such as polyethylene glycol, polypropylene glycol, polyethylene glycol monoether, and polypropylene glycol monoether, or monoalkyl triphenyl ether, alkyl diphenyl ether, dialkyl diphenyl ether, pentaphenyl ether, tetraphenyl ether, and monoalkyl. Examples thereof include phenyl ether oils such as tetraphenyl ether and dialkyl tetraphenyl ether. These base oils can be used alone or as a mixture.

  The thickener is not particularly limited as long as it can form and maintain a grease with the above base oil. For example, a metal soap or a composite metal soap having a metal species of Li, Na, Ba, Ca, Al, etc., Non-soaps such as benton, silica gel, urea compounds, urea / urethane compounds, urethane compounds can be appropriately selected and used. However, considering the heat resistance of the grease, a urea compound, a urea / urethane compound, a urethane compound, or a mixture thereof is preferable. In view of heat resistance performance and acoustic characteristics, a diurea compound is particularly preferable. For high-speed rotation applications, barium composite soap is particularly desirable, and the amount of thickener necessary to form grease is relatively large compared to other types (about 30% by mass of the total amount of grease). Good oil retention performance and moderate oil release characteristics even under high speed rotation. Therefore, a large amount of grease can be retained inside the bearing without washing away the grease adhering to the outer ring portion of the bearing with the oil released during rotation. The amount of the thickener is not particularly limited as long as it can form grease with the base oil, and is generally 10 to 30% by mass of the total amount of grease.

  To the grease, fine particles made of an inorganic compound having an average particle size of 2 μm or less are added in an amount of 0.1 to 7% by mass, preferably 0.1 to 5% by mass, based on the total amount of grease. It is thought that the fine particles made of the inorganic compound enter the concave and convex portions of the friction portion, smooth the friction surface to increase the surface hardness, and extend the life of the bearing by preventing the sliding members from directly contacting each other. It is done. However, if the addition amount exceeds 7% by mass, secondary aggregation of the fine particles tends to occur, dispersibility in the grease is lowered, and the effect of improving the wear resistance is reduced.

Specific examples of the fine particles made of inorganic compounds include metal oxides such as SiO ₂ , Al ₂ O ₃ , MgO, TiO ₂ , PZT, and ZnO, metal nitrides such as Si ₃ N ₄ , ZrN, CrN, and TiAlN, SiC Metal carbides such as TiC and WC, (synthetic) clay minerals such as bentonite, smectite and mica, and diamond. Further, mention may be made of graphite, BN, a solid lubricant such as WS _2. Furthermore, in order to improve the affinity with a base oil or a urea-based thickener, those whose surfaces are modified to be lipophilic may be used. Among these inorganic compounds, metal oxides and clay minerals that have a thickening action themselves are preferable.

  As described above, the particle size of the fine particles made of the inorganic compound needs to enter the minute irregularities of the sliding surface, so that it is desirable that the particle size is smaller. In the present invention, the average particle size is 2 μm or less. If it exists, the favorable seizure prevention effect will express. When the average particle diameter is larger than 2 μm, the number of particles acting as foreign matters (dust) increases, which promotes wear of the sliding surface and causes an early failure of the bearing. Further, the smaller the average particle, the better, and 1 μm or less is more preferable. Further, considering the lubrication life of the sliding portion, the oil film thickness in actual use is about 0.2 μm, so the average particle size is smaller than this, and particularly preferably 0.1 μm or less. The shape of the fine particles is preferably closer to a sphere, but may be a polyhedron (cube, rectangular parallelepiped, etc.) or extremely acicular as long as it is within the above average particle diameter.

  You may add antioxidant, a rust preventive agent, an oiliness agent, an extreme pressure agent, etc. to the said grease as needed. Any of these may be known ones. The content of these additives is preferably individually 0.05% by mass or more of the total amount of grease, and the total amount is preferably in the range of 0.15 to 10% by mass of the total amount of grease. In particular, when the total amount exceeds 10% by mass, not only an effect commensurate with the increase in content cannot be expected, but the content of other components is relatively reduced, and these additives aggregate in the grease, Undesirable phenomena such as an increase in torque may be caused.

  EXAMPLES Hereinafter, although a test example is given and this invention is demonstrated further, this invention is not restrict | limited at all by this.

(Test-1: Bearing durability life test)
An angular contact ball bearing for machine tools (inner diameter 65 mm, outer diameter 100 mm, width 18 mm, silicon nitride sphere) was filled with 2.3 g of grease (occupying 15% of the bearing space volume) to produce a test bearing. The grease is a base grease in which barium composite soap is blended at a ratio of 30% by mass with an ester oil having a kinematic viscosity of 20 mm ² / s at 40 ° C., and MgO (average particle size is changed by changing the addition amount to this base grease. 0.3 μm) was used. In addition, the amount of the thickener was adjusted so that the penetration of each grease was 280.

  And about each test bearing, the bearing durability test was done using the testing apparatus shown in FIG. The illustrated test apparatus has a spindle structure in which a main shaft 6 is supported by two test bearings 20 arranged in a rear combination, and the main shaft 6 is rotationally driven via a motor and a transmission (not shown). In addition, a through hole for supplying a lubricant in the radial direction is provided at a substantially central portion in the axial direction of the test apparatus main body 24 and at an intermediate position between the two test bearings 20, and an oil air nozzle 21 is inserted into the through hole. The oil air nozzle 21 has a structure replaceable with a grease nipple. Further, the test apparatus main body 24 is provided with an exhaust path 23 for exhausting air in the setting space of the bearing 20. Further, a thermocouple 25 is installed in the test apparatus main body 24 with its detection part in contact with the outer ring 20 a of the test bearing 20.

  In the test, the time until the bearing was seized was measured under the conditions of an atmospheric temperature of 25 ° C., a preload of 98 N, and a dmn of 1,500,000 to evaluate the bearing durability life. The result is shown in FIG. 3 as a relative value with respect to the life of the test bearing in which the base grease not added with MgO is encapsulated, and it can be seen that the effect of improving the endurance life is exhibited by containing 0.1 mass% or more of MgO. . In particular, the range of 0.1 to 5% by mass is the most effective. If the content exceeds 3% by mass, the durability improving effect tends to be gradually weakened. If the content exceeds 7% by mass, the durability improving effect hardly appears. From this, in the present invention, it can be seen that the content of fine particles comprising an inorganic compound in the grease is 0.1 to 7% by mass, preferably 0.1 to 5% by mass.

(Test-2 Bearing outer ring temperature rise comparison)
In the same manner as in Test-1 above, grease prepared with various contents of MgO (average particle size 0.3 μm) 0.5 mass% and various base oil kinematic viscosities was sealed in a test bearing, and dmn 1.5 million The rise from the ambient temperature of the outer ring of the bearing when rotated at a speed was measured. The test is performed using the test apparatus shown in FIG. 2, and the spindle rotation speed is increased stepwise by 500 rpm under the conditions of an atmospheric temperature of 25 ° C. and a preload of 98 N. After reaching 1.5 million, the bearing outer ring temperature is reached. Was measured. The results are shown in FIG. 4 as a relative value to the temperature increase of the outer ring of the test bearing in which grease having a base oil dynamic viscosity at 40 ° C. of 25 mm ² / s is sealed. When the base oil kinematic viscosity at 40 ° C. exceeds 40 mm ² / s, the temperature increases rapidly. This shows that the upper limit of the kinematic viscosity of the base oil is 40 mm ² / s in the present invention.

(Test-3: Verification of MgO particle size)
An angular contact ball bearing for machine tools (inner diameter 65 mm, outer diameter 100 mm, width 18 mm, silicon nitride sphere) was filled with 2.3 g of grease (occupying 15% of the bearing space volume) to produce a test bearing. The grease is a base grease in which barium composite soap is blended at a ratio of 30% by mass with an ester oil having a kinematic viscosity of 20 mm ² / s at 40 ° C., and 3% by mass of MgO having a different average particle diameter. The one prepared by blending at a ratio of In addition, the amount of the thickener was adjusted so that the penetration of each grease was 280.

  Then, using the test apparatus shown in FIG. 2, the test bearing was rotated under the conditions of an atmospheric temperature of 20 ° C., a preload of 98 N, and a dmn of 1.5 million. After 10,000 hours had elapsed, the rotation was stopped, the bearing was removed from the test apparatus, and the shape of the inner ring raceway surface was observed using a surface roughness and shape measuring instrument (Form Talysurf PGI Plus manufactured by Taylor Hobson Co., Ltd.). The depth of the V-shaped wear scar on the transfer surface of the bearing inner ring was used as an index of wear. The result is shown in FIG. 5 as a relative value with respect to the wear depth of the test bearing in which the base grease not added with MgO is encapsulated. If the average particle diameter of MgO is 2 μm or less, the effect of reducing the wear is exhibited, and 1 μm or less is exhibited. It is more preferable that 0.2 μm or less is particularly preferable. Moreover, although the wear depth ratio is discontinuously sharply reduced when the average particle size is 0.15 μm or less, this phenomenon causes abrasive wear from the MgO fine particle group by extremely reducing the average particle size. It seems to suggest that the factor has been completely removed.

It is sectional drawing which shows an example of the rolling bearing for machine tool spindles of this invention. It is sectional drawing which shows the testing apparatus used by each test of the Example. It is a graph which shows the relationship between content of magnesium oxide obtained by Test-1, and a seizure life ratio. It is a graph which shows the relationship between the base oil kinematic viscosity obtained by Test-2, and the outer ring temperature rise ratio. It is a graph which shows the relationship between the average particle diameter of magnesium oxide obtained by Test-3, and the wear depth ratio.

Explanation of symbols

10 Angular Contact Ball Bearing 11 Inner Ring 12 Outer Ring 13 Ball 14 Cage

Claims (1)

Between the inner ring and the outer ring, a plurality of rolling elements are rotatably held by a cage, and fine particles made of an inorganic compound having an average particle size of 2 μm or less are 0.1 to 7 mass based on the total amount of grease. A rolling bearing for a spindle of a machine tool characterized by containing a grease containing a base oil of a lubricating oil having a kinematic viscosity at 40 ° C. of 15 to 40 mm ² / s.

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