Patent Application
20140363112
This invention relates to a rolling bearing which is lubricated with lubricating oil whose kinematic viscosity at 40° C. is 1 to 5×10−5 m2/s (10 to 50 cSt) or grease whose kinematic viscosity of base oil at 40° C. is 1 to 5×10−5 m2/s (10 to 50 cSt) and is used on rotation condition that a PV value of rolling contact between a raceway surface of an inner ring or an outer ring and a rolling surface of a rolling element is 100 MPa·m/s or more and a Dmn value is 800000 or more.
A rolling bearing used in a machine tool is used in a high-speed rotation range in which a Dmn value (the product of an average dimension Dm of an inside diameter and an outside diameter of the bearing a diameter Dp (mm) of a pitch circle of a rolling element multiplied by a rotational speed n (min−1)) exceeds 800000. Recently, the Dmn value may exceed 1000000. Further, a previous high pressure is applied in order to increase rigidity of a main spindle. With this, in the rolling bearing for the main spindle of the machine tool, a PV value (P: surface pressure (Pa), V: slip speed (m/s)) often becomes 100 MPa·m/s or more.
Also, a low-viscosity lubricant is used in order to decrease torque while reducing heat generation of the rolling bearing in use. For oil lubrication, kinematic viscosity of lubricating oil at 40° C. is set at 1 to 5×10−5 m2/s (10 to 50 cSt), and for grease lubrication, kinematic viscosity of base oil at 40° C. is set at 1 to 5×10−5 m2/s (10 to 50 cSt). When the kinematic viscosity is less than 1×10−5 m2/s, an oil film is resistant to being formed and metal contact occurs on a rolling contact surface with a slip under high-speed rotation and there is a high possibility of burning. When the kinematic viscosity is more than 5×10−5 m2/s, the oil film is well formed, but viscous resistance or stirring resistance of the oil increases the temperature of the bearing. This increases thermal displacement of the main spindle and results in insufficient machining accuracy of the machine tool.
Since the rolling bearing for the machine tool is used on condition that the PV value is high and the oil film is thin thus, the rolling bearing is used in a clean environment, but the metal contact still tends to occur on the rolling contact surface.
Patent Document 1 describes a rolling bearing for supporting a main spindle of a machine tool, the rolling bearing in which lubrication performance in the case of being used under high-speed rotation is improved and in order to decrease torque and the amount of heat generation, grooves for lubricant holding are formed in a rolling surface and raceway surfaces of an inner ring and an outer ring and an oil-repellent film is formed in the grooves.
Patent Document 2 describes a rolling bearing used in a high-speed rotation environment in which a Dmn value becomes 1.0×106 or more, the rolling bearing in which a PV value becomes high and sliding friction produced between a rolling element and a raceway surface increases and wear or burning occurs before reaching a rolling fatigue life. Also, as described in Patent Document 2, the rolling element shall satisfy the following configurations (a) to (c) in order to well prevent the burning even for grease lubrication.
(a) A nitride precipitate containing 5% or more Si by weight is had on a surface layer part of a rolling surface, and a surface coating ratio of the nitride precipitate is 10% or more.
(b) Steel forming raw materials used contains 0.3 to 1.2% C, 0.5 to 2.0% Si, 0.2 to 2.0% Mn and 0.5 to 2.0% Cr by weight, and contains 0.05 to 0.2% one or more kind of Mo, V and Nb in total by weight, and includes residues having Fe and inevitable impurities.
(c) A retained austenite amount of the surface layer part of the rolling surface is 5% or less by volume.
Patent Document 1: JP-A-2007-192330
Patent Document 2: JP-A-2004-353742
The rolling bearing described in Patent Documents 1 and 2 has room for improvement in an increase in burning resistance performance of the rolling bearing for the machine tool.
Also, metal contact tends to occur on the rolling contact surface since the rolling bearing for the machine tool is used on condition that the PV value is high and the oil film is thin as described above. Then, when the metal contact occurs on the rolling contact surface, wear, adhesion, transfer of indentations, etc. may occur on a rolling surface of a rolling element and raceway surfaces of inner and outer rings, and a strip-shaped travel mark may be left under severe conditions. Inside the travel mark, surface roughness becomes rougher than the beginning.
That is, in the rolling bearing for the machine tool, deterioration in the surface roughness may occur on the rolling surface of the rolling element and the raceway surfaces of the inner and outer rings. The metal contact tends to occur more due to a decrease in an oil film parameter A value (oil film thickness to surface roughness) with such deterioration in the surface roughness. Also, due to progress of the deterioration in the surface roughness, burning tends to occur and a life decreases and vibration increases.
Further, since the rolling bearing for the main spindle of the machine tool is used on rotation condition that the Dmn value is 800000 or more as described above, the rolling bearing is used under condition that a surface pressure P is relatively low and a slip speed V is very high (for example, 0.080 m/s or more, 0.100 m/s or more) when a PV value is equal as compared with rolling bearings for other industrial machines (for example, railroad vehicles, industrial vehicles, construction machines, pumps or turbines).
When the rolling bearing is used on condition that the Dmn value is high and the slip speed V is high, stiff shear resistance on the rolling contact surface locally increases the temperature of lubricating oil and decreases viscosity of the lubricating oil. Since the oil film becomes thin accordingly, the oil film parameter A value becomes smaller. As a result, in the rolling bearing for the main spindle of the machine tool, the metal contact or the burning tends to occur more as compared with the rolling bearings for other industrial machines in which the PV value is equal.
On the other hand, in a machine tool having an ATC (automatic tool changer), a very large axial load is applied to a rolling bearing for supporting a main spindle at the time of unclamping, with the result that a rolling surface of a rolling element and a raceway surface of a bearing ring constructing the rolling bearing require high indentation resistance.
Also, a machine tool with complicated operation such as a five-axis working machine has a problem of causing an unexpected collision in the top of a main spindle including a tool, and with such a collision, indentations may occur in a rolling surface of a rolling element and a raceway surface of a bearing ring constructing the rolling bearing for supporting the main spindle. When the indentations occur in the rolling surface of the rolling element and the raceway surface of the bearing ring, vibration occurs in the rolling bearing or acoustic characteristics of the rolling bearing decrease and machining performance by the machine tool may decrease.
A problem of this invention is to provide a rolling bearing for a machine tool with better burning resistance performance than that of a conventional rolling bearing.
In order to solve the problem described above, one aspect of this invention is a rolling bearing having an inner ring, an outer ring and a rolling element, in which a use condition satisfies the following configuration (1), and is characterized in that retained austenite amounts (γRAB) of surface layer parts of the inner ring and the outer ring are more than 0% by volume and the rolling element has the following configurations (2) to (5).
(1) The rolling bearing is lubricated with lubricating oil whose kinematic viscosity at 40° C. is 1 to 5×10−5 m2/s (10 to 50 cSt) or grease whose kinematic viscosity of base oil at 40° C. is 1 to 5×10−5 m2/s (10 to 50 cSt) and is used on rotation condition that a PV value of rolling contact between a raceway surface of the inner ring or the outer ring and a rolling surface of the rolling element is 100 MPa·m/s or more and a Dmn value is 800000 or more.
(2) The rolling element is obtained by processing a raw material including alloy steel in which an Si content is 0.3 to 2.2% (both inclusive) by weight and an Mn content is 0.3 to 2.0% (both inclusive) by weight and also a content ratio (Si/Mn) of Si to Mn is 5 or less by mass in a predetermined shape and then performing thermal treatment including carbonitriding or nitriding. The Si improves quenching properties and also enhances martensite, but in order to obtain an effect of increasing a life and ensure burning resistance on a rolling contact surface (under conditions of high PV and high V), the Si content is set at 0.3% or more by weight. However, the Si content is set at 2.2% or less by weight since machinability is decreased and a failure risk caused by a shock load due to a program error etc. is increased by a decrease in toughness when the Si content is too high. Also, the Mn content is set at 0.3% or more by weight in order to efficiently precipitate Si.Mn nitride. However, the Mn content is set at 2.0% or less by weight since hardness, shock resistance and indentation resistance are decreased due to the too large retained austenite amounts of the surface layer parts after the thermal treatment when the Mn content is too high. Further, the ratio (Si/Mn) is set at 5 or less since it becomes difficult to promote precipitation of the Si.Mn nitride even in the case of sufficiently diffusing nitrogen when the Mn content is low to the Si content.
(3) The Si.Mn nitride including nitride of silicon (Si) and nitride of manganese (Mn) is present in the rolling surface in a range of 1.0 to 20.0% (both inclusive) by area. The Si.Mn nitride has functions of improving wear resistance or indentation resistance and also improving a rolling fatigue life, and the area ratio of the Si.Mn nitride is set at 1.0% or more in order to effectively obtain the functions. However, the area ratio is set at 20% or less since strength or toughness necessary for a rolling member cannot be obtained when the Si.Mn nitride is too large.
(4) An N content of a surface layer part (range from a surface to a depth of 50 μm) of the rolling surface is 0.2 to 2.0% (both inclusive) by weight. Nitrogen present in the surface layer part has functions of enhancing solution of martensite and ensuring stability of retained austenite and decreasing a tangential force acting on the rolling surface by forming nitride or carbonitride and improving wear resistance and indentation resistance, and the N content of a surface layer part is set at 0.2% or more by weight in order to effectively obtain such functions. However, the N content is set at 2.0% or less by weight since strength or toughness necessary for the rolling member cannot be obtained when the N content is too high.
(5) A retained austenite amount (γRC) of the surface layer part (range from a surface to a depth of 50 μm) of the rolling surface is 0 (exclusive) to 50% (inclusive) by volume and the following formula (1) is satisfied.
γRAB−15≦γRC≦γRAB+15 (1)
In the rolling bearing used on the condition shown in the above configuration (1), metal contact tends to occur on a rolling contact surface (a surface of contact between the raceway surface of the inner ring or the outer ring and the rolling surface of the rolling element) as described above, but the rolling element has the above configurations (2) to (5) to thereby improve wear resistance and indentation resistance of the rolling element. As a result, the strip-shaped travel mark as described above becomes resistant to being formed on the raceway surface of the inner ring or the outer ring and the rolling surface of the rolling element, and deterioration in surface roughness is reduced and therefore, burning resistance performance of the rolling bearing is improved.
The rolling bearing for a main spindle of a machine tool is used on condition that the above configuration (1) and the following configuration (6) are satisfied. Even in the case of being used on condition that the above configuration (1) and the following configuration (6) are satisfied, the rolling bearing of this aspect can obtain good burning resistance performance.
(6) A slip speed V of rolling contact between a raceway surface of the inner ring or the outer ring and a rolling surface of the rolling element is 0.080 m/s or more (0.100 m/s or more).
The rolling bearing of this aspect preferably has the following configuration (7), and more preferably has the following configuration (8).
(7) The retained austenite amounts (γRAB) of the surface layer parts of the inner ring and the outer ring and the retained austenite amount (γRC) of the surface layer part of the rolling surface of the rolling element are 0 (exclusive) to 30% (inclusive) by volume.
(8) The retained austenite amounts (γRAB) of the surface layer parts of the inner ring and the outer ring and the retained austenite amount (γRC) of the surface layer part of the rolling surface of the rolling element are 0 (exclusive) to 20% (inclusive) by volume.
As the retained austenite amount is larger, indentations tend to occur on the rolling surface of the rolling element and the raceway surface of the bearing ring constructing the rolling bearing, and burning resistance and wear resistance are decreased.
Since the rolling bearing of this aspect has the above configuration (7) (more preferably has the above configuration (8)) to thereby improve indentation resistance of the rolling surface of the rolling element and the raceway surface of the bearing ring constructing the rolling bearing, the rolling bearing can also be suitably used as applications for supporting a main spindle of a machine tool such as a five-axis working machine or a machine tool having an ATC (automatic tool changer).
Also, since the rolling bearing of this aspect has the above configuration (7) (more preferably has the above configuration (8)) to thereby improve burning resistance and wear resistance of the rolling bearing, the rolling bearing can be suitably used as applications (for the main spindle of the machine tool) of the condition that the above configuration (1) and the above configuration (6) are satisfied.
This invention provides the rolling bearing for the machine tool with better burning resistance performance than that of a conventional rolling bearing.
An embodiment of this invention will hereinafter be described.
An angular ball bearing having a shape shown in
The inner ring 1 and the outer ring 2 were obtained by performing a normal processing method and a thermal treatment method using a raw material made of SUJ2. The ball 3 of Comparative Example was obtained by performing processing by a normal method and thermal treatment by the following condition I using a raw material made of SUJ2. The ball 3 of Example was obtained by performing processing by a normal method and thermal treatment by the following condition II using the following raw material A.
Composition of alloy steel forming a raw material A: a C content of 1.01% by weight, a Cr content of 1.10% by weight, an Si content of 0.56% by weight, an Mn content of 1.10% by weight, a ratio of the Si content to the Mn content (Si/Mn)=0.51, and residues having Fe and inevitable impurities
After the raw material is held in an atmosphere of Rx gas for a predetermined time, the raw material is quenched with oil and is tempered and then is cooled by air.
After performing carbonitriding treatment for holding the raw material in an atmosphere of Rx gas plus propane gas plus ammonia gas for a predetermined time, the raw material is quenched with oil and is tempered and then is cooled by air.
As a result of measuring retained austenite amounts (γRAB) of surface layer parts of the inner ring 1 and the outer ring 2, the retained austenite amounts were 5.1% by volume.
As a result of measuring Si.Mn nitride including nitride of silicon (Si) and nitride of manganese (Mn) present in a surface (rolling surface) of the ball 3 of Example obtained, a presence ratio of the Si.Mn nitride was 2% by area. As a result of measuring an N content present in a surface layer part of the ball 3 of Example obtained, the N content was 0.5% by weight.
As a result of measuring a retained austenite amount (γRC) of the surface layer part of the ball 3 of Example obtained, the retained austenite amount was 10% by volume. In the ball 3 of Example, a relation between the retained austenite amount (γRC) of the surface layer part of the ball 3 and the retained austenite amounts (γRAB) of the surface layer parts of the inner ring 1 and the outer ring 2 satisfied a formula (1).
As a result of measuring Si.Mn nitride including nitride of silicon (Si) and nitride of manganese (Mn) present in a surface (rolling surface) of the ball 3 of Comparative Example obtained, a presence ratio of the Si.Mn nitride was 0.5% by area. As a result of measuring an N content present in a surface layer part of the ball 3 of Comparative Example obtained, the N content was 0.1% by weight.
As a result of measuring a retained austenite amount (γRC) of the surface layer part of the ball 3 of Comparative Example obtained, the retained austenite amount was 10% by volume.
An angular ball bearing of Example assembled using the ball 3 of Example and an angular ball bearing of Comparative Example assembled using the ball 3 of Comparative Example were attached as a test bearing J of a tester for evaluating burning resistance performance shown in
The tester of
Lubricant: grease containing a thickener including barium complex soap and base oil whose kinematic viscosity at 40° C. is 2.3×10−5 m2/s (23 cSt)
Rotational speed: 18000 min−1 (Dmn value: 1150000, PV value: 500 MPa·m/s)
Axial load: 1400 N
Test results are shown in a graph of
Also, an L50 life of the angular ball bearing of Example was 485.0 hours, and an L50 life of the angular ball bearing of Comparative Example was 55.3 hours. That is, burning resistance performance of the angular ball bearing of Example was 8.8 times that of the angular ball bearing of Comparative Example, and this angular ball bearing of Example was good in the burning resistance performance.
An angular ball bearing (an inside diameter of 50 mm, an outside diameter of 80 mm, a width of 16 mm, and a ball diameter of 6.35 mm) having a shape shown in
The inner ring 1 and the outer ring 2 were obtained by performing a normal processing method and a thermal treatment method using a raw material made of SUJ2. The ball 3 of Comparative Example was obtained by performing processing by a normal method and thermal treatment by the condition I described above using a raw material made of SUJ2. The ball 3 of Example was obtained by performing processing by a normal method and thermal treatment by the condition II described above using the raw material A described above.
An angular ball bearing of Example assembled using the ball 3 of Example and an angular ball bearing of Comparative Example assembled using the ball 3 of Comparative Example were attached as a test bearing J of a tester shown in
The tester of
Lubricant: grease containing a thickener including barium complex soap and base oil whose kinematic viscosity at 40° C. is 2.3×10−5 m2/s (23 cSt)
Rotational speed: 8000 min−1
Previous pressure method: fixed-position previous pressure of DB combination
Previous pressure load at the time of assembly: 1180 N
During test, an external pipe of the test spindle 9 is cooled by oil, and torque (a torque value detected by the torque meter 10) and an outer ring temperature of the test bearing J are measured. A rotational speed of the test spindle 9 is increased by 1000 min−1 every 20 hours and constant-speed rotation is performed and during the constant-speed rotation, it is examined whether a big change such as wobble occurs in the test bearing J, and the product (PV value) of a surface pressure (P) and a rotational speed (V) at a point in time of the occurrence is set at a limit PV value.
In the bearings of Example and Comparative Example, the same test bearings J were prepared by seven sets and each of the tests was performed seven times. The results are shown in
That is, according to the bearing of Example, the limit PV value having 90% reliability can be increased by 40% or more (about 47%) of the limit PV value of the bearing of Comparative Example. Also, in the case of considering a safety factor, an effect by the bearing of Example can probably be exerted when the PV value is 100 Pa·m/s or more.
Also, a relation between the average torque in each of the seven-time tests and the rotational speed of constant-speed rotation in this test is shown by a graph in
Thus, the bearing of Example can reduce an increase in torque in the case of being used on condition that the PV value is high. This is probably because deterioration (a strip-shaped travel mark etc.) becomes resistant to occurring on a surface of the ball and the raceway surfaces of the inner and outer rings by improving wear resistance of the ball.
In addition, this embodiment has been described by taking the angular ball bearing as an example, but this invention can also be applied to rolling bearings other than the angular ball bearing, for example, a single-row cylindrical roller bearing or a double-row cylindrical roller bearing.
The invention has been described in detail with reference to the specific embodiment, but it is apparent to those skilled in the art that various changes or modifications can be made without departing from the spirit and scope of the invention.
The present application is based on Japanese patent application (patent application No. 2012-100027) filed on Apr. 25, 2012, Japanese patent application (patent application No. 2012-233165) filed on Oct. 22, 2012 and Japanese patent application (patent application No. 2013-80642) filed on Apr. 8, 2013, and the contents of the patent applications are hereby incorporated by reference.
A rolling bearing of the invention is particularly useful for support of a main spindle of a machine tool used on condition that a Dmn value is 800000 or more.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2012-100027 | Apr 2012 | JP | national |
| 2012-233165 | Oct 2012 | JP | national |
| 2013-080642 | Apr 2013 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2013/061813 | 4/22/2013 | WO | 00 | 6/27/2014 |
