WO2024101268A1 - Bearing device with sensor, and spindle device for machine tool - Google Patents

Abstract

In this bearing device with a sensor, configured such that a preload is transmitted to a first inner race (31), a first rolling element (32), a first outer race (30), an outer race spacer (26), a second outer race (35), a second rolling element (37), and a second inner race (36), a pressing force in a direction causing the first outer race (30) and the second outer race (35) to approach one another is applied to an axial direction end surface of the first outer race (30) on the opposite side to the second outer race (35) and to an axial direction end surface of the second outer race (35) on the opposite side to the first outer race (30), and the magnitude of the pressing force is set to be larger than the preload.

Description

Sensor-equipped bearing device and machine tool spindle device

This invention relates to a sensor-equipped bearing device and a machine tool spindle device that uses the sensor-equipped bearing device.

　Spindle devices are used in machine tools such as machining centers and lathes, and other industrial machines, to rotatably support a rotating shaft (main shaft) on which tools, workpieces, and other objects are attached. In recent years, there has been a demand in fields where such spindle devices are used to strengthen status monitoring functions to reduce manpower and achieve automation.

The applicant of this application has already proposed a bearing device with a sensor to meet the need for improved condition monitoring functions (Patent Document 1).

The sensor-equipped bearing device of Patent Document 1 has a first bearing and a second bearing spaced apart in the axial direction, a cylindrical outer ring spacer provided between the first bearing and the second bearing, and a strain sensor attached to the outer ring spacer.

The first bearing has a first outer ring, a first inner ring provided radially inward of the first outer ring, and a plurality of first rolling elements incorporated between the first outer ring and the first inner ring. Similarly, the second bearing has a second outer ring, a second inner ring provided radially inward of the second outer ring, and a plurality of second rolling elements incorporated between the second outer ring and the second inner ring. The outer ring spacer is arranged by being sandwiched between the first outer ring and the second outer ring in the axial direction.

　A preload nut is fastened to the axial end face of the first inner ring opposite the second inner ring, and the axial end face of the second inner ring opposite the first inner ring, to bring the first inner ring and the second inner ring closer together. The first bearing and the second bearing are configured to transmit this preload through the first inner ring, the first rolling element, the first outer ring, the outer ring spacer, the second outer ring, the second rolling element, and the second inner ring. The first bearing is an angular ball bearing configured so that the axial preload between the first outer ring and the first inner ring generates a radial component force that causes the first rolling element to press the first outer ring, and the second bearing is also an angular ball bearing configured so that the axial preload between the second outer ring and the second inner ring generates a radial component force that causes the second rolling element to press the second outer ring.

The strain sensor is attached to the outer periphery (or inner periphery) of the outer ring spacer, and the preload of the first bearing and second bearing (hereinafter referred to as "bearing preload") can be detected based on the output of this strain sensor.

JP 2021-014886 A

However, in the sensor-equipped bearing device of Patent Document 1, an error could occur in the magnitude of the bearing preload detected based on the output of the strain sensor in the outer ring spacer. If this error could be eliminated and the bearing preload could be detected with high accuracy, it would be possible to improve the reliability of condition monitoring for labor-saving or unmanned operation.

The problem that the first invention aims to solve is to provide a bearing device with a sensor that can detect bearing preload with high accuracy.

The bearing device in Patent Document 1 supports the main shaft of a spindle device, and is arranged in a back-to-back combination on the inside of a cylindrical bearing housing. The first and second bearings are angular contact ball bearings each having an outer ring, an inner ring rotatably arranged radially inside the outer ring, and multiple rolling elements assembled between the outer and inner rings. A strain sensor is attached to the outer ring spacer assembled between the outer ring of the first bearing and the outer ring of the second bearing.

Then, by tightening the preload nut that is screwed onto the outer circumference of the main shaft supported by both bearings, the axial force of the preload nut is transmitted to the second bearing, the outer ring spacer, and the first bearing in that order, applying a preload to both bearings, and the axial preload load can be calculated based on the output of a strain sensor attached to the outer ring spacer.

Therefore, a spindle device using this bearing device can detect an increase in preload caused by heat generation in each bearing supporting the spindle, and has an enhanced condition monitoring function. Furthermore, when the spindle device is incorporated into a machine tool, changes in cutting load can also be detected, making it possible to monitor the machining condition.

In the bearing device of Patent Document 1, the load acting on the outer ring spacer is determined from the strain caused by deformation of the outer ring spacer when subjected to axial force. Therefore, in order to detect changes in load with high sensitivity, it is desirable to make the outer ring spacer easily deformable within a practical range.

However, if an outer ring spacer that is easily deformed is used, when an excessive load is applied, the outer circumference of the outer ring spacer expands and deforms, coming into contact with the inner circumference of the bearing housing, changing the deformation pattern of the outer ring spacer, which in turn reduces the sensitivity of detecting changes in load.

The problem that the second invention aims to solve is to provide a bearing device with a sensor that can detect load changes stably and with high sensitivity.

The inventors of the present application conducted an evaluation test in which the bearing preload was changed in the sensor-equipped bearing device of Patent Document 1 and the bearing preload was detected based on the output of the strain sensor in the outer ring spacer. They discovered that, even if the magnitude of the bearing preload was the same, the output of the strain sensor was not the same during the process of increasing the bearing preload and the process of decreasing the bearing preload, and that hysteresis occurred, resulting in a certain difference between the former and the latter, and that this hysteresis was the cause of an error in the bearing preload detected based on the output of the strain sensor.

The reasons for the above hysteresis are thought to be as follows:

When the bearing preload increases, the first outer ring elastically deforms in the radial expansion direction due to an increase in the radial component of the force it receives from the first rolling element, and this elastic deformation causes a small amount of slippage between the contact surfaces of the first outer ring and the outer ring spacer, where the first outer ring moves radially outward relative to the outer ring spacer. On the other hand, when the bearing preload decreases, the first outer ring elastically restores in the radial contraction direction due to a decrease in the radial component of the force it receives from the first rolling element, and this elastic restoration causes a small amount of slippage between the contact surfaces of the first outer ring and the outer ring spacer, where the first outer ring moves radially inward relative to the outer ring spacer.

Similarly, when the bearing preload increases, the second outer ring elastically deforms in the radial expansion direction due to an increase in the radial component of the force it receives from the second rolling element, and this elastic deformation causes a small amount of slippage between the contact surfaces of the second outer ring and the outer ring spacer, where the second outer ring moves radially outward relative to the outer ring spacer. On the other hand, when the bearing preload decreases, the second outer ring elastically restores in the radial contraction direction due to a decrease in the radial component of the force it receives from the second rolling element, and this elastic restoration causes a small amount of slippage between the contact surfaces of the second outer ring and the outer ring spacer, where the second outer ring moves radially inward relative to the outer ring spacer.

If the above-mentioned radial slippage occurs between the contact surfaces of the first outer ring and the outer ring spacer, or between the contact surfaces of the second outer ring and the outer ring spacer, the deformation of the outer ring spacer will not be the same when the bearing preload is increasing and when the bearing preload is decreasing. As a result, even if the magnitude of the bearing preload is the same when the bearing preload is increasing and when the bearing preload is decreasing, the output of the strain sensor will not be the same, and it is thought that hysteresis will occur, resulting in a certain difference between the former and the latter. This hysteresis in the strain sensor output will cause an error in the bearing preload detected based on the strain sensor output.

Furthermore, the inventors of the present application came up with the idea that if the surface pressure between the contact surfaces of the first outer ring and the outer ring spacer is increased, the frictional force between the contact surfaces of the first outer ring and the outer ring spacer will increase, making it possible to suppress radial slippage between the contact surfaces of the first outer ring and the outer ring spacer when the radial component of the force that the first outer ring receives from the first rolling element changes in response to changes in the bearing preload; similarly, if the surface pressure between the contact surfaces of the second outer ring and the outer ring spacer is increased, the frictional force between the contact surfaces of the second outer ring and the outer ring spacer will increase, making it possible to suppress radial slippage between the contact surfaces of the second outer ring and the outer ring spacer when the radial component of the force that the second outer ring receives from the second rolling element changes in response to changes in the bearing preload; as a result, the hysteresis in the output of the strain sensor of the outer ring spacer is reduced, making it possible to keep errors due to hysteresis to a minimum.

Based on this idea, in order to solve the above problems, a first aspect of the present invention provides a sensor-equipped bearing device having the following configuration.
[Configuration 1]
The bearing has a first bearing and a second bearing spaced apart in the axial direction, a cylindrical outer ring spacer provided between the first bearing and the second bearing, and a strain sensor attached to the outer ring spacer,
the first bearing includes a first outer ring, a first inner ring provided radially inside the first outer ring, and a plurality of first rolling elements assembled between the first outer ring and the first inner ring,
the second bearing includes a second outer ring, a second inner ring provided radially inside the second outer ring, and a plurality of second rolling elements assembled between the second outer ring and the second inner ring,
a preload is applied to an axial end face of the first inner ring opposite to a side of the second inner ring and an axial end face of the second inner ring opposite to a side of the first inner ring in a direction to bring the first inner ring and the second inner ring closer to each other,
In a sensor-equipped bearing device configured so that the preload is transmitted through the first inner ring, the first rolling element, the first outer ring, the outer ring spacer, the second outer ring, the second rolling element, and the second inner ring,
a pressing force in a direction bringing the first outer ring and the second outer ring closer together is applied to an axial end face of the first outer ring opposite to a side of the second outer ring and an axial end face of the second outer ring opposite to a side of the first outer ring,
A sensor-equipped bearing device, characterized in that the magnitude of the pressing force is set to be greater than the inner ring preload.

When this configuration is adopted, a surface pressure of the sum of the preload applied to the axial end faces of the first inner ring and the second inner ring and the pressing force applied to the axial end faces of the first outer ring and the second outer ring acts between the contact surfaces of the first outer ring and the outer ring spacer, so the surface pressure between the contact surfaces of the first outer ring and the outer ring spacer is large. Therefore, the friction force between the contact surfaces of the first outer ring and the outer ring spacer is large, and when the radial component of the force that the first outer ring receives from the first rolling element changes in response to the change in the bearing preload, it is possible to suppress radial slip between the contact surfaces of the first outer ring and the outer ring spacer. Similarly, a surface pressure of the sum of the preload applied to the axial end faces of the first inner ring and the second inner ring and the pressing force applied to the axial end faces of the first outer ring and the second outer ring acts between the contact surfaces of the second outer ring and the outer ring spacer, so the surface pressure between the contact surfaces of the second outer ring and the outer ring spacer is large. Therefore, the frictional force between the contact surfaces of the second outer ring and the outer ring spacer increases, and when the radial component of the force that the second outer ring receives from the second rolling element changes in response to a change in the bearing preload, it becomes possible to suppress radial slippage between the contact surfaces of the second outer ring and the outer ring spacer. As a result, the hysteresis of the output of the strain sensor of the outer ring spacer is reduced, making it possible to detect the bearing preload with high accuracy.

[Configuration 2]
2. The sensor-equipped bearing device according to claim 1, wherein the magnitude of the pressing force is set to be 10 times or more the magnitude of the preload.

　When this configuration is adopted, a pressing force significantly larger than the preload is applied to the axial end faces of the first outer ring and the second outer ring, so that the surface pressure between the contact surfaces of the first outer ring and the outer ring spacer and the surface pressure between the contact surfaces of the second outer ring and the outer ring spacer are particularly large. Therefore, the frictional force between the contact surfaces of the first outer ring and the outer ring spacer is particularly effectively large, and when the radial component of the force that the first outer ring receives from the first rolling element changes in response to a change in the bearing preload, it is possible to particularly effectively suppress radial slippage between the contact surfaces of the first outer ring and the outer ring spacer. Similarly, the frictional force between the contact surfaces of the second outer ring and the outer ring spacer is particularly effectively large, and when the radial component of the force that the second outer ring receives from the second rolling element changes in response to a change in the bearing preload, it is possible to particularly effectively suppress radial slippage between the contact surfaces of the second outer ring and the outer ring spacer.

[Configuration 3]
3. The sensor-equipped bearing device of configuration 1 or 2, in which the pressing force is applied by assembling the first outer ring and the second outer ring with an axial interference between an annular outer ring positioning step provided on the inner circumference of a cylindrical bearing housing that fits onto the outer circumference of the first outer ring and the outer circumference of the second outer ring, and a cover member fixed to the axial end face of the bearing housing by a screw member.

By adopting this configuration, a large pressing force can be applied to the axial end faces of the first and second outer rings using a simple method of tightening the screw members.

[Configuration 4]
4. The sensor-equipped bearing device according to configuration 3, wherein the screw member is disposed at the same circumferential position as the strain sensor.

By adopting this configuration, the screw member is positioned at the same circumferential position as the strain sensor, so the surface pressure between the contact surfaces of the first outer ring and the outer ring spacer and the surface pressure between the contact surfaces of the second outer ring and the outer ring spacer can be effectively increased at the same circumferential position as the strain sensor. This makes it possible to effectively reduce hysteresis in the output of the strain sensor.

[Configuration 5]
5. The sensor-equipped bearing device according to configuration 3 or 4, wherein the axial interference is set to 10 μm or more.

In order to solve the above-mentioned problems, a second aspect of the present invention provides a sensor-equipped bearing device having the following configuration.
[Configuration 6]
A plurality of rolling bearings having a non-zero contact angle are arranged in a back-to-back arrangement in two or more rows inside a cylindrical bearing housing,
The plurality of rolling bearings include a first bearing and a second bearing opposed to each other in a back-to-back arrangement,
the first bearing includes a first outer ring, a first inner ring rotatably provided radially inside the first outer ring, and a plurality of first rolling elements assembled between the first outer ring and the first inner ring,
the second bearing includes a second outer ring, a second inner ring rotatably provided radially inside the second outer ring, and a plurality of second rolling elements assembled between the second outer ring and the second inner ring,
a cylindrical outer ring spacer is disposed between the first outer ring and the second outer ring in a state of being sandwiched in the axial direction;
In a bearing device with a sensor, a load sensor is attached to the outer ring spacer to determine a load acting on the outer ring spacer from a strain of the outer ring spacer,
the first outer ring, the second outer ring and the outer ring spacer are each fitted with a clearance fit onto an inner periphery of the bearing housing,
a fitting clearance Δ1 between the outer ring spacer and the bearing housing is larger than an amount of radial expansion of the outer ring spacer due to a load acting on the outer ring spacer, and is also larger than a fitting clearance Δ2 between an outer ring of one of the first bearing and the second bearing, which is subjected to an external force.

　By adopting this configuration, even if the axial force applied to the outer ring spacer increases and the radial expansion deformation of the outer ring spacer increases, the outer circumference of the outer ring spacer is less likely to come into contact with the inner circumference of the bearing housing. Therefore, by using an outer ring spacer that is easily deformed within a practical range, changes in the load acting on the outer ring spacer can be detected stably and with high sensitivity.

[Configuration 7]
A bearing device with a sensor as described in configuration 6, wherein a fitting clearance Δ2 between an outer ring of one of the first bearing and the second bearing, which is subjected to an external axial force, and the bearing housing is 40 μm or less in diameter, and a fitting clearance Δ1 between the outer ring spacer and the bearing housing is Δ2 + 50 μm or less (preferably Δ2 + 30 μm or less) in diameter.

Even if Δ2 is set to 40 μm or less as in the past, if the difference between Δ1 and Δ2 exceeds 50 μm, it becomes difficult to align the outer ring spacer when installing it in a spindle device, etc. In other words, to improve the load measurement accuracy, it is necessary to align the outer ring spacer as accurately as possible, and to make centering easier, the difference between Δ1 and Δ2 should be 50 μm or less, preferably 30 μm or less.

[Configuration 8]
The sensor-equipped bearing device of configuration 6 or 7, wherein the outer ring spacer comprises a metal outer ring that is loosely fitted onto the inner circumference of the bearing housing and that contacts the first outer ring and the second outer ring in the axial direction, and a resin inner ring that is arranged radially inside the outer ring, and the load sensor is attached to the outer ring.

By adopting this configuration, it is possible to position the load sensor in a protected state between the outer and inner rings of the outer ring spacer while suppressing any increase in manufacturing costs of the outer ring spacer.

[Configuration 9]
9. The sensor-equipped bearing device according to any one of configurations 6 to 8, wherein a preload is applied to the first bearing and the second bearing.

[Configuration 10]
10. The sensor-equipped bearing device according to any one of configurations 1 to 9, wherein the first bearing and the second bearing are angular contact ball bearings.

The present invention also provides a machine tool spindle device using the above-mentioned sensor-equipped bearing device, which has the following configuration.
[Configuration 11]
A sensor-equipped bearing device according to any one of configurations 1 to 10,
a main spindle of a machine tool rotatably supported by the sensor-equipped bearing device;
a motor that drives and rotates the main shaft.

By adopting this configuration, it becomes possible to stably monitor the status of machine tools to reduce the number of workers required and to automate the operation. It also becomes possible to detect the processing load acting on the spindle of the machine tool during cutting operations.

In the sensor-equipped bearing device of the first invention, a surface pressure acts between the contact surfaces of the first outer ring and the outer ring spacer, which is equal to the sum of the preload applied to the axial end faces of the first inner ring and the second inner ring and the pressing force applied to the axial end faces of the first outer ring and the second outer ring. Therefore, the frictional force between the contact surfaces of the first outer ring and the outer ring spacer is large, and when the radial component of the force that the first outer ring receives from the first rolling element changes in response to the change in the bearing preload, it is possible to suppress radial slip between the contact surfaces of the first outer ring and the outer ring spacer. Similarly, a surface pressure acts between the contact surfaces of the second outer ring and the outer ring spacer, which is equal to the sum of the preload applied to the axial end faces of the first inner ring and the second inner ring and the pressing force applied to the axial end faces of the first outer ring and the second outer ring. Therefore, the surface pressure between the contact surfaces of the second outer ring and the outer ring spacer is large. Therefore, when the frictional force between the contact surfaces of the second outer ring and the outer ring spacer is large and the radial component of the force that the second outer ring receives from the second rolling element changes in response to changes in the bearing preload, it is possible to suppress radial slippage between the contact surfaces of the second outer ring and the outer ring spacer. As a result, the hysteresis of the output of the strain sensor of the outer ring spacer is reduced, making it possible to detect the bearing preload with high accuracy.

As described above, the sensor-equipped bearing device of the second invention specifies the fit clearance between the two bearings and the outer ring spacer that are each loosely fitted into the housing, so that the outer ring spacer to which the load sensor is attached is unlikely to come into contact with the inner circumference of the housing even if it expands and deforms radially due to an axial force, making it possible to detect load changes stably and with high sensitivity.

FIG. 1 is a cross-sectional view showing a machine tool spindle device using a sensor-equipped bearing device according to a first embodiment of the present invention; FIG. 2 is an enlarged view of the sensor-equipped bearing device of FIG. 1 and its surroundings; 3 is a cross-sectional view taken along line III-III in FIG. 2 . FIG. 3 is a diagram showing a transmission path of an axial preload applied between the first inner ring and the second inner ring shown in FIG. 2 and a transmission path of an axial pressing force applied between the first outer ring and the second outer ring. 1 is a cross-sectional view of a machine tool spindle device using a sensor-equipped bearing device according to a second embodiment of the present invention; FIG. 6 is an enlarged cross-sectional view of a main part of FIG. FIG. 6 is an explanatory diagram of a force transmission path caused by a cutting load applied to the spindle device for a machine tool of FIG. FIG. 6 is an explanatory diagram of a force transmission path caused by a preload applied to the sensor-equipped bearing device of the machine tool spindle device of FIG. 5; FIG. 8 is a schematic diagram illustrating a force transmission mode of a comparative example corresponding to FIG. 7 . FIG. 8 is a schematic diagram illustrating a force transmission mode of the embodiment corresponding to FIG. 7 .

Figure 1 shows a spindle device for a machine tool that uses a sensor-equipped bearing device 1 (hereinafter simply referred to as "bearing device 1") according to a first embodiment of the invention. This spindle device has a main spindle 2 of the machine tool, a main spindle housing (outer cylinder) 3 that accommodates the main spindle 2, a motor 4 that drives and rotates the main spindle 2, a bearing device 1 of the embodiment that rotatably supports the main spindle 2 axially forward of the motor 4 (left side in the figure), and a rear bearing device 5 that rotatably supports the main spindle 2 axially rearward of the motor 4 (right side in the figure).

The spindle housing 3 is formed in a hollow cylinder with both ends open. The spindle housing 3 accommodates the bearing device 1 and the motor 4 in that order from the front to the rear in the axial direction. In the figure, the part of the spindle housing 3 that accommodates the bearing device 1 and the part of the spindle housing 3 that accommodates the motor 4 are formed as a seamless integrated unit, but the part of the spindle housing 3 that accommodates the bearing device 1 and the part of the spindle housing 3 that accommodates the motor 4 may be formed separately and then connected to integrate the two parts.

The spindle 2 is inserted into the spindle housing 3 with the front end of the spindle 2 protruding from the front end opening of the spindle housing 3. A chuck (not shown) for gripping a tool or workpiece is removably attached to the front end of the spindle 2. A through hole 6 is formed axially through the spindle 2 to accommodate a drawbar (not shown) of the machine tool so that it can slide axially.

The motor 4 has a rotor 7 attached to the outer periphery of the main shaft 2 and an annular stator 8 that applies a rotational force to the rotor 7. The rotor 7 has a rotor sleeve 9 that fits onto the outer periphery of the main shaft 2 and a rotor core 10 fixed to the outer periphery of the rotor sleeve 9. The rotor core 10 is, for example, a laminate of electromagnetic steel sheets. The rotor sleeve 9 is prevented from rotating on the main shaft 2 so as to rotate integrally with the main shaft 2. The axial front end of the rotor sleeve 9 contacts a step portion 11 that faces the axial rear side and is formed on the outer periphery of the main shaft 2, and is positioned in the axial direction by the contact with the step portion 11.

The stator 8 has a stator core 12 fixed to the inner circumference of the spindle housing 3, and electromagnetic coils 13 wound around a number of teeth formed at intervals in the circumferential direction on the stator core 12. When current is applied to the electromagnetic coil 13, a rotational force is generated in the rotor core 10 by the electromagnetic force acting between the stator core 12 and the rotor core 10, causing the rotor 7 and the spindle 2 to rotate integrally. Here, an electric motor that generates rotational force using electricity is used as the motor 4, but instead of an electric motor, it is also possible to use a motor that generates rotational force using another power source such as compressed air.

The rear bearing device 5 has an annular bearing support member 14 that is coaxially fixed to the rear end of the spindle housing 3, and a rolling bearing 15 that is assembled into the bearing support member 14. The rolling bearing 15 is a cylindrical roller bearing that has an outer ring 16 that fits into the inner circumference of the bearing support member 14, an inner ring 17 that fits into the outer circumference of the spindle 2, and a number of cylindrical rollers 18 that are assembled between the outer ring 16 and the inner ring 17.

An outer ring pressing member 19 is attached to the bearing support member 14. The outer ring pressing member 19 fixes the axial position of the outer ring 16 by contacting the axial rear end face of the outer ring 16. A nut member 20 that presses the inner ring 17 axially forward and an annular spacer 21 assembled between the inner ring 17 and the nut member 20 are attached to the outer periphery of the main shaft 2. The nut member 20 is threadedly engaged with a male thread 22 formed on the outer periphery of the rear end of the main shaft 2. The axial front end face of the spacer 21 contacts the axial rear end face of the inner ring 17, and the axial rear end face of the spacer 21 contacts the axial front end face of the nut member 20. The axial front end face of the inner ring 17 contacts the axial rear end of the rotor sleeve 9.

The bearing device 1 includes:
The bearing has a cylindrical bearing housing 23 fixed to the spindle housing 3, a first bearing 24 and a second bearing 25 assembled into the bearing housing 23 at an axial distance from each other, an outer ring spacer 26 and an inner ring spacer 27 provided between the first bearing 24 and the second bearing 25, and a strain sensor 28 attached to the outer ring spacer 26.

The bearing housing 23 is fitted to the inner circumference of the spindle housing 3. A cooling groove 29 is formed on the outer circumference of the bearing housing 23, through which a refrigerant for cooling the bearing device 1 flows. The cooling groove 29 is a plurality of annular grooves formed at intervals in the axial direction on the outer circumference of the bearing housing 23, or a spiral groove extending in a spiral shape around the outer circumference of the bearing housing 23. The inner diameter of the bearing housing 23 is larger than the outer diameter of the rotor 7.

As shown in FIG. 2, the first bearing 24 has a first outer ring 30 that fits into the inner circumference of the bearing housing 23, a first inner ring 31 that is rotatably arranged radially inside the first outer ring 30, and a plurality of first rolling elements 32 that are assembled between the first outer ring 30 and the first inner ring 31. The first rolling elements 32 are balls here. The inner circumference of the first outer ring 30 is provided with a first outer ring raceway surface 33 that has an arc-shaped cross section with which the first rolling elements 32 roll and contact. The first outer ring 30 is a shoulder-dropped outer ring that has a shape in which the axially front outer ring shoulder is removed from the outer ring shoulder on the axially front side and the outer ring shoulder on the axially rear side with respect to the first outer ring raceway surface 33. The outer circumference of the first outer ring 30 fits into the inner circumference of the bearing housing 23 with a gap. The outer circumference of the first inner ring 31 is provided with a first inner ring raceway surface 34 that has an arc-shaped cross section with which the first rolling elements 32 roll and contact. The first inner ring 31 is a shoulder-reduced inner ring with a shape in which the axially rear inner ring shoulder is removed from the axially front inner ring shoulder and the axially rear inner ring shoulder with respect to the first inner ring raceway surface 34 with which the first rolling element 32 rolls. The first inner ring 31 is fitted to the outer periphery of the main shaft 2 with a tightening margin.

The second bearing 25 has a second outer ring 35 that fits into the inner circumference of the bearing housing 23, a second inner ring 36 that is rotatably arranged radially inside the second outer ring 35, and a plurality of second rolling elements 37 that are assembled between the second outer ring 35 and the second inner ring 36. The second rolling elements 37 are balls here. The inner circumference of the second outer ring 35 is provided with a second outer ring raceway surface 38 that has an arc-shaped cross section with which the second rolling elements 37 roll and contact. The second outer ring 35 is spaced axially rearward from the first outer ring 30, and the second inner ring 36 is also spaced axially rearward from the first inner ring 31. The second outer ring 35 is a shoulder-dropped outer ring that has a shape in which the axially rear outer ring shoulder is removed from the outer ring shoulder on the axial front side and the outer ring shoulder on the axial rear side relative to the second outer ring raceway surface 38. The outer periphery of the second outer ring 35 is fitted with a gap to the inner periphery of the bearing housing 23. The outer periphery of the second inner ring 36 is provided with a second inner ring raceway 39 with an arc-shaped cross section, with which the second rolling element 37 rolls. The second inner ring 36 is a shoulder-reduced inner ring with a shape in which the axially front inner ring shoulder is removed from the axially front inner ring shoulder and the axially rear inner ring shoulder with respect to the second inner ring raceway 39 with which the second rolling element 37 rolls. The second inner ring 36 is fitted with a tightening margin to the outer periphery of the main shaft 2.

Here, the first bearing 24 is configured so that the first rolling body 32 generates a radial component force pressing the first outer ring 30 under axial preload, and similarly, the second bearing 25 is configured so that the second rolling body 37 generates a radial component force pressing the second outer ring 35 under axial preload. In this embodiment, the first bearing 24 is an angular ball bearing in which the line connecting the contact point of the first inner ring 31 and the first rolling body 32 and the contact point of the first outer ring 30 and the first rolling body 32 is inclined axially backward from the radial inside toward the radial outside. Also, the second bearing 25 is an angular ball bearing in which the line connecting the contact point of the second inner ring 36 and the second rolling body 37 and the contact point of the second outer ring 35 and the second rolling body 37 is inclined axially forward from the radial inside toward the radial outside. In other words, the first bearing 24 and the second bearing 25 are a pair of angular contact ball bearings arranged back-to-back with a gap in the axial direction.

The outer ring spacer 26 is a hollow cylindrical member with both ends open. The outer ring spacer 26 is fitted with a gap around the inner circumference of the bearing housing 23. The outer ring spacer 26 is axially sandwiched between the axial rear end face of the first outer ring 30 (the axial end face of the first outer ring 30 on the side of the second outer ring 35) and the axial front end face of the second outer ring 35 (the axial end face of the second outer ring 35 on the side of the first outer ring 30).

Like the outer ring spacer 26, the inner ring spacer 27 is a hollow cylindrical member with both ends open. The inner ring spacer 27 fits onto the outer periphery of the main shaft 2 with a gap. The inner ring spacer 27 is sandwiched axially between the first inner ring 31 and the second inner ring 36.

As shown in FIG. 4, an annular inner ring positioning step 40 is formed on the outer periphery of the main shaft 2, facing the axial front end face of the first inner ring 31 (the axial end face of the first inner ring 31 opposite the second inner ring 36 side). The inner ring positioning step 40 positions the first inner ring 31 in the axial direction by restricting the movement of the first inner ring 31 forward in the axial direction (in the direction away from the second outer ring 35). In addition, a preload nut 41 is attached to the outer periphery of the main shaft 2, axially rearward of the second inner ring 36. The preload nut 41 is threadedly engaged with a male thread 42 formed on the outer periphery of the main shaft 2. An annular spacer 43 is assembled between the second inner ring 36 and the preload nut 41. The axial front end face of the spacer 43 contacts the axial rear end face of the second inner ring 36, and the axial rear end face of the spacer 43 contacts the axial front end face of the preload nut 41.

The preload nut 41 is tightened with a predetermined force, and the axial force of the preload nut 41 applies a preload to the axial front end face of the first inner ring 31 (the axial end face of the first inner ring 31 opposite the second inner ring 36 side) and the axial rear end face of the second inner ring 36 (the axial end face of the second inner ring 36 opposite the first inner ring 31 side) in a direction that brings the first inner ring 31 and the second inner ring 36 closer together. In other words, as shown by the thick solid line in the figure, the axial force of the preload nut 41 applies an axial preload between the first inner ring 31 and the second inner ring 36 so that the preload is transmitted through the first inner ring 31, the first rolling element 32, the first outer ring 30, the outer ring spacer 26, the second outer ring 35, the second rolling element 37, and the second inner ring 36 in that order.

The strain sensor 28 is attached to the axial center of the outer ring spacer 26. Specifically, the strain sensor 28 is attached to the outer ring spacer 26 so that the strain sensor 28 fits within an area within an axial distance of 1/4 of the total axial length of the outer ring spacer 26 from the axial center position of the outer ring spacer 26.

As shown in FIG. 3, multiple strain sensors 28 (three in this example) are provided at equal intervals in the circumferential direction on the inner circumference of the outer ring spacer 26. Axial grooves 44, the same number as the strain sensors 28, are formed at equal intervals in the circumferential direction on the inner circumference of the outer ring spacer 26. Each of the axial grooves 44 has a planar groove bottom surface parallel to the axis, and a strain sensor 28 is attached to each of the groove bottom surfaces.

As shown in FIG. 4, each strain sensor 28 has a strain detection unit 45 and a processing unit 46 connected to the strain detection unit 45. The strain detection unit 45 is a strain gauge whose electrical resistance changes according to strain. The processing unit 46 has a strain detection circuit that detects strain based on the change in the electrical resistance of the strain gauge, and an AD conversion circuit that converts the strain detected by the detection circuit from an analog signal to a digital signal and outputs it.

Here, the strain detection unit 45 employs an axial strain detection unit (strain gauge arranged with the axial direction as the strain detection direction) that detects the axial strain of the outer ring spacer 26 at the mounting position of the strain sensor 28, and a circumferential strain detection unit (strain gauge arranged with the circumferential direction as the strain detection direction) that detects the circumferential strain of the outer ring spacer 26 at the mounting position of the strain sensor 28. The processing unit 46 is configured to take the difference between the axial strain detected by the axial strain detection unit and the circumferential strain detected by the circumferential strain detection unit (i.e., the sum of the absolute value of the axial strain and the absolute value of the circumferential strain), and to use this difference as the output of the strain sensor 28.

A ring-shaped outer ring positioning step 50 is formed on the inner circumference of the bearing housing 23, axially facing the axial rear end face of the second outer ring 35 (the axial end face of the second outer ring 35 opposite the first outer ring 30 side). The outer ring positioning step 50 positions the second outer ring 35 in the axial direction by restricting movement of the second outer ring 35 axially rearward (away from the first outer ring 30).

An annular cover member 51 is fixed to the axial front end face of the bearing housing 23. The cover member 51 has a cylindrical portion 52 that fits into the inner circumference of the bearing housing 23, and a ring-shaped flange portion 53 that extends radially outward from the axial front end of the cylindrical portion 52.

As shown in Figures 2 and 3, the flange portion 53 is fixed to the axial front end face of the bearing housing 23 by a plurality of screw members 54 arranged at equal intervals in the circumferential direction. Here, the screw members 54 are bolts. The screw members 54 are arranged at the same circumferential position as the strain sensor 28. That is, in Figure 3, the strain sensor 28 is arranged at circumferential positions of 0°, 120°, and 240° clockwise from the upper side of the outer ring spacer 26, and the screw members 54 are arranged at circumferential positions that include all of the circumferential positions of the strain sensor 28 (in the figure, the screw members 54 are arranged at circumferential positions of 0°, 60°, 120°, 180°, 240°, and 300°).

The flange portion 53 has a plurality of through holes 55 formed at equal intervals in the circumferential direction, through which the screw members 54 are inserted. Furthermore, the axial front end face of the bearing housing 23 has a plurality of screw holes 56 formed at equal intervals in the circumferential direction, into which the screw members 54 are screwed. By tightening the screw members 54, the flange portion 53 is pressed against the axial front end face of the bearing housing 23. Furthermore, the axial rear end of the tubular portion 52 is in contact with the axial front end face of the first outer ring 30 (the axial end face of the first outer ring 30 opposite the side of the second outer ring 35).

As shown in FIG. 4, the first outer ring 30 and the second outer ring 35 are assembled between the cover member 51 and the outer ring positioning step 50 with an axial interference, and the axial interference applies a pressing force to the axial front end face of the first outer ring 30 (the axial end face of the first outer ring 30 opposite the second outer ring 35 side) and the axial rear end face of the second outer ring 35 (the axial end face of the second outer ring 35 opposite the first outer ring 30 side) in a direction that brings the first outer ring 30 and the second outer ring 35 closer together.

In other words, when the first outer ring 30, outer ring spacer 26, and second outer ring 35 are removed from the bearing housing 23 shown in Figure 4 and the cover member 51 is fixed to the bearing housing 23, the distance between the axial opposing surfaces of the cover member 51 and the outer ring positioning step 50 is set to be shorter by an amount equivalent to the axial tightening margin than the distance from the axial front end face of the first outer ring 30 to the axial rear end face of the second outer ring 35 when the first outer ring 30, outer ring spacer 26, and second outer ring 35 are removed from the bearing housing 23 and the first outer ring 30, outer ring spacer 26, and second outer ring 35 are aligned in the axial direction without any gaps. Then, the first outer ring 30, the outer ring spacer 26, and the second outer ring 35 are assembled into the bearing housing 23, and then the cover member 51 is tightened in the axial direction with a plurality of screw members 54. This causes the first outer ring 30, the outer ring spacer 26, and the second outer ring 35 to be compressed in the axial direction between the cover member 51 and the outer ring positioning step 50 by the amount of the axial interference. As a result, as shown by the thick dashed line in the figure, an axial pressing force is applied between the first outer ring 30 and the second outer ring 35. The axial interference is set in the range of 10 μm to 50 μm (preferably 20 μm or less).

The magnitude of the axial pressing force applied to the axial end faces of the first outer ring 30 and the second outer ring 35 by the axial interference is significantly greater than the magnitude of the axial preload applied to the axial end faces of the first inner ring 31 and the second inner ring 36 by tightening the preload nut 41.

In other words, the magnitude of the axial preload applied to the axial end faces of the first inner ring 31 and the second inner ring 36 by tightening the preload nut 41 is less than 1 kN (approximately several tens to several hundreds of N). On the other hand, an axial pressing force of 10 kN or more is applied to the axial end faces of the first outer ring 30 and the second outer ring 35 by the axial interference. The magnitude of the axial pressing force applied to the axial end faces of the first outer ring 30 and the second outer ring 35 by the axial interference is set to 10 times or more (preferably 50 times or more) the magnitude of the axial preload applied to the axial end faces of the first inner ring 31 and the second inner ring 36 by tightening the preload nut 41.

In the bearing device 1 shown in FIG. 2, the preload of the first bearing 24 and the second bearing 25 (hereinafter referred to as "bearing preload") changes depending on the rotational speed of the main shaft 2 when the spindle device is in operation. The bearing preload can be detected based on the strain of the outer ring spacer 26 detected by the strain sensor 28.

In other words, when the rotational speed of the main shaft 2 changes during operation of the spindle device, the centrifugal force of the first rolling body 32 and the second rolling body 37 changes, and in accordance with the change in centrifugal force, the load with which the first rolling body 32 presses against the first outer ring raceway surface 33 (preload load of the first bearing 24) and the load with which the second rolling body 37 presses against the first outer ring raceway surface 33 (preload load of the second bearing 25) also change. Here, the first outer ring raceway surface 33 and the second outer ring raceway surface 38 are in contact with the first rolling body 32 and the second rolling body 37 at an angle inclined with respect to the axial direction, so when the load with which the first rolling body 32 presses the first outer ring raceway surface 33 and the load with which the second rolling body 37 presses the first outer ring raceway surface 33 change, the axial preload load applied from the first outer ring 30 and the second outer ring 35 to the outer ring spacer 26 changes, and the strain of the outer ring spacer 26 changes. Therefore, it is possible to detect the preload (bearing preload) of the first bearing 24 and the second bearing 25 based on the strain of the outer ring spacer 26 detected by the strain sensor 28.

In addition, when a moment load acts on the spindle 2, loads of different magnitudes are applied to the positions of the strain sensors 28 arranged at equal intervals in the circumferential direction. Therefore, it is possible to detect the moment load acting on the spindle 2 of the machine tool during cutting processing based on the output of each of the multiple strain sensors 28.

However, when the bearing preload increases, the first outer ring 30 elastically deforms in the radial expansion direction due to the increasing radial component force it receives from the first rolling body 32. At this time, the radial expansion force acting on the first outer ring 30 becomes greater than the maximum static friction force between the contact surfaces of the first outer ring 30 and the outer ring spacer 26. As a result, there is a possibility that minute slippage will occur between the contact surfaces of the first outer ring 30 and the outer ring spacer 26, causing the first outer ring 30 to move radially outward relative to the outer ring spacer 26. On the other hand, when the bearing preload decreases, the first outer ring 30 elastically recovers in the radial contraction direction due to the reduction in the radial component of the force it receives from the first rolling element 32. At this time, the radial contraction force acting on the first outer ring 30 becomes greater than the maximum static friction force between the contact surfaces of the first outer ring 30 and the outer ring spacer 26. This may cause a slight slippage between the contact surfaces of the first outer ring 30 and the outer ring spacer 26, causing the first outer ring 30 to move radially inward relative to the outer ring spacer 26.

Similarly, when the bearing preload increases, the second outer ring 35 elastically deforms in the radial expansion direction due to an increase in the radial component force it receives from the second rolling element 37, and this elastic deformation may cause slight slippage between the contact surfaces of the second outer ring 35 and the outer ring spacer 26, where the second outer ring 35 moves radially outward relative to the outer ring spacer 26. On the other hand, when the bearing preload decreases, the second outer ring 35 elastically restores in the radial contraction direction due to a decrease in the radial component force it receives from the second rolling element 37, and this elastic restoration may cause slight slippage between the contact surfaces of the second outer ring 35 and the outer ring spacer 26, where the second outer ring 35 moves radially inward relative to the outer ring spacer 26.

If the above-mentioned radial slippage occurs between the contact surfaces of the first outer ring 30 and the outer ring spacer 26, or between the contact surfaces of the second outer ring 35 and the outer ring spacer 26, the deformation of the outer ring spacer 26 will not be the same in the process of increasing the bearing preload and in the process of decreasing the bearing preload. As a result, even if the magnitude of the bearing preload is the same, the output of the strain sensor 28 will not be the same in the process of increasing the bearing preload and in the process of decreasing the bearing preload, and hysteresis will occur in which a certain difference occurs between the former and the latter, and this hysteresis will cause an error in the bearing preload detected based on the output of the strain sensor 28.

To address this problem, as shown in Figure 4, in the bearing device 1 of this embodiment, an axial tightening margin is set between the cover member 51 and the outer ring positioning step 50, and a pressing force is applied to the axial end faces of the first outer ring 30 and the second outer ring 35 in a direction that brings the first outer ring 30 and the second outer ring 35 closer together.Therefore, a surface pressure acts between the contact surfaces of the first outer ring 30 and the outer ring spacer 26 on the basis of the sum of the preload applied to the axial end faces of the first inner ring 31 and the second inner ring 36 (thick solid line in the figure) and the pressing force applied to the axial end faces of the first outer ring 30 and the second outer ring 35 (thick dashed line in the figure), resulting in a large surface pressure between the contact surfaces of the first outer ring 30 and the outer ring spacer 26. Therefore, the friction force between the contact surfaces of the first outer ring 30 and the outer ring spacer 26 is large, and it is possible to suppress radial slip between the contact surfaces of the first outer ring 30 and the outer ring spacer 26 when the radial component of force that the first outer ring 30 receives from the first rolling element 32 changes in response to a change in the bearing preload. Similarly, a surface pressure acts between the contact surfaces of the second outer ring 35 and the outer ring spacer 26 on the basis of the sum of the preload (thick solid line in the figure) applied to the axial end faces of the first inner ring 31 and the second inner ring 36 and the pressing force (thick dashed line in the figure) applied to the axial end faces of the first outer ring 30 and the second outer ring 35, so that the surface pressure between the contact surfaces of the second outer ring 35 and the outer ring spacer 26 is large. Therefore, the frictional force between the contact surfaces of the second outer ring 35 and the outer ring spacer 26 is large, and when the radial component of the force that the second outer ring 35 receives from the second rolling element 37 changes in response to a change in the bearing preload, it is possible to suppress radial slippage between the contact surfaces of the second outer ring 35 and the outer ring spacer 26. As a result, the hysteresis of the output of the strain sensor 28 of the outer ring spacer 26 is reduced, making it possible to detect the bearing preload with high accuracy.

In particular, in this bearing device 1, the axial pressing force (thick broken line in the figure) applied to the axial end faces of the first outer ring 30 and the second outer ring 35 is set to be significantly larger, at 10 times or more (preferably 50 times or more) the magnitude of the axial preload (thick solid line in the figure) applied to the axial end faces of the first inner ring 31 and the second inner ring 36 by tightening the preload nut 41, so that the surface pressure between the contact surfaces of the first outer ring 30 and the outer ring spacer 26 and the surface pressure between the contact surfaces of the second outer ring 35 and the outer ring spacer 26 are particularly large. Therefore, the friction force between the contact surfaces of the first outer ring 30 and the outer ring spacer 26 is particularly effectively large, and when the radial component of the force that the first outer ring 30 receives from the first rolling element 32 changes in response to a change in the bearing preload, it is possible to particularly effectively suppress radial slip between the contact surfaces of the first outer ring 30 and the outer ring spacer 26. Similarly, the friction force between the contact surfaces of the second outer ring 35 and the outer ring spacer 26 is particularly effectively large, and when the radial component of the force that the second outer ring 35 receives from the second rolling element 37 changes in response to a change in the bearing preload, it is possible to particularly effectively suppress radial slippage between the contact surfaces of the second outer ring 35 and the outer ring spacer 26.

In addition, this bearing device 1 utilizes the axial interference as a method for applying a pressing force to the axial end faces of the first outer ring 30 and the second outer ring 35, so it is possible to apply a large pressing force to the axial end faces of the first outer ring 30 and the second outer ring 35 by simply tightening the screw member 54.

In addition, as shown in FIG. 3, in this bearing device 1, the screw member 54 is disposed at the same circumferential position as the strain sensor 28, so that the surface pressure between the contact surfaces of the first outer ring 30 and the outer ring spacer 26 and the surface pressure between the contact surfaces of the second outer ring 35 and the outer ring spacer 26 shown in FIG. 4 can be effectively increased at the same circumferential position as the strain sensor 28. This makes it possible to effectively reduce the hysteresis in the output of the strain sensor 28.

The axial clamping margin can be set by applying a pressing force to at least one of the axial end faces of the first outer ring 30 or the second outer ring 35 using a test machine or the like that can load and measure a specified pressing amount (μm) and pressing force (N), and deriving the relationship between the pressing amount (μm) and pressing force (N).

In the above embodiment, the strain sensor 28 is disposed on the inner circumference of the outer ring spacer 26, but the strain sensor 28 may be disposed on the outer circumference of the outer ring spacer 26.

In the above embodiment, angular contact ball bearings have been used as the first bearing 24 and the second bearing 25, but it is also possible to use other types of rolling bearings, such as tapered roller bearings or deep groove ball bearings, as the first bearing 24 and the second bearing 25, which generate a radial component force due to an axial preload.

In addition, in the above embodiment, the strain sensor 28 is exemplified as having a strain detection unit 45 and a processing unit 46, but the strain sensor 28 can also be configured with only the strain detection unit 45 (only a strain gauge).

FIG. 5 shows a spindle device for a machine tool that uses a bearing device 1 according to the second embodiment of the invention. In the following, parts corresponding to the first embodiment of the invention are given the same reference numerals and their explanations are omitted. Parts given the same reference numerals are basically configured the same as the first embodiment of the invention.

As shown in Figures 5 and 6, the bearing device 1 has a bearing housing 23 fixed to the spindle housing 3, a first bearing 24 fitted to the inner periphery of the bearing housing 23, a second bearing 25 fitted to the inner periphery of the bearing housing 23 axially rearward of the first bearing 24, and an outer ring spacer 26 and an inner ring spacer 27 sandwiched axially between the first bearing 24 and the second bearing 25, and the first bearing 24 and the second bearing 25 support the spindle 2.

The first bearing 24 is an angular ball bearing having a non-rotating first outer ring 30 that is loosely fitted to the inner circumference of the bearing housing 23, a first inner ring 31 that is rotatably arranged radially inside the first outer ring 30, and a plurality of first rolling elements (here, balls) 32 assembled between the first outer ring 30 and the first inner ring 31. The inner circumference of the first outer ring 30 is provided with a first outer ring raceway surface 33 that has an arc-shaped cross section with which the first rolling elements 32 roll and contact, and the outer circumference of the first inner ring 31 is provided with a first inner ring raceway surface 34 that has an arc-shaped cross section with which the first rolling elements 32 roll and contact. The first outer ring 30 is a shoulder-dropped outer ring that has a shape in which the axially front outer ring shoulder is removed from the axially front outer ring shoulder and the axially rear outer ring shoulder relative to the first outer ring raceway surface 33. In addition, the first inner ring 31 is a shoulder-reduced inner ring in which the axially rear inner ring shoulder is removed from the axially front inner ring shoulder and the axially rear inner ring shoulder relative to the first inner ring raceway surface 34.

The second bearing 25 is an angular ball bearing having a non-rotating second outer ring 35 that is spaced axially rearward from the first outer ring 30 and is loosely fitted to the inner circumference of the bearing housing 23, a second inner ring 36 that is rotatably provided radially inside the second outer ring 35, and a plurality of second rolling elements (here, balls) 37 that are assembled between the second outer ring 35 and the second inner ring 36. The inner circumference of the second outer ring 35 is provided with a second outer ring raceway surface 38 that has an arc-shaped cross section with which the second rolling elements 37 roll and contact, and the outer circumference of the second inner ring 36 is provided with a second inner ring raceway surface 39 that has an arc-shaped cross section with which the second rolling elements 37 roll and contact. The second outer ring 35 is a shoulder-dropped outer ring that has a shape in which the axially rear outer ring shoulder is removed from the outer ring shoulder on the axial front side and the outer ring shoulder on the axial rear side relative to the second outer ring raceway surface 38. The second inner ring 36 is a shoulder-reduced inner ring in which the axially front inner ring shoulder is removed from the axially rear inner ring shoulder relative to the second inner ring raceway surface 39.

Here, in the first bearing 24, a straight line connecting the contact point of the first inner ring 31 and the first rolling element 32 and the contact point of the first outer ring 30 and the first rolling element 32 is inclined axially backward from the radially inner side toward the radially outer side. On the other hand, in the second bearing 25, a straight line connecting the contact point of the second inner ring 36 and the second rolling element 37 and the contact point of the second outer ring 35 and the second rolling element 37 is inclined axially forward from the radially inner side toward the radially outer side. In other words, the first bearing 24 and the second bearing 25 are arranged in a back-to-back arrangement.

The outer ring spacer 26 consists of a metal outer ring 26a that fits loosely around the inner circumference of the bearing housing 23, and a resin inner ring 26b that is disposed radially inward of the outer ring 26a. The outer ring 26a is formed in a hollow cylindrical shape with both ends open. Meanwhile, the inner ring 26b is formed in an L-shaped cross section with an outward flange portion at the axial rear end of the hollow cylindrical portion that is open at both ends, and the flange portion is fixed to the inner peripheral surface of the outer ring 26a. The fixing method can be a method of pressing the flange portion of the inner ring 26b into the inner peripheral surface of the outer ring 26a or a method of gluing, or these methods can be used in combination.

The outer ring 26a is formed so that its radial thickness is approximately the same as the thickness of the first outer ring 30 and the second outer ring 35, and its axial dimension is larger than the axial dimension of the inner ring 26b. As a result, the outer ring 26a's axial front end contacts the axial rear end face of the first outer ring 30, and its axial rear end contacts the axial front end face of the second outer ring 35, but the inner ring 26b does not contact the first outer ring 30 or the second outer ring 35.

Then, multiple load sensors 28 (preferably three or more, here four) are attached at equal intervals in the circumferential direction to the axial center of the inner peripheral surface of the outer ring 26a. The load sensor 28 is a printed circuit board 28a fixed to the outer ring 26a with adhesive or the like, and is equipped with a strain gauge 28b that detects the strain of the outer ring 26a and a processing circuit 28c that determines the load acting on the outer ring 26a from the strain detected by the strain gauge 28b. The space between the cylindrical parts of the outer ring 26a and the inner ring 26b (the space around the load sensor 28) is filled with a sealant 47 such as a resin material. This ensures that the load sensor 28 is fixed securely in a protected state with guaranteed insulation.

On the other hand, the inner ring spacer 27, like the outer ring 26a of the outer ring spacer 26, is formed as a hollow cylinder with both ends open, with its axial front end contacting the axial rear end face of the first inner ring 31 and its axial rear end contacting the axial front end face of the second inner ring 36.

An outer ring pressing member 60 is fixed to the axial front end of the spindle housing 3, which fixes the axial position of the first outer ring 30 by contacting the axial front end face of the first outer ring 30. The outer ring pressing member 60 has a cylindrical portion 61 that fits into the inner circumference of the bearing housing 23, and a flange portion 62 that extends radially outward from the axial front end of the cylindrical portion 61. The flange portion 62 is fixed to the axial front end face of the bearing housing 23. In addition, a step portion 63 that contacts the axial front end face of the first inner ring 31 is formed on the outer periphery of the axial front end of the spindle 2. The first inner ring 31 is positioned in the axial direction by contacting this step portion 63.

A preload nut 64 that presses the second inner ring 36 axially forward is attached to the outer periphery of the spindle 2, and an annular spacer 65 is fitted between the second inner ring 36 and the preload nut 64. The preload nut 64 is threadedly engaged with a male thread 66 formed on a portion extending axially forward from a stepped portion 11 (see FIG. 5) on the outer periphery of the spindle 2. The spacer 65 has its axial front end in contact with the axial rear end face of the second inner ring 36, and its axial rear end in contact with the axial front end face of the preload nut 64. A stepped portion 67 that contacts the axial rear end face of the second outer ring 35 is formed on the inner periphery of the bearing housing 23. The second outer ring 35 is positioned in the axial direction by contact with this stepped portion 67.

The bearing housing 23 has a cylindrical portion 68 that fits into the inner circumference of the spindle housing 3, and a flange portion 69 that extends radially outward from the axial front end of the cylindrical portion 68. A cooling groove 70 is formed on the outer circumference of the cylindrical portion 68, through which a refrigerant for cooling the bearing device 1 flows. The cooling groove 70 is a plurality of annular grooves formed at intervals in the axial direction on the outer circumference of the cylindrical portion 68, or a spiral groove that extends spirally around the outer circumference of the cylindrical portion 68. The flange portion 69 is fixed in contact with the axial front end of the spindle housing 3.

This spindle device for machine tools has the above-mentioned configuration. When an axial force (cutting load) is applied to the main shaft 2 by cutting, the axial force is transmitted in the order of the first inner ring 31, the first rolling element 32, the first outer ring 30, the outer ring 26a of the outer ring spacer 26, and the second outer ring 35 of the bearing device 1, as shown in FIG. 7, and is received by the step portion 67 of the bearing housing 23. At this time, the outer ring 26a of the outer ring spacer 26 that receives the axial force is compressed between the first outer ring 30 and the second outer ring 35 and undergoes radial expansion deformation. Therefore, the load sensors 28 attached at equal intervals around the outer ring 26a can detect the load acting on the outer ring 26a, that is, the cutting load acting on the main shaft 2, from the strain at each circumferential position of the outer ring 26a.

When the bearing device 1 is assembled, the axial force is transmitted in sequence to the spacer 65, second inner ring 36, second rolling element 37, second outer ring 35, outer ring 26a of the outer ring spacer 26, first outer ring 30, first rolling element 32, and first inner ring 31 by tightening the preload nut 64, as shown in FIG. 8, and is received by the stepped portion 63 of the main shaft 2, so that a preload is applied to the first bearing 24 and the second bearing 25. The load acting on the outer ring 26a at this time, i.e., the preload load, can also be detected by the load sensor 28.

Here, in the bearing device 1 of this embodiment, the fit clearance Δ1 between the outer ring 26a of the outer ring spacer 26 and the bearing housing 23 is set to be larger than the amount of radial expansion of the outer ring 26a due to the load acting on the outer ring 26a, and larger than the fit clearance Δ2 between the first outer ring 30 and the second outer ring 35 and the bearing housing 23 (here, the fit clearances of the first outer ring 30 and the second outer ring 35 are set to be the same). Also, Δ2 is set to 40 μm or less in diameter as in the conventional case, but the difference between Δ1 and Δ2 is set to 50 μm or less in diameter (preferably 30 μm or less). If the difference between Δ1 and Δ2 exceeds 50 μm in diameter, it becomes difficult to align the outer ring spacer 26 when assembling the bearing device 1, and the measurement accuracy of the load decreases.

By setting the fit clearances Δ1 and Δ2 between the outer ring 26a, first outer ring 30, and second outer ring 35 of the outer ring spacer 26 and the bearing housing 23 as described above, it is possible to detect changes in the cutting load and preload acting on the outer ring 26a stably and with high sensitivity. This will be explained below with reference to Figures 9 (Comparative Example) and 10. Note that in Figures 9 and 10, the gaps between the outer ring 26a and the first outer ring 30 and the bearing housing 23 are exaggerated.

First, when a cutting load is applied to the spindle 2, the force transmission path is basically as shown in Figure 7. However, in this case, as a comparative example, if the fit clearance Δ1 between the outer ring 26a of the outer ring spacer 26 and the bearing housing 23 is set smaller than the above-mentioned setting range, as shown in Figure 9, the outer ring 26a of the outer ring spacer 26 will come into contact with the inner circumference of the bearing housing 23 before the first outer ring 30, which has expanded in diameter due to the radial component of the force transmitted from the first rolling element 32, comes into contact with the inner circumference of the bearing housing 23, causing the deformation mode of the outer ring 26a to change, which may reduce the sensitivity of detecting load changes.

In contrast, if Δ1 and Δ2 are set as described above, when a cutting load is applied to the spindle 2, as shown in FIG. 10, the first outer ring 30, which has expanded in diameter due to the radial component of the force transmitted from the first rolling element 32, contacts the inner circumference of the bearing housing 23 before the outer ring 26a of the outer ring spacer 26, so that the outer ring 26a is less likely to contact the inner circumference of the bearing housing 23. Therefore, by using an outer ring 26a that is easily deformed within a practical range, changes in the cutting load can be detected stably and with high sensitivity, regardless of the level of the cutting load acting on the outer ring 26a.

Here, the outside diameter of the outer ring 26a of the outer ring spacer 26 and the outside diameters of the first outer ring 30 and the second outer ring 35 can be measured using a dial gauge or a micrometer, etc. The inside diameter of the bearing housing 23 can be measured using a cylinder gauge, an inside micrometer, etc. The fit clearances Δ1 and Δ2 between the outer ring 26a, the first outer ring 30, and the second outer ring 35 of the outer ring spacer 26 and the bearing housing 23 can be derived from the difference between the measured outside diameter and inside diameter. The amount of radial expansion of the outer ring spacer 26 can be estimated by measuring the amount of displacement of the outer diameter side of the outer ring spacer 26 when an axial pressing force is applied to the outer ring spacer 26 using a dial gauge or a micrometer, etc., and deriving the relationship between the pressing force and the amount of radial expansion of the outer ring spacer 26.

Similarly, when preload is applied to the first bearing 24 and the second bearing 25 by tightening the preload nut 64 of the bearing device 1, the second outer ring 35, which has been expanded in diameter by the radial component of the force transmitted from the second rolling element 37, contacts the inner circumference of the bearing housing 23 before the outer ring 26a, and the outer ring 26a is less likely to contact the inner circumference of the bearing housing 23, so that changes in the preload can be detected stably and with high sensitivity.

Therefore, this spindle device for machine tools can quickly respond to cases where the cutting load increases suddenly or where the preload increases due to heat generation in the first bearing 24 and the second bearing 25, etc., and can improve the reliability of condition monitoring. In addition, the preload can be set efficiently and accurately when assembling the bearing device 1.

The embodiments disclosed herein should be considered in all respects as illustrative and not restrictive. The scope of the invention is indicated by the claims rather than the above description, and is intended to include all modifications within the meaning and scope of the claims.

For example, in the embodiment, angular contact ball bearings are used for the first and second bearings that make up the bearing device, but any rolling bearing with a non-zero contact angle, such as a tapered roller bearing, may be used. In addition, the outer ring spacer is not limited to a double structure with an outer ring and an inner ring as in the embodiment, but may be a single cylinder with a load sensor attached to its inner or outer circumferential surface.

In addition, in the embodiment, since both the first bearing and the second bearing are subjected to an external axial force, the fit clearance between the outer ring spacer and the bearing housing is made larger than the fit clearance between the outer rings of both bearings and the bearing housing. However, if only one of the first bearing and the second bearing is assembled so that an external axial force is applied to it, the fit clearance between the outer ring spacer and the bearing housing can be made larger than the fit clearance between that one bearing and the bearing housing.

In addition, in each of the above embodiments, the bearing device 1 that rotatably supports the spindle 2 of a machine tool (such as a machining center or a lathe) has been described as an example, but the present invention can also be applied to a bearing device that rotatably supports the rotating shaft of other devices, such as the spindle of a wind power generation device.

Reference Signs List 1 Sensor-equipped bearing device 2 Spindle 3 Spindle housing 4 Motor 23 Bearing housing 24 First bearing 25 Second bearing 26 Outer ring spacer 26a Outer ring 26b Inner ring 28 Strain sensor, load sensor 30 First outer ring 31 First inner ring 32 First rolling element 35 Second outer ring 36 Second inner ring 37 Second rolling element 50 Outer ring positioning step 51 Cover member 54 Screw member

Claims (11)

1. The bearing has a first bearing (24) and a second bearing (25) that are spaced apart in the axial direction, a cylindrical outer ring spacer (26) provided between the first bearing (24) and the second bearing (25), and a strain sensor (28) attached to the outer ring spacer (26),
   The first bearing (24) has a first outer ring (30), a first inner ring (31) provided radially inside the first outer ring (30), and a plurality of first rolling elements (32) assembled between the first outer ring (30) and the first inner ring (31),
   The second bearing (25) has a second outer ring (35), a second inner ring (36) provided radially inside the second outer ring (35), and a plurality of second rolling elements (37) assembled between the second outer ring (35) and the second inner ring (36),
   a preload is applied to an axial end face of the first inner ring (31) opposite to the second inner ring (36) side and to an axial end face of the second inner ring (36) opposite to the first inner ring (31) side, in a direction to bring the first inner ring (31) and the second inner ring (36) closer to each other;
   In the sensor-equipped bearing device, the preload is transmitted through the first inner ring (31), the first rolling element (32), the first outer ring (30), the outer ring spacer (26), the second outer ring (35), the second rolling element (37), and the second inner ring (36),
   a pressing force in a direction to bring the first outer ring (30) and the second outer ring (35) closer to each other is applied to an axial end face of the first outer ring (30) opposite to the second outer ring (35) and an axial end face of the second outer ring (35) opposite to the first outer ring (30);
   A sensor-equipped bearing device, characterized in that the pressing force is set to be greater than the preload.
2. The sensor-equipped bearing device according to claim 1, wherein the magnitude of the pressing force is set to 10 times or more the preload.
3. The sensor-equipped bearing device according to claim 1 or 2, in which the pressing force is applied by assembling the first outer ring (30) and the second outer ring (35) with an axial tightening margin between an annular outer ring positioning step (50) provided on the inner circumference of a cylindrical bearing housing (23) that fits onto the outer circumference of the first outer ring (30) and the outer circumference of the second outer ring (35), and a cover member (51) fixed to the axial end face of the bearing housing (23) by a screw member (54).
4. The sensor-equipped bearing device according to claim 3, wherein the screw member (54) is disposed at the same circumferential position as the strain sensor (28).
5. The sensor-equipped bearing device according to claim 3 or 4, in which the axial interference is set to 10 μm or more.
6. A plurality of rolling bearings having a non-zero contact angle are arranged in a back-to-back arrangement in two or more rows inside a cylindrical bearing housing (23);
   The plurality of rolling bearings include a first bearing (24) and a second bearing (25) opposed to each other in a back-to-back arrangement,
   The first bearing (24) has a first outer ring (30), a first inner ring (31) rotatably provided radially inside the first outer ring (30), and a plurality of first rolling elements (32) assembled between the first outer ring (30) and the first inner ring (31),
   The second bearing (25) has a second outer ring (35), a second inner ring (36) rotatably provided radially inside the second outer ring (35), and a plurality of second rolling elements (37) assembled between the second outer ring (35) and the second inner ring (36),
   A cylindrical outer ring spacer (26) is disposed between the first outer ring (30) and the second outer ring (35) in a state of being sandwiched in the axial direction,
   In the sensor-equipped bearing device, a load sensor (28) is attached to the outer ring spacer (26) for determining a load acting on the outer ring spacer (26) from a strain of the outer ring spacer (26),
   the first outer ring (30), the second outer ring (35) and the outer ring spacer (26) are each fitted with a clearance fit onto the inner periphery of the bearing housing (23);
   A bearing device with a sensor, characterized in that a fitting clearance Δ1 between the outer ring spacer (26) and the bearing housing (23) is larger than an amount of radial expansion of the outer ring spacer (26) due to a load acting on the outer ring spacer (26), and is also larger than a fitting clearance Δ2 between an outer ring (30, 35) of one of the first bearing (24) and the second bearing (25) to which an external force is applied, and the bearing housing (23).
7. The sensor-equipped bearing device according to claim 6, wherein the fitting clearance Δ2 between the outer ring (30, 35) of the bearing to which an external axial force is applied between the first bearing (24) and the second bearing (25) and the bearing housing (23) is 40 μm or less in diameter, and the fitting clearance Δ1 between the outer ring spacer (26) and the bearing housing (23) is Δ2 + 50 μm or less in diameter.
8. The sensor-equipped bearing device according to claim 6 or 7, wherein the outer ring spacer (26) is comprised of a metal outer ring (26a) that is loosely fitted around the inner circumference of the bearing housing (23) and that contacts the first outer ring (30) and the second outer ring (35) in the axial direction, and a resin inner ring (26b) that is disposed radially inside the outer ring (26a), and the load sensor (28) is attached to the outer ring (26a).
9. A sensor-equipped bearing device according to any one of claims 6 to 8, in which a preload is applied to the first bearing (24) and the second bearing (25).
10. A sensor-equipped bearing device according to any one of claims 1 to 9, wherein the first bearing (24) and the second bearing (25) are angular ball bearings.
11. A sensor-equipped bearing device (1) according to any one of claims 1 to 10,
    a main spindle (2) of a machine tool rotatably supported by the sensor-equipped bearing device (1);
    and a motor (4) for rotating the main shaft (2).

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