Bearing device with sensor and spindle device for machine tool

Abstract

A bearing device with a sensor is configured so as to transmit preload on a first inner ring (31), a first rolling body (32), a first outer ring (30), an outer ring spacer (26), a second outer ring (35), a second rolling body (37), and a second inner ring (36), a pressing force in a direction in which the first outer ring (30) and the second outer ring (35) approach each other is applied to an axial end surface of the first outer ring (30) on the opposite side to the second outer ring (35) side and an axial end surface of the second outer ring (35) on the opposite side to the first outer ring (30) side, and the magnitude of the pressing force is set to be greater than the preload.

Description

Technical Field

[0001] The present invention relates to a bearing device with a sensor and a spindle device for a machine tool using the bearing device with a sensor. Background Art

[0002] In machine tools such as machining centers and lathes, and other industrial machines, a spindle device is used that supports a rotating shaft (spindle) for mounting an object such as a tool or a workpiece so as to be rotatable. In the field of use of such a spindle device, in recent years, there has been a demand for strengthening the condition monitoring function for labor saving and unmanned operation.

[0003] Therefore, in order to meet the demand for strengthening the condition monitoring function, the applicant of the present application has proposed a bearing device with a sensor (Patent Document 1).

[0004] The bearing device with a sensor of Patent Document 1 includes: a first bearing and a second bearing, which are arranged at an axial interval; a cylindrical outer ring spacer, which is provided between the first bearing and the second bearing; and a strain sensor, which is mounted on the outer ring spacer.

[0005] The first bearing includes: a first outer ring; a first inner ring, which is provided radially inside the first outer ring; and a plurality of first rolling elements, which are assembled between the first outer ring and the first inner ring. Similarly, the second bearing includes: a second outer ring; a second inner ring, which is provided radially inside the second outer ring; and a plurality of second rolling elements, which are assembled between the second outer ring and the second inner ring. The outer ring spacer is arranged to be axially sandwiched between the first outer ring and the second outer ring.

[0006] By tightening a preload nut, a preload in a direction in which the first inner ring and the second inner ring approach each other is applied to the axial end face of the first inner ring on the side opposite to the second inner ring side and the axial end face of the second inner ring on the side opposite to the first inner ring side. The first bearing and the second bearing are configured such that this preload is transmitted through the first inner ring, the first rolling elements, the first outer ring, the outer ring spacer, the second outer ring, the second rolling elements, and the second inner ring. The first bearing is an angular contact ball bearing configured to generate a radial component force of the first rolling elements pressing the first outer ring through the axial preload between the first outer ring and the first inner ring, and the second bearing is also an angular contact ball bearing configured to generate a radial component force of the second rolling elements pressing the second outer ring through the axial preload between the second outer ring and the second inner ring.

[0007] The strain sensor is mounted on the outer periphery (or inner periphery) of the outer ring spacer, and can detect the preload of the first bearing and the second bearing (hereinafter referred to as "bearing preload") based on the output of the strain sensor.

[0008] Patent Document 1: Japanese Unexamined Patent Application Publication No. 2021-014886

[0009] However, in the bearing device with a sensor in Patent Document 1, sometimes an error occurs in the magnitude of the bearing preload detected based on the output of the strain sensor of the outer ring spacer. If this error can be eliminated to detect the bearing preload with high precision, the reliability of condition monitoring for labor saving and unmanned operation can be improved.

[0010] Therefore, the problem to be solved by the first invention is to provide a bearing device with a sensor that can detect the bearing preload with high precision.

[0011] In addition, the bearing device of Patent Document 1 supports the main shaft of the main shaft device. A first bearing and a second bearing are arranged in a back-to-back combination inside a cylindrical bearing housing. The first bearing and the second bearing are respectively composed of angular contact ball bearings having an outer ring, an inner ring rotatably provided on the radial inner side of the outer ring, and a plurality of rolling elements assembled between the outer ring and the inner ring. A strain sensor is installed on the outer ring spacer assembled between the outer ring of the first bearing and the outer ring of the second bearing.

[0012] Moreover, by tightening the preload nut threadedly engaged with the outer periphery of the main shaft supported by the two bearings, the axial force of the preload nut is sequentially transmitted to the second bearing, the outer ring spacer, and the first bearing to apply preload to the two bearings, and the axial preload load is obtained based on the output of the strain sensor installed on the outer ring spacer.

[0013] Therefore, the main shaft device using this bearing device can detect an increase in the preload load caused by heat generation or the like of each bearing supporting the main shaft, and the condition monitoring function is strengthened. In addition, when this main shaft device is assembled in a machine tool, the change in the cutting load is also detected, so the machining state can also be monitored.

[0014] In the bearing device of the above Patent Document 1, the load acting on the outer ring spacer is obtained based on the strain caused by the deformation of the outer ring spacer subjected to an axial force. Therefore, in order to detect the change in the load with high sensitivity, it is desirable to make the outer ring spacer easily deformable within the practical range.

[0015] However, in the case of adopting an outer ring spacer that is easily deformable, when an excessive load acts, the outer periphery of the expanded and deformed outer ring spacer comes into contact with the inner periphery of the bearing housing, and thus the deformation mode of the outer ring spacer changes, and conversely, the sensitivity for detecting the change in the load becomes low.

[0016] Therefore, the problem to be solved by the second invention is to provide a bearing device with a sensor that can stably detect the change in the load with high sensitivity. Summary of the Invention

[0017] The inventors of the present application conducted an evaluation test in the bearing device with a sensor in Patent Document 1 mentioned above, in which the bearing preload was changed and the bearing preload was detected based on the output of the strain sensor of the outer ring spacer. It was found that: during the process of increasing the bearing preload and the process of decreasing the bearing preload, even if the magnitude of the bearing preload is the same, the output of the strain sensor is not the same, and there is a certain difference, namely hysteresis, between the former and the latter. This hysteresis becomes the reason for the error of the bearing preload detected based on the output of the strain sensor.

[0018] The reason for the above-mentioned hysteresis is considered as follows.

[0019] When the bearing preload increases, the first outer ring elastically deforms in the diameter-expanding direction due to the increased radial component force received from the first rolling element. Due to this elastic deformation, a small sliding occurs between the contact surfaces of the first outer ring and the outer ring spacer, where the first outer ring moves relatively outward in the radial direction with respect to the outer ring spacer. On the other hand, when the bearing preload decreases, the first outer ring elastically recovers in the diameter-reducing direction due to the decreased radial component force received from the first rolling element. Due to this elastic recovery, a small sliding occurs between the contact surfaces of the first outer ring and the outer ring spacer, where the first outer ring moves relatively inward in the radial direction with respect to the outer ring spacer.

[0020] Similarly, when the bearing preload increases, the second outer ring elastically deforms in the diameter-expanding direction due to the increased radial component force received from the second rolling element. Due to this elastic deformation, a small sliding occurs between the contact surfaces of the second outer ring and the outer ring spacer, where the second outer ring moves relatively outward in the radial direction with respect to the outer ring spacer. On the other hand, when the bearing preload decreases, the second outer ring elastically recovers in the diameter-reducing direction due to the decreased radial component force received from the second rolling element. Due to this elastic recovery, a small sliding occurs between the contact surfaces of the second outer ring and the outer ring spacer, where the second outer ring moves relatively inward in the radial direction with respect to the outer ring spacer.

[0021] Moreover, if the above-mentioned radial sliding occurs between the contact surfaces of the first outer ring and the outer ring spacer and between the contact surfaces of the second outer ring and the outer ring spacer, the deformation of the outer ring spacer is not the same during the process of increasing the bearing preload and the process of decreasing the bearing preload. Therefore, during the process of increasing the bearing preload and the process of decreasing the bearing preload, even if the magnitude of the bearing preload is the same, the output of the strain sensor is not the same, and it is considered that there is a certain difference, namely hysteresis, between the former and the latter. This hysteresis of the output of the strain sensor becomes the reason for the error of the bearing preload detected based on the output of the strain sensor.

[0022] Moreover, the inventors of the present application have come up with the following idea: if the surface pressure between the first outer ring and the outer ring spacer is increased, the frictional force between the first outer ring and the outer ring spacer becomes larger. Therefore, when the radial component force received by the first outer ring from the first rolling elements changes according to the change in bearing preload, the radial sliding between the contact surfaces of the first outer ring and the outer ring spacer can be suppressed. Similarly, if the surface pressure between the second outer ring and the outer ring spacer is increased, the frictional force between the second outer ring and the outer ring spacer becomes larger. Therefore, when the radial component force received by the second outer ring from the second rolling elements changes according to the change in bearing preload, the radial sliding between the contact surfaces of the second outer ring and the outer ring spacer can be suppressed. As a result, the hysteresis of the output of the strain sensor of the outer ring spacer can be reduced, and the error caused by the hysteresis can be suppressed to a small value.

[0023] Based on this idea, in order to solve the above problems, the first invention provides a bearing device with a sensor having the following structure.

[0024] [Structure 1]

[0025] A bearing device with a sensor, configured to include: a first bearing and a second bearing, arranged at an axial interval; a cylindrical outer ring spacer, provided between the first bearing and the second bearing; and a strain sensor, mounted on the outer ring spacer.

[0026] 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.

[0027] 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.

[0028] Preload is applied to the axial end face of the first inner ring on the side opposite to the second inner ring side and the axial end face of the second inner ring on the side opposite to the first inner ring side in a direction to bring the first inner ring and the second inner ring closer.

[0029] The preload is transmitted through the first inner ring, the first rolling elements, the first outer ring, the outer ring spacer, the second outer ring, the second rolling elements, and the second inner ring.

[0030] The bearing device with a sensor is characterized in that

[0031] A pressing force is applied to the axial end face of the first outer ring on the side opposite to the second outer ring side and the axial end face of the second outer ring on the side opposite to the first outer ring side in a direction to bring the first outer ring and the second outer ring closer.

[0032] The magnitude of the pressing force is set to be greater than the inner ring preload described above.

[0033] If this structure is adopted, a surface pressure that is the sum of the preload applied to the axial end faces of the first inner ring and the second inner ring and the magnitude of 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. Therefore, the surface pressure between the contact surfaces of the first outer ring and the outer ring spacer increases. As a result, the frictional force between the contact surfaces of the first outer ring and the outer ring spacer increases, and when the radial component force received by the first outer ring from the first rolling element changes according to the change in the bearing preload, radial sliding between the contact surfaces of the first outer ring and the outer ring spacer can be suppressed. Similarly, a surface pressure that is the sum of the preload applied to the axial end faces of the first inner ring and the second inner ring and the magnitude of 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. Therefore, the surface pressure between the contact surfaces of the second outer ring and the outer ring spacer increases. As a result, the frictional force between the contact surfaces of the second outer ring and the outer ring spacer increases, and when the radial component force received by the second outer ring from the second rolling element changes according to the change in the bearing preload, radial sliding between the contact surfaces of the second outer ring and the outer ring spacer can be suppressed. As a result, the hysteresis of the output of the strain sensor of the outer ring spacer can be reduced, and the bearing preload can be detected with high precision.

[0034] [Structure 2]

[0035] In the bearing device with a sensor described in Structure 1, the magnitude of the pressing force is set to be 10 times or more the magnitude of the preload.

[0036] If this structure is adopted, a pressing force significantly greater than the preload is 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 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 become particularly large. As a result, the frictional force between the contact surfaces of the first outer ring and the outer ring spacer increases particularly effectively, and when the radial component force received by the first outer ring from the first rolling element changes according to the change in the bearing preload, radial sliding between the contact surfaces of the first outer ring and the outer ring spacer can be particularly effectively suppressed. Similarly, the frictional force between the contact surfaces of the second outer ring and the outer ring spacer increases particularly effectively, and when the radial component force received by the second outer ring from the second rolling element changes according to the change in the bearing preload, radial sliding between the contact surfaces of the second outer ring and the outer ring spacer can be particularly effectively suppressed.

[0037] [Structure 3]

[0038] The bearing device with a sensor according to Structure 1 or 2, wherein the pressing force is applied by assembling the first outer ring and the second outer ring with an axial interference amount between a ring-shaped outer ring positioning step portion and a cover member, the outer ring positioning step portion is provided on the inner circumference of a cylindrical bearing housing that fits with the outer circumferences of the first outer ring and the second outer ring, and the cover member is fixed to the axial end surface of the bearing housing by a threaded member.

[0039] If this structure is adopted, a large pressing force can be applied to the axial end surfaces of the first outer ring and the second outer ring by a simple method of tightening the threaded member.

[0040] [Structure 4]

[0041] The bearing device with a sensor according to Structure 3, wherein the threaded member is arranged at the same circumferential position as the strain sensor.

[0042] If this structure is adopted, since the threaded member is arranged at the same circumferential position as the strain sensor, 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. Therefore, the hysteresis of the output of the strain sensor can be effectively reduced.

[0043] [Structure 5]

[0044] The bearing device with a sensor according to Structure 3 or 4, wherein the axial interference amount is set to be 10 μm or more.

[0045] In addition, in order to solve the above problems, the second invention provides a bearing device with a sensor having the following structure.

[0046] [Structure 6]

[0047] A bearing device with a sensor, in which a plurality of rolling bearings with a contact angle not equal to 0 are arranged in a back-to-back combination of two or more rows on the inner side of a cylindrical bearing housing.

[0048] The plurality of rolling bearings include a first bearing and a second bearing that are opposed in a back-to-back combination.

[0049] The first bearing has: 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.

[0050] The second bearing has: 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.

[0051] A cylindrical outer ring spacer is disposed between the first outer ring and the second outer ring in an axially clamped state.

[0052] A load sensor is mounted on the outer ring spacer, and the load sensor calculates the load acting on the outer ring spacer based on the strain of the outer ring spacer.

[0053] The bearing device with a sensor is characterized in that

[0054] The first outer ring, the second outer ring, and the outer ring spacer are respectively fitted into the inner circumference of the bearing housing with a clearance fit.

[0055] The fitting clearance Δ1 between the outer ring spacer and the bearing housing is larger than the radial expansion amount of the outer ring spacer caused by the load acting on the outer ring spacer, and is larger than the fitting clearance Δ2 between the outer ring of the bearing that is externally applied with force among the first bearing and the second bearing and the bearing housing.

[0056] With this structure, even if the axial force applied to the outer ring spacer increases and the radial expansion deformation of the outer ring spacer becomes larger, the outer circumference of the outer ring spacer is not likely to contact the inner circumference of the bearing housing. Therefore, by adopting a structure that is easily deformable within the practical range as the outer ring spacer, it is possible to stably detect changes in the load acting on the outer ring spacer with high sensitivity.

[0057] [Structure 7]

[0058] According to the bearing device with a sensor described in Structure 6, wherein

[0059] The fitting clearance Δ2 between the outer ring of the bearing that is externally applied with an axial force among the first bearing and the second bearing and the bearing housing is 40 μm or less in diameter, and the fitting clearance Δ1 between the outer ring spacer and the bearing housing is Δ2 + 50 μm or less in diameter (preferably Δ2 + 30 μm or less).

[0060] 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 assembling it into a main shaft device or the like. That is, in order to improve the load measurement accuracy, it is necessary to align the outer ring spacer as accurately as possible. In order to facilitate alignment, the difference between Δ1 and Δ2 is 50 μm or less, preferably 30 μm or less.

[0061] [Structure 8]

[0062] According to the bearing device with a sensor described in Structure 6 or 7, wherein

[0063] The above outer ring spacer is composed of an outer ring made of metal and an inner ring made of resin. The outer ring is fitted into the inner circumference of the bearing housing with a clearance fit and contacts the first outer ring and the second outer ring axially. The inner ring is disposed radially inside the outer ring, and the load sensor is mounted on the outer ring.

[0064] If this structure is adopted, it is possible to suppress an increase in the manufacturing cost of the outer ring spacer, and the load sensor is arranged in a protected state between the outer ring and the inner ring of the outer ring spacer.

[0065] [Structure 9]

[0066] In the bearing device with a sensor according to any one of Structures 6 to 8,

[0067] Preload is applied to the first bearing and the second bearing.

[0068] [Structure 10]

[0069] In the bearing device with a sensor according to any one of Structures 1 to 9,

[0070] The first bearing and the second bearing are angular contact ball bearings.

[0071] In addition, in this invention, as a machine tool spindle device using the above bearing device with a sensor, a machine tool spindle device having the following structure is provided together.

[0072] [Structure 11]

[0073] A machine tool spindle device, which has:

[0074] The bearing device with a sensor according to any one of Structures 1 to 10;

[0075] A spindle of a machine tool, which is supported by the bearing device with a sensor so as to be rotatable; and

[0076] A motor, which rotationally drives the spindle.

[0077] If this structure is adopted, it is possible to stably perform state monitoring for labor saving and unmanned operation of the machine tool. In addition, it is also possible to detect the machining load acting on the spindle of the machine tool during cutting.

[0078] In the bearing device with a sensor of the first invention, a surface pressure obtained by summing 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. Therefore, the surface pressure between the contact surfaces of the first outer ring and the outer ring spacer is large. As a result, the frictional force between the contact surfaces of the first outer ring and the outer ring spacer is large. When the radial component force received by the first outer ring from the first rolling elements changes according to the change in the bearing preload, radial sliding between the contact surfaces of the first outer ring and the outer ring spacer can be suppressed. Similarly, a surface pressure obtained by summing 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. Therefore, the surface pressure between the contact surfaces of the second outer ring and the outer ring spacer is large. As a result, the frictional force between the contact surfaces of the second outer ring and the outer ring spacer is large. When the radial component force received by the second outer ring from the second rolling elements changes according to the change in the bearing preload, radial sliding between the contact surfaces of the second outer ring and the outer ring spacer can be suppressed. As a result, hysteresis in the output of the strain sensor of the outer ring spacer can be reduced, and the bearing preload can be detected with high precision.

[0079] As described above, in the bearing device with a sensor of the second invention, by defining the fitting clearances of the two bearings and the outer ring spacer that are respectively in clearance fit with the housing, even when the outer ring spacer equipped with the load sensor expands and deforms radially due to an axial force, it is not likely to come into contact with the inner circumference of the housing. Therefore, load changes can be stably detected with high sensitivity. BRIEF DESCRIPTION OF THE DRAWINGS

[0080] Figure 1 It is a cross-sectional view showing a machine tool spindle device using the bearing device with a sensor according to an embodiment of the first invention.

[0081] Figure 2 is Figure 1 an enlarged view of the vicinity of the bearing device with a sensor.

[0082] Figure 3 is a cross-sectional view taken along line III-III of Figure 2 the same.

[0083] Figure 4 It is a diagram showing the transmission path of the axial preload applied between the first inner ring and the second inner ring shown in Figure 2 the same, and the transmission path of the axial pressing force applied between the first outer ring and the second outer ring.

[0084] Figure 5 It is a cross-sectional view showing a machine tool spindle device using the bearing device with a sensor according to an embodiment of the second invention.

[0085] Figure 6 is an enlarged view Figure 5 showing a cross-section of the main part.

[0086] Figure 7 is a diagram showing the force transmission path Figure 5 generated by the cutting load applied to the spindle device for machine tools.

[0087] Figure 8 is a diagram showing the force transmission path Figure 5 generated by the preload applied to the sensor-equipped bearing device of the spindle device for machine tools.

[0088] Figure 9 is a schematic diagram corresponding to Figure 7 illustrating the force transmission method of the comparative example.

[0089] Figure 10 is a schematic diagram corresponding to Figure 7 illustrating the force transmission method of the embodiment. Detailed Embodiment

[0090] Figure 1 This shows a spindle device for machine tools that uses the sensor-equipped bearing device 1 (hereinafter simply referred to as "bearing device 1") according to the embodiment of the first invention. This spindle device has: a spindle 2 of the machine tool; a spindle housing (outer cylinder) 3 that houses the spindle 2; a motor 4 that rotationally drives the spindle 2; the bearing device 1 of the embodiment that rotatably supports the spindle 2 on the axially front side (left side in the figure) of the motor 4; and a rear bearing device 5 that rotatably supports the spindle 2 on the axially rear side (right side in the figure) of the motor 4.

[0091] The spindle housing 3 is formed in a hollow cylindrical shape that is open at both ends. The spindle housing 3 houses the bearing device 1 and the motor 4 in order from the front side to the rear side in the axial direction. In the figure, the part of the spindle housing 3 that houses the bearing device 1 and the part of the spindle housing 3 that houses the motor 4 are formed as a seamless integral body, but the part of the spindle housing 3 that houses the bearing device 1 and the part of the spindle housing 3 that houses the motor 4 may also be formed as separate bodies and the two parts may be connected and integrated.

[0092] The spindle 2 is inserted into the spindle housing 3 in a state where the front end of the spindle 2 protrudes from the front end opening of the spindle housing 3. A chuck (not shown) for holding a tool or a workpiece is detachably attached to the front end of the spindle 2. In the spindle 2, a through hole 6 is formed axially therethrough, and a drawbar (not shown) of the machine tool is housed in the through hole 6 so as to be slidable in the axial direction.

[0093] The motor 4 has: a rotor 7 mounted on 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 fitted 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 rotationally locked to the main shaft 2 so as to rotate integrally with the main shaft 2. The axially front end of the rotor sleeve 9 contacts a stepped portion 11 formed on the outer periphery of the main shaft 2 and facing the rear side in the axial direction, and is axially positioned by contacting the stepped portion 11.

[0094] The stator 8 has: a stator core 12 fixed to the inner periphery of the main shaft housing 3; and electromagnetic coils 13 wound around a plurality of tooth portions formed on the stator core 12 at circumferential intervals. When the electromagnetic coils 13 are energized, 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, and the rotor 7 rotates integrally with the main shaft 2. Here, as the motor 4, an electric motor that generates a rotational force using electricity is adopted, but a motor that generates a rotational force using other power sources such as compressed air may be adopted instead of the electric motor.

[0095] The rear bearing device 5 has: an annular bearing support member 14 coaxially fixed to the rear end of the main shaft housing 3; and a rolling bearing 15 assembled to the bearing support member 14. The rolling bearing 15 is a cylindrical roller bearing having an outer ring 16 fitted onto the inner periphery of the bearing support member 14, an inner ring 17 fitted onto the outer periphery of the main shaft 2, and a plurality of cylindrical rollers 18 assembled between the outer ring 16 and the inner ring 17.

[0096] An outer ring pressing member 19 is mounted on the bearing support member 14. The outer ring pressing member 19 fixes the axial position of the outer ring 16 by contacting the axially rear end face of the outer ring 16. A nut member 20 that presses the inner ring 17 forward in the axial direction and an annular spacer 21 assembled between the inner ring 17 and the nut member 20 are mounted on the outer periphery of the main shaft 2. The nut member 20 is threadedly engaged with an external thread 22 formed on the outer periphery of the rear end portion of the main shaft 2. The axially front end face of the spacer 21 contacts the axially rear end face of the inner ring 17, and the axially rear end face of the spacer 21 contacts the axially front end face of the nut member 20. The axially front end face of the inner ring 17 contacts the axially rear end of the rotor sleeve 9.

[0097] The bearing device 1 has: a cylindrical bearing housing 23 fixed to the main shaft housing 3; a first bearing 24 and a second bearing 25 assembled to the bearing housing 23 at axial intervals; 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 mounted on the outer ring spacer 26.

[0098] The bearing housing 23 is fitted into the inner circumference of the main shaft housing 3. A cooling groove 29 for the refrigerant for cooling the bearing device 1 is formed on the outer circumference of the bearing housing 23. 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 spirally extending on 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.

[0099] As Figure 2 shown, the first bearing 24 has: a first outer ring 30 fitted into the inner circumference of the bearing housing 23; a first inner ring 31 rotatably provided on the radially inner side of 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. Here, the first rolling elements 32 are balls. A first outer ring raceway surface 33 having a circular arc cross-section for the first rolling elements 32 to roll on is provided on the inner circumference of the first outer ring 30. The first outer ring 30 is a reduced-shoulder outer ring whose shape is such that the front outer ring shoulder on the axial front side and the rear outer ring shoulder on the axial rear side are removed with respect to the first outer ring raceway surface 33. The outer circumference of the first outer ring 30 is fitted into the inner circumference of the bearing housing 23 with a clearance. A first inner ring raceway surface 34 having a circular arc cross-section for the first rolling elements 32 to roll on is provided on the outer circumference of the first inner ring 31. The first inner ring 31 is a reduced-shoulder inner ring whose shape is such that the rear inner ring shoulder on the axial rear side and the front inner ring shoulder on the axial front side are removed with respect to the first inner ring raceway surface 34 where the first rolling elements 32 roll. The first inner ring 31 is fitted onto the outer circumference of the main shaft 2 with an interference fit.

[0100] The second bearing 25 has: a second outer ring 35 fitted into the inner circumference of the bearing housing 23; a second inner ring 36 rotatably provided on the radially inner side of 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. Here, the second rolling elements 37 are balls. A second outer ring raceway surface 38 having a circular arc cross-section for the second rolling elements 37 to roll on is provided on the inner circumference of the second outer ring 35. The second outer ring 35 is arranged at an interval axially rearward from the first outer ring 30, and the second inner ring 36 is also arranged at an interval axially rearward from the first inner ring 31. The second outer ring 35 is a reduced-shoulder outer ring whose shape is such that the rear outer ring shoulder on the axial rear side and the front outer ring shoulder on the axial front side are removed with respect to the second outer ring raceway surface 38. The outer circumference of the second outer ring 35 is fitted into the inner circumference of the bearing housing 23 with a clearance. A second inner ring raceway surface 39 having a circular arc cross-section for the second rolling elements 37 to roll on is provided on the outer circumference of the second inner ring 36. The second inner ring 36 is a reduced-shoulder inner ring whose shape is such that the front inner ring shoulder on the axial front side and the rear inner ring shoulder on the axial rear side are removed with respect to the second inner ring raceway surface 39 where the second rolling elements 37 roll. The second inner ring 36 is fitted onto the outer circumference of the main shaft 2 with an interference fit.

[0101] Here, the first bearing 24 is configured to generate a radial component force by which the first rolling elements 32 press against the first outer ring 30 through axial preloading. Similarly, the second bearing 25 is also configured to generate a radial component force by which the second rolling elements 37 press against the second outer ring 35 through axial preloading. In this embodiment, the first bearing 24 is an angular contact ball bearing in which a straight line connecting the contact point between the first inner ring 31 and the first rolling elements 32 and the contact point between the first outer ring 30 and the first rolling elements 32 inclines axially rearward from the inner side in the radial direction toward the outer side in the radial direction. Further, the second bearing 25 is an angular contact ball bearing in which a straight line connecting the contact point between the second inner ring 36 and the second rolling elements 37 and the contact point between the second outer ring 35 and the second rolling elements 37 inclines axially forward from the inner side in the radial direction toward the outer side in the radial direction. That is, the first bearing 24 and the second bearing 25 are a pair of angular contact ball bearings arranged at an interval in the axial direction in a back-to-back relationship.

[0102] The outer ring spacer 26 is a hollow cylindrical member with both ends open. The outer ring spacer 26 is fitted into the inner circumference of the bearing housing 23 with a clearance. The outer ring spacer 26 is axially sandwiched between the axially rear end face of the first outer ring 30 (the axial end face on the second outer ring 35 side of the first outer ring 30) and the axially front end face of the second outer ring 35 (the axial end face on the first outer ring 30 side of the second outer ring 35).

[0103] Similar to the outer ring spacer 26, the inner ring spacer 27 is also a hollow cylindrical member with both ends open. The inner ring spacer 27 is fitted into the outer circumference of the main shaft 2 with a clearance. The inner ring spacer 27 is axially sandwiched between the first inner ring 31 and the second inner ring 36.

[0104] As Figure 4 shown, a ring-shaped inner ring positioning step portion 40 is formed on the outer circumference of the main shaft 2, and the inner ring positioning step portion 40 is axially opposed to the axially front end face of the first inner ring 31 (the axial end face of the first inner ring 31 on the side opposite to the second inner ring 36 side). The inner ring positioning step portion 40 positions the first inner ring 31 in the axial direction by restricting the movement of the first inner ring 31 in the axially forward direction (the direction away from the second outer ring 35). Further, on the outer circumference of the main shaft 2, a preload nut 41 is mounted on the axially rear side of the second inner ring 36. The preload nut 41 is threadedly engaged with an external thread 42 formed on the outer circumference of the main shaft 2. A ring-shaped gasket 43 is assembled between the second inner ring 36 and the preload nut 41. The axially front end face of the gasket 43 contacts the axially rear end face of the second inner ring 36, and the axially rear end face of the gasket 43 contacts the axially front end face of the preload nut 41.

[0105] The preloading nut 41 is fastened with a specified force, and the axial force of the preloading nut 41 applies preloading in the direction of approaching the first inner ring 31 to the axially front end face of the first inner ring 31 (the axial end face of the first inner ring 31 on the side opposite to the second inner ring 36 side) and the axially rear end face of the second inner ring 36 (the axial end face of the second inner ring 36 on the side opposite to the first inner ring 31 side). That is, as shown by the thick solid line in the figure, a state is achieved in which axial preloading is applied in such a manner that the axial force of the preloading nut 41 sequentially passes through the first inner ring 31, the first rolling elements 32, the first outer ring 30, the outer ring spacer 26, the second outer ring 35, the second rolling elements 37, and the second inner ring 36 through preloading between the first inner ring 31 and the second inner ring 36.

[0106] The strain sensor 28 is installed at the axially central portion of the outer ring spacer 26. Specifically, the strain sensor 28 is installed on the outer ring spacer 26 in such a manner that the strain sensor 28 is housed in a region within an axial distance of 1 / 4 of the total axial length of the outer ring spacer 26 from the central position in the axial direction of the outer ring spacer 26.

[0107] As Figure 3 shown, a plurality (here, three) of strain sensors 28 are provided at equal intervals in the circumferential direction along the inner circumference of the outer ring spacer 26. Axial grooves 44 are formed at equal intervals in the circumferential direction along the inner circumference of the outer ring spacer 26 in the same number as the number of strain sensors 28. Each of the axial grooves 44 has a planar groove bottom surface parallel to the axis, and the strain sensor 28 is installed on each of the groove bottom surfaces.

[0108] As Figure 4 shown, each strain sensor 28 has a strain detection portion 45 and a processing portion 46 connected to the strain detection portion 45. The strain detection portion 45 is a strain gauge whose resistance changes according to the strain. The processing portion 46 has: a strain detection circuit that detects the strain based on the change in the resistance of the strain gauge; and an AD conversion circuit that converts the strain detected by the detection circuit from an analog signal into a digital signal and outputs it.

[0109] Here, the strain detection portion 45 employs a structure having an axial strain detection portion (a strain gauge configured with the axial direction as the strain detection direction) that detects the axial strain of the outer ring spacer 26 at the installation position of the strain sensor 28 and a circumferential strain detection portion (a strain gauge configured with the circumferential direction as the strain detection direction) that detects the circumferential strain of the outer ring spacer 26 at the installation position of the strain sensor 28. Moreover, the processing portion 46 is configured to obtain the difference between the axial strain detected by the axial strain detection portion and the circumferential strain detected by the circumferential strain detection portion (that is, the sum of the absolute value of the axial strain and the absolute value of the circumferential strain), and use this difference as the output of the strain sensor 28.

[0110] An annular outer ring positioning step portion 50 is formed on the inner circumference of the bearing housing 23, and the outer ring positioning step portion 50 is axially opposed to the axial rear end face of the second outer ring 35 (the axial end face of the second outer ring 35 on the side opposite to the first outer ring 30 side). The outer ring positioning step portion 50 positions the second outer ring 35 axially by restricting the movement of the second outer ring 35 in the axial rear direction (the direction away from the first outer ring 30).

[0111] A ring-shaped 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 fitted to the inner circumference of the bearing housing 23; and a flange portion 53 in the shape of a circular ring plate extending radially outward from the axial front end of the cylindrical portion 52.

[0112] As Figure 2 、 Figure 3 shown, the flange portion 53 is fixed to the axial front end face of the bearing housing 23 by a plurality of threaded members 54 arranged at equal intervals in the circumferential direction. Here, the threaded member 54 is a bolt. The threaded member 54 is arranged at the same circumferential position as the strain sensor 28. That is, in Figure 3 , the strain sensor 28 is arranged at the circumferential positions of 0°, 120°, and 240° clockwise from the upper side of the outer ring spacer 26, and the threaded member 54 is arranged at the circumferential positions including all the circumferential positions of the strain sensor 28 (in the figure, at the circumferential positions of 0°, 60°, 120°, 180°, 240°, and 300°).

[0113] On the flange portion 53, a plurality of through holes 55 for inserting the threaded member 54 are formed at equal intervals in the circumferential direction. In addition, on the axial front end face of the bearing housing 23, a plurality of threaded holes 56 for screwing in the threaded member 54 are formed at equal intervals in the circumferential direction. The flange portion 53 is pressed against the axial front end face of the bearing housing 23 by tightening the threaded member 54. In addition, the axial rear end of the cylindrical portion 52 contacts the axial front end face of the first outer ring 30 (the axial end face of the first outer ring 30 on the side opposite to the second outer ring 35 side).

[0114] As Figure 4 shown, the first outer ring 30 and the second outer ring 35 are assembled between the cover member 51 and the outer ring positioning step portion 50 with an axial interference amount, and an axial pressing force in the direction of approaching the first outer ring 30 and the second outer ring 35 is applied to the axial front end face of the first outer ring 30 (the axial end face of the first outer ring 30 on the side opposite to 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 on the side opposite to the first outer ring 30 side) by this axial interference amount.

[0115] That is, in from Figure 4When the cover member 51 is fixed to the bearing housing 23 with the first outer ring 30, the outer ring spacer 26, and the second outer ring 35 removed from the shown bearing housing 23, the distance between the axially opposed surfaces of the cover member 51 and the outer ring positioning step portion 50 is set to be shorter than the distance from the axially front end surface of the first outer ring 30 to the axially rear end surface of the second outer ring 35 when the first outer ring 30, the outer ring spacer 26, and the second outer ring 35 are arranged axially without clearance in the state where the first outer ring 30, the outer ring spacer 26, and the second outer ring 35 are removed from the bearing housing 23 by an amount corresponding to the axial interference amount. Then, the first outer ring 30, the outer ring spacer 26, and the second outer ring 35 are assembled to the bearing housing 23, and then the cover member 51 is tightened axially with a plurality of threaded members 54, so that the first outer ring 30, the outer ring spacer 26, and the second outer ring 35 are compressed axially by the amount of the axial interference amount between the cover member 51 and the outer ring positioning step portion 50. As a result, as shown by the thick dashed line in the figure, a state is formed in which an axial pressing force is applied between the first outer ring 30 and the second outer ring 35. The axial interference amount is set in the range of 10 μm or more and 50 μm or less (preferably 20 μm or less).

[0116] Here, 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 amount is significantly larger 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 the tightening of the preload nut 41.

[0117] That is, 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 the tightening of the preload nut 41 is less than 1 kN (about several tens to several hundreds of N). On the other hand, an axial pressing force of a magnitude 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 amount. 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 amount is set to be 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 the tightening of the preload nut 41.

[0118] In Figure 2 In the shown bearing device 1, the preload of the first bearing 24 and the second bearing 25 (hereinafter referred to as "bearing preload") varies according to the rotational speed of the main shaft 2 during the operation of the main shaft device. Moreover, the bearing preload can be detected based on the strain of the outer ring spacer 26 detected by the strain sensor 28.

[0119] That is, when the main shaft device is in operation, if the rotational speed of the main shaft 2 changes, the centrifugal forces of the first rolling elements 32 and the second rolling elements 37 change, and the loads applied by the first rolling elements 32 to the first outer ring raceway surface 33 (preload of the first bearing 24) and the loads applied by the second rolling elements 37 to the first outer ring raceway surface 33 (preload of the second bearing 25) also change according to the change in the centrifugal force. Here, the first outer ring raceway surface 33 and the second outer ring raceway surface 38 are in contact with the first rolling elements 32 and the second rolling elements 37 at an angle inclined with respect to the axial direction. Therefore, if the loads applied by the first rolling elements 32 to the first outer ring raceway surface 33 and the loads applied by the second rolling elements 37 to the first outer ring raceway surface 33 change, the axial preload applied to the outer ring spacer 26 from the first outer ring 30 and the second outer ring 35 changes, and the strain of the outer ring spacer 26 changes. Therefore, it is possible to detect the preloads of the first bearing 24 and the second bearing 25 (bearing preload) based on the strain of the outer ring spacer 26 detected by the strain sensor 28.

[0120] In addition, if a moment load acts on the main shaft 2, loads of different magnitudes are applied to the positions of the respective strain sensors 28 arranged at equal intervals in the circumferential direction. Therefore, it is also possible to detect the moment load acting on the main shaft 2 of the machine tool during cutting based on the outputs of the plurality of strain sensors 28.

[0121] However, when the bearing preload increases, the first outer ring 30 elastically deforms in the diameter-expanding direction due to an increase in the radial component force received from the first rolling elements 32. At this time, the force in the diameter-expanding direction acting on the first outer ring 30 is 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 of a slight slip in which the first outer ring 30 relatively moves radially outward with respect to the outer ring spacer 26 between the contact surfaces of the first outer ring 30 and the outer ring spacer 26. On the other hand, when the bearing preload decreases, the first outer ring 30 elastically returns in the diameter-reducing direction due to a decrease in the radial component force received from the first rolling elements 32. At this time, the force in the diameter-reducing direction acting on the first outer ring 30 is 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 of a slight slip in which the first outer ring 30 relatively moves radially inward with respect to the outer ring spacer 26 between the contact surfaces of the first outer ring 30 and the outer ring spacer 26.

[0122] Similarly, when the bearing preload increases, the second outer ring 35 elastically deforms in the diameter-expanding direction due to the increased radial component force received from the second rolling elements 37, and there is a possibility of a slight slip in which the second outer ring 35 relatively moves radially outward with respect to the outer ring spacer 26 between the contact surfaces of the second outer ring 35 and the outer ring spacer 26 due to this elastic deformation. On the other hand, when the bearing preload decreases, the second outer ring 35 elastically returns in the diameter-reducing direction due to the decreased radial component force received from the second rolling elements 37, and there is a possibility of a slight slip in which the second outer ring 35 relatively moves radially inward with respect to the outer ring spacer 26 between the contact surfaces of the second outer ring 35 and the outer ring spacer 26 due to this elastic return.

[0123] Moreover, if the above-described radial slip occurs between the contact surfaces of the first outer ring 30 and the outer ring spacer 26 and between the contact surfaces of the second outer ring 35 and the outer ring spacer 26, the deformation of the outer ring spacer 26 is different during the process of increasing the bearing preload and during the process of decreasing the bearing preload. Therefore, even if the magnitude of the bearing preload is the same during the process of increasing the bearing preload and during the process of decreasing the bearing preload, the output of the strain sensor 28 is different, resulting in a hysteresis with a certain difference between the former and the latter. Due to this hysteresis, there is a problem that an error occurs in the bearing preload detected based on the output of the strain sensor 28.

[0124] Regarding this problem, as Figure 4As shown, in the bearing device 1 of this embodiment, an axial interference amount is set between the cover member 51 and the outer ring positioning step portion 50 to apply a pressing force in the direction of approaching the first outer ring 30 and the second outer ring 35 to the axial end faces of the first outer ring 30 and the second outer ring 35. Therefore, a surface pressure that combines 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 acts 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 first outer ring 30 and the outer ring spacer 26 becomes large. Therefore, the frictional force between the contact surfaces of the first outer ring 30 and the outer ring spacer 26 is large, and when the radial component force received by the first outer ring 30 from the first rolling element 32 changes according to the change in bearing preload, radial sliding between the contact surfaces of the first outer ring 30 and the outer ring spacer 26 can be suppressed. Similarly, a surface pressure that combines 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 acts between the contact surfaces of the second outer ring 35 and the outer ring spacer 26. Therefore, 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 force received by the second outer ring 35 from the second rolling element 37 changes according to the change in bearing preload, radial sliding between the contact surfaces of the second outer ring 35 and the outer ring spacer 26 can be suppressed. As a result, hysteresis in the output of the strain sensor 28 of the outer ring spacer 26 can be reduced, and bearing preload can be detected with high precision.

[0125] Particularly in this bearing device 1, the axial 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 is set to be significantly more than 10 times (preferably more than 50 times) 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. Therefore, 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 frictional force between the contact surfaces of the first outer ring 30 and the outer ring spacer 26 becomes particularly effectively large, and when the radial component force received by the first outer ring 30 from the first rolling element 32 changes according to the change in bearing preload, radial sliding between the contact surfaces of the first outer ring 30 and the outer ring spacer 26 can be particularly effectively suppressed. Similarly, the frictional force between the contact surfaces of the second outer ring 35 and the outer ring spacer 26 becomes particularly effectively large, and when the radial component force received by the second outer ring 35 from the second rolling element 37 changes according to the change in bearing preload, radial sliding between the contact surfaces of the second outer ring 35 and the outer ring spacer 26 can be particularly effectively suppressed.

[0126] In addition, as a method of applying a pressing force to the axial end faces of the first outer ring 30 and the second outer ring 35, the bearing device 1 utilizes an axial interference amount, and thus a large pressing force can be applied to the axial end faces of the first outer ring 30 and the second outer ring 35 by a simple method of tightening the threaded member 54.

[0127] In addition, as Figure 3 shown, the bearing device 1 is provided with the threaded member 54 at the same circumferential position as the strain sensor 28, and thus 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 can be effectively increased at the same circumferential position as the strain sensor 28. Therefore, the hysteresis of the output of the strain sensor 28 can be effectively reduced. Figure 4 shown

[0128] The axial interference amount can be set by applying a pressing force to at least one of the axial end face of the first outer ring 30 or the axial end face of the second outer ring 35 using a testing machine or the like that can load and measure a specified press-fitting amount (μm) and a pressing force (N) and deriving the relationship between the press-fitting amount (μm) and the pressing force (N).

[0129] In the above-described embodiment, the case where the strain sensor 28 is disposed on the inner circumference of the outer ring spacer 26 is taken as an example, but the strain sensor 28 may also be disposed on the outer circumference of the outer ring spacer 26.

[0130] In the above-described embodiment, angular contact ball bearings are taken as examples for the first bearing 24 and the second bearing 25, but as the first bearing 24 and the second bearing 25, other types of rolling bearings such as tapered roller bearings and deep groove ball bearings that generate radial component forces by axial preloading can also be used, for example.

[0131] In addition, in the above-described embodiment, a structure having a strain detection unit 45 and a processing unit 46 is taken as an example for the strain sensor 28, but the strain sensor 28 can also be composed of only the strain detection unit 45 (only a strain gauge).

[0132] Figure 5 The spindle device for a machine tool using the bearing device 1 according to the embodiment of the second invention is shown. Hereinafter, parts corresponding to the embodiment of the first invention are denoted by the same reference numerals and description thereof is omitted. The parts denoted by the same reference numerals have substantially the same structure as the embodiment of the first invention.

[0133] As Figure 5 and Figure 6As shown, the bearing device 1 has: a bearing housing 23 fixedly provided on the main shaft 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 on the axially rear side of the first bearing 24; and an outer ring spacer 26 and an inner ring spacer 27 axially sandwiched between the first bearing 24 and the second bearing 25, and the main shaft 2 is supported by the first bearing 24 and the second bearing 25.

[0134] The first bearing 24 is an angular contact ball bearing having a non-rotating first outer ring 30 fitted to the inner periphery of the bearing housing 23 with a clearance fit, a first inner ring 31 rotatably provided on the radially inner side of 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. A first outer ring raceway surface 33 having a cross-sectional arc shape for the first rolling element 32 to rollingly contact is provided on the inner periphery of the first outer ring 30, and a first inner ring raceway surface 34 having a cross-sectional arc shape for the first rolling element 32 to rollingly contact is provided on the outer periphery of the first inner ring 31. The first outer ring 30 is a reduced-shoulder outer ring whose shape has removed the axially front outer ring shoulder among the axially front outer ring shoulder and the axially rear outer ring shoulder with respect to the first outer ring raceway surface 33. In addition, the first inner ring 31 is a reduced-shoulder inner ring whose shape has removed the axially rear inner ring shoulder among the axially front inner ring shoulder and the axially rear inner ring shoulder with respect to the first inner ring raceway surface 34.

[0135] The second bearing 25 is an angular contact ball bearing having a non-rotating second outer ring 35, a second inner ring 36, and a plurality of second rolling elements (here, balls) 37. The second outer ring 35 is arranged at an interval axially rearward from the first outer ring 30 and fitted to the inner periphery of the bearing housing 23 with a clearance fit. The second inner ring 36 is rotatably provided on the radially inner side of the second outer ring 35, and the plurality of second rolling elements 37 are assembled between the second outer ring 35 and the second inner ring 36. A second outer ring raceway surface 38 having a cross-sectional arc shape for the second rolling element 37 to rollingly contact is provided on the inner periphery of the second outer ring 35, and a second inner ring raceway surface 39 having a cross-sectional arc shape for the second rolling element 37 to rollingly contact is provided on the outside of the second inner ring 36. The second outer ring 35 is a reduced-shoulder outer ring whose shape has removed the axially rear outer ring shoulder among the axially front outer ring shoulder and the axially rear outer ring shoulder with respect to the second outer ring raceway surface 38. In addition, the second inner ring 36 is a reduced-shoulder inner ring whose shape has removed the axially front inner ring shoulder among the axially front inner ring shoulder and the axially rear inner ring shoulder with respect to the second inner ring raceway surface 39.

[0136] Here, for the first bearing 24, a straight line connecting the contact point between the first inner ring 31 and the first rolling element 32 and the contact point between the first outer ring 30 and the first rolling element 32 inclines rearward in the axial direction from the inner side in the radial direction toward the outer side in the radial direction. On the other hand, for the second bearing 25, a straight line connecting the contact point between the second inner ring 36 and the second rolling element 37 and the contact point between the second outer ring 35 and the second rolling element 37 inclines forward in the axial direction from the inner side in the radial direction toward the outer side in the radial direction. That is, the first bearing 24 and the second bearing 25 are arranged in a back-to-back combination.

[0137] The outer ring spacer 26 is composed of a metal outer ring 26a fitted to the inner circumference of the bearing housing 23 with a clearance fit and a resin inner ring 26b disposed on the radially inner side of the outer ring 26a. The outer ring 26a is formed in a hollow cylindrical shape with both ends open. On the other hand, the inner ring 26b is formed in an L-shaped cross section having an outward flange portion at the rear end in the axial direction of a hollow cylindrical portion with both ends open, and the flange portion is fixed to the inner circumferential surface of the outer ring 26a. As a fixing means, a method of pressing the flange portion of the inner ring 26b into the inner circumferential surface of the outer ring 26a, a method of bonding, or a combination of these methods can be adopted.

[0138] The radial thickness of the outer ring 26a is formed to be almost the same as the thicknesses of the first outer ring 30 and the second outer ring 35, and the axial dimension is formed to be larger than the axial dimension of the inner ring 26b. Thus, the front end in the axial direction of the outer ring 26a contacts the rear end face in the axial direction of the first outer ring 30, and the rear end in the axial direction contacts the front end face in the axial direction of the second outer ring 35, but the inner ring 26b does not contact the first outer ring 30 and the second outer ring 35.

[0139] Moreover, a plurality (preferably three or more, four here) of load sensors 28 are installed at equal intervals in the circumferential direction at the axial center portion of the inner circumferential surface of the outer ring 26a. The load sensor 28 has a strain gauge 28b for detecting the strain of the outer ring 26a and a processing circuit 28c for obtaining the load acting on the outer ring 26a based on the strain detected by the strain gauge 28b, which are installed on a printed circuit board 28a fixed to the outer ring 26a by an adhesive or the like. In addition, a sealing agent 47 such as a resin material is filled in the space between the outer ring 26a and the cylindrical portion of the inner ring 26b (the space around the load sensor 28). Thus, the load sensor 28 is reliably fixed in a protected state with insulation ensured.

[0140] On the other hand, the inner ring spacer 27, like the outer ring 26a of the outer ring spacer 26, is formed in a hollow cylindrical shape with both ends open, and its front end in the axial direction contacts the rear end face in the axial direction of the first inner ring 31, and the rear end in the axial direction contacts the front end face in the axial direction of the second inner ring 36.

[0141] An outer ring pressing member 60 is fixed to the axial front end of the main shaft housing 3. The outer ring pressing member 60 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 fitted into the inner circumference of the bearing housing 23, and a flange portion 62 extending 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 stepped portion 63 is formed on the outer circumference of the axial front end portion of the main shaft 2, and the stepped portion 63 contacts the axial front end face of the first inner ring 31. The first inner ring 31 is axially positioned by contacting the stepped portion 63.

[0142] A preload nut 64 that presses the second inner ring 36 forward in the axial direction and an annular spacer 65 assembled between the second inner ring 36 and the preload nut 64 are mounted on the outer circumference of the main shaft 2. The preload nut 64 is threadedly engaged with an external thread 66 formed on a portion of the main shaft 2 that extends forward in the axial direction from a stepped portion 11 (see Figure 5 ) on the outer circumference. The axial front end of the spacer 65 contacts the axial rear end face of the second inner ring 36, and the axial rear end contacts the axial front end face of the preload nut 64. A stepped portion 67 is formed on the inner circumference of the bearing housing 23, and the stepped portion 67 contacts the axial rear end face of the second outer ring 35. The second outer ring 35 is axially positioned by contacting the stepped portion 67.

[0143] The bearing housing 23 has a cylindrical portion 68 fitted into the inner circumference of the main shaft housing 3, and a flange portion 69 extending radially outward from the axial front end of the cylindrical portion 68. Cooling grooves 70 for the refrigerant flow for cooling the bearing device 1 are formed on the outer circumference of the cylindrical portion 68. The cooling grooves 70 are a plurality of annular grooves formed at intervals in the axial direction on the outer circumference of the cylindrical portion 68 or spiral grooves extending spirally on the outer circumference of the cylindrical portion 68. The flange portion 69 contacts and is fixed to the axial front end of the main shaft housing 3.

[0144] When an axial force (cutting load) is applied to the main shaft 2 by cutting in the above-described structure of the machine tool main shaft device, as Figure 7 shown, this axial force is sequentially transmitted to the first inner ring 31, the first rolling elements 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, and is received by the stepped portion 67 of the bearing housing 23. At this time, the outer ring 26a of the outer ring spacer 26 that has received 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 acting on the outer ring 26a, that is, the cutting load acting on the main shaft 2, can be detected based on the strain at each circumferential position of the outer ring 26a by load sensors 28 installed at equal intervals in the circumferential direction of the outer ring 26a.

[0145] In addition, when assembling the bearing device 1, by tightening the preload nut 64, asFigure 8 As shown, the axial force is sequentially transmitted to the gasket 65, the second inner ring 36, the second rolling elements 37, the second outer ring 35, the outer ring 26a of the outer ring spacer 26, the first outer ring 30, the first rolling elements 32, and the first inner ring 31, and becomes a state borne by the stepped portion 63 of the main shaft 2, applying preload to the first bearing 24 and the second bearing 25. At this time, the load acting on the ring 26a, i.e., the preload, can also be detected by the load sensor 28.

[0146] Here, in the bearing device 1 of this embodiment, the fitting 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 radial expansion amount of the outer ring 26a caused by the load acting on the outer ring 26a, and larger than the fitting clearance Δ2 between the first outer ring 30 and the second outer ring 35 and the bearing housing 23 (here, the fitting clearances of the first outer ring 30 and the second outer ring 35 are set to be the same). In addition, although Δ2 is set to be 40 μm or less in diameter as in the past, the difference between Δ1 and Δ2 is set to be 50 μm or less in diameter (preferably 30 μm or less). This is because: if the difference between Δ1 and Δ2 exceeds 50 μm in diameter, it becomes difficult to center the outer ring spacer 26 when assembling the bearing device 1, and the measurement accuracy of the load decreases.

[0147] By setting the fitting clearances Δ1 and Δ2 between the outer ring 26a of the outer ring spacer 26, the first outer ring 30, and the second outer ring 35 and the bearing housing 23 as described above, it is possible to stably detect changes in the cutting load and preload acting on the outer ring 26a with high sensitivity. The following is based on Figure 9 (Comparative example) and Figure 10 this will be described. In addition, in Figure 9 、 Figure 10 the gaps between the outer ring 26a and the first outer ring 30 and the bearing housing 23 are exaggeratedly shown.

[0148] First, if a cutting load is applied to the main shaft 2, the force transmission path is basically as Figure 7 shown, but at this time, as a comparative example, it is assumed that the fitting 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. Then, as Figure 9 shown, before the first outer ring 30, which has been expanded in diameter due to the radial component of the force transmitted from the first rolling elements 32, contacts the inner circumference of the bearing housing 23, the outer ring 26a of the outer ring spacer 26 contacts the inner circumference of the bearing housing 23, and the deformation mode of the outer ring 26a changes, so there is a concern that the sensitivity to detect load changes becomes low.

[0149] In contrast, if Δ1 and Δ2 are set as described above, when a cutting load is applied to the main shaft 2, as Figure 10As shown, the first outer ring 30 that has been expanded in diameter due to the radial component of the force transmitted from the first rolling elements 32 comes into contact with the inner circumference of the bearing housing 23 before the outer ring 26a of the outer ring spacer 26. Thus, it is difficult for the outer ring 26a to come into contact with the inner circumference of the bearing housing 23. Therefore, by using the outer ring 26a that is easily deformable within the practical range, it is possible to stably detect changes in the cutting load with high sensitivity regardless of the level of the cutting load acting on the outer ring 26a.

[0150] Here, the outer diameter dimensions of the outer ring 26a of the outer ring spacer 26, the first outer ring 30, and the second outer ring 35 can be measured using a micrometer or a dial indicator, etc. The inner diameter dimension of the bearing housing 23 can be measured using a bore gauge, an inside micrometer, etc. The clearance fits Δ1 and Δ2 between the outer ring 26a of the outer ring spacer 26, the first outer ring 30, and the second outer ring 35 and the bearing housing 23 can be derived from the difference between the measured outer diameter dimension and the inner diameter dimension. In addition, the radial expansion amount of the outer ring spacer 26 can be inferred by measuring the displacement amount on 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 micrometer or a dial indicator, etc., and deriving the relationship between the pressing force and the radial expansion amount of the outer ring spacer 26.

[0151] Similarly, when preloading 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 that has been expanded in diameter due to the radial component of the force transmitted from the second rolling elements 37 also comes into contact with the inner circumference of the bearing housing 23 before the outer ring 26a, and it is difficult for the outer ring 26a to come into contact with the inner circumference of the bearing housing 23. Therefore, it is possible to stably detect changes in the preload with high sensitivity.

[0152] Therefore, in this spindle device for a machine tool, it is possible to quickly respond to a situation where the cutting load suddenly increases, or a situation where the preload increases due to heat generation of the first bearing 24 and the second bearing 25, etc., and the reliability of condition monitoring can be improved. In addition, it is also possible to set the preload during the assembly of the bearing device 1 efficiently and with high precision.

[0153] It should be considered that the embodiments disclosed herein are illustrative in all aspects and not restrictive. The scope of the present invention is not the above description, but is shown by the scope claimed in this application, and is intended to include the meaning equivalent to the scope claimed in this application and all changes within the scope.

[0154] For example, the first bearing and the second bearing constituting the bearing device are angular contact ball bearings in the embodiment, but as long as they are rolling bearings with a contact angle other than 0 such as tapered roller bearings. In addition, the outer ring spacer is not limited to the double-layer structure having an outer ring and an inner ring as in the embodiment, and an outer ring spacer composed of a single cylindrical body can also be used, and a load sensor can be installed on its inner circumferential surface or outer circumferential surface.

[0155] In addition, in the embodiments, axial forces are applied to both the first bearing and the second bearing from the outside, so that the fitting clearance between the outer ring spacer and the bearing housing is larger than the fitting clearances between the outer rings of the two bearings and the bearing housing. However, in the case where only one of the first bearing and the second bearing is assembled in such a way that an axial force is applied from the outside, it is sufficient to make the fitting clearance between the outer ring spacer and the bearing housing larger than the fitting clearance between this one bearing and the bearing housing.

[0156] In addition, in each of the above embodiments, the bearing device 1 that supports the spindle 2 of a machine tool (such as a machining center or a lathe) so as to be rotatable has been described as an example. However, the present invention can also be applied, for example, to a bearing device that supports the spindle of a wind power generation device or the rotating shaft of other devices so as to be rotatable.

[0157] Description of reference numerals

[0158] 1... Bearing device with sensor; 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 portion; 51... Cover member; 54... Threaded member.

Claims

1. A bearing device with a sensor, configured to have: a first bearing (24) and a second bearing (25), arranged at an axial interval; a cylindrical outer ring spacer (26), provided between the first bearing (24) and the second bearing (25); and a strain sensor (28), mounted on the outer ring spacer (26), The first bearing (24) has: a first outer ring (30); a first inner ring (31), arranged 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), arranged 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 in the direction of approaching the first inner ring (31) and the second inner ring (36) is applied to the axial end face of the first inner ring (31) on the side opposite to the second inner ring (36) side and the axial end face of the second inner ring (36) on the side opposite to the first inner ring (31) side, The preload is transmitted among the first inner ring (31), the first rolling elements (32), the first outer ring (30), the outer ring spacer (26), the second outer ring (35), the second rolling elements (37), and the second inner ring (36), The bearing device with a sensor is characterized in that, A pressing force in the direction of approaching the first outer ring (30) and the second outer ring (35) is applied to the axial end face of the first outer ring (30) on the side opposite to the second outer ring (35) side and the axial end face of the second outer ring (35) on the side opposite to the first outer ring (30) side, The magnitude of the pressing force is set to be greater than the preload.

2. The bearing device with a sensor according to claim 1, characterized in that, The magnitude of the pressing force is set to be 10 times or more the magnitude of the preload.

3. The bearing device with a sensor according to claim 1 or 2, characterized in that, The pressing force is applied by assembling the first outer ring (30) and the second outer ring (35) with an axial interference amount between an annular outer ring positioning step portion (50) and a cover member (51). The outer ring positioning step portion (50) is provided on the inner circumference of a cylindrical bearing housing (23) that fits with the outer circumferences of the first outer ring (30) and the second outer ring (35), and the cover member (51) is fixed to the axial end face of the bearing housing (23) by a threaded member (54).

4. The bearing device with a sensor according to claim 3, characterized in that, The threaded member (54) is arranged at the same circumferential position as the strain sensor (28).

5. The bearing device with a sensor according to claim 3 or 4, characterized in that, The axial interference amount is set to be 10 μm or more.

6. A bearing device with a sensor, in which a plurality of rolling bearings with a non-zero contact angle are arranged in a back-to-back combination of two or more rows on the inner side of 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 combination, The first bearing (24) has: a first outer ring (30); a first inner ring (31) rotatably provided on the radially inner side of 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 on the radially inner side of 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 arranged between the first outer ring (30) and the second outer ring (35) in a state of being axially clamped, A load sensor (28) is installed on the outer ring spacer (26), and the load sensor (28) obtains the load acting on the outer ring spacer (26) according to the strain of the outer ring spacer (26), The bearing device with a sensor is characterized in that, The first outer ring (30), the second outer ring (35) and the outer ring spacer (26) are respectively fitted into the inner circumference of the bearing housing (23) with a clearance fit, The fit clearance Δ1 between the outer ring spacer (26) and the bearing housing (23) is greater than the radial expansion amount of the outer ring spacer (26) caused by the load acting on the outer ring spacer (26), and is greater than the fit clearance Δ2 between the outer ring (30, 35) of the bearing on the side where an external force is applied from the outside among the first bearing (24) and the second bearing (25) and the bearing housing (23).

7. The bearing device with a sensor according to claim 6, Characterized in that, The fit clearance Δ2 between the outer ring (30, 35) of the bearing on the side where an axial force is applied from the outside among the first bearing (24) and the second bearing (25) and the bearing housing (23) is 40 μm or less in diameter, and the fit clearance Δ1 between the outer ring spacer (26) and the bearing housing (23) is Δ2 + 50 μm or less in diameter.

8. The bearing device with a sensor according to claim 6 or 7, Characterized in that, The outer ring spacer (26) is composed of a metal outer ring (26a) and a resin inner ring (26b). The outer ring (26a) is fitted into the inner circumference of the bearing housing (23) with a clearance fit and contacts the first outer ring (30) and the second outer ring (35) axially. The inner ring (26b) is arranged on the radially inner side of the outer ring (26a), and the load sensor (28) is installed on the outer ring (26a).

9. The bearing device with a sensor according to any one of claims 6 to 8, Characterized in that, A preload is applied to the first bearing (24) and the second bearing (25).

10. The bearing device with a sensor according to any one of claims 1 to 9, characterized in that the first bearing (24) and the second bearing (25) are angular contact ball bearings.

11. A spindle device for a machine tool, characterized in that it has: the bearing device (1) with a sensor according to any one of claims 1 to 10; a spindle (2) of the machine tool, which is supported by the bearing device (1) with a sensor so as to be rotatable; and a motor (4) that rotationally drives the spindle (2).

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