Bearing device with strain sensor, outer ring spacer with strain sensor, and spindle device for machine tool

Abstract

A bearing device (1) with a strain sensor is configured such that a preload applied from a preload nut (30) is transmitted through a first inner ring (34), a first rolling element (35), a first outer ring (33), an outer ring spacer (26), a second outer ring (38), a second rolling element (40), and a second inner ring (39). The strain sensor is constituted of a central strain sensor (28) disposed at the axial center of the outer ring spacer (26), and an offset strain sensor (29) disposed offset in the axial direction from the axial center of the outer ring spacer (26).

Description

Bearing device with strain sensor, outer ring spacer with strain sensor, and spindle device for machine tool

[0001] The present invention relates to a bearing device with a strain sensor, an outer ring spacer with a strain sensor used in the bearing device with a strain sensor, and a spindle device for a machine tool that uses the bearing device with a strain sensor.

[0002] Machine tools such as machining centers and lathes, as well as other industrial machinery, use spindle devices that rotatably support a rotating shaft on which a tool, workpiece, or other object is attached. In recent years, in fields where such spindle devices are used, there has been a demand for enhanced status monitoring functions to reduce manpower and enable unmanned operation.

[0003] Therefore, the inventors of the present application have already proposed a bearing device with a strain sensor in order to meet the need for enhanced condition monitoring functions (Patent Document 1).

[0004] The bearing device with strain sensor of Patent Document 1 has a first bearing and a second bearing arranged at an axial distance from each other, a cylindrical outer ring spacer provided between the first bearing and the second bearing, and a strain sensor attached to the outer ring spacer, and this strain sensor makes it possible to monitor the magnitude of the preload.

[0005] Here, the first bearing has a first outer ring, a first inner ring, and a plurality of first rolling elements installed between the first outer ring and the first inner ring. Similarly, the second bearing has a second outer ring, a second inner ring, and a plurality of second rolling elements installed between the second outer ring and the second inner ring. The outer ring spacer is arranged axially sandwiched between the first outer ring and the second outer ring.

[0006] A preload is applied between 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 by tightening a preload nut. The first bearing and the second bearing are configured so that the preload load applied by tightening the preload nut is transmitted through the first inner ring, first rolling elements, first outer ring, outer ring spacer, second outer ring, second rolling elements, and second inner ring. The first bearing and the second bearing are a pair of angular contact ball bearings arranged in a back-to-back relationship.

[0007] The strain sensor is attached to the outer periphery (or inner periphery) of the outer ring spacer, and the magnitude of the preload applied to the first bearing and the second bearing can be detected based on the output of this strain sensor.

[0008] The first outer ring and the second outer ring are incorporated into a hollow cylindrical bearing support cylinder, and are fixed by being pressed in the axial direction by a cover member attached to the axial end face of the bearing support cylinder with a bolt.

[0009] Japanese Patent Application Laid-Open No. 2021-014886

[0010] In the bearing device with a strain sensor disclosed in Patent Document 1, two types of loads are simultaneously applied to the outer ring spacer: a preload load from the preload nut and an outer ring fixing load from the cover member.

[0011] In other words, the outer ring spacer is simultaneously subjected to a preload load transmitted through the first inner ring, first rolling element, first outer ring, outer ring spacer, second outer ring, second rolling element, and second inner ring by tightening the preload nut, and an outer ring fixing load transmitted through the first outer ring, outer ring spacer, and second outer ring by fixing the cover member.

[0012] As a result, strain due to the preload load applied by the preload nut and strain due to the outer ring fixing load applied by the cover member simultaneously occur in the outer ring spacer, and the combined strain of these two is detected by the strain sensor in the outer ring spacer.

[0013] Here, if the magnitude of the outer ring fixing load applied by the cover member is a known constant value, the magnitude of the preload can be accurately detected by subtracting the axial load applied by the cover member from the load value detected based on the output of the strain sensor.

[0014] However, in reality, the magnitude of the outer ring fixing load applied by the cover member is not a constant value, and it is difficult to know its magnitude accurately. Therefore, it has not been easy to accurately detect the magnitude of the preload based on the output of a strain sensor attached to the outer ring spacer.

[0015] The problem to be solved by the present invention is to provide a bearing device with a strain sensor that can detect the magnitude of a preload with high accuracy.

[0016] The inventors of the present application considered how to accurately detect the magnitude of the preload based on the output of a strain sensor attached to the outer ring spacer, and noticed that the deformation mode caused in the outer ring spacer by the preload load applied from the preload nut is different from the deformation mode caused in the outer ring spacer by the outer ring fixing load applied from the cover member.

[0017] That is, assuming that the preload applied by the preload nut is transmitted through the first inner ring, first rolling elements, first outer ring, outer ring spacer, second outer ring, second rolling elements, and second inner ring, the axial compressive load causes the outer ring spacer to deform in a manner that expands its axial center (a barrel-shaped deformation). At this time, the first outer ring undergoes elastic deformation in the radial expansion direction due to the radial component force received from the first rolling elements, and the second outer ring also undergoes elastic deformation in the radial expansion direction due to the radial component force received from the second rolling elements. Therefore, the cylindrical outer ring spacer assembled between the first and second outer rings simultaneously undergoes deformation that expands the diameter of both axial ends of the outer ring spacer (deformation such that the axial center of the outer ring spacer becomes constricted relative to the axial ends). In other words, the preload applied by the preload nut simultaneously causes deformation in the outer ring spacer that expands its axial center and expands its diameter at both axial ends.

[0018] On the other hand, when it is assumed that the outer ring fixing load applied from the cover member is transmitted through the first outer ring, outer ring spacer, and second outer ring, the axial compressive load causes deformation in the outer ring spacer that bulges the axial center (barrel-shaped deformation), but no deformation occurs that causes the outer ring spacer to expand in diameter at both axial ends.

[0019] As described above, the deformation mode that occurs in the outer ring spacer due to the preload load applied from the preload nut is a combination of deformation that expands the axial center and deformation that expands the axial ends, while the deformation mode that occurs in the outer ring spacer due to the outer ring fixing load applied from the cover member is composed only of deformation that expands the axial center and does not include deformation that expands the axial ends, and the inventors of the present application have noticed that the former deformation mode and the latter deformation mode are different from each other.

[0020] The inventors of the present application then came up with the idea that if the amount of deformation at the axial center of the outer ring spacer and the amount of deformation at a position axially offset from the axial center of the outer ring spacer are determined simultaneously, then the proportion of the deformation of the outer ring spacer that is due to the deformation mode caused by the preload load applied from the preload nut and the proportion of the deformation mode caused by the outer ring fixed load applied from the cover member will be uniquely determined. Therefore, if strain sensors are placed at the axial center of the outer ring spacer and at a position axially offset from the axial center of the outer ring spacer, it will be possible to accurately detect the magnitude of the preload load applied by the preload nut based on the outputs of these strain sensors.

[0021] Based on this idea, in order to solve the above problems, the present invention provides a bearing device with a strain sensor having the following configuration. [Configuration 1] A bearing device with a strain sensor, comprising: a first bearing and a second bearing arranged at an interval 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, wherein the first bearing has a first outer ring, a first inner ring, and a plurality of first rolling elements incorporated between the first outer ring and the first inner ring; and the second bearing has a second outer ring, a second inner ring, and a plurality of second rolling elements incorporated between the second outer ring and the second inner ring; and further comprising a preload applying portion that applies a preload between an axial end face of the first inner ring opposite to the second inner ring side and an axial end face of the second inner ring opposite to the first inner ring side, and an outer ring pressing portion that applies an outer ring fixing load between the axial end face of the first outer ring opposite to the second outer ring side and an axial end face of the second outer ring opposite to the first outer ring side, A bearing device with strain sensors, characterized in that the strain sensors are composed of a central strain sensor arranged at the axial center of the outer ring spacer, and an offset strain sensor arranged axially offset from the axial center of the outer ring spacer.

[0022] By adopting this configuration, it is possible to eliminate the influence of the outer ring fixed load applied from the outer ring pressing portion and accurately detect the magnitude of the preload applied from the preload applying portion based on the output of the central strain sensor, which detects the magnitude of strain at the axial center of the outer ring spacer, and the output of the offset strain sensor, which detects the magnitude of strain at a position axially offset from the axial center of the outer ring spacer.

[0023] [Configuration 2] The bearing device with strain sensors according to Configuration 1, wherein three or more of the central strain sensors are provided at intervals in the circumferential direction, and three or more of the offset strain sensors are also provided at intervals in the circumferential direction.

[0024] By adopting this configuration, the magnitude of the preload can be accurately detected even when a circumferential bias occurs in the load applied to the outer ring spacer. In other words, even when a circumferential bias occurs in the magnitude of strain at the axial center of the outer ring spacer, the magnitude of strain at the axial center of the outer ring spacer can be detected based on the outputs of three or more central strain sensors arranged at intervals in the circumferential direction, eliminating the influence of the circumferential bias. Similarly, even when a circumferential bias occurs in the magnitude of strain at a position axially offset from the axial center of the outer ring spacer, the magnitude of strain at the position axially offset from the axial center of the outer ring spacer can be detected based on the outputs of three or more offset strain sensors arranged at intervals in the circumferential direction, eliminating the influence of the circumferential bias. Therefore, even when a circumferential bias occurs in the preload or outer ring fixing load applied to the outer ring spacer, the magnitude of the preload can be accurately detected based on the outputs of three or more offset strain sensors arranged at intervals in the circumferential direction, eliminating the influence of the circumferential bias.

[0025] [Configuration 3] A bearing device with a strain sensor according to Configuration 1 or 2, wherein the axial distance from the position of the offset strain sensor to the axial end of the outer ring spacer is set to ¼ or less of the overall length of the outer ring spacer.

[0026] When this configuration is adopted, the offset strain sensor is positioned sufficiently close to the axial end of the outer ring spacer, making it possible to detect with particularly high accuracy strain associated with deformation that expands the diameter of both axial ends of the outer ring spacer (deformation that occurs in the outer ring spacer due to the preload load applied from the preload application section).

[0027] [Configuration 4] The bearing device with a strain sensor according to any one of Configurations 1 to 3, wherein the first bearing and the second bearing are a pair of angular contact ball bearings arranged in a back-to-back relationship.

[0028] [Configuration 5] The bearing device with strain sensors according to any one of Configurations 1 to 4, further comprising a preload calculation processing unit that calculates the magnitude of the preload based on the output of the central strain sensor and the output of the offset strain sensor.

[0029] The present invention also provides an outer ring spacer with a strain sensor for use in the above-mentioned bearing device with a strain sensor, having the following configuration: [Configuration 6] An outer ring spacer with a strain sensor, comprising: a cylindrical outer ring spacer provided between a first bearing and a second bearing that are spaced apart in the axial direction; and a strain sensor attached to the outer ring spacer, wherein the strain sensor is made up of a central strain sensor located in the axial center of the outer ring spacer; and an offset strain sensor located axially offset from the axial center of the outer ring spacer.

[0030] [Configuration 7] The outer ring spacer with strain sensors according to Configuration 6, wherein three or more of the central strain sensors are provided at intervals in the circumferential direction, and three or more of the offset strain sensors are also provided at intervals in the circumferential direction.

[0031] [Configuration 8] The outer ring spacer with strain sensor according to Configuration 6 or 7, wherein the axial distance from the position of the offset strain sensor to the axial end of the outer ring spacer is set to ¼ or less of the overall length of the outer ring spacer.

[0032] The present invention also provides a spindle device for a machine tool that uses the above-mentioned bearing device with a strain sensor, having the following configuration: [Configuration 9] A spindle device for a machine tool, comprising: the bearing device with a strain sensor according to any one of configurations 1 to 5; a main spindle of a machine tool that is rotatably supported by the bearing device with a strain sensor; and a motor that rotationally drives the main spindle.

[0033] This configuration enables stable status monitoring for reducing the number of workers required for machine tools and for unmanned operation. It also makes it possible to detect the machining load acting on the spindle of the machine tool during cutting.

[0034] The bearing device with strain sensors of this invention is able to eliminate the influence of the outer ring fixed load applied from the outer ring pressing portion and accurately detect the magnitude of the preload applied from the preload applying portion, based on the output of the central strain sensor, which detects the magnitude of strain at the axial center of the outer ring spacer, and the output of the offset strain sensor, which detects the magnitude of strain at a position axially offset from the axial center of the outer ring spacer.

[0035] 2 is a cross-sectional view showing a spindle device for a machine tool that uses a bearing device with a strain sensor according to an embodiment of the present invention; FIG. 1 is an enlarged view of the vicinity of the bearing device with a strain sensor of FIG. 1; FIG. 2 is a cross-sectional view taken along line III-III of FIG. 2; FIG. 3 is a diagram showing the transmission path of the preload load applied from the preload nut shown in FIG. 2 and the transmission path of the outer ring fixing load applied from the cover member;

[0036] 1 shows a spindle device for a machine tool that uses a bearing device with a strain sensor 1 according to an embodiment of the present invention (hereinafter simply referred to as "bearing device 1"). This spindle device has a main spindle 2 of the machine tool, a main spindle housing 3 that accommodates the main spindle 2, a motor 4 that rotates and drives the main spindle 2, the bearing device 1 of the embodiment that rotatably supports the main spindle 2 axially forward of the motor 4, and a rear bearing device 5 that rotatably supports the main spindle 2 axially rearward of the motor 4.

[0037] The axial direction is a direction parallel to the central axis of the main shaft 2, the radial direction is a direction perpendicular to the central axis of the main shaft 2, and the circumferential direction is a direction along the circumference revolving around the central axis of the main shaft 2. The central axes of a first bearing 24, a second bearing 25, and an outer ring spacer 26, which will be described later, coincide with the central axis of the main shaft 2.

[0038] The spindle housing 3 is formed in the shape of 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 drawing, the portion of the spindle housing 3 that accommodates the bearing device 1 and the portion of the spindle housing 3 that accommodates the motor 4 are formed seamlessly as a single unit, but the portion of the spindle housing 3 that accommodates the bearing device 1 and the portion of the spindle housing 3 that accommodates the motor 4 may be formed separately and then connected together to form a single unit.

[0039] The spindle 2 is inserted into the spindle housing 3 with its front end protruding from the front-end opening of the spindle housing 3. A chuck (not shown) for gripping a tool or a 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 a machine tool so that the drawbar can slide axially therethrough.

[0040] Motor 4 has a rotor 7 attached to the outer periphery of main shaft 2 and an annular stator 8 that applies a rotational force to rotor 7. Rotor 7 has a rotor sleeve 9 that fits around the outer periphery of main shaft 2 and a rotor core 10 fixed to the outer periphery of rotor sleeve 9. Rotor core 10 is, for example, a laminate of electromagnetic steel sheets. Rotor sleeve 9 is prevented from rotating by main shaft 2 so as to rotate integrally with main shaft 2. The axial front end of rotor sleeve 9 contacts a step 11 formed on the outer periphery of main shaft 2, and is positioned axially by this contact with step 11.

[0041] Stator 8 has a stator core 12 fixed to the inner periphery of spindle housing 3, and electromagnetic coils 13 wound around a plurality of teeth formed at intervals in the circumferential direction on stator core 12. When current is applied to electromagnetic coil 13, a rotational force is generated in rotor core 10 due to the electromagnetic force acting between stator core 12 and rotor core 10, causing rotor 7 and spindle 2 to rotate integrally. Here, an electric motor that generates rotational force using electricity is used as motor 4, but instead of an electric motor, a motor that generates rotational force using another power source such as compressed air can also be used.

[0042] Rear bearing device 5 has an annular bearing support member 14 coaxially fixed to the rear end of spindle housing 3, and a rolling bearing 15 incorporated in bearing support member 14. Rolling bearing 15 is a cylindrical roller bearing having an outer ring 16 fitted onto the inner periphery of bearing support member 14, an inner ring 17 fitted onto the outer periphery of spindle 2, and a plurality of cylindrical rollers 18 incorporated between outer ring 16 and inner ring 17.

[0043] 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 surface of the outer ring 16. A nut member 20 that presses the inner ring 17 axially forward and an annular spacer 21 incorporated 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 surface of the spacer 21 contacts the axial rear end surface of the inner ring 17, and the axial rear end surface of the spacer 21 contacts the axial front end surface of the nut member 20. The axial front end surface of the inner ring 17 contacts the axial rear end of the rotor sleeve 9.

[0044] The bearing device 1 has a hollow cylindrical bearing support tube 23 fixed to the spindle housing 3, a first bearing 24 and a second bearing 25 assembled into the bearing support tube 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, a central strain sensor 28 and an offset strain sensor 29 attached to the outer ring spacer 26, a preload nut 30 that applies a preload to the first bearing 24 and the second bearing 25, and a cover member 31 that fixes the first bearing 24 and the second bearing 25 to the bearing support tube 23.

[0045] The bearing support cylinder 23 is fitted onto the inner periphery of the spindle housing 3. Cooling grooves 32, through which a refrigerant for cooling the bearing device 1 flows, are formed on the outer periphery of the bearing support cylinder 23. The cooling grooves 32 are a plurality of annular grooves formed at intervals in the axial direction on the outer periphery of the bearing support cylinder 23, or spiral grooves that extend spirally around the outer periphery of the bearing support cylinder 23.

[0046] 2, the first bearing 24 has a first outer ring 33 fitted onto the inner periphery of the bearing support cylinder 23, a first inner ring 34 arranged coaxially radially inside the first outer ring 33, and a plurality of first rolling elements 35 assembled at intervals in the circumferential direction between the first outer ring 33 and the first inner ring 34. The first outer ring 33 rotatably supports the first inner ring 34 via the first rolling elements 35.

[0047] In this example, the first rolling elements 35 are balls. The inner circumference of the first outer ring 33 is provided with a first outer ring raceway surface 36 having an arcuate cross section, with which the first rolling elements 35 roll. The first outer ring 33 is a shouldered outer ring having an axially front outer ring shoulder and an axially rear outer ring shoulder with respect to the first outer ring raceway surface 36, with the axially front outer ring shoulder removed. The outer circumference of the first outer ring 33 is fitted into the inner circumference of the bearing support sleeve 23 with a gap. The axial rear end face of the first outer ring 33 (the axial end face of the first outer ring 33 on the second outer ring side) is in contact with the axial front end of the outer ring spacer 26.

[0048] A first inner ring raceway surface 37 having an arcuate cross section with which the first rolling elements 35 make rolling contact is provided on the outer periphery of the first inner ring 34. The first inner ring 34 is a shouldered inner ring having a shape in which the axially rear inner ring shoulder is removed from an inner ring shoulder on the axially front side and an inner ring shoulder on the axially rear side with respect to the first inner ring raceway surface 37 with which the first rolling elements 35 make rolling contact. The first inner ring 34 is fitted onto the outer periphery of the main shaft 2 with interference.

[0049] Like the first bearing 24, the second bearing 25 has a second outer ring 38 that fits onto the inner periphery of the bearing support cylinder 23, a second inner ring 39 that is coaxially arranged radially inside the second outer ring 38, and a plurality of second rolling elements 40 that are incorporated at intervals in the circumferential direction between the second outer ring 38 and the second inner ring 39. The second outer ring 38 rotatably supports the second inner ring 39 via the second rolling elements 40.

[0050] In this example, the second rolling elements 40 are balls. The inner circumference of the second outer ring 38 is provided with a second outer ring raceway surface 41 with an arcuate cross section, with which the second rolling elements 40 roll. The second outer ring 38 is spaced axially rearward from the first outer ring 33, and the second inner ring 39 is also spaced axially rearward from the first inner ring 34. The second outer ring 38 is a shouldered outer ring, with the axially rear outer ring shoulder removed from the outer ring shoulders on the axial front and rear sides of the second outer ring raceway surface 41. The outer circumference of the second outer ring 38 is fitted with a gap to the inner circumference of the bearing support sleeve 23. The axial front end face of the second outer ring 38 (the axial end face of the second outer ring 38 on the first outer ring 33 side) contacts the axial rear end of the outer ring spacer 26.

[0051] A second inner ring raceway surface 42 having an arcuate cross section with which the second rolling elements 40 make rolling contact is provided on the outer periphery of the second inner ring 39. The second inner ring 39 is a shouldered inner ring having an inner ring shoulder on the axially front side and an inner ring shoulder on the axially rear side with respect to the second inner ring raceway surface 42 with which the second rolling elements 40 make rolling contact, with the axially front inner ring shoulder removed. The second inner ring 39 is fitted onto the outer periphery of the spindle 2 with interference.

[0052] Here, the first bearing 24 is configured so that when an axial preload is applied between the first outer ring 33 and the first inner ring 34, the first rolling element 35 generates a radial component force that presses the first outer ring 33 radially outward; similarly, the second bearing 25 is configured so that when an axial preload is applied between the second outer ring 38 and the second inner ring 39, the second rolling element 40 generates a radial component force that presses the second outer ring 38 radially outward.

[0053] In this embodiment, the first bearing 24 and the second bearing 25 are a pair of angular contact ball bearings arranged in a back-to-back relationship. That is, the first bearing 24 is an angular contact ball bearing arranged so that a line connecting the contact point between the first inner ring 34 and the first rolling element 35 and the contact point between the first outer ring 33 and the first rolling element 35 is inclined axially rearward from the radially inner side to the radially outer side, and the second bearing 25 is an angular contact ball bearing arranged so that a line connecting the contact point between the second inner ring 39 and the second rolling element 40 and the contact point between the second outer ring 38 and the second rolling element 40 is inclined axially forward from the radially inner side to the radially outer side.

[0054] The outer ring spacer 26 is a hollow cylindrical member with both ends open. The outer ring spacer 26 is fitted onto the inner periphery of the bearing support cylinder 23 with a gap therebetween. The outer ring spacer 26 is sandwiched in the axial direction between the first outer ring 33 and the second outer ring 38. The axial front end of the outer ring spacer 26 contacts the axial rear end face of the first outer ring 33 (the axial end face of the first outer ring 33 on the side of the second outer ring 38), and the axial rear end of the outer ring spacer 26 contacts the axial front end face of the second outer ring 38 (the axial end face of the second outer ring 38 on the side of the first outer ring 33).

[0055] Like 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 onto the outer periphery of the spindle 2 with a gap therebetween. The inner ring spacer 27 is sandwiched between the first inner ring 34 and the second inner ring 39 in the axial direction.

[0056] 4, an annular inner race positioning step 43 is formed on the outer periphery of the spindle 2, axially facing the axial front end face of the first inner race 34 (the axial end face of the first inner race 34 opposite the side of the second inner race 39). The inner race positioning step 43 positions the first inner race 34 in the axial direction by restricting movement of the first inner race 34 forward in the axial direction (in the direction away from the second outer race 38).

[0057] A preload nut 30 is attached to the outer periphery of the main shaft 2, axially rearward of the second inner ring 39. The preload nut 30 is threadedly engaged with a male thread 44 formed on the outer periphery of the main shaft 2. An annular spacer 45 is fitted between the second inner ring 39 and the preload nut 30. The axial front end face of the spacer 45 contacts the axial rear end face of the second inner ring 39, and the axial rear end face of the spacer 45 contacts the axial front end face of the preload nut 30.

[0058] The preload nut 30 is tightened with a predetermined force, and the axial force of the preload nut 30 applies a preload load between the axial front end face of the first inner ring 34 (the axial end face of the first inner ring 34 opposite the second inner ring 39 side) and the axial rear end face of the second inner ring 39 (the axial end face of the second inner ring 39 opposite the first inner ring 34 side) in a direction that brings the first inner ring 34 and the second inner ring 39 closer together. Here, the preload applied by the preload nut 30 is transmitted through the first inner ring 34, first rolling element 35, first outer ring 33, outer ring spacer 26, second outer ring 38, second rolling element 40, and second inner ring 39, as shown by the thick solid line in the figure.

[0059] An annular outer ring positioning step 46 is formed on the inner periphery of the bearing support cylinder 23, axially facing the axial rear end face of the second outer ring 38 (the axial end face of the second outer ring 38 on the opposite side from the first outer ring 33). The outer ring positioning step 46 positions the second outer ring 38 in the axial direction by restricting movement of the second outer ring 38 axially rearward (in the direction away from the first outer ring 33).

[0060] A cover member 31 is attached to the axial front end surface of the bearing support cylinder 23 with bolts 47, and the first outer ring 33 and the second outer ring 38 are pressed and fixed in the axial direction by the cover member 31. The cover member 31 has a circular plate-shaped flange portion 48 fixed to the axial front end surface of the bearing support cylinder 23 with the bolts 47, and an annular protrusion portion 49 protruding in the axial direction from the radial inner end of the flange portion 48 along the inner circumference of the bearing support cylinder 23. The tip of the annular protrusion portion 49 abuts against the axial front end surface of the first outer ring 33 (the axial end surface of the first outer ring 33 opposite the side where the second outer ring 38 is located). As shown in FIG. 3 , a plurality of bolts 47 are provided at equal intervals around the circumferential direction.

[0061] 4, the first outer ring 33 and the second outer ring 38 are fitted between the cover member 31 and the outer ring positioning step 46 with an axial interference, and this axial interference applies an outer ring fixing load between the axial front end face of the first outer ring 33 (the axial end face of the first outer ring 33 opposite the second outer ring 38 side) and the axial rear end face of the second outer ring 38 (the axial end face of the second outer ring 38 opposite the first outer ring 33 side). Here, the outer ring fixing load applied by the cover member 31 is transmitted through the first outer ring 33, the outer ring spacer 26 and the second outer ring 38, as shown by the thick dashed line in the figure.

[0062] The central strain sensor 28 is disposed in the axial center of the outer ring spacer 26. Specifically, the central strain sensor 28 is disposed so that the center of the strain detection portion of the central strain sensor 28 is located at a position that bisects the distance between the axial front end and the axial rear end of the outer ring spacer 26, or so that the axial deviation from that position to the center of the strain detection portion of the central strain sensor 28 is less than 10% of the entire axial length of the outer ring spacer 26.

[0063] The offset strain sensor 29 is disposed offset axially forward from the axial center of the outer ring spacer 26. As shown in Fig. 6 , the axial distance d from the position of the offset strain sensor 29 to the axial front end of the outer ring spacer 26 is ¼ or less (preferably ⅕ or less) of the overall length L of the outer ring spacer 26. Specifically, the offset strain sensor 29 is disposed so that the axial distance d from the axial front end of the outer ring spacer 26 to the center of the strain detection portion of the offset strain sensor 29 is ¼ or less (preferably ⅕ or less) of the length from the axial front end to the axial rear end of the outer ring spacer 26.

[0064] 3, three or more (three in this example) central strain sensors 28 are provided at equal intervals in the circumferential direction on the inner circumference of the outer ring spacer 26. Axial grooves 50, the same number as the central 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 50 has a planar groove bottom surface extending in the axial direction, and a central strain sensor 28 is fixedly attached to each groove bottom surface.

[0065] 5, three or more (three in this example) offset strain sensors 29 are also provided at equal intervals in the circumferential direction on the inner circumference of the outer ring spacer 26. Each offset strain sensor 29 is disposed at the same circumferential position as each central strain sensor 28. Like the central strain sensors 28, each offset strain sensor 29 is fixedly attached to the flat groove bottom surface of each axial groove 50.

[0066] The central strain sensor 28 uses a strain gauge whose electrical resistance changes in response to axial strain at the bonded portion. Like the central strain sensor 28, the offset strain sensor 29 also uses a strain gauge whose electrical resistance changes in response to axial strain at the bonded portion. The outputs of these strain gauges change in response to axial compressive deformation, bending deformation, radial expansion deformation, etc. at the locations of the outer ring spacer 26 where the strain gauges are attached. As shown in FIG. 7 , the central strain sensor 28 and the offset strain sensor 29 are electrically connected to a preload calculation processor 51, and the output signals of the central strain sensor 28 and the offset strain sensor 29 are input to the preload calculation processor 51. The preload calculation processor 51 is, for example, an electronic circuit or electronic control device provided outside the bearing device 1.

[0067] As shown in FIG. 4, two types of loads are simultaneously applied to the outer ring spacer 26: a preload load from the preload nut 30 and an outer ring fixing load from the cover member 31.

[0068] That is, as shown by the thick solid line in FIG. 4 , the outer ring spacer 26 is simultaneously subjected to a preload load that is transmitted through the first inner ring 34, first rolling element 35, first outer ring 33, outer ring spacer 26, second outer ring 38, second rolling element 40, and second inner ring 39 by tightening the preload nut 30, and an outer ring fixing load that is transmitted through the first outer ring 33, outer ring spacer 26, and second outer ring 38 by fixing the cover member 31, as shown by the thick dashed line in FIG. 4 .

[0069] Therefore, strain due to the preload load applied from the preload nut 30 and strain due to the outer ring fixing load applied from the cover member 31 simultaneously occur in the outer ring spacer 26. The combined strain of these two strains is detected by the central strain sensor 28.

[0070] Here, if the magnitude of the outer ring fixing load applied by the cover member 31 is a known constant value, the magnitude of the preload can be detected by subtracting the axial load applied by the cover member 31 from the load value detected based on the output of the central strain sensor 28.

[0071] However, in reality, the magnitude of the outer ring fixing load applied by the cover member 31 is not a constant value, and it is difficult to know the magnitude of the load accurately. Therefore, it is not easy to accurately detect the magnitude of the preload based on the output of the center strain sensor 28.

[0072] Therefore, in order to accurately detect the magnitude of the preload, this embodiment takes note of the fact that the deformation mode caused in the outer ring spacer 26 by the preload load applied from the preload nut 30 is different from the deformation mode caused in the outer ring spacer 26 by the outer ring fixing load applied from the cover member 31, and employs a configuration in which, in addition to a central strain sensor 28 placed in the axial center of the outer ring spacer 26, an offset strain sensor 29 is provided that is positioned axially offset from the axial center of the outer ring spacer 26.

[0073] 4 , assuming that the preload applied from the preload nut 30 is transmitted through the first inner ring 34, the first rolling elements 35, the first outer ring 33, the outer ring spacer 26, the second outer ring 38, the second rolling elements 40, and the second inner ring 39, the axial compressive load causes deformation (barrel-shaped deformation) in the outer ring spacer 26, expanding the axial center thereof. At this time, the first outer ring 33 undergoes elastic deformation in the radial expansion direction due to the radial component force received from the first rolling elements 35, and the second outer ring 38 also undergoes elastic deformation in the radial expansion direction due to the radial component force received from the second rolling elements 40. Therefore, the cylindrical outer ring spacer 26 assembled between the first outer ring 33 and the second outer ring 38 simultaneously undergoes deformation that expands the diameter of both axial ends of the outer ring spacer 26 (deformation such that the axial center of the outer ring spacer 26 becomes constricted relative to the axial ends). In other words, the preload applied by the preload nut 30 simultaneously causes the outer ring spacer 26 to deform in such a way that the axial center expands and the diameter of both axial ends increases.

[0074] On the other hand, as shown by the thick dashed line in Figure 4, when it is assumed that the outer ring fixing load applied from the cover member 31 is transmitted through the first outer ring 33, the outer ring spacer 26, and the second outer ring 38, the axial compressive load causes deformation in the outer ring spacer 26 that expands the axial center (barrel-shaped deformation), but does not cause deformation that expands the diameter of both axial ends of the outer ring spacer 26.

[0075] As described above, the deformation mode that occurs in the outer ring spacer 26 due to the preload load applied from the preload nut 30 is a combination of deformation that expands the axial center and deformation that expands the axial ends. On the other hand, the deformation mode that occurs in the outer ring spacer 26 due to the outer ring fixing load applied from the cover member 31 is composed only of deformation that expands the axial center and does not include deformation that expands the axial ends, and the former deformation mode and the latter deformation mode are different from each other.

[0076] Therefore, if the amount of deformation at the axial center position of the outer ring spacer 26 and the amount of deformation at a position axially offset from the axial center of the outer ring spacer 26 are determined simultaneously, the proportion of the deformation of the outer ring spacer 26 that is due to the deformation mode caused by the preload load applied from the preload nut 30 and the proportion of the deformation mode caused by the outer ring fixing load applied from the cover member 31 can be uniquely determined.

[0077] Therefore, in this embodiment, in addition to the central strain sensor 28 arranged in the axial center of the outer ring spacer 26, an offset strain sensor 29 is provided that is arranged offset in the axial direction from the axial center of the outer ring spacer 26, and the preload calculation processing unit 51 shown in FIG. 7 detects the magnitude of the preload applied by the preload nut 30 based on the output of the central strain sensor 28 and the output of the offset strain sensor 29, eliminating the effect of the outer ring fixing load applied by the cover member 31.

[0078] 7 calculates the magnitude of the preload based on the output of the center strain sensor 28 and the output of the offset strain sensor 29. This calculation of the preload can be performed, for example, as follows.

[0079] First, a test bearing device 1 is prepared that replicates the same configuration as the bearing device 1 shown in Fig. 4. Then, using this bearing device 1, a test is performed in which an outer ring fixing load by the lid member 31 and a preload load by the preload nut 30 are simultaneously applied, the magnitude of the outer ring fixing load and the magnitude of the preload load are changed independently, and the changes in the output of the central strain sensor 28 and the offset strain sensor 29 are recorded. This test determines the correspondence between the outputs of the central strain sensor 28 and the offset strain sensor 29 and the combination of the magnitude of the outer ring fixing load by the lid member 31 and the magnitude of the preload by the preload nut 30, making it possible to create conversion data for converting the preload (and outer ring fixing load) from the outputs of the central strain sensor 28 and the offset strain sensor 29.

[0080] Next, the conversion data obtained as described above is stored in the preload calculation processor 51 shown in Fig. 7. Based on the conversion data, the preload calculation processor 51 converts the outputs of the central strain sensor 28 and the offset strain sensor 29 into the magnitude of the preload. The conversion data may be a conversion map that stores the individual relationships between the outputs of the central strain sensor 28, the offset strain sensor 29, and the magnitude of the preload applied by the preload nut 30, or may be a mathematical approximation of the relationship between the outputs of the central strain sensor 28, the offset strain sensor 29, and the magnitude of the preload applied by the preload nut 30.

[0081] Here, the outputs of the multiple central strain sensors 28 spaced apart in the circumferential direction can be used as the sum (or average) of their outputs, and similarly, the outputs of the offset strain sensors 29 spaced apart in the circumferential direction can be used as the sum (or average) of their outputs.

[0082] As shown in Figure 4, this bearing device 1 is able to eliminate the influence of the outer ring fixed load applied from the cover member 31 and accurately detect the magnitude of the preload applied from the preload nut 30 based on the output of the central strain sensor 28, which detects the magnitude of strain at the axial center of the outer ring spacer 26, and the output of the offset strain sensor 29, which detects the magnitude of strain at a position axially offset from the axial center of the outer ring spacer 26.

[0083] Furthermore, as shown in Fig. 5, this bearing device 1 has three or more (three in this embodiment) central strain sensors 28 provided at equal intervals in the circumferential direction, and three or more (three in this embodiment) offset strain sensors 29 also provided at equal intervals in the circumferential direction, so it is possible to accurately detect the magnitude of the preload even if a circumferential bias occurs in the load applied to the outer ring spacer 26 shown in Fig. 4. In other words, even if a circumferential bias occurs in the magnitude of strain at the axial center of the outer ring spacer 26, it is possible to detect the magnitude of strain at the axial center of the outer ring spacer 26 based on the output of each central strain sensor 28 arranged at intervals in the circumferential direction, with the influence of the circumferential bias eliminated. Similarly, even if a circumferential bias occurs in the magnitude of strain at a position axially offset from the axial center of the outer ring spacer 26, the magnitude of strain at the position axially offset from the axial center of the outer ring spacer 26 can be detected without the influence of the circumferential bias based on the output of each offset strain sensor 29 arranged at intervals in the circumferential direction. Therefore, even if a circumferential bias occurs in the preload load or outer ring fixing load applied to the outer ring spacer 26, it is possible to accurately detect the magnitude of the preload without the influence of the circumferential bias.

[0084] 6, in this bearing device 1, the axial distance d from the position of the offset strain sensor 29 to the axial end of the outer ring spacer 26 is set to ¼ or less (preferably ⅕ or less) of the overall length L of the outer ring spacer 26, and the position of the offset strain sensor 29 is sufficiently close to the axial end of the outer ring spacer 26. Therefore, it is possible to detect with particularly high accuracy strain associated with deformation that expands the diameter of both axial ends of the outer ring spacer 26 (deformation occurring in the outer ring spacer 26 due to the preload load applied by the preload nut 30).

[0085] Furthermore, as shown in FIG. 5 , this bearing device 1 has three or more (three in this embodiment) central strain sensors 28 provided at equal intervals around the circumferential direction, and three or more (three in this embodiment) offset strain sensors 29 also provided at equal intervals around the circumferential direction, so it is also possible to detect the magnitude and direction of the moment load acting on the spindle 2 of the machine tool during cutting processing based on the output of each central strain sensor 28 and each offset strain sensor 29.

[0086] In the above embodiment, as shown in FIG. 4 , the offset strain sensor 29 is provided only at the front of the two axially forward and rearward positions from the axial center of the outer ring spacer 26. However, as shown in FIG. 8 , the offset strain sensor 29 may be provided on both axial sides of the axial center of the outer ring spacer 26.

[0087] In the above embodiment, as shown in Fig. 4, the central strain sensor 28 and the offset strain sensor 29 are provided on the inner periphery of the outer ring spacer 26, but as shown in Fig. 9, the central strain sensor 28 and the offset strain sensor 29 may be provided on the outer periphery of the outer ring spacer 26. In Fig. 9, the central strain sensor 28 and the offset strain sensor 29 are fixedly attached to D-cut portions 52 formed at equal intervals in the circumferential direction on the outer periphery of the outer ring spacer 26. The D-cut portions 52 are flat surfaces shaped by cutting the outer periphery of the outer ring spacer 26 along a plane parallel to the axis.

[0088] In the above embodiment, angular contact ball bearings have been used as examples of 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.

[0089] The embodiments disclosed herein should be considered to be illustrative in all respects and not restrictive. The scope of the present invention is defined by the claims, not by the above description, and is intended to include all modifications within the meaning and scope of the claims.

[0090] REFERENCE SIGNS LIST 1 Bearing device with strain sensor 2 Main shaft 4 Motor 24 First bearing 25 Second bearing 26 Outer ring spacer 28 Central strain sensor 29 Offset strain sensor 30 Preload nut (preload applying portion) 31 Cover member (outer ring pressing portion) 33 First outer ring 34 First inner ring 35 First rolling element 38 Second outer ring 39 Second inner ring 40 Second rolling element 51 Preload calculation processing unit d Axial distance L Overall length

Claims

1. A bearing comprising a first bearing (24) and a second bearing (25) 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 attached to the outer ring spacer (26), wherein the first bearing (24) comprises a first outer ring (33), a first inner ring (34), and a plurality of first rolling elements (35) incorporated between the first outer ring (33) and the first inner ring (34), and the second bearing (25) comprises a second outer ring (38), a second inner ring (39), and a plurality of second rolling elements (40) incorporated between the second outer ring (38) and the second inner ring (39), a preload applying portion (30) that applies a preload between an axial end face of the first inner ring (34) opposite to the second inner ring (39) side and an axial end face of the second inner ring (39) opposite to the first inner ring (34) side, and an outer ring pressing portion (31) that applies an outer ring fixing load between an axial end face of the first outer ring (33) opposite to the second outer ring (38) side and an axial end face of the second outer ring (38) opposite to the first outer ring (33) side, wherein the strain sensor is composed of a central strain sensor (28) arranged in the axial center of the outer ring spacer (26), and an offset strain sensor (29) arranged offset in the axial direction from the axial center of the outer ring spacer (26).

2. A bearing device with strain sensors as described in claim 1, wherein three or more of the central strain sensors (28) are provided at intervals in the circumferential direction, and three or more of the offset strain sensors (29) are also provided at intervals in the circumferential direction.

3. A bearing device with a strain sensor as described in claim 1 or 2, wherein the axial distance (d) from the position of the offset strain sensor (29) to the axial end of the outer ring spacer (26) is set to 1 / 4 or less of the total length (L) of the outer ring spacer (26).

4. A bearing device with a strain sensor as described in any one of claims 1 to 3, wherein the first bearing (24) and the second bearing (25) are a pair of angular contact ball bearings arranged in a back-to-back combination relationship.

5. A bearing device with a strain sensor as described in any one of claims 1 to 4, which has a preload calculation processing unit (51) that calculates the magnitude of the preload based on the output of the central strain sensor (28) and the output of the offset strain sensor (29).

6. An outer ring spacer with strain sensor, comprising a cylindrical outer ring spacer (26) provided between a first bearing (24) and a second bearing (25) spaced apart in the axial direction, and a strain sensor attached to the outer ring spacer (26), wherein the strain sensor is composed of a central strain sensor (28) located in the axial center of the outer ring spacer (26) and an offset strain sensor (29) located offset in the axial direction from the axial center of the outer ring spacer (26).

7. An outer ring spacer with strain sensors as described in claim 6, wherein three or more of the central strain sensors (28) are provided at intervals in the circumferential direction, and three or more of the offset strain sensors (29) are also provided at intervals in the circumferential direction.

8. An outer ring spacer with a strain sensor as described in claim 6 or 7, wherein the axial distance (d) from the position of the offset strain sensor (29) to the axial end of the outer ring spacer (26) is set to 1 / 4 or less of the total length (L) of the outer ring spacer (26).

9. A spindle device for a machine tool comprising: a bearing device with a strain sensor (1) according to any one of claims 1 to 5; a main spindle (2) of a machine tool rotatably supported by the bearing device with a strain sensor (1); and a motor (4) that rotates and drives the main spindle (2).

Images