Sensor-equipped storage device and spindle device for machine tools

Abstract

A sensor-equipped bearing device is provided in which a preload is transmitted to a first inner ring (31), first rolling elements (32), a first outer ring (30), an outer ring spacer (26), a second outer ring (35), second rolling elements (37), and a second inner ring (36). In the bearing device, a compressive force is exerted on the axial end surface of the first outer ring (30) remote from the second outer ring (35) and on the axial end surface of the second outer ring (35) remote from the first outer ring (30), so that the first outer ring (30) and the second outer ring (35) approach each other, and the compressive force is greater than the preload.

Description

Technical area

[0001] The present invention relates to a sensor-equipped bearing device and a spindle device for a machine tool in which the sensor-equipped bearing device is used. State of the art

[0002] In machine tools such as machining centers and lathes, as well as other industrial machines, a device is used that rotatably supports a rotating shaft (spindle) to which a target object, such as a tool or workpiece, is attached. In the field where such a device is used, there has recently been a need to improve a condition monitoring function for labor-saving and automated operation.

[0003] In order to meet the requirements for improving condition monitoring, the applicant of this application has already proposed a sensor-equipped bearing device (hereinafter referred to as Patent Document 1).

[0004] The sensor-equipped bearing device disclosed in Patent Document 1 includes a first bearing and a second bearing axially spaced from each other, a tubular outer ring spacer disposed between the first bearing and the second bearing, and a stress sensor attached to the outer ring spacer.

[0005] The first bearing includes a first outer ring, a first inner ring disposed radially inward of the first outer ring, and a plurality of first rolling elements disposed between the first outer ring and the first inner ring. Likewise, the second bearing includes a second outer ring, a second inner ring disposed radially inward of the second outer ring, and a plurality of second rolling elements disposed between the second outer ring and the second inner ring. The outer ring spacer is axially embedded between the first outer ring and the second outer ring.

[0006] By tightening a preload nut, a preload is applied to the axial end surface of the first inner ring remote from the second inner ring and to the axial end surface of the second inner ring remote from the first inner ring, so that the first inner ring and the second inner ring approach each other. The first bearing and the second bearing are designed so that this preload is transmitted to the first inner ring, the first spacer, 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 so that a radial component force with which the first rolling elements press on the first outer ring is generated by the axial preload between the first outer ring and the first inner ring.The second bearing is also an angular contact ball bearing configured so that a radial component force with which the second rolling elements press on the second outer ring is generated by the axial preload between the second outer ring and the second inner ring.

[0007] The tension sensor is mounted on the outer circumference (or inner circumference) of the outer ring spacer. A preload applied to the first bearing and the second bearing (hereinafter referred to as "bearing preload") can be detected based on an output signal from the tension sensor. State of the art documentPatent specification

[0008] Patent Document 1: Japanese Unopposed Patent Application Publication No. 2021-014886 Summary of the inventionProblems to be solved by the invention

[0009] In the sensor-equipped bearing device of Patent Document 1, the magnitude of the bearing preload determined based on the output signal of the outer ring spacer tension sensor is prone to error. If the bearing preload can be determined with high accuracy by avoiding such error, it can increase the reliability of condition monitoring for labor-saving and automated operation.

[0010] It is therefore an object of the first invention to provide a sensor-equipped bearing device capable of detecting a bearing preload with high accuracy.

[0011] Furthermore, the bearing device of Patent Document 1 supports a spindle of a spindle device. This device includes a first bearing and a second bearing arranged back-to-back in a tubular bearing housing, each comprising an angular contact ball bearing with an outer ring, an inner ring arranged radially inside the outer ring, and rolling elements arranged between the outer ring and the inner ring. A stress sensor is attached to an outer ring spacer, which is arranged between the outer ring of the first bearing and the outer ring of the second bearing.

[0012] An axial force is generated by tightening a preload nut bolted to the outer periphery of the spindle supported by the first and second bearings. The axial force is transmitted first to the second bearing, then to the outer ring spacer, and then to the first bearing, thus exerting a preload on the first and second bearings. An axial preload force is determined based on an output of the stress sensor mounted on the outer ring spacer.

[0013] Therefore, it is possible to detect an increase in preload in the spindle device in which this bearing device is used, caused, for example, by heat generation in the bearings supporting the spindle, and to ensure improved condition monitoring. When this device is used in a machine tool, a change in the cutting load is also detected, allowing the machining condition to be monitored.

[0014] In the device of Patent Document 1, a load acting on the outer ring spacer is determined from the stress generated by the deformation of the outer ring spacer that receives an axial force. Therefore, it is desirable to use an outer ring spacer that is easily deformable within a practically available range in order to detect a load change with high sensitivity.

[0015] However, when an easily deformed outer ring spacer is used, when an excessive load is applied to the outer ring spacer, the outer circumference of the stress-deformed outer ring spacer comes into contact with the inner circumference of the bearing housing, so that the deformation state of the outer ring spacer changes, thereby deteriorating the sensitivity of detecting load changes.

[0016] It is therefore an object of the second invention to provide a sensor-equipped bearing device capable of stably detecting a load change with high sensitivity. Means to solve the problems

[0017] The inventors of the present application conducted an evaluation test on the sensor-equipped bearing device of Patent Document 1, in which a bearing preload was varied and the bearing preload was determined based on the output of the outer ring spacer tension sensor. As a result, the inventors of the present application discovered that hysteresis occurs, causing a certain difference between the output of the tension sensor in the process where the bearing preload increases and the output of the tension sensor in the process where the bearing preload decreases, even when the bearing preload of the same magnitude is applied; and the hysteresis causes an error in the bearing preload detected based on the output of the tension sensor.

[0018] The reasons for the occurrence of hysteresis are described as follows.

[0019] As the bearing preload increases, the first outer ring is elastically deformed to radially expand due to an increase in the radial component force absorbed by the first rolling elements. This elastic deformation causes microslip at the contact surfaces of the first outer ring and the outer ring spacer, so that the first outer ring moves radially outward relative to the outer ring spacer. On the other hand, as the bearing preload decreases, the first outer ring is elastically restored to radially contract due to a decrease in the radial component force absorbed by the first rolling elements. This elastic recovery causes microslip at the contact surfaces of the first outer ring and the outer ring spacer, so that the first outer ring moves radially inward relative to the outer ring spacer.

[0020] Likewise, with increasing bearing preload, the second outer ring is elastically deformed, causing it to expand radially due to an increase in the radial component force absorbed by the second rolling elements. This elastic deformation causes microslip at the contact surfaces of the second outer ring and the outer ring spacer, causing the second outer ring to move radially outward relative to the outer ring spacer. On the other hand, with decreasing bearing preload, the second outer ring is elastically restored, causing it to contract radially due to a decrease in the radial component force absorbed by the second rolling elements. This elastic recovery causes microslip at the contact surfaces of the second outer ring and the outer ring spacer, causing the second outer ring to move radially inward relative to the outer ring spacer.

[0021] When radial slippage occurs at the contact surfaces of the first outer ring and the outer ring spacer or at the contact surfaces of the second outer ring and the outer ring spacer as described above, this causes a difference between the deformation of the outer ring spacer in the process where the bearing preload increases and the deformation of the outer ring spacer in the process where the bearing preload decreases. Therefore, hysteresis is considered to occur, causing a certain difference between the output of the stress sensor in the process where the bearing preload increases and the output of the stress sensor in the process where the bearing preload decreases, even if the bearing preload is applied in the same order of magnitude. The hysteresis of the stress sensor output causes an error in the bearing preload determined based on the stress sensor output.

[0022] The inventors of the present application came up with the idea that (i) when the surface pressure at the contact surfaces of the first outer ring and the outer ring spacer is increased, this increases the frictional force at the contact surfaces of the first outer ring and the outer ring spacer, so that when the force of the radial component that the first outer ring receives from the first rolling elements varies in accordance with a variation in the bearing preload, it is possible to reduce the amount of radial slip occurring at the contact surfaces of the first outer ring and the outer ring spacer; (ii) when the surface pressure at the contact surfaces of the second outer ring and the outer ring spacer is increased, this increases the frictional force at the contact surfaces of the second outer ring and the outer ring spacer, so that when the force of the radial member that the second outer ring receives from the second rolling elements varies in accordance with a change in the bearing preload, it is possible to reduce the amount of radial slip occurring at the contact surfaces of the second outer ring and the outer ring spacer; and (iii) As a result, the hysteresis of the output signal of the outer ring spacer stress sensor decreases and the hysteresis error decreases.

[0023] In order to achieve the above-mentioned purpose, the first invention based on this idea provides a sensor-equipped bearing device having the following arrangements. [Arrangement 1]

[0024] A sensor-equipped bearing device comprising: a first bearing and a second bearing axially spaced from each other; a tubular outer ring spacer disposed between the first bearing and the second bearing; and a stress sensor attached to the outer ring spacer, wherein the first bearing comprises: a first outer ring; a first inner ring disposed radially inward of the first outer ring; and a plurality of first rolling elements disposed between the first outer ring and the first inner ring, wherein the second bearing comprises: a second outer ring; a second inner ring disposed radially inward of the second outer ring;and a plurality of second rolling elements arranged between the second outer ring and the second inner ring, wherein a preload is to be applied to an axial end surface of the first inner ring remote from the second inner ring and an axial end surface of the second inner ring remote from the first inner ring so that the first inner ring and the second inner ring approach each other, and wherein the preload is to be transmitted to 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, characterized in that a compressive force is to be exerted on an axial end surface of the first outer ring remote from the second outer ring and an axial end surface of the second outer ring remote from the first outer ring so that the first outer ring and the second outer ring approach each other;and wherein the pressing force is greater than the preload of the inner ring.;

[0025] With this arrangement, the surface pressure corresponding to the sum of the preload applied to the axial end surfaces of the first and second inner rings and the pressing force applied to the axial end surfaces of the first and second outer rings acts on the contact surfaces of the first outer ring and the outer ring spacer, thereby increasing the surface pressure at the contact surfaces of the first outer ring and the outer ring spacer. This increases the friction force at the contact surfaces of the first outer ring and the outer ring spacer. It is therefore possible to reduce the radial slippage occurring at the contact surfaces of the first outer ring and the outer ring spacer when the radial component force received by the first outer ring from the first rolling elements varies according to a variation in the bearing preload.Likewise, the surface pressure, the magnitude of which corresponds to the sum of the preload exerted on the axial end surfaces of the first and second inner rings and the pressing force exerted on the axial end surfaces of the first and second outer rings, acts on the contact surfaces of the second outer ring and the outer ring spacer, thereby increasing the surface pressure at the contact surfaces of the second outer ring and the outer ring spacer. This increases the friction force at the contact surfaces of the second outer ring and the outer ring spacer. Therefore, it is possible to reduce the radial slip occurring at the contact surfaces of the second outer ring and the outer ring spacer when the radial component force absorbed by the second outer ring from the second rolling elements varies according to a change in the bearing preload.This makes it possible to reduce the hysteresis of an output of the outer ring spacer tension sensor and to detect a bearing preload with high accuracy. [Arrangement 2]

[0026] The sensor-equipped bearing device according to arrangement 1, wherein a magnitude of the pressing force is not less than 10 times a magnitude of the preload.

[0027] In this arrangement, a pressing force that is significantly greater than the preload is applied to the axial end faces of the first and second outer rings, thereby increasing, in particular, the surface pressure at the contact surfaces of the first outer ring and the outer ring spacer and the surface pressure at the contact surfaces of the second outer ring and the outer ring spacer. This particularly effectively increases the frictional force at the contact surfaces of the first outer ring and the outer ring spacer. It is therefore possible to particularly effectively reduce the radial slip occurring at the contact surfaces of the first outer ring and the outer ring spacer if the radial component force absorbed by the first outer ring from the first rolling elements varies according to a variation in the bearing preload. Likewise, the frictional force at the contact surfaces of the second outer ring and the outer ring spacer increases particularly effectively.It is therefore possible to particularly effectively reduce the radial slip occurring at the contact surfaces of the second outer ring and the outer ring spacer when the radial component force absorbed by the second outer ring from the second rolling elements varies according to a variation of the bearing preload. [Arrangement 3]

[0028] The sensor-equipped bearing device according to arrangement 1 or 2 further comprises: a tubular bearing housing attached to an outer periphery of the first outer ring and an outer periphery of the second outer ring, the tubular bearing housing having an inner periphery provided with an annular outer ring positioning step; and a cover member fixed to an axial end surface of the bearing housing with a screw member, wherein the compressive force is to be applied by disposing the first outer ring and the second outer ring between the annular outer ring positioning step and the cover member with axial interference.

[0029] With this arrangement, it is possible to exert a large compressive force on the axial end surfaces of the first and second outer rings by slightly tightening the screw element. [Arrangement 4]

[0030] The sensor-equipped bearing device according to arrangement 3, wherein the screw element is arranged at the same circumferential position as the tension sensor.

[0031] With this arrangement, since the screw element is located at the same circumferential position as the tension sensor, it is possible to effectively increase the contact pressure at the contact surfaces of the first outer ring and the outer ring spacer and the contact pressure at the contact surfaces of the second outer ring and the outer ring spacer at the same circumferential position as the tension sensor. This effectively reduces the hysteresis of the tension sensor's output signal. [Order 5]

[0032] The sensor-equipped bearing device according to arrangement 3 or 4, wherein the axial interference is 10 µm or more.

[0033] To achieve the above purpose, the second invention provides a sensor-equipped bearing device having the following configurations. [Order 6]

[0034] A sensor-equipped bearing device comprising: a tubular bearing housing; and a plurality of rolling bearings having a contact angle that is not 0 and arranged in the bearing housing so as to be arranged in a back-to-back relationship, the rolling bearings comprising a first bearing and a second bearing that oppose each other in the back-to-back relationship, the first bearing comprising: a first outer ring, a first inner ring rotatably disposed radially inward of the first outer ring, and a plurality of first rolling elements disposed between the first outer ring and the first inner ring, the second bearing comprising: a second outer ring, a second inner ring rotatably disposed radially inward of the second outer ring, and a plurality of second rolling elements disposed between the second outer ring and the second inner ring;wherein a tubular outer ring spacer is disposed axially between the first outer ring and the second outer ring;and wherein a stress sensor is attached to the outer ring spacer to determine a load acting on the outer ring spacer from the stress of the outer ring spacer, characterized in that the first outer ring, the second outer ring, and the outer ring spacer are each attached to an inner circumference of the bearing housing by a clearance fit, wherein a clearance Δ1 between the bearing housing and the outer ring spacer is larger than a radial expansion amount of the outer ring spacer by which the outer ring spacer is radially expanded by the load acting on the outer ring spacer, and the clearance Δ1 is larger than a clearance Δ2 between the bearing housing and the outer ring of one of the first bearing and the second bearing, the one bearing receiving an external force.;

[0035] With this arrangement, even if the axial force acting on the outer ring spacer increases, thereby increasing the deformation of the outer ring spacer due to its radial expansion, the outer periphery of the outer ring spacer is less likely to come into contact with the inner periphery of the bearing housing. Therefore, a change in the load acting on the outer ring spacer can be detected stably and with high sensitivity by using an outer ring spacer that is easily deformable within a practically available range. [Order 7]

[0036] The sensor-equipped bearing device according to arrangement 6, wherein the fit clearance Δ2 between the bearing housing and the outer ring of the one bearing that receives an external axial force has a diameter of 40 μm or less, and wherein the fit clearance Δ1 between the bearing housing and the outer ring spacer is not more than “Δ2+50 μm” (desirably not more than “Δ2+30 μm”) in diameter.

[0037] Even if the fit clearance Δ2 is set to 40 μm or less, which is commonly used, if the difference between the fit clearances Δ1 and Δ2 is more than 50 μm, it is difficult to align the center axes of the outer ring spacer and the bearing housing when inserting the fixture into a spindle or the like. To increase the accuracy of load measurement, it is necessary to align the above-mentioned center axes as accurately as possible. To easily align the above-mentioned center axes, the difference between the fit clearances Δ1 and Δ2 is set to 50 μm or less, preferably 30 μm or less. [Order 8]

[0038] The sensor-equipped bearing device according to arrangement 6 or 7, wherein the outer ring spacer comprises: an outer ring gear made of a metal and mounted with a clearance fit on the inner circumference of the bearing housing, the outer ring gear being in axial contact with the first outer ring and the second outer ring; and an inner ring gear made of a resin and disposed radially inward of the outer ring gear, and wherein the stress sensor is mounted on the outer ring gear.

[0039] With this arrangement, it is possible to place the force sensor in a protected manner between the outer ring gear and the inner ring gear of the outer ring spacer and at the same time reduce the manufacturing costs of the outer ring spacer. [Order 9]

[0040] The sensor-equipped bearing device according to one of the arrangements 6 to 8, wherein the first bearing and the second bearing are to be preloaded. [Order 10]

[0041] The sensor-equipped bearing device according to any one of arrangements 1 to 9, wherein the first bearing and the second bearing are angular contact ball bearings.

[0042] Furthermore, the present invention provides a spindle device of the following arrangement for a machine tool in which the above-mentioned sensor-equipped bearing device is used. [Order 11]

[0043] A spindle device for a machine tool, the spindle device comprising: the sensor-equipped bearing device according to any one of arrangements 1 to 10; a spindle of the machine tool, the spindle being rotatably supported by the sensor-equipped bearing device; and a motor configured to rotatably drive the spindle.

[0044] This arrangement makes it possible to perform stable condition monitoring for labor-saving and automated operation of the machine tool. It is also possible to detect the machining load acting on the machine tool spindle during machining. Effects of the invention

[0045] In the sensor-equipped bearing device of the first invention, the surface pressure, which is the sum of the preload applied to the axial end surfaces of the first and second inner rings and the contact force applied to the axial end surfaces of the first and second outer rings, acts on the contact surfaces of the first outer ring and the outer ring spacer, thereby increasing the surface pressure at the contact surfaces of the first outer ring and the outer ring spacer. This increases the friction force at the contact surfaces of the first outer ring and the outer ring spacer. Therefore, it is possible to reduce the radial slippage occurring at the contact surfaces of the first outer ring and the outer ring spacer when the radial component force absorbed by the first outer ring from the first rolling elements varies according to a variation in the bearing preload.Likewise, the surface pressure, the magnitude of which corresponds to the sum of the preload applied to the axial end surfaces of the first and second inner rings and the pressing force applied to the axial end surfaces of the first and second outer rings, acts on the contact surfaces of the second outer ring and the outer ring spacer, thereby increasing the surface pressure at the contact surfaces of the second outer ring and the outer ring spacer. This increases the friction force at the contact surfaces of the second outer ring and the outer ring spacer. Therefore, it is possible to reduce the radial slip occurring at the contact surfaces of the second outer ring and the outer ring spacer when the radial component force absorbed by the second outer ring from the second rolling elements varies according to a change in the bearing preload.This makes it possible to reduce the hysteresis of an output signal of the outer ring spacer tension sensor and to detect a bearing preload with high accuracy.

[0046] Since, in the sensor-equipped bearing device of the second invention, as described above, fit clearances are defined with respect to the two bearings and the outer ring spacer, which are attached to the housing with a clearance fit, even if the outer ring spacer to which the stress sensor is attached is deformed by an axial force to expand radially, the outer ring spacer is less likely to come into contact with the inner circumference of the housing. Therefore, a load change can be stably detected with high sensitivity. Short description of the characters Fig. 1 is a sectional view of a spindle device for a machine tool in which a sensor-equipped bearing device according to an embodiment of the first invention is used. Fig. 2 is an enlarged view of the sensor-equipped storage device and its surroundings in Fig. 1. Fig. 3 is a sectional view along the line III-III in Fig. 2. Fig. 4 is a view showing the transmission path of an axial preload applied between a first inner ring and a second inner ring as shown in Fig. 2, and the transmission path of an axial compressive force acting between a first outer ring and a second outer ring, as shown in Fig. 2 is shown, is applied. Fig. 5 is a sectional view of a spindle device for a machine tool in which a sensor-equipped bearing device according to an embodiment of the second invention is used. Fig. Figure 6 is an enlarged sectional view of a portion of Fig. 5. Fig. Fig. 7 is a view illustrating the transmission path of a force applied by a force acting on the spindle device of a Fig. 5 shown cutting load exerted by the machine tool. Fig. Fig. 8 is a view illustrating a transmission path of a force caused by a preload applied to the sensor-equipped bearing device of the spindle device for a Fig. 5 shown machine tool is applied. Fig. 9 is a schematic view showing Fig. 7, where the schematic view shows how a force is transmitted in a comparative example. Fig. 10 is a schematic view showing Fig. 7, the schematic view illustrating how a force is transmitted in this embodiment. Best mode for carrying out the invention

[0047] Fig. 1 shows a spindle device for a machine tool, the spindle device comprising a sensor-equipped bearing device 1 (hereinafter referred to simply as "bearing device 1") according to an embodiment of the first invention. This spindle device comprises a spindle 2 of a machine tool, a spindle housing (outer tube) 3 in which the spindle 2 is accommodated, a motor 4 configured to rotate the spindle 2, the bearing device 1 of the embodiment, which is arranged axially in front of the motor 4 (left side in Fig. 1) and rotatably supports the spindle 2, and a rear bearing device 5, which is axially behind the motor 4 (right side in Fig. 1) and supports the spindle 2 in a rotatable manner.

[0048] The spindle housing 3 has a hollow, tubular shape with both ends open. The bearing device 1 and the motor 4 are accommodated in the spindle housing 3 from the axial front to the axial rear in the order of elements 1 and 4. While the spindle housing 3 is in Fig. 1 is a seamlessly integrated housing comprising a portion in which the bearing device 1 is received and a portion in which the motor 4 is received, the spindle housing 3 may comprise these two portions as separate portions integrally connected to each other.

[0049] The spindle 2 is inserted into the spindle housing 3, with the front end of the spindle 2 protruding beyond a front opening of the spindle housing 3. A chuck (not shown) for holding a tool or workpiece is releasably attached to the front end of the spindle 2. The spindle 2 has a through-bore 6 that extends axially through the spindle 2 and in which a drawbar (not shown) of the machine tool is axially displaceably received.

[0050] The motor 4 includes a rotor 7 mounted on the outer circumference of the spindle 2, and an annular stator 8 configured to exert a rotating force on the rotor 7. The rotor 7 includes a rotor sleeve 9 mounted on the outer circumference of the spindle 2, and a rotor core 10 fixed to the outer circumference of the rotor sleeve 9. The rotor core 10 is made of, for example, laminated electromagnetic steel plates. The rotor sleeve 9 is non-rotatably connected to the spindle 2 so that it rotates in step with the spindle 2. The axially front end of the rotor sleeve 9 is in contact with a step 11 formed on the outer circumference of the spindle 2 and facing axially rearward. The rotor sleeve 9 is axially positioned by contact with the step 11.

[0051] The stator 8 includes a stator core 12 fixed to the inner circumference of the spindle housing 3, and electromagnetic coils 13 each wound around a plurality of toothed portions formed on the stator core 12 so as to be circumferentially spaced from each other. When the electromagnetic coils 13 are energized, a rotational force is generated at the rotor core 10 due to the electromagnetic force acting between the stator core 12 and the rotor core 10, so that the rotor 7 and the spindle 2 rotate in unison with each other. While in this embodiment, the motor 4 is an electric motor that generates rotational force due to electrical energy, a motor that generates rotational force due to another energy source, such as compressed air, may be used as the motor 4 instead of such an electric motor.

[0052] The rear bearing device 5 comprises an annular bearing support member 14 coaxially fixed to the rear end of the spindle housing 3, and a rolling bearing 15 arranged in the bearing support member 14. The rolling bearing 15 is a cylindrical roller bearing having an outer ring 16 mounted on the inner circumference of the bearing support member 14, an inner ring 17 mounted on the outer circumference of the spindle 2, and a plurality of cylindrical rollers 18 arranged between the outer ring 16 and the inner ring 17.

[0053] An outer ring pressing element 19 is attached to the bearing support element 14. The outer ring pressing element 19 bears against the axially rear end face of the outer ring 16, thereby fixing the axial position of the outer ring 16. A nut element 20 is attached to the outer circumference of the spindle 2, which presses the inner ring 17 axially forward, and an annular spacer 21 is arranged between the inner ring 17 and the nut element 20. The nut element 20 is in threaded engagement with an external thread 22 formed on the outer circumference of the spindle 2 at its rear end. The axial front end surface of the spacer 21 is in contact with the axial rear end surface of the inner ring 17. The axial rear end surface of the spacer 21 is in contact with the axial front end surface of the nut member 20. The axial front end surface of the inner ring 17 is in contact with the axial rear end of the rotor sleeve 9.

[0054] The bearing device 1 comprises a tubular bearing housing 23 which is fixed to the spindle housing 3, a first bearing 24 and a second bearing 25 which are arranged axially spaced from each other in the bearing housing 23, an outer ring spacer 26 and an inner ring spacer 27 which are arranged between the first bearing 24 and the second bearing 25, and stress sensors 28 which are attached to the outer ring spacer 26.

[0055] The bearing housing 23 is attached to the inner circumference of the spindle housing 3. The bearing housing 23 has at least one cooling groove 29 in its outer circumference, through which a coolant flows to cool the bearing device 1. The at least one cooling groove 29 consists of a plurality of annular grooves formed axially spaced from one another in the outer circumference of the bearing housing 23, or of a helical groove extending helically in 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.

[0056] As in Fig. 2, the first bearing 24 comprises a first outer ring 30 attached to the inner circumference of the bearing housing 23, a first inner ring 31 rotatably disposed radially inside the first outer ring 30, and a plurality of first rolling elements 32 arranged between the first outer ring 30 and the first inner ring 31. The first rolling elements 32 are balls in this embodiment. The first outer ring 30 has, on its inner circumference, a first outer ring raceway surface 33 having a circular arc-shaped cross-section and with which the first rolling elements 32 come into rolling contact. The first outer ring 30 is a shoulder-cut outer ring in which the axially front outer ring shoulders arranged axially in front of and behind the first outer ring raceway surface 33 have been removed. The outer circumference of the first outer ring 30 is attached to the inner circumference of the bearing housing 23 with a clearance.The first inner ring 31 has a first inner ring raceway surface 34 with a circular cross-section on its outer circumference, on which the first rolling elements 32 roll. The first inner ring 31 is a shoulder-cut inner ring, in which the inner ring shoulders located axially behind the first inner ring raceway surface 34, with which the first rolling elements 32 come into rolling contact, have been removed. The first inner ring 31 is mounted with interference on the outer circumference of the spindle 2.

[0057] The second bearing 25 includes a second outer ring 35 mounted on the inner circumference of the bearing housing 23, a second inner ring 36 rotatably disposed radially inside the second outer ring 35, and a plurality of second rolling elements 37 disposed between the second outer ring 35 and the second inner ring 36.

[0058] The second rolling elements 37 are balls in this embodiment. The second outer ring 35 has on its inner circumference a second outer ring raceway surface 38 which has a circular arc-shaped cross-section and with which the second rolling elements 37 come into rolling contact. The second outer ring 35 is arranged axially behind the first outer ring 30 so that it is axially spaced from the first outer ring 30. The second inner ring 36 is also arranged axially behind the first inner ring 31 so that it is axially spaced from the first inner ring 31. The second outer ring 35 is a shoulder-cut outer ring in which the outer ring shoulder axially behind the outer ring shoulders located axially in front of and behind the second outer ring raceway surface 38 has been removed. The outer circumference of the second outer ring 35 is attached to the inner circumference of the bearing housing 23 with a clearance.The second inner ring 36 has a second inner ring raceway surface 39 with a circular cross-section on its outer circumference, on which the second rolling elements 37 roll. The second inner ring 36 is a shoulder-cut inner ring, in which the axially forward inner ring shoulders arranged axially in front of and behind the second inner ring raceway surface 39, with which the second rolling elements 37 come into rolling contact, have been removed. The second inner ring 36 is mounted with interference on the outer circumference of the spindle 2.

[0059] The first bearing 24 is designed such that a radial component force with which the first rolling elements 32 press against the first outer ring 30 is generated by an axial preload. Like the first bearing 24, the second bearing 25 is also designed such that a radial component force with which the second rolling elements 37 press against the second outer ring 35 is generated by an axial preload. In this embodiment, the first bearing 24 is an angular contact ball bearing arranged such that a straight line connecting the contact point between the first inner ring 31 and each first rolling element 32 and the contact point between the first outer ring 30 and the first rolling element 32 is inclined axially backward from the radial inner side to the radial outer side.The second bearing 25 is also an angular contact ball bearing arranged such that a straight line connecting the contact point between the second inner ring 36 and each second rolling element 37 and the contact point between the second outer ring 35 and the second rolling element 37 is inclined axially forward from the radially inner side to the radially outer side. That is, the first bearing 24 and the second bearing 25 are a pair of angular contact ball bearings arranged back to back so as to be axially spaced from each other.

[0060] The outer ring spacer 26 is a hollow, tubular member with both ends open. The outer ring spacer 26 is mounted on the inner circumference of the bearing housing 23 with a clearance. The outer ring spacer 26 is axially disposed between the axial rear end surface of the first outer ring 30 (the axial end surface of the first outer ring 30 closer to the second outer ring 35) and the axial front end surface of the second outer ring 35 (the axial end surface of the second outer ring 35 closer to the first outer ring 30).

[0061] The inner ring spacer 27 is also a hollow, tubular element, both ends of which are open, like the outer ring spacer 26. The inner ring spacer 27 is mounted on the outer circumference of the spindle 2 with a clearance. The inner spacer 27 is arranged axially between the first inner ring 31 and the second inner ring 36.

[0062] As in Fig. As shown in Figure 4, an annular inner ring positioning step 40 is formed on the outer circumference of the spindle 2, axially opposite the axial front end surface of the first inner ring 31 (axial end surface of the first inner ring 31 opposite / away from the second inner ring 36). The inner ring positioning step 40 restricts the axial forward movement of the first inner ring 31 (movement of the first inner ring 31 in the direction away from the second outer ring 35), thereby axially positioning the first inner ring 31. A preload nut 41 is mounted on the outer circumference of the spindle 2 so as to be axially rearward of the second inner ring 36. The preload nut 41 is threadably engaged with an external thread 42 formed on the outer circumference of the spindle 2. An annular spacer 43 is disposed between the second inner ring 36 and the preload nut 41.The axial front end surface of the spacer 43 is in contact with the axial rear end surface of the second inner ring 36. The axial rear end surface of the spacer 43 is in contact with the axial front end surface of the preload nut 41.

[0063] When the preload nut 41 is tightened with a predetermined force, the axial force of the preload nut 41 applies a preload to the axial front end surface of the first inner ring 31 (axial end surface of the first inner ring 31 that is opposite to the second inner ring 36) and the axial rear end surface of the second inner ring 36 (axial end surface of the second inner ring 36 that is opposite to the first inner ring 31), so that the first inner ring 31 and the second inner ring 36 approach each other. That is, the axial force of the preload nut 41 applies an axial preload between the first inner ring 31 and the second inner ring 36, so that, as shown by the bold solid line in Fig. 4, the preload is transmitted to 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 in the order of the elements 31, 32, 30, 26, 35, 37 and 36.

[0064] The stress sensors 28 are mounted on the axial center portion of the outer ring spacer 26. Specifically, the stress sensors 28 are each mounted on the outer ring spacer 26 so as to be located within a range corresponding to one-quarter (1 / 4) of the entire axial length of the outer ring spacer 26 from the axial center position of the outer ring spacer 26.

[0065] As in Fig. As shown in Figure 3, the (plural) stress sensors 28 (three stress sensors 28 in this embodiment) are circumferentially arranged at equal intervals on the inner circumference of the outer ring spacer 26. Axial grooves 44, which are provided in the same number as the stress sensors 28, are circumferentially formed at equal intervals on the inner circumference of the outer ring spacer 26. Each axial groove 44 has a flat bottom surface extending parallel to the central axis, and the stress sensors 28 are attached to the respective flat bottom surfaces of the axial grooves 44.

[0066] As in Fig. As shown in Figure 4, each voltage sensor 28 includes a voltage detector 45 and a processor 46 connected to the voltage detector 45. The voltage detector 45 is a voltage gauge whose electrical resistance changes depending on the voltage. The processor 46 includes a voltage detection circuit configured to detect the voltage based on a change in the electrical resistance of the voltmeter, and an AD converter circuit configured to convert the voltage detected by the voltage detection circuit from an analog signal to a digital signal and output the digital signal.

[0067] The stress detector 45 (each stress sensor 28) includes an axial stress detection part configured to detect the axial stress of the outer ring spacer 26 at the position where the corresponding stress sensor 28 is mounted (a stress gauge part arranged to detect the stress in the axial direction), and a circumferential strain detection part configured to detect the circumferential strain of the outer ring spacer 26 at the position where the corresponding stress sensor 28 is mounted (a stress gauge part arranged to detect the stress in the circumferential direction). The processor 46 (each stress sensor 28) is configured to obtain a difference between the axial stress detected by the corresponding axial stress detection part and the circumferential strain detected by the corresponding circumferential strain detection part (i.e.,the sum of the absolute value of the axial stress and the absolute value of the circumferential strain) and to process this difference as an output signal from the corresponding stress sensor 28.

[0068] On the inner circumference of the bearing housing 23, an annular outer ring positioning step 50 is formed axially opposite the axially rearward end surface of the second outer ring 35 (the axial end surface of the second outer ring 35 opposite / away from the first outer ring 30). The outer ring positioning step 50 restricts the axial rearward movement of the second outer ring 35 (movement of the second outer ring 35 in the direction away from the first outer ring 30), thereby axially positioning the second outer ring 35.

[0069] An annular cover member 51 is attached to the axial end face of the bearing housing 23. The cover member 51 includes a tubular portion 52 attached to the inner periphery of the bearing housing 23 and a flange portion 53 in the shape of a circular annular plate extending radially outward from the axial front end of the tubular portion 52.

[0070] As in the Fig. 2 and Fig. 3, the flange portion 53 is secured to the axial front end surface of the bearing housing 23 by a plurality of screw elements 54 arranged at equal intervals around the circumference. The screw elements 54 are bolts in this embodiment. Some of the screw elements 54 are arranged at the same circumferential positions as the stress sensors 28. In particular, as shown in Fig. 3, the tension sensors 28 are arranged at circumferential positions of 0°, 120° and 240° clockwise from the top of the outer ring spacer 26, and the screw elements 54 are arranged at circumferential positions including all circumferential positions of the tension sensors 28 (circumferential positions of 0°, 60°, 120°, 180°, 240° and 300° in Fig. 3).

[0071] A plurality of through holes 55 are formed at equal intervals on the circumference of the flange portion 53, and the screw members 54 are inserted through the respective through holes 55. A plurality of screw holes 56 are formed at equal intervals in the axial end surface of the bearing housing 23, and the screw members 54 are screwed into the respective screw holes 56. By tightening the screw members 54, the flange portion 53 is pressed against the axial front end surface of the bearing housing 23. In addition, the axial rear end of the tube portion 52 is in contact with the axial front end surface of the first outer ring 30 (axial end surface of the first outer ring 30 opposite / away from the second outer ring 35).

[0072] As in Fig. As shown in Figure 4, the first outer ring 30 and the second outer ring 35 are axially interposed between the cover member 51 and the outer ring positioning step 50. This axial interference exerts a compressive force on the axial front end surface of the first outer ring 30 (the axial end surface of the first outer ring 30 opposite / remote from the second outer ring 35) and the axial rear end surface of the second outer ring 35 (the axial end surface of the second outer ring 35 opposite / remote from the first outer ring 30), causing the first outer ring 30 and the second outer ring 35 to approach each other.

[0073] That is, the distance between the axially opposite surfaces of the cover member 51 and the outer ring positioning step 50 when the cover member 51 is fixed to the bearing housing 23, wherein the first outer ring 30, the outer ring spacer 26 and the second outer ring 35 are separated from the Fig. 4 is fixed to the bearing housing 23, is shorter by an amount corresponding to the axial interference than the distance from the axial front end surface of the first outer ring 30 to the axial 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 detached from the bearing housing 23 and, in this detached state, are axially arranged so that no gaps are defined therebetween. By inserting the first outer ring 30, the outer ring spacer 26 and the second outer ring 35 into the bearing housing 23 and then axially pressing the cover element 51 while simultaneously tightening the screw elements 54, the first outer ring 30, the outer ring spacer 26 and the second outer ring 35 are axially compressed between the cover element 51 and the outer ring positioning step 50 by the amount corresponding to the axial interference.As a result, an axial pressing force is exerted between the first outer ring 30 and the second outer ring 35, as shown by the bold dashed line in . Fig. 4. The axial interference is set in the range of 10 µm or more and 50 µm or less (preferably 20 µm or less).

[0074] The magnitude of the axial compressive force exerted by the axial interference on the axial end surfaces of the first and second outer rings 30 and 35 is significantly larger than the magnitude of the axial preload exerted by tightening the preload nut 41 on the axial end surfaces of the first and second inner rings 31 and 36.

[0075] Specifically, the magnitude of the axial preload applied to the axial end surfaces of the first and second inner rings 31 and 36 by tightening the preload nut 41 is less than 1 kN (about tens to hundreds of Newtons). On the other hand, an axial pressing force of 10 kN or more is applied to the axial end surfaces of the first and second outer rings 30 and 35 by the axial interference. The magnitude of the axial pressing force applied to the axial end surfaces of the first and second outer rings 30 and 35 by the axial interference is at least 10 times (preferably at least 50 times) the magnitude of the axial preload applied to the axial end surfaces of the first and second inner rings 31 and 36 by tightening the preload nut 41.

[0076] At the Fig. In the bearing device 1 shown in Figure 2, the preload of the first bearing 24 and the second bearing 25 (hereinafter referred to as "bearing preload") varies depending on the rotational speed of the spindle 2 during operation of the spindle device. This bearing preload can be determined based on the tension of the outer ring spacer 26 detected by the tension sensors 28.

[0077] During operation of the spindle device, when the rotational speed of the spindle 2 changes, the centrifugal force of the first rolling elements 32 and the centrifugal force of the second rolling elements 37 vary, and the load with which the first rolling elements 32 press the first outer ring raceway surface 33 (preload applied to the first bearing 24) and the load with which the second rolling elements 37 press the first outer ring raceway surface 33 (preload applied to the second bearing 25) also vary in accordance with the changes in the centrifugal forces. The first outer ring raceway surface 33 and the second outer ring raceway surface 38 contact the first rolling elements 32 and the second rolling elements 37, respectively, at an angle inclined to the axial direction.Therefore, when the load with which the first rolling elements 32 press against the first outer ring raceway surface 33 and the load with which the second rolling elements 37 press against the first outer ring raceway surface 33 vary, the axial preload exerted by the first outer ring 30 and the second outer ring 35 on the outer ring spacer 26 also varies, so that the tension of the outer ring spacer 26 changes. Therefore, it is possible to determine the preload exerted on the first bearing 24 and the second bearing 25 (bearing preload) based on the tension of the outer ring spacer 26 detected by the tension sensors 28.

[0078] When a moment load is applied to spindle 2, the positions of the respective tension sensors 28, which are arranged at equal intervals in the circumferential direction, are subjected to different loads. Therefore, it is possible to detect the moment load acting on spindle 2 of the machine tool during cutting from the outputs of the respective tension sensors 28.

[0079] As the bearing preload increases, the first outer ring 30 is elastically deformed, radially expanding due to an increase in the radial component force absorbed by the first rolling elements 32. At this time, if the force radially expanding the first outer ring 30 is greater than the maximum static friction force at the contact surfaces of the first outer ring 30 and the outer ring spacer 26, microslip may occur at the contact surfaces of the first outer ring 30 and the outer ring spacer 26, causing the first outer ring 30 to move radially outward relative to the outer ring spacer 26. On the other hand, as the bearing preload decreases, the first outer ring 30 is elastically restored, radially compressing due to a decrease in the radial component force absorbed by the first rolling elements 32.At this time, if the force radially compressing the first outer ring 30 is greater than the maximum static friction force at the contact surfaces of the first outer ring 30 and the outer ring spacer 26, micro-slip may occur at the contact surfaces of the first outer ring 30 and the outer ring spacer 26, causing the first outer ring 30 to move radially inward relative to the outer ring spacer 26.

[0080] Likewise, with increasing bearing preload, the second outer ring 35 is elastically deformed, so that it expands radially due to an increase in the radial component force absorbed by the second rolling elements 37. This elastic deformation could lead to microslip at the contact surfaces of the second outer ring 35 and the outer ring spacer 26, so that the second outer ring 35 moves radially outward relative to the outer ring spacer 26. On the other hand, with decreasing bearing preload, the second outer ring 35 is elastically restored, so that it is radially compressed due to a decrease in the radial component force absorbed by the second rolling elements 37. This elastic recovery could lead to microslip at the contact surfaces of the second outer ring 35 and the outer ring spacer 26, so that the second outer ring 35 moves radially inward relative to the outer ring spacer 26.

[0081] When radial slippage occurs at the contact surfaces of the first outer ring 30 and the outer ring spacer 26 or at the contact surfaces of the second outer ring 35 and the outer ring spacer 26 as described above, a difference results between the deformation of the outer ring spacer 26 when the bearing preload increases and the deformation of the outer ring spacer 26 when the bearing preload decreases. This results in the following problem. That is, hysteresis occurs, causing a certain difference between the output signal of each stress sensor 28 in the process where the bearing preload increases and the output signal of each stress sensor 28 in the process where the bearing preload decreases, even if the bearing preload is applied in the same order. This hysteresis causes an error in the bearing preload determined based on the outputs of the stress sensors 28.

[0082] To solve this problem, in the storage device 1 of this embodiment, as shown in Fig. 4, an axial interference is set between the cover element 51 and the outer ring positioning step 50, so that a pressing force is exerted on the axial end surfaces of the first and second outer rings 30 and 35, so that the first outer ring 30 and the second outer ring 35 approach each other. Accordingly, the surface pressure, which is equal to the sum of the preload (represented by the bold solid line in Fig. 4) exerted on the axial end surfaces of the first and second inner rings 31 and 36, and the pressing force (shown by the bold dashed line in Fig. 4) exerted on the axial end surfaces of the first and second outer rings 30 and 35, acts on the contact surfaces of the first outer ring 30 and the outer ring spacer 26, so that the surface pressure on the contact surfaces of the first outer ring 30 and the outer ring spacer 26 is high. This increases the frictional force on the contact surfaces of the first outer ring 30 and the outer ring spacer 26. It is therefore possible to reduce the radial slip occurring on the contact surfaces of the first outer ring 30 and the outer ring spacer 26 when the radial component force that the first outer ring 30 receives from the first rolling elements 32 varies depending on a variation in the bearing preload. Likewise, the surface pressure corresponding to the sum of the preload exerted on the axial end surfaces of the first and second inner rings 31 and 36 (in Fig. 4 by the bold solid line) and the contact pressure exerted on the axial end surfaces of the first and second outer rings 30 and 35 (in Fig. 4 by the bold dashed line) on the contact surfaces of the second outer ring 35 and the outer ring spacer 26, so that the surface pressure on the contact surfaces of the second outer ring 35 and the outer ring spacer 26 is high. This increases the frictional force on the contact surfaces of the second outer ring 35 and the outer ring spacer 26. Therefore, it is possible to reduce the radial slippage occurring on the contact surfaces of the second outer ring 35 and the outer ring spacer 26 when the radial component force that the second outer ring 35 receives from the second rolling elements 37 varies according to a variation in the bearing preload. This makes it possible to reduce the hysteresis of the output signals of the stress sensors 28 of the outer ring spacer 26 and to detect the bearing preload with high accuracy.

[0083] Since, in particular, in this bearing device 1, the magnitude of the axial pressing force (represented by the bold dashed line in Fig. 4) exerted on the axial end surfaces of the first and second outer rings 30 and 35 is significantly large, namely not less than 10 times (preferably not less than 50 times) the magnitude of the axial preload (shown by the bold line in Fig. 4) applied to the axial end surfaces of the first and second inner rings 31 and 36 by tightening the preload nut 41, the surface pressure at the contact surfaces of the first outer ring 30 and the outer ring spacer 26 and the surface pressure at the contact surfaces of the second outer ring 35 and the outer ring spacer 26 are particularly high / large. This increases the frictional force at the contact surfaces of the first outer ring 30 and the outer ring spacer 26 particularly effectively. It is therefore possible to particularly effectively reduce the radial slip occurring at the contact surfaces of the first outer ring 30 and the outer ring spacer 26 when the radial component force that the first outer ring 30 receives from the first rolling elements 32 varies depending on a variation in the bearing preload. Likewise, the frictional force at the contact surfaces of the second outer ring 35 and the outer ring spacer 26 increases particularly effectively.The radial slip occurring at the contact surfaces of the second outer ring 35 and the outer ring spacer 26 can therefore be particularly effectively reduced if the radial force component that the second outer ring 35 absorbs from the second rolling elements 37 varies depending on a variation in the bearing preload.

[0084] In addition, since an axial interference is used in this bearing device 1 to apply a compressive force to the axial end surfaces of the first and second outer rings 30 and 35, it is possible to apply a large compressive force to the axial end surfaces of the first and second outer rings 30 and 35 by slightly tightening the screw members 54.

[0085] Since in this storage device 1, as in Fig. 3, some of the screw elements 54 are arranged at the same circumferential positions as the tension sensors 28, it is also possible to effectively increase the surface pressure at the contact surfaces of the first outer ring 30 and the outer ring spacer 26 and the surface pressure at the contact surfaces of the second outer ring 35 and the outer ring spacer 26 at the same circumferential positions as the tension sensors 28 (see Fig. 4). In this way, it is possible to effectively reduce the hysteresis of the output signals of the voltage sensors 28.

[0086] The axial interference can be adjusted by applying a compressive force to at least one of the axial end surfaces of the first outer ring 30 and the axial end surfaces of the second outer ring 35 using a tester or the like capable of applying and measuring a predetermined indentation amount (µm) and a predetermined compressive force (N) to derive the relationship between the indentation amount (µm) and the compressive force (N).

[0087] While in the above embodiment, a case is shown in which the stress sensors 28 are arranged on the inner circumference of the outer ring spacer 26, the stress sensors 28 may also be arranged on the outer circumference of the outer ring spacer 26.

[0088] While in the above embodiment, a case is described as an example in which the first bearing 24 and the second bearing 25 are angular contact ball bearings, the first bearing 24 and the second bearing 25 may also be other rolling bearings configured to generate a radial component force by an axial preload, e.g., tapered roller bearings or deep groove ball bearings.

[0089] Although in the above embodiment, a case is illustrated in which each voltage sensor 28 includes the voltage detector 45 and the processor 46, each voltage sensor 28 may consist of only the voltage detector 45 (voltage gauge).

[0090] Fig. 5 shows a spindle device for a machine tool in which a bearing device 1 according to an embodiment of the second invention is used. The elements of the embodiment of the second invention that correspond to those of the first invention are denoted by the same reference numerals below, and the description thereof is omitted. The elements denoted by the same reference numerals have substantially the same structures as those of the first invention.

[0091] As in the Fig. 5 and Fig. 6, the bearing device 1 includes a bearing housing 23 fixed to the spindle housing 3, a first bearing 24 mounted on the inner circumference of the bearing housing 23, a second bearing 25 mounted on the inner circumference of the bearing housing 23 so as to be axially rearward of the first bearing 24, and an outer ring spacer 26 and an inner ring spacer 27 axially arranged between the first bearing 24 and the second bearing 25, and the spindle 2 is supported by the first bearing 24 and the second bearing 25.

[0092] The first bearing 24 is an angular contact ball bearing having a non-rotatable first outer ring 30 mounted with a clearance fit on the inner circumference of the bearing housing 23, a first inner ring 31 rotatably disposed radially inside the first outer ring 30, and a plurality of first rolling elements (balls in this embodiment) 32 disposed between the first outer ring 30 and the first inner ring 31. The first outer ring 30 has, on its inner circumference, a first outer ring raceway surface 33 having a circular arc-shaped cross-section with which the first rolling elements 32 come into rolling contact. The first inner ring 31 has, on its outer circumference, a first inner ring raceway surface 34 having a circular arc-shaped cross-section with which the first rolling elements 32 come into rolling contact.The first outer ring 30 is a shoulder-cut outer ring in which the axially front outer ring shoulder has been removed from the outer ring shoulders located axially in front of and behind the first outer ring raceway surface 33. The first inner ring 31 is a shoulder-cut inner ring in which the axially rearward inner ring shoulder has been removed from the inner ring shoulders located axially in front of and behind the first inner ring raceway surface 34.

[0093] The second bearing 25 is an angular contact ball bearing with a non-rotatable second outer ring 35, which is arranged axially behind the first outer ring 30 at a distance from the first outer ring 30 and is fastened to the inner circumference of the bearing housing 23 with a clearance fit, a second inner ring 36, which is rotatably arranged radially inside the second outer ring 35, and a plurality of second rolling elements (balls in this embodiment) 37, which are arranged between the second outer ring 35 and the second inner ring 36. The second outer ring 35 has, on its inner circumference, a second outer ring raceway surface 38, which has a circular arc-shaped cross-section and with which the second rolling elements 37 come into rolling contact. The second inner ring 36 has, on its outer circumference, a second inner ring raceway surface 39, which has a circular arc-shaped cross-section and with which the second rolling elements 37 come into rolling contact.The second outer ring 35 is a shoulder-cut outer ring, in which the axially rearward outer ring shoulder has been removed from the outer ring shoulders located axially in front of and behind the second outer ring raceway surface 38. The second inner ring 36 is a shoulder-cut inner ring, in which the axially forward inner ring shoulder has been removed from the inner ring shoulders located axially in front of and behind the second inner ring raceway surface 39.

[0094] The first bearing 24 is arranged such that a straight line connecting the contact point between the first inner ring 31 and each first rolling element 32 and the contact point between the first outer ring 30 and the first rolling element 32 is inclined axially backward from the radially inner side to the radially outer side. On the other hand, the second bearing 25 is arranged such that a straight line connecting the contact point between the second inner ring 36 and each second rolling element 37 and the contact point between the second outer ring 35 and the second rolling element 37 is inclined axially forward from the radially inner side to the radially outer side. That is, the first bearing 24 and the second bearing 25 are arranged back to back.

[0095] The outer ring spacer 26 consists of a metal outer ring gear 26a, which is mounted with a clearance fit on the inner circumference of the bearing housing 23, and a resin inner ring gear 26b, which is arranged radially inside the outer ring gear 26a. The outer ring gear 26a has a hollow cylindrical shape with both ends open. On the other hand, the inner ring gear 26b is a member with an L-shaped cross section, which includes a hollow cylindrical portion with both ends open and an outwardly extending flange portion located at the rear axial end of the hollow cylindrical portion. The flange portion is fixed to the inner peripheral surface of the outer ring gear 26a. This fixing can be achieved by press-fitting or bonding the flange portion of the inner ring gear 26b to the inner peripheral surface of the outer ring gear 26a, or by press-fitting and bonding the flange portion to the inner peripheral surface.

[0096] The radial thickness of the outer ring gear 26a is substantially equal to the radial thickness of the first outer ring 30 and the radial thickness of the second outer ring 35. The axial dimension of the outer ring gear 26a is larger than the axial dimension of the inner ring gear 26b. Therefore, the front axial end and the rear axial end of the outer ring gear 26a abut against the rear axial end surface of the first outer ring 30 and the front axial end surface of the second outer ring 35, respectively, and the inner ring gear 26b does not abut against the first outer ring 30 and the second outer ring 35.

[0097] A plurality of stress sensors 28 (preferably three or more stress sensors 28; four stress sensors 28 in this embodiment) are mounted at equal intervals in the circumferential direction on the axial center portion of the inner peripheral surface of the outer ring gear 26a. Each stress sensor 28 includes a printed circuit board 28a fixed to the outer ring gear 26a with an adhesive or the like, a stress gauge 28b mounted on the printed circuit board 28a and configured to detect the stress of the outer ring gear 26a, and a processing circuit 28c mounted on the printed circuit board 28a and configured to determine a load acting on the outer ring gear 26a from the stress detected by the stress gauge 28b. The outer ring gear 26a and the cylindrical portion of the inner ring gear 26b define a space (space around the force sensors 28) which is sealed with a sealing means 47, e.g.a resin material. Each voltage sensor 28 is thus reliably fixed in a protected state in which insulation is ensured.

[0098] The inner ring spacer 27, on the other hand, has a hollow cylindrical shape with both ends open, like the outer ring gear 26a of the outer ring spacer 26. The axial front end and the axial rear end of the inner spacer 27 are in contact with the axial rear end surface of the first inner ring 31 and the axial front end surface of the second inner ring 36, respectively.

[0099] An outer ring pressing member 60 is attached to the axial front end of the spindle housing 3. The outer ring pressing member 60 comes into contact with the axial front end surface of the first outer ring 30, thereby fixing the axial position of the first outer ring 30. The outer ring pressing member 60 includes a tubular portion 61 attached to the inner periphery of the bearing housing 23 and a flange portion 62 extending radially outward from the axial front end of the tubular portion 61. The flange portion 62 is attached to the axial front end surface of the bearing housing 23. A step 63 is formed on the outer periphery of the axial front end of the spindle 2 and is in contact with the axial front end surface of the first inner ring 31. The first inner ring 31 is axially positioned by coming into contact with the step 63.

[0100] On the outer circumference of the spindle 2, a preload nut 64, which presses the second inner ring 36 axially forward, and an annular spacer 65, which is arranged between the second inner ring 36 and the preload nut 64, are attached. The preload nut 64 is screwed to an external thread 66, which is formed on a part of the outer circumference of the spindle 2 that extends axially forward from the step 11 (see Fig. 5). The axial front end of the spacer 65 is in contact with the axial rear end surface of the second inner ring 36. The axial rear end of the spacer 65 abuts the axial front end surface of the preload nut 64. A step 67 is formed on the inner circumference of the bearing housing 23 and is in contact with the axial rear end surface of the second outer ring 35. The second outer ring 35 is axially positioned by coming into contact with the step 67.

[0101] The bearing housing 23 consists of a tubular portion 68 attached to the inner circumference of the spindle housing 3 and a flange portion 69 extending radially outward from the axial front end of the tubular portion 68. The tubular portion 68 has at least one cooling groove 70 on its outer circumference, in which a coolant flows to cool the bearing device 1. The at least one cooling groove 70 comprises a plurality of annular grooves formed in the outer circumference of the tubular portion 68 such that they are axially spaced from one another, or comprises a spiral groove extending helically in the outer circumference of the tubular portion 68. The flange portion 69 tightly abuts the axial front end of the spindle housing 3.

[0102] In the spindle device for a machine tool having the structure described above, when an axial force (cutting load) is applied to the spindle 2 by machining, the axial force is transmitted to the first inner ring 31, the first rolling elements 32, the first outer ring 30, the outer ring gear 26a of the outer ring spacer 26 and the second outer ring 35 in the order of the elements 31, 32, 30, 26a and 35 of the bearing device 1 and received by the step 67 of the bearing housing 23, as shown in Fig. 7. The outer ring gear 26a of the outer ring spacer 26, which absorbs the axial force, is compressed and deformed, causing it to expand radially between the first outer ring 30 and the second outer ring 35. The stress sensors 28, which are mounted at equal intervals along the circumferential direction on the outer ring gear 26a, are thus able to detect the load acting on the outer ring gear 26a, i.e., the cutting load acting on the spindle 2, from the stress of the outer ring gear 26a at its circumferential positions.

[0103] When inserting the bearing device 1 into the spindle device as shown in Fig. As shown in Fig. 8, the axial force caused by tightening the preload nut 64 is transmitted to the spacer 65, the second inner ring 36, the second rolling elements 37, the second outer ring 35, the outer ring gear 26a of the outer ring spacer 26, the first outer ring 30, the first rolling elements 32, and the first inner ring 31 in the order of the elements 65, 36, 37, 35, 26a, 30, 32, and 31, and the axial force is received by the step 63 of the spindle 2, so that a preload is applied to the first bearing 24 and the second bearing 25. The stress sensors 28 are also capable of detecting the load acting on the outer ring gear 26a at this time, that is, the preload.

[0104] In the bearing device 1 of this embodiment, a fitting clearance Δ1 between the bearing housing 23 and the outer ring gear 26a of the outer ring spacer 26 of the outer ring is larger than a radial expansion amount of the outer ring gear 26a by which the outer ring gear 26a is radially expanded by the load acting on the outer ring gear 26a, and also the fitting clearance Δ1 is larger than a fitting clearance Δ2 between the bearing housing 23 and the first outer ring 30 and the second outer ring 35 (in this embodiment, the fitting clearance between the bearing housing 23 and the first outer ring 30 is equal to the fitting clearance between the bearing housing 23 and the second outer ring 35). The fit clearance Δ2 is set to a diameter of 40 μm or less, which is commonly used, but the difference between the fit clearances Δ1 and Δ2 is set to a diameter of 50 μm or less (desirably 30 μm or less).Because if the difference between the fit distances Δ1 and Δ2 is more than 50 μm in diameter, when inserting the bearing device 1 into the spindle device, it is difficult to align the center axes of the outer ring spacer 26 and the bearing housing 23, so that the accuracy of load measurement deteriorates.

[0105] Since the fit clearance Δ1 between the bearing housing 23 and the outer ring gear 26a of the outer ring spacer 26 and the fit clearance Δ2 between the bearing housing 23 and the first outer ring 30 and the second outer ring 35 are set as described above, fluctuations in the cutting load and preload acting on the outer ring gear 26a can be detected stably and with high sensitivity. This will be explained below with reference to Fig. 9 (comparison example) and Fig. 10. In the Fig. 9 and Fig. 10, the clearance between the bearing housing 23 and the outer ring gear 26a as well as the clearance between the bearing housing 23 and the first outer ring 30 are shown exaggerated.

[0106] First, when a cutting load is applied to spindle 2, the force is essentially as in Fig. 7. If at this point, as in the comparison example of Fig. 9, the fit clearance Δ1 between the bearing housing 23 and the outer ring gear 26a of the outer ring spacer 26 is smaller than the above-specified range, as shown in Fig. As shown in Figure 9, the outer ring gear 26a of the outer ring spacer 26 comes into contact with the inner circumference of the bearing housing 23 before the first outer ring 30, which radially expands due to the radial component force of the force transmitted from the first rolling elements 32, comes into contact with the inner circumference of the bearing housing 23. As a result, the deformation state of the outer ring gear 26a changes, which may affect the sensitivity of the load change detection.

[0107] When the fit clearances Δ1 and Δ2 are set as described above, the first outer ring 30, which expands radially by 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 gear 26a of the outer ring spacer 26 comes into contact with the inner circumference of the bearing housing 23, as shown in Fig.10. Therefore, the outer ring gear 26a is less likely to come into contact with the inner circumference of the bearing housing 23. Therefore, by using an outer ring gear 26a that is easily deformable within a practically available range, a change in the cutting load acting on the outer ring 26a can be detected stably and with high sensitivity regardless of the magnitude of the cutting load.

[0108] The outer diameter of the outer ring gear 26a of the outer ring spacer 26, the outer diameter of the first outer ring 30, and the outer diameter of the second outer ring 35 can be measured with a dial indicator, a micrometer, or the like. The inner diameter of the bearing housing 23 can be measured with a dial indicator, a micrometer, or the like. The fit clearance Δ1 between the bearing housing 23 and the outer ring gear 26a of the outer ring spacer 26, and the fit clearance Δ2 between the bearing housing 23 and the first outer ring 30 and the second outer ring 35, respectively, can be derived from the difference between the measured outer diameter dimensions and the measured inner diameter dimensions.In addition, the radial expansion amount of the outer ring spacer 26 can be estimated by (i) measuring the amount of displacement of the radially outer side of the outer ring spacer 26 when an axial compressive force is applied to the outer ring spacer 26 with a dial indicator, a micrometer, or the like, and (ii) deriving the relationship between the compressive force and the radial expansion amount of the outer ring spacer 26.

[0109] When the first bearing 24 and the second bearing 25 are preloaded by tightening the preload nut 64 in the bearing device 1, the second outer ring 35, which expands radially due to the radial component of the force transmitted by the second rolling elements 37, comes into contact with the inner periphery of the bearing housing 23 before the outer ring gear 26a. As a result, the outer ring gear 26a is less likely to come into contact with the inner periphery of the bearing housing 23. Therefore, a change in the preload can be detected stably and with high sensitivity.

[0110] With regard to the fixture for the spindle of a machine tool, it is thus possible to detect a sudden increase in cutting load or an increase in preload caused, for example, by heat generation of the first bearing 24 and the second bearing 25 early on, and to increase the reliability of condition monitoring. Furthermore, it is possible to efficiently and accurately adjust the preload when inserting the bearing fixture 1 into the spindle fixture.

[0111] The embodiments described above are merely examples in all respects, and the present invention is not limited thereto. The scope of the present invention is indicated not by the above description, but by the claims, and is to be understood as including all modifications within the meaning and scope of the claims.

[0112] For example, while in each of the above embodiments the first bearing and the second bearing of the device are angular contact ball bearings, the first and second bearings may also be rolling bearings with a contact angle other than 0 (zero), e.g., tapered roller bearings. Also, the outer ring spacer is not limited to an outer ring spacer having a dual structure, that is, consisting of an outer ring and an inner ring, as described in the embodiment of the second invention. Instead, the spacer may comprise a single cylindrical member having an inner peripheral surface or an outer peripheral surface to which stress sensors are mounted.

[0113] Since both the first bearing and the second bearing receive an external axial force, the fit clearance between the bearing housing and the outer ring spacer is larger than the fit clearance between the bearing housing and the outer ring of each bearing. However, if the fixture is installed in the spindle fixture so that only one of the first bearing and the second bearing receives an external axial force, the fit clearance between the bearing housing and the outer ring spacer is set to a clearance larger than the fit clearance between the bearing housing and either bearing.

[0114] Although in each of the above embodiments, the bearing device 1 that rotatably supports the spindle 2 of the machine tool (e.g., a machining center or a lathe) is described by way of example, the present invention is also applicable to a bearing device that rotatably supports a rotatable shaft of another device, such as a spindle / main shaft of a wind power generator. Description of reference symbols 1 storage device equipped with a sensor 2 spindles 3 spindle housing 4 Engine 23 Bearing housing 24 first camp 25 second camp 26 outer ring spacers 26a outer ring gear 26b inner ring gear 28 Voltage 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 stage 51 Cover element 54 screw element QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] JP 2021-014886

[0008]

Claims

[1] A sensor-equipped storage device comprising: a first bearing (24) and a second bearing (25) axially spaced from each other; a tubular outer ring spacer (26) arranged between the first bearing (24) and the second bearing (25); and a stress sensor (28) mounted on the outer ring spacer (26), wherein the first bearing (24) comprises: 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) arranged between the first outer ring (30) and the first inner ring (31), wherein the second bearing (25) comprises: 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) arranged between the second outer ring (35) and the second inner ring (36), wherein a preload is to be applied to an axial end surface of the first inner ring (31) remote from the second inner ring (36) and an axial end surface of the second inner ring (36) remote from the first inner ring (31) so that the first inner ring (31) and the second inner ring (36) approach each other, and wherein the preload is to be transmitted to 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), characterized bythat a pressing force is to be applied to an axial end surface of the first outer ring (30) which is remote from the second outer ring (35) and to an axial end surface of the second outer ring (35) which is remote from the first outer ring (30), so that the first outer ring (30) and the second outer ring (35) approach each other, where the pressing force is greater than the preload. [2] The sensor-equipped bearing device according to claim 1, wherein a magnitude of the pressing force is not less than ten times a magnitude of the preload. [3] The sensor-equipped bearing device according to claim 1 or 2, further comprising: a tubular bearing housing (23) mounted on an outer periphery of the first outer ring (30) and an outer periphery of the second outer ring (35), the tubular bearing housing (23) having an inner periphery provided with an annular outer ring positioning step (50); and a cover element (51) which is fastened to an axial end surface of the bearing housing (23) with a screw element (54), wherein the pressing force is to be applied by arranging the first outer ring (30) and the second outer ring (35) with axial interference between the annular outer ring positioning step (50) and the cover element (51). [4] The sensor-equipped bearing device according to claim 3, wherein the screw member (54) is arranged at the same circumferential position as the tension sensor (28). [5] The sensor-equipped bearing device according to claim 3 or 4, wherein the axial interference is 10 µm or more. [6] A sensor-equipped storage device comprising: a tubular bearing housing (23); and a plurality of rolling bearings having a contact angle other than 0 and arranged in the bearing housing (23) so that they are back to back, wherein the rolling bearings comprise a first bearing (24) and a second bearing (25) which are arranged back to back, wherein the first bearing (24) comprises: a first outer ring (30), a first inner ring (31) rotatably arranged radially inside the first outer ring (30), and a plurality of first rolling elements (32) arranged between the first outer ring (30) and the first inner ring (31), wherein the second bearing (25) comprises: a second outer ring (35), a second inner ring (36) rotatably arranged radially inside the second outer ring (35), and a plurality of second rolling elements (37) arranged between the second outer ring (35) and the second inner ring (36); wherein a tubular outer ring spacer (26) is arranged between the first outer ring (30) and the second outer ring (35); and wherein a tension sensor (28) is attached to the outer ring spacer (26) to determine a load acting on the outer ring spacer (26) from the tension of the outer ring spacer (26), characterized by that the first outer ring (30), the second outer ring (35) and the outer ring spacer (26) are each attached to an inner circumference of the bearing housing (23) by a clearance fit, wherein a fit clearance Δ1 between the bearing housing (23) and the outer ring spacer (26) is greater than a radial expansion amount of the outer ring spacer (26) by which the outer ring spacer (26) is radially expanded by the load acting on the outer ring spacer (26), and the fit clearance Δ1 is greater than a fit clearance Δ2 between the bearing housing (23) and the outer ring (30, 35) of a bearing of the first bearing (24) and the second bearing (25), the one bearing receiving an external force. [7] The sensor-equipped bearing device according to claim 6, wherein the fitting clearance Δ2 between the bearing housing (23) and the outer ring (30, 35) of the one bearing receiving an external axial force has a diameter of 40 µm or less, and wherein the fitting clearance Δ1 between the bearing housing (23) and the outer ring spacer (26) is not more than “Δ2+50 µm” in diameter. [8] The sensor-equipped bearing device according to claim 6 or 7, wherein the outer ring spacer (26) comprises: an outer ring gear (26a) made of a metal and mounted on the inner circumference of the bearing housing (23) with a clearance fit, the outer ring gear (26a) being in axial contact with the first outer ring (30) and the second outer ring (35); and an inner ring gear (26b) made of a resin and arranged radially inside the outer ring gear (26a), and wherein the tension sensor (28) is attached to the outer ring gear (26a). [9] The sensor-equipped bearing device according to one of claims 6 to 8, wherein the first bearing (24) and the second bearing (25) are to be preloaded. [10] The sensor-equipped bearing device according to any one of claims 1 to 9, wherein the first bearing (24) and the second bearing (25) are angular contact ball bearings. [11] A spindle device for a machine tool, the spindle device comprising: the sensor-equipped bearing device (1) according to one of claims 1 to 10; a spindle (2) of the machine tool, wherein the spindle (2) is rotatably supported by the sensor-equipped bearing device (1); and a motor (4) configured to rotate the spindle (2).

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