Motor bearing structure, motor, and rotary actuator

Abstract

Provided is a motor bearing structure in which a front-side shaft part (222) of a motor shaft (22) is supported by a first rolling bearing (27), and a rear-side shaft part (223) is supported by a second rolling bearing (28). At the time of assembly to a motor housing (21), a state in which there is no initial gap is formed by preloading by means of a low-rigidity first spring member (6) mounted on the side of the first rolling bearing (27). At the time of assembly, a low-rigidity spring member functions as a preloading mechanism, and after assembly, only a high-rigidity spring member functions as a preloading mechanism. Due to this configuration, restriction of an axial direction movement amount of the motor shaft (22) and suppression of bearing gap fluctuations caused by a temperature change can both be realized at low cost.

Description

Bearing structure for motor, motor and rotary actuator

[0001] The present invention relates to a motor bearing structure equipped with a preload mechanism that applies a preload to a bearing that rotatably supports a motor shaft, and also to a motor and a rotary actuator equipped with such a motor bearing structure.

[0002] A known rotary actuator includes a motor and a reducer that reduces the output rotation of the motor before outputting it. For example, as described in Patent Documents 1 and 2, a rotary actuator that includes a strain wave gear device as the reducer is known. A motor encoder is generally attached to the motor shaft to detect the motor rotation speed. The motor shaft is rotatably supported on both axial ends of the motor housing via bearings.

[0003] In such rotary actuators, a motor bearing structure in which a motor shaft is supported by a pair of bearings is known that includes a constant-pressure preload mechanism that preloads the bearings using a spring member such as a wave washer. By applying axial preload, axial play in the motor shaft supported by the rolling bearing is eliminated. Furthermore, fluctuations in the axial gap at the motor shaft support portion due to temperature changes are absorbed by the elastic deformation of the spring member.

[0004] Patent Document 3 proposes a bearing structure for a flywheel that uses a constant-pressure preload mechanism to apply preload to a pair of bearings. In this bearing structure, a high-rigidity (large spring constant) disc spring is installed as a preload-applying member between the inner ring of a ball bearing supporting one side of the shaft and the shaft, and a low-rigidity (small spring constant) disc spring is installed between the inner ring of a ball bearing supporting the other side of the shaft and the shaft. The expansion and contraction of the low-rigidity disc spring absorbs axial gaps in the bearing portion caused by temperature changes, and the preload force of the high-rigidity disc spring can withstand large impact forces applied to the shaft.

[0005] Japanese Patent Application Laid-Open No. 2008-115896 Japanese Patent Application Laid-Open No. 2001-304382 Japanese Patent Application Laid-Open No. 2003-194052

[0006] In a rotary actuator equipped with a motor and a reducer, such as a strain wave gearing, a thrust load acts on the motor shaft from the reducer during operation. The magnitude of this thrust load varies depending on the operating state of the reducer. Furthermore, the amount of axial movement of the motor shaft caused by the applied thrust load, temperature changes, and other factors must be kept below the allowable axial movement of the motor encoder attached to the motor shaft. These factors must be taken into consideration when designing a motor bearing structure. This means that the size of the rolling bearing, the preload mechanism, and other factors must be selected. Furthermore, when designing a motor bearing structure, there is a demand for it to be small, compact, and inexpensive.

[0007] Known preload mechanisms include a constant pressure preload mechanism, which uses a spring component to apply preload to the rolling bearing, and a fixed position preload mechanism. When these preload mechanisms are used in motor bearing structures, the following problems arise:

[0008] Issues with using a constant pressure preload mechanism: Regarding the restriction of the axial movement of the motor shaft, the movement can be suppressed by setting the preload load to be equal to or greater than the external force (axial load applied from the reducer). However, if the preload is increased, the bearing size must be selected to a relatively high capacity, otherwise the bearing fatigue life may not meet the required time. If the axial clearance can be reduced by increasing the machining precision of the parts, it is possible to reduce the movement by increasing the rigidity of the spring parts, but this increases the cost of the parts.

[0009] Issues with using a fixed position preload mechanism: Temperature changes cause differences in the expansion rates of the motor shaft and motor housing, which changes the axial clearance. If the clearance decreases, the axial load on the bearing increases, which can shorten bearing life or cause damage, so it is necessary to select a bearing size with a relatively high capacity. If the clearance increases, the bearing preload will be released, shortening bearing life or the amount of movement may increase beyond the allowable movement of the motor encoder.

[0010] In view of the above, it is an object of the present invention to provide a motor bearing structure with an improved preload mechanism that enables miniaturization, compactness, and cost reduction, particularly in terms of reducing the size of the bearing, and also to provide a motor and a rotary actuator that use this new motor bearing structure.

[0011] In order to solve the above-mentioned problems, the motor bearing structure of the present invention comprises: a first rolling bearing and a second rolling bearing that are mounted between a motor shaft and a motor housing and support the motor shaft rotatably relative to the motor housing; a first spring member that is mounted in a compressed state between the motor housing and the outer ring of the first rolling bearing so as to apply a first preload force to the first rolling bearing toward the second rolling bearing along the axial direction; and a second spring member that is mounted in a compressed state between the motor housing and the outer ring of the second rolling bearing so as to apply a second preload force to the second rolling bearing toward the first rolling bearing along the axial direction, wherein the second spring member has a larger spring constant than the first spring member, and wherein when the first preload force of a predetermined magnitude is applied to the first rolling bearing by the first spring member, the outer ring of the first rolling bearing is fixed to the motor housing so as not to move in the axial direction.

[0012] In the motor bearing structure of the present invention, a low-rigidity first spring member is attached as a preload mechanism to a first rolling bearing supporting one side of the motor shaft, and a high-rigidity second spring member is attached as a preload mechanism to a second rolling bearing supporting the other side of the motor shaft. When assembled into the motor housing, the first and second rolling bearings are assembled without an initial gap due to elastic deformation of the low-rigidity first spring member. In this state, the outer ring of the first rolling bearing is fixed to the motor housing using an adhesive or the like. Therefore, after assembly, the low-rigidity first spring member does not function as a preload mechanism, and only the high-rigidity second spring member functions as a preload mechanism.

[0013] In this way, the first spring member, which has a small spring constant and low rigidity, eliminates the initial gap that occurs when the first and second rolling bearings are assembled. The second spring member, which has a large spring constant and high rigidity, expands and contracts to absorb changes in the bearing clearance caused by temperature changes and other factors during motor operation. The spring constant of the second spring member can be set based on the expected range of gap changes. The spring constant is set so that the gap can be absorbed and the fatigue life of the second rolling bearing satisfies the required time.

[0014] In this invention, two types of spring members with different spring constants are used in the preload mechanism, and the low-rigidity spring member is used during assembly, and after assembly, only the high-rigidity spring member functions as the preload mechanism.This invention makes it possible to achieve a motor bearing structure that can both limit the axial movement of the motor shaft and absorb bearing clearance fluctuations due to temperature changes at low cost, and is also advantageous for downsizing the rolling bearings.

[0015] 1 is a longitudinal sectional view showing a rotary actuator according to an embodiment of the present invention;

[0016] Hereinafter, embodiments of the present invention will be described with reference to the drawings. The embodiments are merely examples of the present invention, and are not intended to limit the present invention to the embodiments.

[0017] (Overall Configuration) Fig. 1 is a schematic longitudinal cross-sectional view showing a rotary actuator according to this embodiment. The rotary actuator 1 is composed of a motor 2 and a strain wave gear device 4 that functions as a reducer. The motor 2 has a motor shaft 22 that is coaxially arranged inside a cylindrical motor housing 21, and the outer peripheral portion of the motor shaft 22 forms a motor rotor 24 with a ring-shaped rotor magnet coaxially fixed thereto. The motor rotor 24 faces, across a minute gap, a ring-shaped motor stator 26 around which a stator coil is wound. The motor stator 26 is fixed to the inner peripheral portion of the motor housing 21.

[0018] A disk-shaped mounting flange 211 is formed at the front end of the motor housing 21, and a disk-shaped end plate 212 is formed at the rear end. The motor shaft 22 includes a central shaft portion 221 on which the motor rotor 24 is formed, a front shaft portion 222 extending forward from the front end of the central shaft portion 221, and a rear shaft portion 223 extending rearward from the center of a circular recess formed at the rear end of the central shaft portion 221. The front shaft portion 222 is a cylindrical shaft portion with a smaller diameter than the central shaft portion 221 and is rotatably supported by the mounting flange 211 via a first rolling bearing 27 made of a ball bearing. The front end of the front shaft portion 222 protrudes from a central opening of the mounting flange 211 toward the front strain wave gear device 4. The rear shaft portion 223 is rotatably supported by the end plate 212 via a second rolling bearing 28 made of a ball bearing. The rear end of the rear shaft portion 223 protrudes rearward from the central opening of the end plate 212. An encoder chamber is formed on the rear side of the end plate 212 and is sealed by the end plate 212 and an encoder cover 29 that seals off the rear end of the end plate 212. A motor encoder 30 that detects the rotation of the motor shaft 22 is installed in this encoder chamber.

[0019] The wave gearing 4, which is located in front of the mounting flange 211, comprises a rigid annular internal gear 41, a top-hat-shaped flexible external gear 42 coaxially arranged inside the internal gear 41, a wave generator 43 fitted coaxially inside the internal gear 41, a cross roller bearing 44 that supports the internal gear 41 and the external gear 42 in a state where they can rotate freely relative to each other, and a disk-shaped output shaft 45 that seals off the front end of the wave gearing 4. The wave generator 43 comprises a cylindrical input shaft 432 that is coaxially fixed to the front end of the motor shaft 22 by a fastening bolt 431, a cam plate 433 that is integrally formed with the front end portion of the input shaft 432, and a wave bearing 434 that is attached to the elliptical outer circumferential surface of the cam plate 433 and is elliptically deflected.

[0020] The external gear 42 comprises a cylindrical body 421 that is flexible in the radial direction, external teeth 422 formed on the outer peripheral surface portion of the front end side of this cylindrical body 421, a disk-shaped diaphragm 423 that extends radially outward from the rear end of the cylindrical body 421 along the mounting flange 211, and a rigid annular boss 424 formed continuously with the outer peripheral edge of the diaphragm 423. The front end side portion of the cylindrical body 421 where the external teeth 422 are formed is bent into an elliptical shape by the wave generator 43, and the external teeth 422 mesh with the internal teeth 411 of the internal gear 41 at portions located at both ends of the major axis of the ellipse.

[0021] A cross roller bearing 44 is disposed on the outer periphery of the cylindrical body portion 421 of the external gear 42, between the internal gear 41 and the diaphragm 423 and boss 424 of the external gear 42. The boss 424 of the external gear 42 is sandwiched between the outer ring 441 of the cross roller bearing 44 and the outer periphery of the mounting flange 211, and these three members are fastened and fixed in the axial direction by fastening bolts 46. The internal gear 41 is sandwiched between the inner ring 442 of the cross roller bearing 44 and the output shaft 45, and these three members are fastened and fixed in the axial direction by fastening bolts 46.

[0022] In the rotary actuator 1 having this configuration, the high-speed rotation of the motor shaft 22 is input to the wave generator 43 of the wave gear device 4. When the wave generator 43 rotates, the meshing position of the external gear 42 with respect to the internal gear 41 moves in the circumferential direction. The motor output rotation is reduced at a predetermined reduction ratio according to the difference in the number of teeth between the two gears, and reduced rotation is taken out from the internal gear 41. The reduced rotation is output from the output shaft 45 to the load side (not shown).

[0023] As the external gear 42 meshes with the internal gear 41, a thrust force is generated between the two gears. The thrust force is applied to the motor shaft 22 from the wave generator 43. The magnitude of the thrust force varies depending on the driving state. Thermal expansion and contraction of the motor shaft due to temperature changes can cause fluctuations in the axial clearance at the first and second rolling bearings 27, 28 that constitute the motor bearing structure supporting the motor shaft 22, resulting in problems such as excessive force acting on the first and second rolling bearings 27, 28 and rattle. Furthermore, in the motor encoder 30 attached to the rear end of the motor shaft 22, the rotating member attached to the motor shaft has a limited allowable axial movement relative to the fixed member attached to the end plate in order to maintain its detection accuracy.

[0024] For this reason, the motor bearing structure of this example, in which the motor shaft 22 is supported by the first and second rolling bearings 27, 28, is equipped with a preload mechanism that applies a predetermined preload in the axial direction to the first and second rolling bearings 27, 28.

[0025] (Motor bearing structure) The motor bearing structure supporting the motor shaft 22 includes a preload mechanism, which includes a first spring member 6 that applies a preload to the first rolling bearing 27 and a second spring member 8 that applies a preload to the second rolling bearing 28. The first spring member 6 is a low-rigidity spring member, and its spring constant is smaller than the spring constant of the second spring member 8. For example, a wave washer is used for the low-rigidity first spring member 6, and a disc spring is used for the high-rigidity second spring member 8.

[0026] First, we will explain the high-rigidity second spring member 8. The second spring member 8 is a member that applies a second preload force to the second rolling bearing 28 in the axial direction from the rear side of the motor to the front side of the motor. The inner ring 282 of the second rolling bearing 28 is attached to the circular outer peripheral surface 223a of the rear shaft portion 223 of the motor shaft 22 in an interference fit. A stepped end surface 223b for retaining the inner ring is formed at the axial front end of the circular outer peripheral surface 223a of the rear shaft portion 223, facing the axial rear side, and the inner ring 282 abuts against the stepped end surface 223b from the axial rear side. The outer ring 281 of the second rolling bearing 28 is attached to the circular inner peripheral surface 212a of the end plate 212 formed in a position surrounding the circular outer peripheral surface 223a of the motor shaft 22 in a clearance fit.

[0027] An annular stepped end surface 212b for a spring seat that protrudes radially inward is formed at the axial rear end of the circular inner peripheral surface 212a of the end plate 212. The second spring member 8 is mounted in a predetermined compressed state between this stepped end surface 212b for a spring seat and the outer ring 281. The elastic force of the second spring member 8 is applied to the outer ring 281 as a second preload force that is directed from the rear side to the front side in the axial direction.

[0028] In contrast, the low-rigidity first spring member 6 applies a first preload force to the first rolling bearing 27 in the axial direction from the front side of the motor to the rear side of the motor. The first preload force is smaller than the second preload force. The inner ring 272 of the first rolling bearing 27 is attached to the circular outer peripheral surface 222a of the front shaft portion 222 of the motor shaft 22 in an interference fit state. An annular stepped end surface 222b facing the axial front is formed at the axial rear end of the circular outer peripheral surface 222a of the front shaft portion 222 for retaining the inner ring, and the inner ring 272 abuts against the stepped end surface 222b from the axial front side. The outer ring 271 of the first rolling bearing 27 is attached to the circular inner peripheral surface 211a of the mounting flange 211 formed in a position surrounding the circular outer peripheral surface 222a of the motor shaft 22 in a clearance fit state.

[0029] An annular spring bearing stepped end surface 211b that protrudes radially inward is formed at the axial front end of the circular inner peripheral surface 211a of the mounting flange 211. A first spring member 6 is mounted in a predetermined compressed state between this spring bearing stepped end surface 211b and an inner ring 272 that is mounted on the circular outer peripheral surface 222a of the motor shaft 22. The elastic force of the first spring member 6 is applied to the inner ring 272 as a first preload force that acts from the front to the rear in the axial direction.

[0030] When assembling the motor shaft 22, first and second rolling bearings 27, 28, and first and second spring members 6, 8 into the motor housing 21, the low-rigidity first spring member 6 expands and contracts to remove initial gaps between the first and second rolling bearings 27, 28. With a predetermined magnitude of first preload force acting on the first spring member 6, the outer ring 271 of the first rolling bearing 27 is adhesively fixed to the circular inner circumferential surface 211a of the mounting flange 211 using an adhesive. This fixes the first rolling bearing 27 to the motor housing 21 so that it does not move axially.

[0031] In this way, the low-rigidity first spring member 6 absorbs the initial gap between the first and second rolling bearings 27, 28, creating a state in which the minimum required preload is applied to the first and second rolling bearings 27, 28. After this state is created, the first rolling bearing 27 is adhesively fixed so as not to move in the axial direction, and therefore the first spring member 6 does not contribute to the preload thereafter.

[0032] When the motor is in operation, temperature changes cause gaps to form in the first and second rolling bearings 28 and other locations, and these gaps change. The gaps are absorbed by the elastic deformation of the highly rigid second spring member 8. By setting the spring constant of the second spring member 8 based on the expected range of gap changes, it is possible to absorb the axial gap and suppress the load acting on the second rolling bearing 28 so that the fatigue life of the second rolling bearing 28 falls within a range that satisfies the required time.

[0033] As described above, in the motor bearing structure of the rotary actuator 1 of this example, the first and second spring members 6 and 8, which have two different types of rigidity, are used in combination in the preload mechanism. After the motor is assembled, a preload is applied to the first and second rolling bearings 27 and 28, and the first rolling bearing 27, which is preloaded by the low-rigidity first spring member 6, is adhesively fixed so as not to move in the axial direction. After this, only the high-rigidity second spring member 8 functions as the preload mechanism. This makes it possible to limit the amount of axial movement and absorb changes in axial clearance due to temperature changes at a low cost.

[0034] While the above example relates to a rotary actuator configured with a motor and a strain wave gearing, the present invention is also applicable to a rotary actuator configured with a motor and a planetary reducer other than a strain wave gearing, or a reducer other than that.

Claims

1. A motor bearing structure comprising: a first rolling bearing and a second rolling bearing mounted between a motor shaft and a motor housing to support the motor shaft rotatably relative to the motor housing; a first spring member mounted in a compressed state between the motor housing and the outer ring of the first rolling bearing to apply a first preload force to the first rolling bearing in the axial direction toward the second rolling bearing; and a second spring member mounted in a compressed state between the motor housing and the outer ring of the second rolling bearing to apply a second preload force to the second rolling bearing in the axial direction toward the first rolling bearing; wherein the second spring member has a larger spring constant than the first spring member, and when a predetermined magnitude of first preload force is applied to the first rolling bearing by the first spring member, the outer ring of the first rolling bearing is fixed to the motor housing so as not to move in the axial direction.

2. A motor bearing structure according to claim 1, wherein the outer ring of the first rolling bearing is fixed to the motor housing with an adhesive.

3. A motor bearing structure according to claim 1, wherein the first spring member is a wave washer and the second spring member is a disc spring.

4. A motor equipped with a motor encoder that detects the rotational state of the motor shaft, the motor having the motor bearing structure according to any one of claims 1 to 3 that supports the motor shaft.

5. A rotary actuator comprising: a motor equipped with a motor encoder that detects the rotational state of the motor shaft; and a reducer that decelerates the rotation of the motor shaft, wherein the motor is equipped with the motor bearing structure described in any one of claims 1 to 3 that supports the motor shaft.

6. A rotary actuator according to claim 5, wherein the reducer is a wave gear device.

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