Strain wave gear device

Abstract

Provided is a strain wave gear device that has high durability and is capable of achieving high torque while achieving weight reduction using a resin material. A strain wave gear device (1) comprises: an internally toothed gear (2); an annular externally toothed gear (3) having flexibility and disposed inside the internally toothed gear (2); a rotation support body (6) disposed inside the externally toothed gear (3); and a non-circular cam (5) capable of rotating with respect to the externally toothed gear (3) via the rotation support body (6). At least one of the internally toothed gear (2) and the externally toothed gear (3) is made of a resin material, and at least an outer contact portion (6a) of the rotation support body (6) that comes into contact with the externally toothed gear (3) is made of a flexible metal material. The linear expansion coefficient A of the non-circular cam (5) and the linear expansion coefficient B of at least the outer contact portion (6a) of the rotation support body (6) satisfy the relationship 0.4 < A / B < 4.0.

Description

Harmonic drive

[0001] The present invention relates to a harmonic drive. This application claims the benefit of priority of Japanese Patent Application No. 2025-020291 filed on February 10, 2025, the content of which is incorporated herein by reference.

[0002] Since harmonic drives can achieve a high reduction ratio with high accuracy while achieving weight reduction and downsizing, they are used, for example, in reduction devices of robots. However, when a harmonic drive is disposed at the tip (e.g., hand) of a robot arm, for example, further weight reduction is required for the purpose of reducing the load applied to the robot arm.

[0003] On the other hand, since the Young's modulus of resin is generally lower than that of metal, resin gears are more likely to wear compared to metal gears. In particular, when the amount of heat generated increases, there are problems that the maximum torque of the harmonic drive decreases and the amount of wear also increases.

[0004] Therefore, as a conventional harmonic drive, while further weight reduction is achieved by making one of the internal gear and the external gear of the harmonic drive a resin material, and further, by making the other of the internal gear and the external gear a high thermal conductivity material with high thermal conductivity, heat generation that may occur at the meshing portion between the internal gear and the external gear is suppressed, thereby suppressing a decrease in product life (see, for example, Patent Document 1).

[0005] On the other hand, as a lubricant used in a harmonic drive (gear device) having an internal gear (rigid gear), an external gear (flexible gear), a bearing (roller), and a non-circular cam (main body), there is also known a technique for reducing seizure and wear of the harmonic drive by adjusting the change rate of the kinematic viscosity of the lubricant (see, for example, Patent Document 2).

[0006] Japanese Patent Application Laid-Open No. 2018-155313 Japanese Patent Application Laid-Open No. 2018-53942

[0007] However, the inventors of this application have diligently conducted tests and research and found that even when at least one of the internal and external gears is made of resin for the purpose of weight reduction, and when heat dissipation is performed using a high thermal conductivity material and cooling is performed using a lubricant, there is still room for improvement in terms of the reduction in maximum torque and durability of the harmonic drive gear.

[0008] The objective of the present invention is to provide a harmonic drive gear that is lightweight using resin materials, highly durable, and capable of high torque.

[0009] (1) The wave drive gear of the present invention comprises an internal gear, a flexible annular external gear disposed inside the internal gear, a rotating support disposed inside the external gear, and a non-circular cam rotatable relative to the external gear via the rotating support, wherein at least one of the internal gear and the external gear is made of a resin material, and at least the outer contact portion of the rotating support that contacts the external gear is made of a flexible metal material, and the coefficient of linear expansion A of the non-circular cam and the coefficient of linear expansion B of at least the outer contact portion of the rotating support satisfy the relationship 0.4 < A / B < 4.0.

[0010] (2) In the harmonic drive gear apparatus described in (1) above, the resin material is preferably a thermoplastic resin having a glass transition temperature Tg (°C) of 50 or higher.

[0011] (3) In the harmonic drive gear apparatus of (1) or (2) above, the lubricant used in both the meshing portion between the internal gear and the external gear and the sliding portion between the rotating support and at least one of the external gear and the non-circular cam contains a solid lubricant, and it is preferable that the solid lubricant contains a molybdenum compound and at least one of melamine cyanurate and polytetrafluoroethylene.

[0012] (4) In the wave gear device described in (3) above, the lubricant is the kinematic viscosity (mm) of the base oil contained in the lubricant. ２When the kinematic viscosity coefficient a (mm / s) is obtained by dividing the value of the base oil by the tip circle diameter (mm) of the external gear, it is preferable that the kinematic viscosity coefficient a of the base oil is 1.0 ≤ a ≤ 6.0.

[0013] (5) In the harmonic drive gear apparatus described in (3) or (4) above, it is preferable that the base oil contained in the lubricant is a synthetic oil and the thickener contained in the lubricant is a lithium-based soap.

[0014] (6) In any one of the above (1) to (5) harmonic drive gears, it is preferable that the non-circular cam is made of a material with a thermal conductivity (W / (m·K)) of 50 or more.

[0015] (7) In any one of the harmonic drive gears of claims (1) to (6) above, the non-circular cam is preferably made of an aluminum alloy, or a material which is an aluminum base material with ceramics or metallic silicon composite or alloyed with it.

[0016] According to the present invention, it is possible to provide a harmonic drive gear that is lightweight using resin materials, yet highly durable and capable of high torque.

[0017] This is a schematic front view showing a harmonic drive gear according to one embodiment of the present invention, viewed from the axial input side. This is a front view showing a harmonic drive gear used in tests according to embodiments and comparative examples of the present invention.

[0018] Hereinafter, with reference to the drawings, a harmonic drive gear according to an exemplary embodiment of the present invention will be described.

[0019] Here, the axial direction refers to the direction in which the axis (central axis) extends. The "perpendicular direction" refers to the direction perpendicular to the axial direction. Furthermore, the "perpendicular direction" is also called the "radial direction" when the axis is used as the reference (center). In particular, when referring to the radial direction, the side closer to the axis is called the "inner radial direction," and the side further from the axis is called the "outer radial direction."

[0020] Figure 1 schematically shows a harmonic drive gear 1 according to one embodiment of the present invention, viewed from the axial input side.

[0021] In Figure 1, the symbol O represents the central axis of the harmonic drive gear 1. The harmonic drive gear 1 outputs input from one side in the axial direction to the other side in the axial direction along its central axis O (hereinafter also simply referred to as "axis O"). In this embodiment, the front side of the drawing is the one side in the axial direction (input side in the axial direction), and the back side of the drawing is the other side in the axial direction (output side in the axial direction).

[0022] The wave drive gear unit 1 comprises a circular spline (internal gear) 2, a flexspline (annular external gear) 3, and a wave generator 4.

[0023] The circular spline 2 is an annular ring member having a plurality of internal teeth 2a and an annular body 2b. The circular spline 2 is positioned such that its central axis is coaxial with axis O. A plurality of internal teeth 2a are formed on the inner circumference of the circular spline 2, spaced apart in the circumferential direction around axis O. In this embodiment, the circular spline 2 is fixed to the case (not shown) of the wave drive gear 1. The circular spline 2 is preferably made of a highly rigid material, such as a rigid body. The circular spline 2 can be made of, for example, a metal material or a resin material such as engineering plastic.

[0024] The flexspline 3 is a cup-shaped member having an opening A3. The flexspline 3 is also positioned such that its central axis is coaxial with axis O. Multiple external teeth 3a are formed on the outer circumference of the flexspline 3, spaced apart in the circumferential direction around axis O. The flexspline 3 is a flexible and deformable member. The external teeth 3a of the flexspline 3 can locally mesh with multiple internal teeth 2a formed on the circular spline 2 by the deformation of the flexspline 3. The flexspline 3 can also be formed from, for example, a metal material or a resin material such as engineering plastic.

[0025] In this disclosure, the flexspline 3 includes a cylindrical portion 3b with axis O as its central axis. The external teeth 3a are formed on the outer circumference of the cylindrical portion 3b on the axial input side. Furthermore, the flexspline 3 has a bottom portion (not shown) on the far side of the drawing. In this disclosure, the flexspline 3 is formed into a cup shape by closing one end (axial output side end) of the cylindrical portion 3b with the bottom portion. In the axial direction, the flexspline 3 has an opening A3 on the opposite side of the bottom portion (forward side of the drawing). A wave generator 4 can be housed in the opening A3. In this disclosure, the output shaft (not shown) of the wave drive gear 1 is connected to the bottom portion.

[0026] The wave generator 4 generates wave motion relative to the flexspline 3 in the circumferential direction around the axis O. As shown in Figure 1, the wave generator 4 comprises a cam member (non-circular cam) 5 having a non-circular contour and a rotating support 6 mounted on the outer circumferential surface of the cam member 5.

[0027] In this disclosure, an input shaft 7 is connected to the cam member 5. Power from a power source such as a motor (not shown) is transmitted to the input shaft 7. As a result, the cam member 5 can rotate about axis O. The central axis (rotation axis) of the input shaft 7 is coaxial with axis O. That is, in this disclosure, the wave generator 4 is positioned on the same axis as axis O, with axis O as its central axis.

[0028] As shown in Figure 1, the cam member 5 has a non-circular shape when viewed in the axial direction (when viewed from the direction of extension of the axis O). The cam member 5, like the circular spline 2, is preferably a highly rigid member, such as a rigid body. The cam member 5 can also be formed from, for example, a metal material or a resin material such as engineering plastic. The outer circumferential surface F5 of the cam member 5 functions as a cam surface.

[0029] In this disclosure, the cam member 5 is two-lobe shaped. As shown in Figure 1, the two-lobe shape is elliptical in axial view. However, the cam member 5 is not limited to two-lobe shape. For example, the cam member 5 may have multiple lobe shapes. Other specific examples of the cam member 5 include, for example, a triangular three-lobe shape and a square four-lobe shape.

[0030] In this disclosure, the rotating support 6 is a radial bearing. The rotating support 6 is a member capable of receiving forces applied radially. The rotating support 6 comprises a plurality of rolling elements 6c arranged circumferentially around an axis O. The rolling elements 6c are members having high rigidity, for example, rigid bodies. The rolling elements 6c can be formed from, for example, metal materials, resin materials such as engineering plastics. In addition, in this disclosure, the rotating support 6 comprises an inner ring 6b and an outer ring 6a. The inner ring 6b and the outer ring 6a are circular raceways that hold the rolling elements 6c so that they can roll. In this embodiment, the rolling elements 6c are balls (spheres). The rolling elements 6c can be held by a cage. However, the rolling elements 6c are not limited to balls, but can be rollers, etc. Also, at least one of the outer ring 6a and the inner ring 6b can be omitted as appropriate.

[0031] As shown in Figure 1, the rotating support 6 can be bent to conform to the contour shape (elliptical in this embodiment) of the outer circumferential surface F5 of the cam member 5 by fitting its inner ring 6b to the outer circumferential surface F5 of the cam member 5. Therefore, the flexspline 3 can also be bent to conform to the contour shape (elliptical in this embodiment) of the outer circumferential surface F5 of the cam member 5 by fitting its inner circumferential surface 3 to the outer ring 6a of the rotating support 6.

[0032] The wave generator 4 can engage the external teeth 3a of the flexus spline 3 with the internal teeth 2a of the circular spline 2 by bending the flexus spline 3 in a non-circular shape. In this disclosure, as shown in Figure 1, the internal teeth 2a of the circular spline 2 and the external teeth 3a of the flexus spline 3 engage at two locations on the long axis portion of the cam member 5. There is a difference in the number of teeth (a difference of 2 in this disclosure) between the number of teeth ZR of the internal teeth 2a of the circular spline 2 and the number of teeth ZF of the external teeth 3a of the flexus spline 3. In Figure 1, the symbol P indicates the engagement portion between the internal teeth 2a of the circular spline 2 and the external teeth 3a of the flexus spline 3. As shown in Figure 1, in this disclosure, the engagement portion P is located on the two long axis sides of the cam member 5.

[0033] The wave drive gear unit 1 converts the rotational motion of the wave generator 4 into wave motion of the flexspline 3 generated between the wave generator 4 and the circular spline 2. This allows the wave drive gear unit 1 to decelerate the input rotation from the wave generator 4 and then output the decelerated input rotation as the output rotation from the flexspline 3.

[0034] Specifically, when the cam member 5 rotates together with the input shaft 7 around axis O, the cam member 5 can rotate the flexspline 3 relative to the circular spline 2 via the rotating support 6. As a result, when the cam member 5 rotates, the meshing portion P moves in the opposite direction to the rotation direction of the cam member 5 relative to the circular spline 2. For example, when the cam member 5 rotates clockwise around axis O, the meshing portion P moves counterclockwise around axis O relative to the circular spline 2. In this disclosure, the meshing portion P moves one mesh relative to the circular spline 2 in the opposite direction to the rotation direction of the cam member 5 for every 180-degree rotation of the cam member 5 around axis O. Therefore, according to the harmonic drive gear 1, the input rotation from the input shaft 7 is output in reverse as a reduced rotation from the flexspline 3.

[0035] In this disclosure, both the circular spline 2 and the flexspline 3 are made of a resin material, which generally has a lower specific gravity than metal materials, for the purpose of reducing weight.

[0036] Here, when a harmonic drive gear is operated at its rated torque, the temperature of the meshing portion P is generally around 40°C. Therefore, if the glass transition temperature Tg of the resin material used in the circular spline 2 and flexspline 3 is 40°C or lower, there is a risk that the power transmission efficiency of the harmonic drive gear will decrease drastically. For this reason, in this disclosure, the glass transition temperature Tg of the resin material used in the circular spline 2 and flexspline 3 is set to 50°C or higher, taking safety into consideration. In this case, even when the harmonic drive gear is operated continuously at its rated torque, it is possible to prevent a decrease in power transmission efficiency. In addition, in this disclosure, the resin material used in the circular spline 2 and flexspline 3 is a thermoplastic resin. In this case, since the circular spline 2 and flexspline 3 can be formed by injection molding, it is possible to manufacture the circular spline 2 and flexspline 3 at low cost.

[0037] However, in the case of a harmonic drive gear, the temperature of the meshing portion P reaches a maximum of approximately 100°C during operation, so it is desirable that the glass transition temperature Tg of the resin material used for the circular spline 2 and flexspline 3 be Tg = 100 (°C) or higher. Also, from a cost standpoint, injection-molded thermoplastic resins, specifically super engineering plastics such as PEEK (polyether ether ketone), polyimide, and PPS (polyphenylene sulfide), are suitable. However, when considering their use as circular splines 2 and flexsplines 3 in the harmonic drive gear 1, PEEK, which has superior mechanical properties among super engineering plastics, is the most desirable.

[0038] On the other hand, when the flexspline 3 is an external gear made of resin, the resin flexspline 3 has a lower Young's modulus compared to a metal flexspline 3. Therefore, when a large torque (load) is applied to the flexspline 3, the cylindrical portion (rim portion) 3b of the flexspline 3 may deform in a wave-like manner in the circumferential direction. Consequently, when the flexspline 3 is an external gear made of resin, the maximum torque obtainable by the harmonic drive gear as a reduction device decreases. To prevent this decrease in maximum torque, in this disclosure, the outer ring 6a of the rotating support 6 located radially inward of the flexspline 3 is made of metal. As a result, the outer ring 6a of the rotating support 6 functions as a back metal for the flexspline 3, thereby suppressing the wave-like deformation of the cylindrical portion 3b of the flexspline 3. Examples of metal materials that can be used for the outer ring 6a include high-carbon chromium bearing steel, and from the viewpoint of weight reduction, light metals such as titanium and aluminum are suitable as metal materials for the outer ring 6a.

[0039] Incidentally, if the cam member 5, which is a component of the wave drive gear 1, is also made of resin, the wave drive gear can be made even lighter.

[0040] However, through diligent testing and research, the inventors of this application have come to realize that when a metal bearing is used as the rotating support 6 for the purpose of improving the maximum torque, durability may decrease if the materials of the cam member 5 and the rotating support 6 are different. Specifically, if the difference in the coefficient of linear expansion between the cam member 5 and the rotating support 6 causes the thermal expansion of the cam member 5 to be greater than that of the rotating support 6, the cam member 5 will push the rotating support 6 outward from the radially inward direction, thereby tightening the rotating support 6 and consequently shortening its lifespan. Furthermore, if dimensional changes occur due to the thermal expansion of the wave generator 4 (for example, the cam member 5), the position of the meshing portion P between the circular spline 2 and the flexspline 3 will change, which may consequently reduce the durability of the wave drive gear.

[0041] That is, as a result of intensive research and tests, the inventor of the present application has found that when at least one of the circular spline 2 and the flex spline 3 is resinized for the purpose of weight reduction, there is still room for improvement in terms of the reduction of the maximum torque and durability of the harmonic gear device, whether heat dissipation is performed using a high thermal conductivity material as described in Patent Document 1 or cooling is performed using a lubricant as described in Patent Document 2.

[0042] Therefore, for the purpose of weight reduction, the harmonic gear device 1 is configured such that at least one of the circular spline 2 and the flex spline 3 is made of a resin material. On the other hand, as the back metal of the flex spline 3, the outer ring 6a, which is the outer contact portion that at least contacts the flex spline 3 of the rotary support 6, is made of a flexible metal material. Furthermore, by paying attention to the difference in the linear expansion coefficients, the linear expansion coefficient A of the cam member 5 and the linear expansion coefficient B of at least the outer ring 6a of the rotary support 6 satisfy the relationship of 0.4 < A / B < 4.0. According to the harmonic gear device 1, it is possible to provide a harmonic gear device that realizes weight reduction using a resin material, has high durability, and can achieve high torque.

[0043] Here, specific materials for the cam member 5 include, for example, resin materials in which fillers and the like are blended to suppress the linear expansion coefficient, CFRP (carbon fiber composite material), aluminum alloys, and iron-based materials.

[0044] Furthermore, the inventor of the present application has confirmed that if the thermal conductivity of the cam member 5 is 50 (W / (m·K)) or more, more preferably 70 (W / (m·K)) or more, the heat generated in the rotary support 6 can be sufficiently drawn into the cam member 5, and as a result, the life of the harmonic gear device 1 as a speed reduction device is improved. Therefore, more preferably, the material of the cam member 5 is an aluminum alloy or an iron-based material. From the perspective of weight reduction, an aluminum alloy is desirable for the cam member 5.

[0045] Furthermore, in the harmonic drive device 1, it is more desirable that the cam member 5 be made of an aluminum alloy, or a material in which ceramics (such as silicon carbide, alumina, boron carbide, aluminum nitride, carbon, etc.) or metallic silicon having a low coefficient of linear expansion is compounded or alloyed with aluminum as a base material. However, as described above, from the viewpoint of weight reduction, it is desirable that the cam member 5 be made of an aluminum alloy, but the coefficient of linear expansion of the aluminum alloy is high. Therefore, in the harmonic drive device 1, it is more desirable that the cam member 5 be made of a material in which ceramics (such as silicon carbide, alumina, boron carbide, aluminum nitride, carbon, etc.) or metallic silicon having a low coefficient of linear expansion is compounded or alloyed with aluminum as a base material.

[0046] What has been described above focuses on the material of the cam member 5 and mainly devises the material of the cam member 5. However, in the present disclosure, further, by focusing on the lubricant L used in the harmonic drive device 1, the lubricant L is also devised.

[0047] In the case where the circular spline (internal gear) 2 and the flex spline (external gear) 3 of the harmonic drive device 1 are made of a resin material while the rotary support 6 is made of a metal material, due to its configuration, there are sliding portions between resins such as the meshing portion P between the circular spline 2 and the flex spline 3, and sliding portions between metals such as between the rolling elements (for example, balls) 6c of the rotary support 6 and the inner ring 6b and the outer ring 6a.

[0048] Here, since the pressure generated at the sliding points between metals is high, a molybdenum-based compound with high load-bearing capacity is suitable as the solid lubricant SL contained in the lubricant L used at the sliding points between metals. Examples of molybdenum-based compounds include MoS2 (molybdenum disulfide), MoDTC (molybdenum dialkyldithiocarbamate), MoDTP (molybdenum dialkyldithiophosphate), and MoAmn (molybdenamine). On the other hand, since the pressure generated at the sliding points between resins or between resin and metal is low, a solid lubricant SL contained in the lubricant L used at the sliding points between resins is suitable as MCA (melamine cyanurate) or PTFE (polytetrafluoroethylene), which have low load-bearing capacity but a low coefficient of friction.

[0049] Therefore, in cases where there are sliding parts for both resin and metal parts, such as in the wave drive gear 1 according to this disclosure, it is desirable that the solid lubricant SL contained in the lubricant L be a combination of a molybdenum-based compound with high load-bearing capacity and at least one of PTFE and MCA, which have low load-bearing capacity but a low coefficient of friction.

[0050] In this disclosure, the solid lubricant SL contained in the lubricant L used in both the meshing portion P between the circular spline 2 and the flexspline 3, and the sliding portion between the rotating support 6 and at least one of the flexspline 3 and the cam member 5, includes a molybdenum-based compound and at least one of MCA and PTFE. In this case, in the sliding portion between metal parts, the molybdenum-based compound, which has high load-bearing capacity, acts effectively as the solid lubricant SL, while in the sliding portion between resin parts and metal parts or resin parts, at least one of PTFE and MCA acts effectively as the solid lubricant SL. Therefore, in this case, the lubricant L used in the interior of the wave generator 4 (sliding portion between the cam member 5 and the rotating support 6), the sliding portion between the wave generator 4 and the flexspline 3 (sliding portion between the outer ring 6a and the flexspline 3), and the sliding portion between the circular spline 2 and the flexspline 3 can be supplied by a single common lubricant.

[0051] Furthermore, the kinematic viscosity of the lubricant L used in the wave drive gear 1 is also important.

[0052] The harmonic drive gear unit 1 uses resin material for both the circular spline 2 and the flexspline 3. Therefore, the surface energy (surface tension) of the teeth (2a, 3a) of the circular spline 2 and flexspline 3 is low, resulting in poor wettability with the lubricant L. Consequently, if the kinematic viscosity of the lubricant L is too low, there is a risk that the lubricant L may be depleted from the teeth due to the centrifugal force during rotation of the circular spline 2 or flexspline 3. On the other hand, the tooth module of the harmonic drive gear unit is smaller than that of a general speed reducer and has a backlash-free structure, resulting in small gaps between the individual teeth (2a, 3a). Therefore, if the kinematic viscosity of the lubricant L is too high, there is a risk that the lubricant L may have difficulty entering the gaps between the teeth. Therefore, the kinematic viscosity of the lubricant L used in the harmonic drive gear 1 needs to be adjusted to an optimal value such that it is maintained at the sliding surface between the circular spline 2 and the flexspline 3 even during the operation of the harmonic drive gear 1. Furthermore, although the rated rotational speed of the harmonic drive gear remains constant, the peripheral speed of the sliding surface varies greatly depending on the size of the harmonic drive gear. As a result, the centrifugal force during rotation changes significantly when the peripheral speed of the sliding surface changes. Therefore, the kinematic viscosity coefficient a (kinematic viscosity (mm)) of the lubricant L used in the harmonic drive gear 1 must be defined by parameters that take the above effects into account. ２ A lubricant using a base oil that falls within the range of 1.0 ≤ a ≤ 6.0 (mm / s) in terms of ( / s) / (external tooth tip diameter (mm)) is desirable.

[0053] In this disclosure, the lubricant L is defined as having a kinematic viscosity coefficient a, where the kinematic viscosity of the base oil contained in the lubricant L is divided by the tip circle diameter of the flexspline 3, and the kinematic viscosity coefficient a is set to 1.0 ≤ a ≤ 6.0. In this case, the lubricant L is more likely to flow into the gap between the meshing portion of the internal teeth 2a of the circular spline 2 and the external teeth 3a of the flexspline 3, while being less likely to be scattered by the centrifugal force generated when the flexspline 3 rotates. Therefore, it is effective as a lubricant L used for resin materials, which have lower surface wettability compared to metal materials.

[0054] Furthermore, in this disclosure, since the circular spline 2 and flexspline 3 are made of resin material with low thermal conductivity, the internal temperature of the harmonic drive gear 1 tends to rise more easily compared to a harmonic drive gear

[0055] In this disclosure, the base oil contained in the lubricant L is a synthetic oil, and the thickener contained in the lubricant L is a lithium-based soap. For example, mineral oil or synthetic oil can be used as the base oil of the lubricant L used in the harmonic drive gear 1, but in this disclosure, synthetic oil is used as the base oil of the lubricant L. In particular, in this disclosure, it is desirable that the base oil of the lubricant L is a synthetic oil with a viscosity index of 120 or higher. In this disclosure, the base oil of the lubricant L is PAO (polyalphaolefin) with a viscosity index of 120 or higher. If the viscosity index of the base oil is less than 120, when the internal temperature rises due to the operation of the harmonic drive gear 1 at high speed, the viscosity of the lubricant L decreases, which may cause the lubricant L to scatter easily. Therefore, if the viscosity index of the base oil is 120, the scattering of lubricant L that may occur when the harmonic drive gear 1 is operated at high speed can be suppressed. Furthermore, in this disclosure, the thickener of the lubricant L is preferably a lithium-based soap that has a heat resistance temperature of 100°C or higher and can maintain its heat resistance over a long period of time. Here, "lithium-based soap" refers to lithium soap with a maximum operating temperature exceeding 100°C, or lithium complex soap which generally has a higher maximum operating temperature than lithium soap. When the harmonic drive gear 1 is operated at high speed, the temperature of the meshing portion P between the circular spline 2 and the flexspline 3 may reach nearly 100°C. For this reason, if the thickener contained in the lubricant L used in the harmonic drive gear 1 is a lithium-based soap with a heat resistance temperature of 100°C or higher and can maintain its heat resistance over a long period of time, the lubrication can be maintained without the mesh of the thickener being destroyed.

[0056] Table 1 shows the results of a test conducted to determine how changing the material of the cam member affects the durability and heat generation of the harmonic drive gear 1 when using a harmonic drive gear 1 with a similar configuration as a reduction gear. In this test, the harmonic drive gear 1 was used as a reduction gear and a 100-hour durability test was conducted under rated conditions.

[0057] In this test, a harmonic drive gear 10 with the configuration shown in Figure 2 was used. In this harmonic drive gear 10, the circular spline 2 and the flexspline 3 were each made of PEEK, a thermoplastic resin. The rotating support 6 was a bearing equipped with an outer ring 6a, an inner ring 6b, and multiple rolling elements 6c. The bearing is a ball bearing, using balls (spheres) as the rolling elements 6c. Each of the multiple rolling elements 6c is held by a cage 6d so as to be spaced apart in the circumferential direction. The bearing was made of SUJ2, a metal. Here, SUJ2 is a bearing steel specified in "JIS G 4805 High Carbon Chromium Bearing Steel Materials". Note that in Figure 2, the flexspline 3 is shown not meshing with the circular spline 2 in the longitudinal direction, but here, the flexspline 3 is assumed to be meshing with the circular spline 2 in the longitudinal direction by deforming outward in the longitudinal direction together with the rotating support 6, as in Figure 1.

[0058] Furthermore, the following materials were used for the cam components: In Example 1, a component made of titanium was used; in Example 2, a component made of iron was used; in Example 3, a component made of ceramic-metal composite material (in this example, a composite material of ceramics and aluminum alloy) was used; and in Example 4, a component made of A7075 (super duralumin), an aluminum alloy, was used. In Comparative Example 1, a component made of PEEK, a resin, was used; and in Comparative Example 2, a component made of silicon carbide was used.

[0059] Rank A: Very good Rank B: Good Rank C: Slightly good Rank D: Bad

[0060] The overall evaluation shown in Table 1 is the result of a comprehensive evaluation that takes into account durability and heat generation. In Table 1, ranks A to D indicate that, in the case of the evaluation of "durability," the amount of wear decreases as the rank increases from D to A, and in the case of the evaluation of "heat generation," the amount of heat generated decreases as the rank increases from D to A. In the overall evaluation of Table 1, those with a rank of C or higher are considered to be practical examples. Note that in Table 1, the unit of the coefficient of linear expansion is "×10⁻⁶ (1 / K)".

[0061] For the evaluation of Examples 1-4 and Comparative Examples 1-2, a 100-hour durability test was conducted using a harmonic drive gear as a reduction gear under rated conditions. Durability was compared by measuring the amount of wear on each sliding part, while heat generation was compared by measuring the amount of heat generated by the cam member. Specifically, wear was evaluated by comparing the dimensions of the sliding parts of each component before and after the test, and heat generation was compared by attaching a temperature-indicating material to the cam member and measuring the temperature change. Table 1 also includes, for reference, the coefficient of linear expansion ratio obtained by dividing the coefficient of linear expansion of the outer ring material of the bearing by the coefficient of linear expansion of the cam member material.

[0062] [Examples 1-4] Examples 1-4 are embodiments relating to the inventions described in claims 1 and 6 of this application. In the harmonic drive gear 1, the durability and heat generation properties of the harmonic drive gear 1 were evaluated by selecting titanium, iron, a composite material of ceramics and aluminum alloy, and aluminum alloy as the material of the cam member 5.

[0063] Referring to Table 1, the results from Examples 1 to 4 show that the amount of wear was lowest when the material of the cam member 5 was made of iron, or a composite material of ceramics and aluminum alloy. Also referring to Table 1, the results from Comparative Examples 1 to 2 show that in both the case of Comparative Example 2, where the coefficient of linear expansion was smaller than that of Examples 1 to 4, and the case of Comparative Example 1, where the coefficient of linear expansion was larger than that of Examples 1 to 4, the durability performance was poor. This is thought to be because, as shown in Comparative Example 2, when the coefficient of linear expansion becomes too small, a gap is created between the cam member 5 and the rotating support 6 due to the difference in thermal expansion coefficients with the rotating support 6, and the internal teeth 2a of the circular spline 2 and the external teeth 3a of the flexspline 3 do not mesh in the correct position. Because they do not mesh in the correct position, the meshing area becomes smaller, stress concentration occurs, and it can be inferred that the amount of wear increases. Conversely, as in Comparative Example 1, when the coefficient of linear expansion becomes too large, the cam member 5 expands more than the rotating support 6, so the cam member 5 puts pressure on the rotating support 6. In this case, even when the internal teeth 2a of the circular spline 2 and the external teeth 3a of the flexspline 3 mesh, pre-pressure is applied to the flexspline 3 from the rotating support 6, which increases the stress generated on the tooth surface of at least the external teeth 3a of the flexspline 3, thus reducing its durability. For this reason, Table 1 shows that the linear expansion coefficient of the cam member 5 should be as close as possible to the linear expansion coefficient of the rotating support 6, and it is desirable that the linear expansion coefficient A of the cam member 5 and the linear expansion coefficient B of the rotating support 6 satisfy the relationship 0.4 < A / B < 4.0.

[0064] Furthermore, the results from Examples 1 to 4 show that when the thermal conductivity of the cam member is low, the amount of heat generated when the harmonic drive gear is decelerated increases. This is thought to be because the low thermal conductivity makes it difficult to dissipate the heat generated inside the wave generator 4. In addition, if the amount of heat generated by the cam member increases, it may cause dimensional changes due to thermal expansion, or their strength may decrease due to the temperature rise caused by the fact that the circular spline 2 and flexspline 3 are made of resin material. For this reason, it is preferable that the material of the harmonic drive gear 1 has a thermal conductivity (W / (m·K)) of 50 or more, more preferably 100 or more. Specific examples include iron, composite materials of ceramics and aluminum alloys, and light metals. Furthermore, considering that the objective of the present invention is to reduce the weight of the harmonic drive gear, the material of the cam member 5 is suitable to be any of aluminum alloy, ceramics, or fine ceramics.

[0065] Based on the results in Table 1, and considering the overall evaluation results of durability and heat generation in Examples 1 to 4, it is desirable that the material of the cam member 5 be an aluminum alloy. Note that the evaluation in Table 1 is for the case where the cam member 5 is a single component. However, if the cam member 5 is a cam member composed of multiple components, including a self-aligning mechanism, then durability and heat generation are considered to be improved if at least the outer contact portion of the cam member 5 that contacts the rotating support 6 (specifically, the cam portion (main body) of the cam member 5) satisfies the linear expansion coefficient ratio and thermal conductivity as in Examples 1 to 4. For this reason, it is desirable that at least the cam portion (main body) of the cam member satisfies the physical property value of 0.4 < A / B < 4.0 in relation to the outer ring of the bearing.

[0066] Next, Tables 2 to 4 show the results of evaluating the durability (amount of wear) and heat generation of the harmonic drive gear 1 as a reduction gear by changing each parameter of the lubricant L.

[0067] The same lubricant L was used in each sliding part of the harmonic drive gear. The evaluation of the base oil, base oil kinematic viscosity, thickener, and solid lubricant was carried out according to the product properties table.

[0068] Rank A: Very good Rank B: Good Rank C: Slightly good Rank D: Bad

[0069] Table 2 shows the results of tests on durability and heat generation when PTFE and MCA are added to organic molybdenum as a solid lubricant in lubricant L, and when PTFE and MCA are not added to organic molybdenum.

[0070] In this test, Example 5 was defined as a solid lubricant containing organic molybdenum with PTFE and MCA added, while Comparative Example 3 was defined as a solid lubricant containing organic molybdenum without PTFE and MCA added. The evaluation of durability and heat generation in this test was the same as in the test shown in Table 1.

[0071] Rank A: Very good Rank B: Good Rank C: Slightly good Rank D: Bad

[0072] Table 3 shows the results of tests on durability and heat generation when only the kinematic viscosity of the base oil was changed, without altering the base oil and solid lubricant of lubricant L. The evaluation of durability and heat generation in this test was the same as in the test in Table 1.

[0073] Rank A: Very good Rank B: Good Rank C: Slightly good Rank D: Bad

[0074] Table 4 shows the results of tests on durability and heat generation by changing the base oil and thickener of lubricant L. The evaluation of durability and heat generation in this test is the same as in the test in Table 1.

[0075] The overall evaluations in Tables 2-4 are the results of a comprehensive evaluation that takes into account durability and heat generation. In the overall evaluations in Tables 2-4, those with a rank of B or higher were considered practical examples.

[0076] For the evaluation of durability and heat generation in Examples 5-8 and Comparative Examples 3-8, a 100-hour durability test was conducted under rated conditions using a harmonic drive gear as a reduction gear, similar to the tests for the cam members in Table 1. For durability, the influence of each material was compared by measuring the amount of wear on each sliding part, while for heat generation, the influence of the lubricant L used on each sliding part was compared by measuring the temperature change from the temperature of the casing in which the harmonic drive gear 1 is incorporated.

[0077] [Example 5] Referring to Table 2, Example 5 is an example relating to the invention of claim 3. In Example 5, in addition to organic molybdenum, PTFE and MCA are added to lubricant L as the solid lubricant SL. From Table 2, it can be seen that by adding PTFE and MCA to lubricant L in addition to organic molybdenum, durability is improved and heat generation is reduced compared to Comparative Example 3, which does not contain these additives. This is thought to be because the friction coefficient is lowered by introducing PTFE and MCA as additives in addition to organic molybdenum, improving wear resistance, and thus reducing wear and heat generation.

[0078] [Examples 6-7] Examples 6-7 in Table 3 are embodiments relating to the invention of claim 4. Examples 6-7 and Comparative Examples 4-5 each evaluate durability and heat generation by changing only the kinematic viscosity of the base oil while keeping the base oil and solid lubricant SL the same.

[0079] Referring to Table 3, the results from Examples 6-7 show that the kinematic viscosity of the base oil should not be too high or too low. This is because, if the kinematic viscosity of the base oil is too high, it becomes difficult for the grease (lubricant L) to flow into the gap between the internal teeth 2a of the circular spline 2 and the external teeth 3a of the flexspline 3. On the other hand, if the kinematic viscosity of the base oil is too low, the grease is scattered by the centrifugal force generated when the flexspline 3 rotates, causing grease depletion in the sliding parts inside the wave drive gear 1. This is thought to be due to the fact that, in the case of resin components, the wettability of the surface of the component is lower compared to metal components, making it difficult for grease to remain on the sliding surface. Furthermore, considering the results of Example 5, the highest evaluation was obtained when the kinematic viscosity coefficient a was a = 4.0. Therefore, in relation to Example 6 and Example 7, it is preferable that the kinematic viscosity coefficient a be at least greater than a = 1.4, for example a = 1.5 or higher, while at most not exceeding a = 5.4, for example a = 4.5 or lower, i.e., 1.5 ≤ a ≤ 4.5, and that a maximum of a = 4.5 is considered optimal.

[0080] [Example 8] Example 8 in Table 4 is an example relating to the invention of claim 5. In Example 8 and Comparative Examples 6 to 8, the durability and calorific value were evaluated by changing the base oil and thickener, respectively.

[0081] Referring to Table 4, it can be seen that, regarding the base oil, using synthetic oil yielded better evaluation results than using mineral oil. Also, referring to Table 4, it can be seen that, regarding the thickener, using lithium-based soap yielded better evaluation results than using calcium-based soap. In cases where there are sliding parts where resins slide against each other, such as in the wave drive gear 1, heat tends to be generated between these resins. Therefore, greases with low heat resistance, such as mineral oil and calcium-based soap, deteriorate more quickly. In contrast, greases with high heat resistance, such as synthetic oil and lithium-based soap, are less prone to deterioration, and as a result, it can be inferred that durability and heat generation improved. Here, "lithium-based soap" refers to lithium soap or lithium complex soap. When lithium complex soap is used as the lithium-based soap, the heat resistance is further improved compared to when lithium soap is used.

[0082] Based on the results of Examples 5 to 8, when a harmonic drive gear is used as a speed reducer, it is desirable to adjust the grease used as lubricant L to the following conditions: (1) Use synthetic oil as the base oil and lithium soap as the thickener. (2) The solid lubricant SL is a molybdenum-based compound with PTFE and MCA added. (3) The kinematic viscosity coefficient a of the base oil is adjusted to within the range of 1.5 ≤ a ≤ 4.5 (a maximum of 4.5 is considered optimal).

[0083] The above describes only exemplary embodiments of the present invention, and various modifications are possible according to the claims. In this disclosure, the rotating support includes a roller rotatably supported on the outer circumferential surface F5 of the cam member 5, as described in Patent Document 2. When the rotating support 6 is a roller, at least the outer contact portion of the rotating support 6 that contacts the flexspline 3 corresponds to the roller. In this case, the roller may be made of a flexible metallic material. In the above description, the power transmission path of the harmonic drive gear 1 has the wave generator 4 as the input and the flexspline 3 as the output, but it is not limited to this. For example, the power transmission path of the harmonic drive gear 1 may have the flexspline 3 as the input and the wave generator 4 as the output.

[0084] 1: Harmonic drive gear, 2: Circular spline (internal gear), 2a: Internal tooth, 2b: Annular body, 3: Flexspline (external gear), 3a: External tooth, 3b: Cylindrical part, 4: Wave generator, 5: Cam member (non-circular cam), 6: Rotating support (bearing), 6a: Outer ring (outer contact part), 6b: Inner ring, 6c: Roller (rolling element), 6d: Cage, 7: Input shaft, 10: Harmonic drive gear (experimental machine), A3: Opening, F5: Outer surface of cam member (cam surface), L: Lubricant, SL: Solid lubricant, O: Axis (central axis), P: Meshing part

Claims

1. A harmonic drive gear comprising: an internal gear; a flexible annular external gear disposed inside the internal gear; a rotating support disposed inside the external gear; and a non-circular cam rotatable relative to the external gear via the rotating support, wherein at least one of the internal gear and the external gear is made of a resin material; at least the outer contact portion of the rotating support that contacts the external gear is made of a flexible metal material; and the coefficient of thermal expansion A of the non-circular cam and the coefficient of thermal expansion B of at least the outer contact portion of the rotating support satisfy the relationship 0.4 < A / B < 4.

0.

2. The harmonic drive gear device according to claim 1, wherein the resin material is a thermoplastic resin having a glass transition temperature Tg (°C) of 50 or higher.

3. The harmonic drive gear apparatus according to claim 1, wherein the lubricant used in both the meshing portion between the internal gear and the external gear and the sliding portion between the rotating support and at least one of the external gear and the non-circular cam comprises a solid lubricant, the solid lubricant comprising a molybdenum compound and at least one of melamine cyanurate and polytetrafluoroethylene.

4. The lubricant has a kinematic viscosity (mm²) of the base oil contained in the lubricant. ２ The harmonic drive gear according to claim 3, wherein when the value obtained by dividing the ratio ( / s) by the tip circle diameter (mm) of the external gear is taken as the kinematic viscosity coefficient a (mm / s), the kinematic viscosity coefficient a of the base oil is 1.0 ≤ a ≤ 6.

0.

5. The harmonic drive gear apparatus according to claim 3, wherein the base oil contained in the lubricant is a synthetic oil, and the thickener contained in the lubricant is a lithium-based soap.

6. The harmonic drive gear apparatus according to claim 1, wherein the non-circular cam is made of a material having a thermal conductivity (W / (m·K)) of 50 or more.

7. The harmonic drive gear according to any one of claims 1 to 6, wherein the non-circular cam is made of an aluminum alloy, or a material which is a composite or alloy of ceramics or metallic silicon on an aluminum base.

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