Flat type wave gear device

Abstract

Flat-type wave gear device comprising an annular fixed inner gear (2, 3), an annular flexible outer gear (4) located within the fixed inner gear (2, 3), and a wave generator (5) for bending the flexible outer gear (4) into an elliptical shape so that it partially engages with the fixed inner gear (2, 3), and for moving the positions at which the gears (2, 3, 4) engage circumferentially, wherein the flat-type wave gear device is characterized in that if d is an amount of radial deformation at a point on the major axis of a neutral circle of the edge of the flexible external gear (4) which is deformed into an elliptical shape, and t is the thickness of the edge of the flexible external gear (4), then ( 0.5237 Ln ( R ) − 1.32 ) d ≤ t ≤ ( 0.8728 Ln ( R ) − 2.2 ) d , if the reduction ratio R of the wave gear device is less than 80, and ( 1.5499 Ln ( R ) − 5.8099 ) d ≤ t ≤ ( 2.5832 Ln ( R ) − 9.6832 ) d , when the reduction ratio R of the wave gear device is 80 or greater.

Description

[0001] The present invention relates to a flat-type wave gear device equipped with an annular flexible gear with external teeth, and in particular to an improved technology to increase the tooth root strength with the aim of increasing the load-bearing capacity of the flat-type wave gear device.

[0002] A wave gear device consists of a fixed or rigid inner gear, a flexible outer gear located within the inner gear, and a wave generator that bends the flexible outer gear into an elliptical shape, causing it to partially engage with the fixed inner gear. When the wave generator is rotated by a motor or similar device, the position at which the two gears engage moves circumferentially, generating a relative rotation at a reduced speed between them. This rotation corresponds to the difference in the number of teeth on the two gears. By fixing one gear so that it does not rotate, it is possible to output a reduced-speed rotation from the other gear and transmit it to the load.

[0003] Depending on the shape of the flexible external gear, wave gear devices can be classified into flat types, cup-shaped types, and cylindrical hat-shaped types. As in Fig. As shown in Figure 6, a flat type wave gear device 1 has two annular fixed internal gears 2 and 3 arranged coaxially parallel, an annular flexible external gear 4 located inside the fixed internal gears 2 and 3, and a wave generator 5 to bend the flexible external gear 4 into an elliptical shape so that it partially engages with the fixed internal gears 2 and 3, and to move the engagement position circumferentially.

[0004] Here, the fixed internal gear 2 has the same number of teeth as the flexible external gear 4, and the other fixed internal gear 3 has n=2k (where k is a positive integer) more teeth than the flexible external gear 4. Or, the fixed internal gear 2 has n more teeth than the flexible external gear 4, and the other fixed internal gear 3 has the same number of teeth as the flexible external gear 4.

[0005] Rotating the shaft generator 5 creates a relative rotation between the fixed internal gear 3 and the flexible external gear 4, which is much slower than the rotation of the shaft generator 5, and this reduced rotation is output by the fixed internal gear 3 on the output side.

[0006] The construction of such a flat-type wave gear device is therefore more compact and simpler than that of a cup-type or top-hat-type device. Wave gear devices of the flat, cup-shaped, and top-hat-shaped types are disclosed, for example, in patent documents 1, 2, and 3, respectively. Patent document 1: JP H05-172 195 A Patent document 2: JP H08-166 052 A Patent Document 3: JP H02-91 238 U

[0007] Compared to other types, such as the cup-shaped and the cylinder-shaped type, the usability of a flat-type wave gear device is limited by its low load-bearing capacity.

[0008] The flexible outer gear transmits the load in mesh with the fixed inner gears when it is deformed elliptically by the elliptical shape of the shaft generator inserted within it. Therefore, it is necessary to increase the tooth root strength to improve load-bearing capacity. A key factor in determining the tooth root strength is the edge thickness of the flexible outer gear. However, no studies have previously focused on optimizing the edge thickness design of the flexible outer gear in a flat-type shaft gear assembly.

[0009] In view of the foregoing, the object of the present invention is to endeavor to optimize the thickness of the edge of the flexible external gear in order to greatly improve the load-bearing capacity of the flat type wave gear device.

[0010] The present invention makes it possible to more than double the previous fatigue strength of the tooth root of the flexible external gear by achieving the optimal thickness of the edge of the flexible external gear, as generated by the bending deformation by the elliptically shaped shaft generator and the tensile deformation due to the load torque, and by specifying a suitable hardness of the material of the flexible external gear.

[0011] That is, according to the present invention, a flat-type wave gear device is provided which has an annular fixed inner gear, an annular flexible outer gear located inside the inner gear, and a wave generator for bending the flexible outer gear into an elliptical shape to cause it to partially engage with the fixed inner gear and to move the position at which the gears engage with each other circumferentially, characterized in that if d is an amount of radial bending at a point on a principal axis of a neutral circle of the edge of the flexible external gear being bent into an elliptical shape, and t is the thickness of the edge of the flexible external gear, then (0.5237 Ln(R)−1.32)d≤t≤(0.8728 Ln(R)−2.2)d, if the reduction ratio R of the wave gear device is less than 80, and (1.5499 Ln(R)−5.8099)d≤t≤(2.5832 Ln(R)−9.6832)d, when the reduction ratio R of the wave gear device is 80 or greater.

[0012] The present invention is also characterized in that the flexible external gear has a material hardness (HRC) value in the range of 40 to 50.

[0013] In accordance with this invention, it is possible to optimize the tooth root strength by optimizing the thickness of the edge of the flexible outer gear of the flat-type wave gear device, which is subject to bending and tensile deformation. It is also possible to significantly increase the tooth root strength by optimizing the edge thickness of the flexible outer gear and its material. As a result, in accordance with this invention, it is possible to realize a flat-type wave gear device that has a much greater load-bearing capacity than previously possible. Fig. Figure 1 is a schematic diagram showing the thickness of the edge of a flat type wave gear device. Fig. Figure 2 is a graph showing the relationship between the quotient of the edge thickness and the amount of radial bending relative to the reduction rate or reduction ratio. Fig. Figure 3 is a graph showing the relationship between bending stress and tensile stress with respect to the thickness of the edge. Fig. Figure 4 is a graph showing the relationship between the fatigue limit load torque and the thickness of the edge. Fig. Figure 5 is a graph showing the relationship between the number of bends and the fatigue limit load torque of the tooth root. Fig. Figure 6 is a general sectional view of a flat type wave gear device.

[0014] Details of the present invention are described below with reference to the drawings.

[0015] Fig. Figure 6 shows the construction of a flat-type wave gear device in which the present invention can be used. Fig. Figure 1 is a concept diagram showing the thickness of the rim of an annular flexible external gear (F / S) 4 of the flat-type wave gear device 1. In the diagram, the rim thickness t is the thickness of the area labeled "rim thickness". Also, the magnitude d of radial bending of the flexible external gear 4, which is bent into an elliptical shape by the wave generator 5, is the magnitude of radial bending at a point on the major axis on the neutral circle of its rim. If n is the difference between the number of teeth of the flexible external gear 4 and the fixed internal gear 3, and m is the module of the flexible external gear 4, then the magnitude of radial bending d = mn.

[0016] Fig. Figure 2 is a graph showing the rim thickness as prescribed by the present invention and the rim thickness of the flexible outer gear (F / S) of an ordinary conventional flat-type wave gear device. In this graph, the horizontal axis is the reduction ratio R of the flat-type wave gear device, and the vertical axis is the ratio t / d of the rim thickness t and the amount of radial bending d.

[0017] In this graph, the dashed line A0 shows the relationship between the reduction ratio and the thickness of the edge of the flexible external gear in a conventional flat-type wave gear device. In contrast, the dashed lines A1 to A4 show the relationship between the reduction ratio and the thickness of the edge of the flexible external gear as prescribed in accordance with the present invention. With respect to the edge thickness value according to the conventional design, which is denoted by the dashed line A0, the dashed lines A1 to A4 represent increases in edge thickness of 20%, 45%, 60%, and 100%, respectively.

[0018] Here, equation 1 expresses the part of the reduction ratio that is less than 80, and equation 2 expresses the part of the reduction ratio that is 80 or greater, where the dashed line A1 describes the lower limit of the thickness t of the edge. t / d=0.5237 Ln(R)−1.32 t / d=1.5499 Ln(R)−5.8099

[0019] Equation 3 also expresses the part of the reduction ratio that is less than 80, and equation 4 expresses the part of the reduction ratio that is 80 or greater, where the dashed line A4 describes the upper limit of the thickness t of the edge. t / d=0.8728 Ln(R)−2.2 t / d=2.5832 Ln(R)−9.6832

[0020] Therefore, in the graph, the area indicated by the slanted lines is the area of ​​the thickness t of the edge according to the present invention, wherein the area of ​​the thickness of the edge according to the present invention lies within the range of approximately 120% to 200% of the thickness of the edge of the design with conventional construction.

[0021] Next, the critical importance of the upper and lower limits of the thickness of the edge t according to the invention will be described.

[0022] Fig. Figure 3 is a graph showing the relationship between the edge thickness t and the bending stress ab, tensile stress σt, and total stress (σb+σt) generated in the flat-type wave gear device. The horizontal axis is the ratio (%) between the edge thickness t of the flexible outer gear and the edge thickness t of the conventional design, as shown in Figure 3. Fig. Figure 1 shows the ratio between the bending stress σb, tensile stress σt, total stress (σb + σt) and the pitch circle diameter (PCD) of the flexible external gear.

[0023] As shown by line segment B1, the bending stress σb increases more or less proportionally to the thickness of the edge, but as shown by line segment B2, the tensile stress σt decreases as the thickness of the edge increases. Because of this, the total stress (σb+σt), as shown by line segment B3, decreases up to an edge thickness that is approximately 160% of the thickness of the edge of the conventionally constructed structure, and thereafter gradually increases, although the increase is small.

[0024] Regarding the thickness of the edge in the conventional design (at the 100% point of the horizontal axis), the overall stress is high, and there is no optimization of the edge thickness in this area. In contrast, in the area covered by the present invention (the range from 120% to 200% of the horizontal axis), the overall stress is kept to a minimum, demonstrating the optimization of the edge thickness. Particularly at the lower limit of 120% or below, the overall stress increases, indicating that it is desirable for the edge thickness t to be at least this value.

[0025] Next up is Fig. Figure 4 shows a graph illustrating the result of calculating the tooth root strength of the flexible external gear when the thickness of the gear's edge is varied. As in Fig. 3 is the horizontal axis, the ratio (%) between the thickness t of the edge of the flexible external gear and the thickness of the edge of the in Fig. In the conventional design shown in Figure 1, the vertical axis represents the ratio between the fatigue limit load torque of the tooth root of the flexible external gear and a nominal torque. In the graph, curve C0 applies when the material hardness is HRC 36, curve C1 applies when the material hardness is HRC 43, and curve C2 applies when the material hardness is HRC 50.

[0026] As can be seen from the curves, the fatigue limit load torque increases with increasing edge thickness, but when the edge thickness exceeds a certain value, a reversal occurs and the fatigue limit load torque decreases. The maximum value of the fatigue limit load torque appears within the range of the present invention (the range from 120% to 200% on the horizontal axis). Furthermore, the point at which the maximum value of the fatigue limit load torque occurs shifts towards a thicker edge with higher material hardness (HRC), with the point at which the maximum value occurs being more or less at the 200% point on the horizontal axis when the material hardness (HRC) is on the order of 50. Currently, it is difficult to machine a flexible external gear with radial flexibility when using material with a material hardness (HRC) exceeding 50.Therefore, a rim thickness of up to 200%, which is the upper limit in the present invention, can be specified in order to achieve the maximum fatigue limit load torque using the hardest material that can be used in practice.

[0027] From the graph in Fig. 4. It can also be seen that the fatigue limit load torque is higher compared to that of the conventionally designed construction (curve C0 in Fig. 4) can be significantly improved by determining the thickness of the rim as above and by manufacturing the flexible outer gear using a material harder than the HRC of 36 previously used.

[0028] For example, it was confirmed that using a design with a structure according to the invention with a thickness of the edge defined by the 145% interrupted line A2 in Fig. Designated as 2, and with a material hardness HRC of 43, the fatigue limit load torque is improved more than twice compared to the design with conventional construction (which has a material hardness HRC of 36). Fig. Figure 5 is a graph showing an example of the results of this experimental test. In this diagram, the horizontal axis represents the number of times the flexible external gear has been bent, and the vertical axis represents the ratio between the load torque and the rated torque. The square dots are the graphical representation of test results for the conventional design, and the round dots are the graphical representation of test results obtained using the present invention.

[0029] In the conventional case, the fatigue strength of the flexible external gear increases up to a material hardness of HRC 40, but if the material used exceeds this hardness, it has been observed that the fatigue strength decreases. However, as in the Fig. 4 and Fig. Figure 5 confirms that the fatigue strength of the flexible external gear can be increased even when the material hardness is HRC 40 or higher. Based on the inventors' test, it was confirmed that when using a rim thickness according to the present invention, it is advantageous to use a material hardness HRC in the range of 40 to 50, and that it is particularly advantageous to use a material hardness HRC in the range of 40 to 43.

Claims

[1] Flat-type wave gear device comprising an annular fixed inner gear (2, 3), an annular flexible outer gear (4) located inside the fixed inner gear (2, 3), and a wave generator (5) to bend the flexible outer gear (4) into an elliptical shape so that it partially engages with the fixed inner gear (2, 3), and to move the positions at which the gears (2, 3, 4) engage circumferentially, wherein the flat-type wave gear device characterized by is that if d is an amount of radial deformation at a point on the major axis of a neutral circle of the edge of the flexible external gear (4) which is deformed into an elliptical shape, and t is the thickness of the edge of the flexible external gear (4), then (0.5237 Ln(R)−1.32)d≤t≤(0.8728 Ln(R)−2.2)d, if the reduction ratio R of the wave gear device is less than 80, and (1.5499 Ln(R)−5.8099)d≤t≤(2.5832 Ln(R)−9.6832)d, when the reduction ratio R of the wave gear device is 80 or greater. [2] Flat type wave gear device according to claim 1, characterized by , that the flexible outer gear (4) has a material hardness (HRC) value in the range of 40 to 50.

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