Cup-shaped flexible externally toothed gear and cup-type strain wave gearing

In the cup-shaped externally toothed gear, the outside end face profile of the diaphragm is defined by a first concave circular arc having a first radius, a second concave circular arc that has a second radius and is smoothly connected to the first concave circular arc, an inclined straight line that is smoothly connected to the second concave circular arc and is inclined toward an inside straight line with respect to the center axis line, the inside straight line defining the outside profile of the diaphragm. The second radius is larger than the first radius, and the thickness of the diaphragm is gradually decreased from the side of the boss to the side of the cylindrical body. The stress concentration in the boss-side joint portion of the diaphragm can be relieved, whereby enhancing fatigue strength of the flexible externally toothed gear.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to cup-type strain wave gearings, and in particular, relates to a small-size, cup-shaped flexible externally toothed gear for a cup-type strain wave gearing.

Description of the Related Art

FIG. 1Ais a longitudinal cross sectional view showing a typical cup-type strain wave gearing, andFIG. 1Bis a schematic diagram thereof when cut along a plane perpendicular to a center axis line of the device. As shown in these drawings, the strain wave gearing1has an annular rigid internally toothed gear2, a cup-shaped flexible externally toothed gear3arranged inside the rigid internally toothed gear in a concentric manner, and an ellipsoidal-contoured wave generator4fitted inside the flexible externally toothed gear3. The flexible externally toothed gear3has a flexible cylindrical body11, a diaphragm12extending radially inward from one end of the cylindrical body in the direction of the center axis line1a, and a rigid boss13continued to the inner peripheral edge of the diaphragm12.

A portion of the cylindrical body11of the flexible externally toothed gear3where external teeth14are formed is flexed by the wave generator4into an ellipsoidal shape, whereby the external teeth14located on both ends in the major-axis direction of the ellipsoidal shape are meshed with internal teeth15of the rigid internally toothed gear2. Since the difference in number of teeth between the both gears2and3is 2n (n is a positive integer), the meshing positions between the both gears2and3move circumferentially to generate relative rotation between the gears according to the difference in number of teeth when the wave generator4is rotated by a motor or another rotational source. Typically, the rigid internally toothed gear2is fixed so as not to rotate, and a greatly reduced-speed rotation is output from the flexible externally toothed gear3.

FIGS. 2A, 2B and 2Care explanatory views showing longitudinal cross sections of the cup-shaped flexible externally toothed gear3before and after it is deformed. The cylindrical body11of the flexible externally toothed gear3has an original cylindrical shape before it is deformed as shown inFIG. 2A. After being deformed into an ellipsoidal shape by the wave generator4, the cylindrical body11becomes a state in which the longitudinal cross sectional shape thereof including the major axis of the ellipsoidal shape is tapered outward from the side of the diaphragm12toward the open end11a, as shown inFIG. 2B. Whereas, the longitudinal cross sectional shape of the cylindrical body11including the minor axis of the ellipsoidal shape is tapered inward from the side of the diaphragm12toward the open end11a, as shown inFIG. 2C.

The diaphragm12is formed between the cylindrical body11and the rigid boss13in order for the portion of the cylindrical body11on the open end11aside to be capable of being deformed into an ellipsoidal shape. When the portion including the open end11aof the cylindrical body11is deformed into an ellipsoidal shape, the diaphragm12is bent backwards as shown by an arrow inFIG. 2Bat a joint portion thereof joining to the rigid boss13in the longitudinal cross section including the major axis of the ellipsoidal shape. Whereas, the diaphragm12is bent forward toward the side of the open end11aas shown by the arrow inFIG. 2Cin the longitudinal cross section including the minor axis of the ellipsoidal shape. Thus, during the operation of the gearing1, the diaphragm12is applied with bending stress in the direction of the center axis line11band, at the same time, is applied with share stress caused by torque transmission.

Taking into consideration of these stresses applied in combination to the diaphragm12, the longitudinal cross sectional shape of the diaphragm12is designed so that the open-end side portion of the cylindrical body11is capable of being deformed into an ellipsoidal shape with a smaller force and that the diaphragm12is capable of transferring a larger torque. In particular, the longitudinal cross sectional shape of the diaphragm is designed so as to avoid stress concentration on the diaphragm in a state in which the combined stresses are applied.

Patent document 1 (Japanese Unexamined Utility Model Application Publication No. 61-173851) discloses a cup-shaped flexible externally toothed gear, in which the longitudinal cross sectional shape of a diaphragm is designed so that the inside end face thereof is defined by a straight line, and the outside end face thereof in the joint portion to the boss is defined by a streamline so as to gradually increase the thickness of the diaphragm.

Patent document 2 (WO 2013/024511) discloses a flexible externally toothed gear, in which the diaphragm as a whole is made slightly inclined with respect to a direction perpendicular to the center axis line, and the outside profile of the joint portion to the boss in the diaphragm is defined by three circular arcs.

The streamline, which is superior in dynamic characteristics, is employed to define the profile of the boss-side joint portion in the diaphragm of the cup-shaped flexible externally toothed gear. The streamline profile is constituted by three or more circular curves having different radii as disclosed in Patent Document 1. The circular curves are arranged so that the radii thereof become smaller toward the boss side.

The flexible externally toothed gear is usually manufactured by lathe turning. When small-size flexible externally toothed gears are concerned, the radii of the curves for constituting the streamline become smaller inevitably. It is therefore difficult to generate a profile shape of the boss-side joint portion in the diaphragm according to the streamline by making use of lathe turning.

Specifically, in commercially available typical lathe turning machines, the minimum value of the nose tip radius is 0.2 mm or larger. It is difficult to generate a profile shape of the boss-side joint portion of the diaphragm in case in which a streamline defined by circular curves including one having a radius smaller than 0.2 mm is employed. For example, when the flexible externally toothed gear is small in size and has a pitch circle diameter of 20 to 40 mm, if the profile shape of the boss-side joint portion in the diaphragm is defined by a streamline, curves that constitute the streamline include curves having a radius smaller than the minimum value of the nose tip radius of lathe turning machines.

Here, as shown inFIG. 3, it is considered to define the profile shape of the outside end face of the boss-side joint portion42ain the diaphragm42by a circular arc54in place of the stream line, the circular arc54having a radius R3that is the same as the minimum nose tip radius and is able to be processed by a commercially available lathe turning machine.

However, in the diaphragm42having the profile shape defined by the circular arc, the boss-side joint portion42amay suffer from stress concentration that is greater than when the streamline profile is employed. This causes to decrease fatigue strength of the flexible externally toothed gear40, and load capacity of the strain wave gearing cannot be enhanced.

SUMMARY OF INVENTION

In view of the above, an object of the present invention is to realize a profile shape of a diaphragm of a flexible externally toothed gear suited for use in a cup-shaped flexible externally toothed gear which is so small in size that it is difficult to generate a profile shape of streamline by lathe turning.

Another object of the present invention is to realize a strain wave gearing provided with a cup-shaped flexible externally toothed gear having a novel profile shape.

In order to realize the above and other objects, according to one aspect of the present invention, there is provided a cup-shaped externally toothed gear for use in a cup-type strain wave gearing, in which the externally toothed gear is deformed by a wave generator into an ellipsoidal shape and is partially mesh with a rigid internally toothed gear. The cup-shaped externally toothed gear includes a flexible cylindrical body having a first end and a second end in a direction of a center axis line, a diaphragm extending radially inward from the first end of the cylindrical body, a rigid boss formed integrally in a center portion of the diaphragm, and external teeth formed on an outer peripheral surface portion of the second end of the cylindrical body. In the cup-shaped externally toothed gear, when cut along a plane including the center axis line, an inside end face profile of the rigid boss and the diaphragm is defined by an inside straight line perpendicular to the center axis line, and an outside end face profile of the diaphragm is defined by a first concave circular arc having a first radius, a second concave circular arc having a second radius, and an inclined straight line. The first concave circular arc is smoothly connected at one end thereof to a parallel straight line parallel to the center axis line and defines an outer peripheral surface of the rigid boss, the second concave circular arc is smoothly connected at one end thereof to the other end of the first concave circular arc, the inclined straight line is smoothly connected to the other end of the second concave circular arc and is inclined toward the inside straight line, the second radius of the second concave circular arc is larger than the first radius of the first concave circular arc, and a thickness of the diaphragm is gradually decreased from a side of the rigid boss to a side of the cylindrical body.

The inside end face profile of the diaphragm is defined by the straight line perpendicular to the center axis line, the outside end face profile is defined by the first and second concave circular arcs and the inclined straight line. According to experiments conducted by the present inventors et. al, it was confirmed that stress concentration on a boss-side joint portion of the diaphragm can be relieved, and fatigue strength of the flexible externally toothed gear can be enhanced to the same extent as a case where the streamline is employed.

The flexible externally toothed gear according to the embodiment of the present invention is in particular suitable for a small-sized flexible externally toothed gear. Specifically, it is suitable for such a small-sized flexible externally toothed gear that the pitch diameter of external teeth thereof is less than 40 mm and the streamline shape is difficult or unable to be generated by lathe turning. For such a small-sized flexible externally toothed gear, it is preferable that the first radius of the first concave circular arc is equal to or more than 0.2 mm so that lathe turning can be employed.

In another aspect of the present invention, there is provided a cup-type strain wave gearing that has the cup-shaped flexible externally toothed gear as constituted above. According to the embodiment of the present invention, a strain wave gearing having a high load capacity can be realized.

DESCRIPTION OF THE EMBODIMENTS

An embodiment of a cup-shape flexible externally toothed gear of a cup-type strain wave gearing to which the present invention is applied will be described below, making reference to the accompanying drawings.

FIG. 4is a longitudinal cross sectional view showing a cup-shaped flexible externally toothed gear of the present embodiment. The flexible externally toothed gear20has the same basic configuration as conventional ones (seeFIGS. 1A, 1B, and 2A to 2C), and is of a small size in which the pitch circle diameter is about 20 to 40 mm.

The flexible externally toothed gear20has a radially flexible cylindrical body21, a discoid diaphragm22extending radially and inward from one end of the cylindrical body21in the direction of the center axis line21a, a ring-shaped rigid boss23integrally formed on the center portion of the diaphragm22in a concentric manner, and external teeth24formed on the outer peripheral surface portion of the other end of the cylindrical body21.

Cross-sectional profile shapes in the respective portions of the diaphragm22will be described. As shown inFIG. 4, when the cup-shaped flexible externally toothed gear20is cut along a plane including the center axis line21a, end faces of the diaphragm22and the boss23that face the inner side of the cup shape of the gear20are called as inside end faces, and opposite end faces thereof that face the outer side of the cup shape are called as outside end faces.

The profiles of the inside end face of the boss23and the inside end face of the diaphragm22are defined by an inside straight line31perpendicular to the center axis line21a. The profile of the outside end face of the boss23is defined by an outside straight line32perpendicular to the center axis line21a. Thus, the boss23has a constant-thick ring shape defined by the two straight lines parallel to each other in the present embodiment.

The outside end face profile for the boss-side joint portion22aof the diaphragm22is defined by a first concave circular arc34having a first radius R1centered on point O1. The first concave circular arc34has one end34asmoothly connected to a parallel straight line33that is parallel to the center axis line21a. The parallel straight line33defines an outer circumferential profile of the boss23.

The outside end face profile for the portion22bof the diaphragm22other than the boss-side joint portion22a, is defined by a second concave circular arc35having a second radius R2centered on point O2and an inclined straight line36smoothly connected to the second concave circular arc35.

The second concave circular arc35is smoothly connected at one end thereof to the end34bof the first concave circular arc34. An inclined straight line36is smoothly connected to the other end35bof the second concave circular arc35. The inclined straight line36is slightly inclined toward the inside straight line31with respect to the direction perpendicular to the center axis line21a.

The diaphragm22is defined by the inside end face portion formed by the inside straight line31and the outside end face portion formed by the first concave circular arc34, the second concave circular arc35and the inclined straight line36. Therefore, the thickness of the diaphragm22is gradually decreased from the side of the boss to the side of the cylindrical body21.

The second radius R2of the second concave circular arc35is much larger than the first radius R1of the first concave circular arc34. The first radius R1of the first concave circular arc34is set to be 0.2 mm, for example, that is the minimum value of the tip nose radius of commercially available typical lathe turning machines.

The outer peripheral edge22cof the diaphragm22is smoothly connected to the inner peripheral edge of an end part21bof the cylindrical body21. The end part21bis curved in a circular-arc shape. For example, the cylindrical body21has an approximately constant thickness that is the same as the thickness of the outer peripheral edge22cof the diaphragm22.

The inventors of the present invention et. al conducted experiments to measure stress distributions during operation in the flexible externally toothed gear20ofFIG. 4and the conventional flexible externally toothed gear40ofFIG. 3. The conventional flexible externally toothed gear40has the same configuration as that of the flexible externally toothed gear20, except that the conventional flexible externally toothed gear40has a profile shape portion50indicated by an imaginary line shown inFIG. 4.

InFIG. 4, the profile shape portion50indicated by the imaginary line is drawn so that, in the conventional flexible externally toothed gear40, the circular arc54thereof (seeFIG. 3) is set to be the same as the first concave circular arc34and the inclined straight line36is extended radially inward so as to be smoothly connect to the end of the arc34.

FIG. 5Ashows positions at which stress is measured, andFIG. 5Bis a graph showing obtained stress distributions. In the graph, a solid line is a curve showing the stress distribution obtained from the flexible externally toothed gear20, while a dotted line is a curve showing the stress distribution obtained from the conventional flexible externally toothed gear40. The stress measurement point p0corresponds to the end34a, the point p2to the end34b, the point p6to the middle position of the second concave circular arc35, the point p8to the end35bof the second concave circular arc35, and the point p10to the outer peripheral end22cof the diaphragm.

As can be seen from the graph, the stress concentration on the boss-side joint portion22aof the diaphragm22is greatly relieved, which shows that the fatigue strength of the externally toothed gear20can be increased. The buckling torque of the flexible externally toothed gear20is also increased, which is not shown in the drawings.

The cup-shaped flexible externally toothed gear20having the above-mentioned structure can be used for the flexible externally toothed gear3shown inFIGS. 1 and 2. A cup-type strain wave gearing in which the flexible externally toothed gear20is assembled is capable of increasing the lord capacity when compared to a case where the flexible externally toothed gear40as shown inFIG. 3is assembled.