Harmonic drive gear reduction mechanism

A harmonic drive gear reduction mechanism includes a rigid inner spline with 2(n+1) teeth and a tooth module of m, a flexible outer spline inserted in the rigid inner spline has 2n teeth and a tooth module of m, a flexible bearing inserted in the flexible outer spline, and a wave generator inserted in the flexible bearing. A thickness between an inner diameter of the flexible bearing and a pitch diameter of the flexible outer spline is t, a cross sectional profile of the wave generator consists four curves, the first and third curves have a radius of curvature R, R=m (n+1)−t, a central angle of the first and third curves is k, and 10<k≦60, the second and fourth curves and conjunction points between the second, fourth curves and the first, third curves can be first-order differentiated.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a gear reduction mechanism, and more particularly to a harmonic drive gear reduction mechanism.

2. Description of the Prior Art

FIG. 1shows a conventional harmonic drive gear reduction mechanism disclosed in U.S. Pat. No. 7,178,427 B, which comprises a rigid inner spline11, a flexible outer spline12disposed in the rigid inner spline11, a flexible bearing13disposed in the flexible outer spline12, and a wave generator14disposed in the flexible bearing13. When the wave generator14rotates, it pushes against and makes the flexible bearing13and the flexible outer spline12deform, and as a result, the teeth at different parts of the deformed flexible outer spline12will be engaged with the teeth of corresponding part of the rigid inner spline11, furthermore, the flexible outer spline12has less teeth than the rigid inner spline11, so that a rotation speed reduction is achieved.

Since the wave generator14is elliptical, the flexible outer spline12driven by the wave generator14will also be deformed into an elliptical shape. However, the rigid inner spline11is circular, which makes the flexible outer spline12only engage at two points with the rigid inner spline11, namely, only a few teeth are engaged, hence, the contact pressure and torque applied to each tooth are relatively big.

Many types of tooth, as shown inFIGS. 2-6, have been studied in the past in order to increase the number of teeth, however, the male tooth15and the female tooth16are much different in configuration from each other, which results in a relatively small contact area, large contact pressure and severe abrasion for each tooth, for example, the conventional harmonic drive gear reduction mechanism disclosed in U.S. Pat. No. 3,996,816 B as shown inFIG. 2,FIG. 3shows a conventional harmonic drive gear reduction mechanism disclosed in U.S. Pat. No. 5,456,139 B,FIG. 4shows a conventional harmonic drive gear reduction mechanism disclosed in U.S. Pat. No. 4,974,470 B,FIG. 5shows a conventional harmonic drive gear reduction mechanism disclosed in U.S. Pat. No. 4,823,638 B, andFIG. 6shows a conventional harmonic drive gear reduction mechanism disclosed in U.S. Pat. No. 8,028,603 B.

FIG. 7shows a conventional harmonic drive gear reduction mechanism disclosed in U.S. Pat. No. 8,011,273 B, andFIG. 8shows a conventional harmonic drive gear reduction mechanism disclosed in JPU-1978165179, wherein the wave generator14is also elliptical, therefore, it also suffers from the disadvantages as mentioned as above.

FIG. 9shows a conventional harmonic drive gear reduction mechanism disclosed in JPA-1994174018,FIG. 10shows a conventional harmonic drive gear reduction mechanism disclosed in JPA-1999094030,FIG. 11shows a conventional harmonic drive gear reduction mechanism disclosed in JPU-1974140583, wherein the part142of the wave generator14abutting against the elongated shaft141is still elliptical, and the radius143of the elliptical part142is smaller than the diameter144of the elongated shaft141, so that the flexible spline12which is driven by the wave generator14only engage at two points with the rigid inner spline11, namely, only a few teeth are engaged. Hence, the contact pressure and torque applied to each tooth are relatively big.

FIGS. 12, 13show a conventional harmonic drive gear reduction mechanism disclosed in JPA-2012251603, wherein the profile of the wave generator14consists of three arc-shaped sections which simulate an ellipse. However, the angle α for the arc-shaped section which has the radius R1 is only 5 degrees, therefore, the structural strength of the wave generator is limited.

SUMMARY OF THE INVENTION

The primary objective of the present invention is to provide a harmonic drive gear reduction mechanism capable of increasing the number of engaged teeth, reducing pitch error for each tooth, and enhancing transmission accuracy.

Another objective of the present invention is to provide a harmonic drive gear reduction mechanism capable of increasing the torque to be transmitted by increasing the number of engaged teeth.

Another objective of the present invention is to provide a harmonic drive gear reduction mechanism, wherein the profile of the wave generator can be first-order differentiated, which allows the flexible bearing to rotate smoothly.

Another objective of the present invention is to provide a harmonic drive gear reduction mechanism, the teeth of the rigid inner spline and the flexible outer spline have the same tooth profile, which increases the contact area when the rigid inner spline and the flexible outer spline engage with each other, thus reducing contact pressure of the engaged teeth.

Yet another objective of the present invention is to provide a harmonic drive gear reduction mechanism capable of preventing the roots of the teeth of the rigid inner spline and the flexible outer spline from coming into contact with each other and generating large torque, causing tooth fracture.

To achieve the above objectives, a harmonic drive gear reduction mechanism in accordance with the present invention comprises: a rigid inner spline, a flexible outer spline, a flexible bearing and a wave generator. The rigid inner spline is hollow circular structure provided with 2(n+1) teeth and has a tooth module of m. The flexible outer spline is a hollow circular structure inserted in the rigid inner spline and provided with 2n teeth and the same tooth module of m as the rigid inner spline. The flexible bearing is a hollow structure inserted in the flexible outer spline. The wave generator is inserted in the flexible bearing. A thickness between an inner diameter of the flexible bearing and a pitch diameter of the flexible outer spline is defined as t, a cross sectional profile of the wave generator consists of a first curve, a second curve, a third curve and a fourth curve, the first and third curves have the same radius of curvature R, it satisfies the equation: R=m (n+1)−t, a central angle of the first and third curves is k degrees, and 10<k≦60, the second and fourth curves can be first-order differentiated, and conjunction points between the second and fourth curves and the first and third curves can also be first-order differentiated.

Preferably, the teeth of the rigid outer spline are involute teeth.

Preferably, the second and fourth curves of the wave generator are semi-elliptical and compressed in an equiangular and equal proportion manner.

Preferably, the second and fourth curves are equiangularly and equiproportionally compressed based on an elliptical equation: (X2/R2+Y2/b2=1), b is a radius of a minor axis of the wave generator.

Preferably, a coordinate for any arbitrary point on the second and fourth curves is {X(θ), Y(θ)}, and it satisfies the relations:
X(θ)=Rcos(θ−α)π/(π−2α);
Y(θ)=bsin(θ−α)π/(π−2α);

α is half of the central angle k of the first or third curve.

Preferably, the first and third curves are symmetrical with respect to the center of the wave generator, and the second and fourth curves are also symmetrical with respect to the center of the wave generator.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring toFIG. 14A, a harmonic drive gear reduction mechanism in accordance with a first embodiment of the present invention comprises: a rigid inner spline20, a flexible outer spline30, a flexible bearing40and a wave generator50.

The rigid inner spline20is a hollow circular structure provided with 2(n+1) teeth and has a tooth module of m. In this embodiment, the teeth of the rigid inner spline20are involute teeth.

The flexible outer spline30is a hollow circular structure inserted in the rigid inner spline20and provided with 2n teeth and the same tooth module of m as the rigid inner spline20. In this embodiment, the teeth of the flexible outer spline30are involute teeth, and in a valid area, the rigid inner spline20and the flexible outer spline30have the same tooth profile. The valid area means the parts of the rigid inner spline20and the flexible outer spline30which are engaged with each other and excludes the root and crown of the teeth21,31of the rigid inner spline20and the flexible outer spline30.

The flexible bearing40is a hollow structure inserted in the flexible outer spline30.

The wave generator50is inserted in the flexible bearing40, a radius of a major axis of the wave generator50is a, and a radius of a minor axis of the wave generator50is b.

A thickness between an inner diameter of the flexible bearing40and a pitch diameter32of the flexible outer spline30is defined as t. A cross sectional profile of the wave generator50consists of a first curve51, a second curve52, a third curve53and a fourth curve54. The first and third curves51,53has the same radius of curvature R and has a center511,531located at the center55of the wave generator50. Namely, the radius R of the first and third curves51,53is equal to the radius a of the wave generator50, and they satisfy the equation: R=m (n+1)−t, wherein b is the radius of the minor axis of the wave generator50, b<R. To balance the load applied to the flexible outer spline30, the first and third curves51,53are symmetrical with respect to the center55of the wave generator50, and the second and fourth curves52,54are also symmetrical with respect to the center55of the wave generator50. A central angle of the first and third curves51,53is k degrees, and 10<k≦60, and preferably, 20≦k≦40. In this embodiment, k=40, and it can also be k=60 as shown inFIG. 15, or k=20, as shown inFIG. 16.

The second and fourth curves52,54can be first-order differentiated since they are a curve with a continuous radius of curvature, and the conjunction points521,541,512and532between the second and fourth curves52,54and the first and third curves51,53can also be first-order differentiated, so that the two conjunction points512,532of the first and third curves51,53have the same slope as the two conjunction points521,541of the second and fourth curves52,54, namely, the radius of curvature is continuous.

Since the cross sectional profile of the wave generator50consists of the first, second, third and fourth curve51,52,53,54, plus the abovementioned limitations, such as, the first and third curves51,53have the same radius of curvature R and have their centers,511,531located at the center55of the wave generator50, and the central angle k of the first and third curves51,53is 40 degrees, these arrangements will increase the number of engaged teeth (contact ratio) of the rigid inner spline20and the flexible outer spline30, when the wave generator50rotates the flexible outer spline30. Since the number of engaged teeth of the present invention is more than one and much bigger than that of conventional design, the relative angle between the rigid inner spline20and the flexible outer spline30will be determined by all the engaged teeth rather than being determined by the pitch or profile of a certain tooth. Namely, the relative angle between the rigid inner spline20and the flexible outer spline30will be determined by average pitch of all the engaged teeth21,31. The teeth maybe different in pitch from one another, and the pitch difference will cause transmission angle error. When the angle errors of all the engaged teeth are averaged, the error will be reduced, and therefore, transmission accuracy will be improved.

Furthermore, since the present invention increases the number of engaged teeth between the rigid inner spline20and the flexible outer spline30, when the flexible outer spline30is loaded, the load on each tooth will be reduced as compared to the conventional design. Hence, the torque that the harmonic drive gear reduce mechanism can transmit is effectively increased.

It is noted that the cross sectional profile of the wave generator50consists of the first, second, third and fourth curve51,52,53,54, and the second and fourth curves52,54and the conjunction points521,541,512and532between the second and fourth curves52,54and the first and third curves51,53can be first-order differentiated, which allows the flexible bearing40to rotate smoothly.

Besides, the teeth of the rigid inner spline20and the flexible outer spline30are involute teeth, the rigid inner spline20and the flexible outer spline30have the same tooth profile, except the root and crown of the teeth, which increases the contact area when the rigid inner spline20and the flexible outer spline30engage with each other, thus reducing contact pressure of the engaged teeth, meanwhile, preventing the roots of the teeth21,31of the rigid inner spline20and the flexible outer spline30from coming into contact with each other and generating large torque, causing tooth fracture.

It is to be noted that the second and fourth curves52,54of the wave generator50are semi-elliptical and compressed in an equiangular and equal proportion manner, as shown inFIG. 14B, the second and fourth curves52,54are equiangularly and equiproportionally compressed based on the elliptical equation: (X2/R2+Y2/b2=1). Therefore, a coordinate for any arbitrary point on the second and fourth curves52,54is {X(θ), Y(θ)}, and it satisfies the relations: X(θ)=R cos(θ−α)π/(π−2α); Y(θ)=b sin(θ−α)π/(π−2α); wherein α is half of the central angle k of the first or third curve51,53; R is the radius of the major axis of the wave generator50, and R=m (n+1)−t, b is the radius of the minor axis of the wave generator50, and b<R.