Wave generator, wave gear device, and method of preventing reduction in efficiency of wave gear device

A wave generator (4) of a wave gear device (1) is formed from a material in which only the minor axis-side portions (51, 52), which include the two end portions (51a, 52a) of an elliptical minor axis Lmin, has a larger coefficient of linear expansion than the other portions. When the temperature of a rigid cam plate (5) increases during high-speed rotational input, the minor axis-side portions (51, 52) thermally expand considerably more than the other portions do, and balls (10) other than several balls (10b) on a minor axis Lmin of a wave bearing (7) enter a tight state and form a locked state in which the wave bearing (7) rotates integrally with the rigid cam plate (5). Since the generation of agitation resistance of the lubricating agent due to the high-speed rolling movement of the balls (10) during high-speed rotational input can be avoided or reduced, a reduction of the torque transmission efficiency caused by the agitation resistance can be prevented or inhibited.

TECHNICAL FIELD

The present invention relates to a wave generator for a wave gear device, and particularly relates to a technique for preventing a reduction in efficiency caused by lubricating agent agitation resistance produced by the wave generator during high-speed rotation.

BACKGROUND ART

A wave gear device is provided with a rigid internally toothed gear, a flexible externally toothed gear, and a wave generator. The flexible externally toothed gear is flexed in the radial direction by the wave generator and made to partially mesh with the rigid internally toothed gear, and the wave generator is rotated to thereby cause the meshing position of the two gears to move in the circumferential direction and generate relative rotation between the two gears due to difference in the number of teeth of the two gears.

A typical wave generator is provided with an elliptically contoured rigid cam plate and a wave bearing mounted on the external peripheral surface of the rigid cam plate. The wave bearing is provided with a radially flexible outer ring and inner ring, and a plurality of balls or other rolling elements that are rollably mounted between the rings, and is elliptically flexed by the rigid cam plate. The wave bearing is fitted inside the flexible externally toothed gear, and the flexible externally toothed gear and the rigid cam plate are held in a rotatable state relative to each other.

The wave gear device uses the rigid cam plate of the wave generator as a rotation input component and uses the rigid internally toothed gear or the flexible externally toothed gear as a reduced-speed rotation output component. The rigid cam plate rotates while repeatedly displacing all parts of the wave bearing and the flexible externally toothed gear in the radial direction. The inner ring of the wave bearing rotates at high speed together with the rigid cam plate, and the outer ring rotates integrally with the flexible externally toothed gear, and the balls inserted between the inner and outer rings roll along a raceway of the outer and inner rings, whereby the rigid cam plate and the flexible externally toothed gear can smoothly rotate relative to each other with little torque.

SUMMARY OF INVENTION

Here, when a plurality of balls roll at high speed, considerable agitation resistance acts on each ball due to the lubricating agent. For this reason, there is a problem of a reduction in the torque transmission efficiency of a wave gear device in an operating state of high-speed rotational input.

An object of the present invention is to provide a wave generator for a wave gear device that can prevent or suppress this kind of reduction in efficiency.

Another object of the present invention is to provide a wave gear device provided with such a novel wave generator.

A further object of the present invention is to provide a method for preventing a reduction in efficiency of the wave gear device provided with such a novel wave generator.

In order to solve the above-stated problems, a wave generator for a wave gear device of the present invention comprising:

a rigid cam plate having an elliptical profile; and

a wave bearing mounted on an external peripheral surface of the rigid cam plate, wherein

the wave bearing is provided with an inner ring and an outer ring capable of flexing in a radial direction, and a plurality of rolling elements rollably inserted therebetween, and the outer ring and the inner ring are elliptically flexed by the rigid cam plate,

the rigid cam plate has a minor axis-side portion, which includes a minor axis-side external peripheral end portion positioned at two ends of a minor axis of the elliptical profile of the rigid cam plate, and

the minor axis-side portion is formed from a material having a large coefficient of linear expansion in comparison with a remaining portion of the rigid cam plate.

Here, the minor axis-side portion can be set to be an area having a crescent shape that extends along the external peripheral surface of the rigid cam plate with the minor axis at the center.

The minor axis-side portion can be formed in an area having an angle range of 55 degrees to the left and right along the circumferential direction with the minor axis at the center.

The material for the minor axis-side portion can be aluminum alloy, 60/40, brass, beryllium copper, or SUS 305. In this case, a material for the remaining portion of the rigid cam plate can be steel material.

Next, the wave gear device of the present invention is characterized in being provided with the wave generator of the above-stated configuration.

A method of preventing a reduction in efficiency of the wave gear device of the present invention is characterized in adopting the above-stated configuration as the wave generator; locking the rolling movement of the rolling elements of the wave bearing with the aid of thermal expansion of the minor axis-side portion when the temperature of the rigid cam plate increases together with an increase in rotation speed of the rigid cam plate, and setting the wave bearing in a locked state of integral rotation with the rigid cam plate; and reducing an agitation resistance of a lubricating agent produced by the rolling movement of the rolling elements, and inhibiting a reduction of the torque transmission efficiency that is caused by the agitation resistance.

Here, one parameter selected from the shape of the minor axis-side portion, the shape formation area, and the coefficient of linear expansion of the material that forms the minor axis-side portion is modified, whereby the operating condition can be modified at the time the wave bearing switches to a locked state.

Examples of the operating conditions include the rotational speed of the rigid cam plate and the value of the temperature increase of the rigid cam plate.

[Advantageous Effect of Invention]

In the wave generator of the present invention, a wave bearing is elliptically flexed with the aid of a rigid cam plate, and the rolling elements positioned at the two ends of an elliptical major axis are tightly sandwiched between an inner and outer rings, and are in a state of rolling point contact with the raceway of the inner and outer rings. The remaining rolling elements positioned in a portion other than the two ends of the major axis have a gap between the internal and external rings and are held in a rollable state.

When the wave generator rotates at high speed and becomes heated, each of the parts of the wave generator undergoes thermal expansion. Since a minor axis-side portion of the rigid cam plate is formed from a first material having a large coefficient of linear expansion, the expansion distance of this portion is larger than the portion of the major axis side.

For this reason, due to the thermal expansion of the minor axis-side portion of the rigid cam plate, the gap between the inner ring raceway part, the rolling elements, and the outer ring raceway part of the wave bearing in the minor axis-side portion becomes narrow, and the number of rolling elements tightly sandwiched between the inner and outer rings increase in accompaniment with the temperature increase. When the number of rolling elements held in a rollable state becomes a prescribed number or less, the inner and outer rings of the wave bearing enter a locked state in which relative rotation is impossible.

The wave bearing integrally rotates with the rigid cam plate when the wave bearing is in a locked state. The wave bearing can smoothly rotate in an integral fashion with the rigid cam plate because sufficient lubricant film is formed between the flexible externally teethed gear and the external ring of the wave bearing during high-speed rotation.

Consequently, in the wave generator of the present invention, since the rolling elements can be locked to be kept from rotating at high speed during high-speed rotation, the agitation resistance of the lubricant generated due to the high-speed rolling movement of the rolling elements or the agitation resistance is reduced. Therefore, in accordance with the present invention, the reduction of the transmission torque efficiency during high-speed rotation can be prevented or suppressed.

Here, in order to lock the wave bearing during high-speed rotation, it is possible to consider forming the entire rigid cam plate from a first material having a high coefficient of linear expansion in comparison with the inner and outer rings of the wave bearing. However, when the entire rigid cam plate is formed from a material having a high coefficient of linear expansion, the elliptical major axis dimension of the rigid cam plate changes considerably due to thermal expansion brought about by heating during high-speed rotation. As a result, there is a possibility that the flexible externally toothed gear will not be flexed to a proper elliptical shape and become unable to adequately mesh with the rigid internally toothed gear.

In the wave generator of the present invention, the elongation due to the thermal expansion of the major axis direction of the rigid cam plate is small, and an adequate meshing state of the flexible externally toothed gear and the rigid internally toothed gear at the two end portions of the major axis is maintained even in a heated state. Therefore, the wave bearing can be kept in a locked state while an adequate meshing state of the two gears is maintained, and a reduction of the torque transmission efficiency caused by the agitation resistance of the lubricant can be prevented or suppressed.

DESCRIPTION OF EMBODIMENTS

Embodiments of a wave gear device to which the present invention has been applied are described below with reference to the drawings.

FIG. 1is a longitudinal sectional view of a wave gear device in accordance with the present embodiments, andFIG. 2is a schematic diagram that indicates the meshing state of the wave gear device. A wave gear device1has a rigid internally toothed gear2, a cup-type flexible externally toothed gear3disposed inside the rigid internally toothed gear, and a wave generator4having an elliptical profile fitted inside the flexible externally toothed gear. A portion in which an external tooth3a, in the circular flexible externally toothed gear3is formed, is elliptically flexed by the wave generator4. The two end portions in the elliptical major axis Lmax direction in the external tooth3a, mesh with an internal tooth2aof the circular rigid internally toothed gear2.

A motor shaft or another high speed rotating input shaft is connected to the wave generator4. When the wave generator4rotates, the meshing location of the two gears2,3moves circumferentially, and relative rotation is generated between the two gears2,3due to the difference in the number of teeth of the two gears. For example, the rigid internally toothed gear2is secured to prevent rotation, the flexible externally toothed gear3is connected to a load member, and reduced-speed rotation is taken from the flexible externally toothed gear3and transmitted to the load member.

FIG. 3is a front view that shows the wave generator4. When described with reference toFIGS. 1 and 3, the wave generator4is provided with an elliptically profiled rigid cam plate5and a wave bearing7mounted on an external peripheral surface6of the rigid cam plate. The wave bearing7is provided with a circular inner ring8and an outer ring9that are flexible in the radial direction, and a plurality of balls10mounted in a rollable state therebetween.

The wave bearing7is fitted inside the flexible externally toothed gear3in an elliptically flexed state with the aid of the rigid cam plate5, and the flexible externally toothed gear3and the rigid cam plate5connected to the high-speed rotating input shaft are held in a rotatable state relative to each other. That is, the balls10inserted between the elliptically flexed inner ring8and outer ring9rollably move along the raceway surfaces of the inner and outer rings8,9, whereby the rigid cam plate5and the flexible externally toothed gear3can smoothly rotate relative to each other with a small amount of torque.

When described in detail, the wave bearing7is elliptically flexed by the rigid cam plate5, and one or a plurality of balls10positioned at the two ends of the elliptical major axis Lmax are tightly sandwiched between the inner and outer rings8,9, and are in a state of rolling point contact with the raceway portion of the inner and outer rings. The remaining balls10positioned in a portion other than the two ends of the major axis Lmax have a gap between the inner and outer rings8,9, and are held in a rollable state. Therefore, the balls10inserted between the inner and outer rings8,9roll along the raceway surfaces of the inner and outer rings, and the rigid cam plate5and the flexible externally toothed gear3can smoothly rotate relative to each other with a small amount of torque.

Here, the rigid cam plate5is formed from a material having substantially the same coefficient of linear expansion as the inner and outer rings8,9with the exception of minor axis-side portions51,52that include the two ends51a,52a, of a minor axis Lmin. The minor axis-side portions51,52are formed from a material having a coefficient of linear expansion greater than the rigid cam plate. The minor axis-side portions51,52are shown by the diagonal line inFIG. 3.

The minor axis-side portions51,52of the present example have crescent shapes that subtend an angle of 55 degrees to the left and right along an external peripheral surface6centered on the two ends51a,52a, of the minor axis Lmin, as seen from the direction of the center axis line (device axis line)1a, of the rigid cam plate5. In other words, an area surrounded by the circular arcs6a,6b, which have an angle range of 110 degrees in the two end portions of the minor axis that define the elliptical profile of the rigid cam plate5, and by the circular arcs61,62, which have less curvature than the circular arcs6a,6b, is defined as the minor axis-side portions51,52.

Steel is a generally used material for the rigid cam plate5. It is possible to use aluminum alloy, 60/40 brass, beryllium copper, or SUS 305 as the material for the minor axis-side portions51,52. The linear coefficients of expansion of the materials are shown below.

The internal temperature of the wave gear device1gradually increases with increasing input rotation speed of the wave generator4of the wave gear device1having this configuration. The graph inFIG. 4shows the relationship between the increase value of the internal temperature and the input rotation speed of three different types of wave gear devices. As shown in the graph, the increase of input rotation speed and the increase value of the internal temperature have a substantially inverse relationship.

When the internal temperature increases, the minor axis-side portions51,52, which include the two ends51a,52a, of the minor axis Lmin of the rigid cam plate5, thermally expand to a greater extent than the other portions in accompaniment with the increase in temperature. The gap between the balls10and the inner and outer rings8,9in the wave bearing7is gradually reduced due to the thermal expansion of the rigid cam plate5that accompanies the temperature increase. As a result, not only the balls10a, positioned at the two ends of the major axis Lmax of the wave bearing7, but also the balls adjacent to the balls10a, become tightly sandwiched between the inner and outer rings8,9. As the temperature increases, the number of balls10in a tight state gradually increases. When the balls10, excluding several balls10b, positioned at the two ends of the minor axis Lmin, are in a tight state, the wave bearing7enters a locked state, becomes integrated with the rigid cam plate5, and begins to rotate.

The graph ofFIG. 5shows the relationship between the rollable balls (loose balls) and the residual gap between the balls10and the inner and outer rings8,9in the wave gear device1of the present example. At room temperature (20° C.), there are approximately 14 loose balls and the wave bearing7rotates smoothly. In contrast, when the residual gap is reduced (the temperature increases) and the number of loose balls is reduced to about nine, it becomes difficult for the wave bearing7to rotate. When the number of loose balls is reduced to about five and the residual gap is reduced, the wave bearing7does not rotate and enters a locked state.

Here, the wave bearing7can be set in a locked state at a stage in which the internal temperature has increased by a prescribed amount by adjusting the shape formation area of the minor axis-side portions51,52and the value of the linear coefficient of the portions. For example, the coefficient of linear expansion may be selected and the formation area and shape of the minor axis-side portions51,52may be set so that the wave bearing7can be set in a locked state when the temperature increase during high-speed rotation reaches approximately 35° C. or higher.