Machine element or mechanism – Gearing
Reexamination Certificate
2002-10-21
2004-05-18
Wright, Dirk (Department: 3681)
Machine element or mechanism
Gearing
Reexamination Certificate
active
06736028
ABSTRACT:
The present invention relates to a harmonic drive and an internal geared wheel for such a drive according to the respective preambles of the main claims.
BACKGROUND OF THE INVENTION
The functionality of such a harmonic drive—also known as a wave drive or ring band drive—as a very strongly reducing, self-limiting system having a driven shaft coaxial to the drive shaft, is based on a rotating wave generator revolvingly radially deforming a hoop, also referred to as a flexible band, and therefore the external peripheral surface of this wheel rim revolvingly pressing locally outward against the hollow cylindrical internal peripheral surface of slightly greater circumference of a stationary, dimensionally stable bearing ring. As a consequence, the internal geared wheel itself or its wheel hoop, mounted thereon as a rotatable wheel rim, rolls in the bearing ring frictionally via friction surfaces or positively via teeth, the wheel and/or its hoop rotating slower than the drive core of the wave generator, which is driven by a motor, according to the difference in circumferences. This rotational movement, which is greatly slowed in relation to the actuation, is preferably transmitted via the external teeth of the wheel rim to the internal teeth of a further external ring, the driven ring, which is concentric to the bearing ring but not stationary, a further reduction of the rotational velocity able to occur due to differing circumferences (number of teeth). The wave generator is typically driven via a low voltage DC motor, coaxially flanged on, which is high-speed and therefore available very cheaply, whose rotation is thus reduced into a much greater torque corresponding to a much slower rotational movement.
In the embodiments of such a harmonic drive known from German Utility Model 2 96 14 738 and described in more detail in the article “Genial einfach” by H. Hirn (KEM Antriebstechnik issue November 1996), a non-round (for example triangular or preferably oval in axial cross-section) drive core is rotated concentrically in the hub of a radially deformable internal geared wheel as a wave generator. Dimensionally stable spokes between the hub, which is radially deformed by the drive core, and the hoop wheel rim of this internal geared wheel, which is also radially deformable and is externally toothed, have the effect that the external teeth of the hoop, corresponding to its revolving radial deformation, only engage over a limited curved segment, which rolls therein, with the inner teeth of the bearing ring. Instead of the internal geared wheel, made of hub, spokes, and externally toothed wheel rim, which rotates in relation to the stationary external bearing ring and in the opposite direction relative to the drive core, according to the utility model cited, individual mushroom-shaped or T-shaped tappets may be guided using their shafts in the channels of a stationary cage so they are radially displaceable in relation to the axis of the shaft generator. The revolving radial displacement of the rolling procedure in the bearing ring then occurs in that the feet of the tappets rest under elastic tension on the non-round external lateral surface of the drive core, which rotates in a coaxial central bore of the cage, driven by a motor. The flexible band, whose external teeth are again locally deformed outward revolvingly by the sequence of the tappets radially displaced outward, in order to thus roll in the bearing ring, runs as a wheel rim over the heads of the tappets, which are expanded in the peripheral or circumferential direction, i.e., tangentially.
In this way, in such a harmonic drive, the very high speed of an input shaft is converted into the, in contrast, very low speed of an output shaft, oriented coaxially to the input shaft, having correspondingly higher torque. To reduce the heat-generating friction between the pairing of the plastic lateral surfaces, of the drive core on the one hand and of the internal geared wheel hub or the sequence of tappet feet on the other hand, rotating relative to one another, in this prior publication, a friction bearing in the form of a metal collar is introduced, which also eccentrically deforms revolvingly under the influence of the eccentric drive core rotated therein. This interposed friction bearing distributes the relative rotation between the drive core and the internal geared wheel hub onto two stages and, merely due to the frictional behavior of plastic, which is generally more favorable than that of metal, also leads to reduction of the friction losses, i.e., the waste heat within this harmonic drive, which is compactly produced almost completely from plastic injection molded parts and is hermetically encapsulated. However, high demands are placed on the alternating stressability of the collar, which continuously deforms under load, and its installation in such a harmonic drive of small dimensions leads to a detectable elevation of the manufacturing costs, because the non-round drive core must be inserted into a metal collar, which is round per se, without play. Probably due to this significant disadvantage, in the refinement of the harmonic drive according to German Patent Application 1 97 33 497 A1—having a continuous sequence of circular ring sectors, which may be displaced radially relative to one another, instead of the tappets distanced from one another around the circumference—such a metal friction collar was again dispensed with and instead (column 2, line 11) importance is placed on the high elasticity of the internal geared wheel hub, in order to be capable of inserting the non-round drive core here as easily as possible during assembly. This is even more significant if (as shown in FIG. 6 of this prior publication), two convex surfaces having parallel axes, namely the external lateral surface of the drive core and the foot end of the respective internal geared wheel sector, are supported on one another, i.e., a load which is now almost only linear occurs, having correspondingly steeply increasing surface pressure. However, the additional heat to be expected due to the continuous frictional movement of the neighboring sectors against one another is also disadvantageous. The refinement then produced according to European Patent Application 0 974 773 A2 also gave cause to fear a significantly worsened thermal fatigue strength in this respect in the support of the internal geared wheel tappets, now again in the form of spokes, on the drive core of the wave generator, because the feet of the tappets, again distanced from one another but now connected to one another by a flexible ring band, are no longer solid, but split in order to produce a radial spring effect through elastic bending. It may therefore be seen that those skilled in the art no longer provide proper consideration to the thermal strain of the internal geared wheel by the wave generator.
The present invention is therefore based on the technical problem of being able to design the very strongly reducing plastic drive, which has proven itself per se, so it is even more functionally reliable and compact, through further reduction of the number of individual parts to be assembled, which are subject to wear, and nonetheless, precisely in consideration of the restricted volume, to further reduce the frictional development of waste heat, since it may be dissipated via the plastic parts only poorly.
The achievement of this object is based on the idea that in the drives in consideration here, compactly produced in plastic injection molding, the functionality of the radially deformable internal geared wheel—whether it is produced integrally or from individual parts—requires greatly differing, even contradictory material properties in radial section. This is because the rolling procedure on the drive core must, in spite of frictional heat, be dimensionally stable and have high fatigue strength; wheel spokes or tappet shafts which are elastically coupled but stiff per se are to be sought for the revolving radial power transmission, in order to be able to avoid the assembly cost
Ruttor Martin
Wolf Wilhelm
Kasper Horst M.
Oechsler Aktiengesellschaft
Wright Dirk
LandOfFree
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