A GEAR ASSEMBLY

Abstract: This invention relates to a gear assembly comprising two parallel dual harmonic gears. The moment transmission ability of the gear assembly is relatively high, if the gears are interconnected and they have a common resilient rim (2) both ends of which are provided with a dual harmonic gear in such a manner that teeth (3a, 3b) on the outer surface of the rim are in meshing engagement with peripheral teeth (4a, 4b) and teeth (5a, 5b) on the inner surface in meshing engagement with shaft teeth (6a, 6b). A ring (7) positioned between the teeth serves as a primary ring from which the torsional moment is transmitted into the resilient rim through a bearing (8) and either one of the interconnected teeth in meshing engagement with the rim teeth may serve as a secondary shaft while the other remains receiving the difference between the secondary and the primary moment.

Description: A gear assembly The invention relates to a gear assembly com¬ prising two double harmonic gears. The double harmonic gear will herein below be referred to as the dual har¬ monic gear. Furthermore, it is possible to connect a motor to the gear assembly according to the invention in a way described below, whereby a compact gear motor is obtained. One advantageous application of this motor is in mechanisms in which a high torsional mo¬ ment, a low speed of rotation, a small need of space and a low weight and possibly also a good parking ac¬ curacy are required from the power device; for example, in the driving means of movement joints in industrial robots. In several motor types, the greatest power is obtained from the motor at a speed of rotation higher than what is required for moving the load. Therefore, a reduction gear is provided between the motor and the load so that the greater power obtained from the motor can be utilized as a high torsional moment at a low speed of rotation. As is known, a harmonic gear pro¬ vides a high reduction ratio while the gear takes up little space and its performance is good. The principle of a harmonic gear is disclosed in U.S. Patent 2,906,143, September 29, 1959, Musser, and that of a dual harmonic gear in U.S. Patent 2,943,513, June 5, I960, Musser. According to the invention two dual harmonic gears can be connected in parallel in such a way that the primary moment can be supplied to the gear assem¬ bly e.g. via an external annular primary shaft po¬ sitioned between the gears, whereby the secondary shaft of the gear assembly may be sleevelike. The rotor in a motor which can be made annular can be di- rectly used as a primary shaft. Such motors include various types of electric motors such as brushless di¬ rect-current motors and stepper motors or motors of some other type, such as pneumatic motors. The moment transmission ability of the gear assembly is very high. As to its need of space and its range of trans¬ mission ratio (about 25 to 100:1), it is well suited for use as a reduction gear in an annular motor with¬ out considerably increasing the need of space thereof. The gear assembly can also be constructed substantial¬ ly without clearances, and its diameter and, as a con¬ sequence, the moment transmission ability can be varied within wide limits while retaining the compact structure. If desired, the primary moment can be supplied to the gear assembly in other ways, too; for instance, via a separate drive shaft to an external ring or via a bearing positioned internally in the rim. For obtaining these effects the invention is mainly characterized by the matters disclosed in claim 1. In the following the invention and the pre¬ ferred embodiments thereof will be described in more detail with reference to the examples of the attached drawings, wherein Figure 1 illustrates the overall structure of a gear assembly according to the invention in a partial longitudinal cross-sectional view; Figure 2 illustrates an end projection of the previous figure; Figure 3 is a partial longitudinal cross- sectional view of a gear assembly whereto a brushless direct-current motor is attached as a motor; Figure 4 is a cross-sectional view of the motor of Figure 3 in the direction of the line A-A; Figure 5 illustrates the magnetic circuits for making the rim elliptical in the gear assembly of Fig¬ ure 3; Figure 6 is a reduced view of the operation of the gear assembly according to the invention as an end projection when the assembly is in motion; Figure 7 illustrates the distribution of tangential stresses in the rim of the gear assembly within the dual harmonic gears as an end projection when the gear assembly is under load. Figures 1 and 2 illustrate the overall struc¬ ture of a gear assembly. The gear assembly can be assumed to comprise dual harmonic gears la and lb having a common resilient rim 2. The rim is provided with external teeth 3a belonging to the gear la and interengageable with peripheral teeth 4a, and internal teeth 5a and shaft teeth 6a interengageable therewith. Correspondingly, the gear lb comprises teeth 3b formed externally on the rim and peripheral teeth 4b interengageable therewith, and internal teeth 5b and shaft teeth 6b interengageable therewith. The rim 2 is made elliptical by means of a primary ring 7 positioned between the gears la and lb, from which ring the primary moment is transmitted to the rim via a bearing 8. The rim can be made elliptical by means of the bearing 8 or magnets disposed in the primary ring 7. -The peripheral teeth 4a and 4b of the gears la and lb on one hand and the shaft teeth 6a and 6b on the other hand are both separately connected together. Either one of these pairs may serve as a secondary shaft while the other remains . receiving the difference between the secondary and primary moment. However, in this connection the shaft connecting the internal teeth 6a and 6b will be consistently referred to as "the secondary shaft". In principle, the moment transmission ability of the gear assembly is doubled as a result of the interconnection of the teeth. Figures 3 and 4 show a gear assembly to which a brushless direct-current motor is attached as a motor. The shaft teeth (6a and 6b) are thereby integral with a common nonferromagnetic secondary shaft 9 and the peripheral teeth 4a and 4b are integral with station¬ ary end flanges 10a and 10b. In addition to the basic parts described above, the structure comprises a body 11 of the motor stator, a stator 12, a stator winding 13 and a gear assembly primary ring 7 serving as the rotor of the motor. Permanent magnets 14 for the motor, permanent magnets 15 for making the rim ellip¬ tical, and support rings 16 thereof are positioned in the primary ring. The nonferromagnetic bearing 8 and air gaps 17 enable the movement of the rotor in relation to the rim. The rotor is mounted to the end flanges by means of bearings 18a and 18b. Figure 5 shows magnetic circuits in the rotor 7 and the rim 2 for giving the rim an elliptical shape in the gear assembly of Figure 3. The direction of the poles of the permanent magnets 15 fastened to the rotor 7 is chosen so that the magnetic flux revolves axially and radially via the rim, the permanent mag¬ nets and the rotor. In this way a strong magnetic at- traction is obtained against the bearing 8 over a de¬ sired length of the periphery of the bearing. By means of the support rings 16 it can be ensured that the magnets 15 remain fast and that the air gap 17 is maintained irrespective of the magnetic attraction. Since the magnetic flux does not penetrate the rim, no major eddy currents are induced in the rim in spite of the rotatory motion between the rotor and the rim. Eddy currents can be decreased by making the rim in the area of the magnetic circuit of a material having high permeability, a good saturation value, and a high specific resistance. One way of further decreasing in¬ duction currents is to manufacture the rim in the area of the magnetic circuit by winding it of a thin band in such a way that a particularly thin insulating ma- terial-layer is provided between the band surfaces for preventing the passage of radial induction currents. The hysteresis loss can be substantially reduced when the directions of motion of the magnetic fluxes in the rim are the same. In Figure 5, the top and bottom fluxes thereby have opposite directions of rotation. The elliptic shape can also be obtained by adjusting the direction of the flux in the rim so that it is parallel with the periphery of the rim. Drawbacks caused thereby include the greater hysteresis losses . Figure 6 is a reduced view of a situation when the gear assembly is in motion. When the rotor 7 ro¬ tates over an angle os, the rim 2 rolls along the out¬ er periphery having the radius R a distance s-^ = o R. The same distance Oό R of the rim thereby rolls over the curved portion s. A shorter length S2 = Otr thereby rolls away from the curved portion s. Since the length of the curved portion s remains the same, the second¬ ary shaft having the radius r also has to turn a distance S3 = fb r in a direction opposite to the di- rection of rotation of the rotor. The incident is il¬ lustrated by the equation: s + Si - s 2 - S3 = s which can be simplified and solved as follows: s-f. - s 2 - s 3 = 0 OCR - cxr - (-/i)r = 0 6 06(R-r) = - fb r (positive clockwise direc¬ tion of rotation) Oύ r i - -r— = - - = reduction ratio 3 R-r The negative sign in the formula of the transmission ratio indicates that the secondary shaft rotates in a direction opposite to the direction of rotation of the primary shaft. In practice, the rim has a certain thickness and the inner and outer surface thereof have different numbers of teeth while the tooth size is the same. The transmission ratio can thereby be calcu- lated by means of the formulae disclosed in the patent relating to the dual harmonic gear. Figure 7 shows the tangential tensile and com¬ pression stresses created in the rim 2 due to the load moment acting on the secondary shaft. When the second- ary shaft rotates in a direction opposite to the di¬ rection of rotation of the primary shaft, a com¬ pression stress prevails within the curve length pi and 2 and a tensile stress within the curve length v and V2- Since the teeth in a harmonic gear are not in contact at the apex points of the rim (even though they are interlocked) , but beginning from a determined point after the tangential point, the zero stress points are positioned at a distance of determined angles v* " - * _ and v*"^ rom the tangential points between the rim and the circle circumferences (apex points). Tangential tensile and compression stress states created in the rim due to the load are such that they tend to make the rim even more elliptical in the area of the dual harmonic gears la and lb and to rotate the rim ellipse against the torsional moment of the primary shaft (of the rotor) . If the rim ellipse of one side were twisted over a determined angle for this reason, this would cause a movement between the outer and inner peripheries within this area due to the gear mechanism. Since the outer and inner peripheries are interconnected on both sides of the gear assembly, the corresponding movement between the outer and inner periphery would also take place on the other side of the gear. This would further cause the rim ellipse to be twisted in a corresponding way also on this side of the gear assembly. Due to the gear mechanism, the twisting of the gear assembly is thus symmetrical on both sides of the gear assembly. Such a deformation of the rim ellipse in relation to the elliptical shape within the area of the primary ring requires shearing of the material, for instance, and not only bending of the rim plate, if no buckling can take place. The twisting requires torsional stresses in the rim. How¬ ever, these can be caused by the primary moment only, not by the secondary moment, wherefore the twistings remain small. The rim is not detached from the ex¬ ternal elliptical shape of the primary ring, which is due to the fact that because it cannot be pressed out of shape within the compression stress areas ]_ and p when the rim rests tightly against the supporting bearing by virtue of the attractive force of the per¬ manent magnets, it cannot, either, be detached out of its shape inwards within the tensile stress areas v* j _ and V2- Further, the rim is not disengaged from the teeth of the secondary shaft, because the rim is pressed against the secondary shaft due to the tensile stresses acting within the areas v * j_ and V2. To sum up the above, it can be mentioned that the gear assembly is able to operate stably with respect to loads and rigidly with respect to resiliences. For obtaining an optimal moment transmission ability, the rigidity of the rim should be dimensioned taking into considera- tion all the stress components exerted thereon. This 8 is concerned with optimizing by known methods, which will not be dealt with in more detail in this connec¬ tion. The rim of the gear assembly may also be given an elliptical shape by means of an external support bearing, whereby no separate pulling magnets are need¬ ed. While the support bearing presses the rim against the teeth of the secondary shaft, the rim is simulta¬ neously pressed against external peripheral teeth in the perpendicular plane. Due to necessary bearing clearances, this kind of gear assembly is more liable to torsional clearances than when the elliptical shape is obtained by magnetism. An advantage is that no eddy currents are therefore induced in the rim and the fer- romagnetic properties need not be taken into account in the choice of material of the parts. The rim can be made elliptical and the supply of primary moment in the rim can also be effected in¬ ternally from the center of the rim in a corresponding way. If desired, the torsional clearances of the gear assembly can be reduced by dividing the teeth of the outer periphery and the secondary shaft in two parts in -the longitudinal direction. By turning the counterteeth portions of each toothing in the rim with respect to each other and by locking the portions to¬ gether, a- contact without any clearance is obtained between the toothings. The gear assembly has been described above when the rim of dual harmonic gears is formed by two lobes. The gear assembly is also operative even though the number of the lobes would differ from ' the above, like dual harmonic gears generally do. The gear assembly can be easily sealed so as to be lubricant-proof and the entrance of external im- purities into the teeth can be prevented, even though this has not been illustrated in the example figures for the sake of the clarity thereof. The secondary shaft of the gear assembly gets a certain radial support through the rim. The radial and axial load capacity of the secondary shaft can further be improved by external bearings; further, the rim can be supported axially. This is not, either, shown in the example figures for the sake of the clarity there- of. The gear assembly can also serve as a promotion gear, whereby the secondary shaft acts as a driving shaft. If desired, a ball nut and a ball screw or the like axially movable element can be positioned in con¬ nection with the secondary shaft, whereby a strong, accurate and compact axial power device is obtained. Gear assemblies can be connected in parallel e.g. on a driven shaft, whereby their moment transmis- sion ability with respect to the shaft is the sum of the moment transmission abilities of individual gears . Gear assemblies can be connected in sequence by connecting them with each other so that the secondary shaft of a preceding gear is connected to the primary rim of the former one. The transmission ratio of the combination is thereby the product of individual transmission ratios. Even though the invention and the applications thereof have been described above with reference to the examples of the attached drawings, the invention is not restricted thereto, but one skilled in the art can, of course, modify it within the inventive idea disclosed in the attached claims as well as within his own knowledge.

Claims: 10 Claims : 1. A gear assembly comprising two parallel dual harmonic gears, c h a r a c t e r i z e d in that the gears are interconnected and that they comprise a com¬ mon resilient rim (2) both ends of which are provided with a dual harmonic gear in such a way that teeth (3a, 3b) provided on the outer surface of the rim are in meshing engagement with peripheral teeth (4a, 4b) and teeth (5a, 5b) on the inner surface are in meshing engagement with shaft teeth (6a, 6b) and that a ring (7) provided between the teeth serves as a primary ring from which the torsional moment is transmitted to the resilient rim through a bearing (8) and that either one of the interconnected teeth in meshing en¬ gagement with the rim teeth may serve as a secondary shaft while the other remain receiving the difference between the secondary and the primary moment. 2. A gear assembly according to claim 1, c h a r a c t e r i z e d in that the rim (2) is made elliptical by means of magnets (15) positioned in the primary ring (7) in such a way that the magnet flux revolves axially and radially via the rim (2), the magnets (15) and the primary ring (7) over a desired length of the primary ring and that the support bear¬ ing (8) positioned in the primary ring retains an air gap between the primary ring and the rim, which air gap enables a relative movement therebetween. 3. A gear assembly according to claim 1, c h a r a c t e r i z e d in that the rim (2) is made elliptical by means of the bearing (8) . 4. A gear assembly according to claim 1, 2 or 3, c h a r a c t e r i z e d in that the primary ring (7) is integral with the rotor of a motor attached to the gear assembly.

Inventors: PORO ANTTI MIKAEL, PORO, ANTTI MIKAEL
Applicants: PORO ANTTI MIKAEL, PORO, ANTTI MIKAEL
Application year: 1988
Main classification: F16H1
IPC codes: F16H1, F16H49, H02K7