A coupling arrangement

Abstract: A mechanism for use in a reduction planetary gearing with an eccentric output gear, comprises a plate (22) with four apertures receiving axially extending projections in diametrically opposed pairs (24a,24b,25a,25b) on the input and output coupling members (21,23). The projections (24a,24b,25a,25b) are radially movable in the apertures which may open into the periphery of the plate.

Description: The present invention relates to a coupling arrangement in an eccentric gear of the type comprising a planetary gearwheel on which a first pair of axially extending and diametrically opposed cams are mounted and which is eccentrically journalled relative to a first shaft; a second disc-like shaft which is journalled on the first shaft and has a second pair of axially extending and diametrically opposed cams; and an intermediate flat plate having four generally equidistant apertures for receiving said first and second pairs of cams, said cams being radially displaceable relative to said apertures, the gear serving to translate low rotational speed of the gearwheel around its centre from a high planetary rotational speed about the first shaft, that has a high planetary rotational speed, to a concentric rotation about the second shaft, and the arrangement being such that all the tangentially acting torque transmission forces will lie in the same axial plane of the plate and thereby avoid distortion of the plate with subsequent lowering of torque stiffness. In its rapid planetary rotation, the eccentric gearwheel rotates relative to an internal stationary gearwheel at a reduced speed about its centre of gravity in an opposite direction of rotation. This reduction in rotational speed is dependent on the difference in teeth number between the teeth of the eccentric gear wheel and those of the stationary gear wheel. See in other respects WO 88/05508. The purpose of the coupling arrangement is to translate this reduced rotational speed about the centre of gravity of the gearwheel that executes the aforesaid rapid planetary movement to solely a slow, centric rotational movement. Reduction gears which achieve large speed reductions are used, for instance, in robot technology. In this respect, high demands are placed on rigidity and on the absence of play. The reduction ratio often lies within the range of 50:1-200:1. The reduction gears will preferably have a small volume and be maintenance free. BACKGROUND ART The output shaft of the previously known so-called Cyclo gear has mounted thereon a plurality of axially directed pins which slide against the surfaces of bores formed in and extending through two or three excentrically journalled gearwheels of the gear. In order for this coupling arrangement to function, it is necessary that at least two gearwheels are phase-shifted through 180° in relation to one another. The drawback with this known coupling arrangement is that the gear components must be manufactured to high degrees of accuracy. The gear also comprises a large number of components. Variants of the so-called Oldham coupling, as disclosed in e.g. US-A-4,429,595 JP-A-01158249, are examples of another coupling arrangement for eccentric gears. All of these types of coupling arrangements are space-consuming and have a large mass, which is liable to cause undesirable vibrations. Another coupling arrangement which functions to translate the aforesaid gearwheel rotary movement to centric rotation is described in WO 88/05509. Another known coupling arrangement includes a circular plate, called a gear plate, which has groups of mutually parallel grooves on each side surface of the plate. The grooves in the groups on one side of the plate extend perpendicularly to the grooves in the groups on the other side of said plate. The gear plate is characterized in that it is highly rigid in relation to its weight. This low weight of the circular plate enables the radially directed, dynamic disturbances that are unavoidable in eccentric gears to be kept at an acceptably low level. The tangentially directed forces of the transmitted torque are transferred from one group of grooves to the group of grooves on the other side of the gear disc. As mentioned, this group of grooves is displaced through 90°. In order to obtain a low surface pressure, and accordingly a long useful life, the forces are distributed over all tooth-surfaces of the groups of grooves. Since the gear plate is relatively thin, the axial distance between the pressure points within respective groove groups will be small, whereby the gear plate will only be arched/distorted from a flat state to a small extent when the output shaft of the gear is subjected to a heavy load. The construction imparts a relatively high rigidity to the gear. This known coupling arrangement has however, certain drawbacks. For instance, when the groove groups are composed of a slide friction element, they will generate a certain power loss. Relatively high demands are placed on manufacturing tolerances, in order to obtain uniform load distribution over all teeth in the groove groups. In order to reduce friction losses, the known arrangement includes balls or rollers disposed in the groups of grooves on both sides of the disc, said balls or rollers being intended to roll in the grooves on the disc and in the grooves on the side of the gearwheel or in the grooves on the axial surface of the output shaft. In order to obtain play-free transmission, however, it is necessary to subject the balls or rollers, to an axially acting spring bias, in order to eliminate play. This requires the provision of a thrust bearing. It is desirable to use rollers when wishing to transmit large torques. For geometrical reasons, the grooves in the disc must then have an angle of 90°, which results in a higher axial force component that must be taken-up by the thrust bearing. Another drawback with this arrangement is that it is difficult to hold the balls or rollers in their correct position in the grooves. SUMMARY OF THE INVENTION The object of the present invention is to provide an improved coupling arrangement, by means of which the aforesaid drawbacks, such as the tendency of the plate to arch at high torque transmissions and thereby decrease the transmission rigidity, the need of thrust bearings, and high tolerance requirements on the components of the coupling arrangement and the friction losses caused thereby, are eliminated. In accordance with the invention this object is achieved by means of the features defined in the characterizing clause of the claim. In order to achieve play-free transmission, the diameters of the rollers may be chosen so as to generate a biassing force. When the gear is subjected to load, the aforedescribed plate will retain its flatness, but will be elastically deformed to a slightly elliptical shape. See Figure 5. When no load acts on the plate, the mutually opposing and mutually parallel surfaces of the plate will separate elastically from one another and also form angles with one another. These deformations are extremely small, in the order of 10 micrometers, while the angular change is in the order of 0.5 arc minutes. However, the deformations are sufficiently large to influence the distribution of load between the loaded rollers and therewith have an effect on the torque transmission capacity. By selecting rollers of mutually different size at each transmission surface when assembling the gear, the plate will obtain a deformation that corresponds, for instance, a gear loading of 80% of the rated torque. When the gear is not subjected to load, the outermost roller, seen radially, will be subjected to the aforesaid biassing force and the rollers that lie inwardly of this outermost roller will be subjected to a low biassing force, or may even present a clearance to the rolling surfaces. When the gear is subjected to a low load, the tangentially acting forces will generally influence these outermost rollers in each plate aperture. As the transmitted torque increases, the plate will spring-out radially and the angle of the rolling surface will change slightly. The load will then be distributed successively more uniformly between the rollers and all of the rollers will be subjected to the same load at the rated torque. The opposing rolling surfaces of the four apertures are ground parallel in pairs. The aforesaid cams have the form of axially extending pins pressed into the gearwheel and in the output shaft, on which bored blocks are mounted with a light press fit. The blocks have two mutually parallel, opposing sides which have been ground to a high degree of accuracy. When the gear is subjected to load and the plate is subsequently deformed, the blocks will adjust so that the load distribution is generally uniform between the loaded rollers. BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described in more detail with reference to the accompanying drawings, in which Figure 1 is a cross-sectional view, partially in side view, of an eccentric gear which includes an inventive coupling arrangement; Figure 2 is a sectional view of the output shaft and its journals, and shows the plate provided with blocks and rollers; Figure 3 is a sectional view of the eccentric gearwheel, provided with pins; Figure 4 is an axial view of Figure 2; Figure 5 illustrates an FEM plate-calculating model; and Figure 6 illustrates the plate in an FEM-calculated deformed state (the deformation being greatly enlarged). Description OF the invention Figure 1 illustrates an eccentric gear which includes an input shaft 11, an internally toothed ring 12, an eccentric gearwheel 13, an output shaft 14 and a plate 15. The eccentric gearwheel 13 is journalled eccentrically on the input shaft 11 and rolls against the internally toothed ring 12. When the difference in teeth number is small, preferably one or two less teeth on the eccentric gearwheel 13, the eccentric gearwheel 13 will rotate slowly in contra-rotation about its centre of gravity. See in other respects WO 88/05508. The eccentric gearwheel 13 thus executes a rapid, planetary movement about the input shaft 11 and rotates slowly about its centre of gravity. This composite movement shall be translated by means of the inventive coupling arrangement to a pure slow rotational movement which is concentrical with the output shaft 14. Figures 2 and 4 illustrate the plate 15, which is provided with apertures or recesses 40. Mounted on the output shaft 14 are two diametrically opposed and axially extending studs 42a, 42b on which blocks 41a, 41b are journalled. Mounted on the gearwheel 13 are two further diametrically opposed and axially extending studs 43a, 43b, of which only stud 43a (can be seen) on which blocks 44a, 44b are journalled. The blocks include holes 45a, 45b into which the studs 43a, 43b are inserted when assembling the gear. Between the parallel surfaces of each block and the parallel surfaces of each associated aperture 40 there are rows of rollers 46. The advantage achieved by the invention is that the arrangement is self-adjusting in a manner to obtain a good distribution of the load between the rollers. Figure 5 illustrates an FEM-calculation model (Finite Element Method). The grid pattern defines the finite elements. The aperture openings are subjected to a uniformly distributed load F. Figure 6 illustrates the deformed plate. It will be seen that the centre hole of the plate is deformed to an elliptical shape and that an angular change has occurred between the initially parallel surfaces. For reasons of a calculation/technical nature, the deformation-figure is not symmetrical, although it will be symmetrical in reality. In order to calculate the outward bulging of the plate, the two diametrically opposed bulges are added together and divided by two. The inventive coupling arrangement provides a highly rigid torque transfer. The plate 15 will execute a linear, reciprocating movement in relation to the output shaft 14. The frequency of this movement is equal to the rotational speed of the input shaft per second, i.e. the movement has a high frequency. The plate mass will thus result in a high-frequency, dynamic radially acting disturbance force which is coupled in the direction of the output shaft 14. The magnitude of this force is a function of plate mass, frequency and amplitude. A characteristic feature of this type of eccentric gear (see WO 88/05508) is that the eccentricity is small, normally in the region of 0.5-1.2 mm (among other things, depending on the transmission ratio). Low level angular vibrations are important with regard to servo-applications. The radially directed vibrations have no great significance, and no appreciable disturbances have been experienced in practice. In order to minimize the mass of the plate 15, the plate is suitably manufactured by precision forging and then subsequently machined, for instance milled, hardened and ground.

Claims: A coupling arrangement in an eccentric gear of the type comprising a planetary gearwheel (13) on which a first pair of axially extending and diametrically opposed cams are mounted and which is eccentrically journalled relative to a first shaft (11); a second disc-like shaft (14) which is journalled on the first shaft and has a second pair of axially extending and diametrically opposed cams; and an intermediate flat plate (15) having four generally equidistant apertures (40) for receiving said first and second pairs of cams, said cams being radially displaceable relative to said apertures (40), the gear serving to translate low rotational speed of the gearwheel around its centre from a high planetary rotational speed about the first shaft, that has a high planetary rotational speed, to a concentric rotation about the second shaft, and the arrangement being such that all the tangentially acting torque transmission forces will lie in the same axial plane of the plate (15) and thereby avoid distortion of the plate with subsequent lowering of torque stiffness, characterized in that said apertures (40) are open not only axially but also radially and that said cams are composed of blocks (41a, 41b; 44a, 44b) which are journalled on axially extending studs (42a, 42b; 43a, 43b) mounted on said gearwheel (13) and said second shaft (14) respectively, and that separate rolling bodies in the form of rollers (46) are placed in rows on each side of the block between outer parallel surfaces of each block and inner parallel surfaces of each associated aperture, said blocks being self-adjusting in order to provide optimal load distribution on said intermediate plate (15).

Inventors: RENNERFELT GUSTAV, RENNERFELT, GUSTAV
Applicants: RENNERFELT GUSTAV, RENNERFELT, GUSTAV
Application year: 1993
Main classification: F16H1
IPC codes: F16H1, F16H3, F16D3