Planetary gear transmission systems or components – Planet periphery surrounds axis of interacting gear
Patent
1989-07-21
1991-07-09
Herrmann, Allan D.
Planetary gear transmission systems or components
Planet periphery surrounds axis of interacting gear
475904, F16H 128
Patent
active
050301845
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to an excentric gear of the kind which allows high reduction, while the volume of the gear is small. A gear of this kind has an excentric spur gear, also known as a satellite wheel or planet wheel, which is freely journalled on an excentric stub shaft rotating together with the input shaft of the gear. The excentric gear wheel is in mesh with a stationary, internally toothed gear wheel and rolls round on the internal surface thereof. The excentric gear wheel has a circumference which is somewhat less than that of the stationary gear wheel, and as it rolls round on the inside of the stationary gear wheel it executes a slow rotation about its journal on the excentric stub shaft simultaneously as its centre of gravity rotates at a great rotational rate about the input shaft.
The excentric wheel has a number of teeth which is less than that of the stationary gear wheel. For one revolution of the input shaft the excentric wheel has rotated in the opposite direction an angle responsive to the relationship between the number of teeth on the respective gear wheel. ##EQU1##
Z.sub.s =number of teeth on stationary gear wheel
Z.sub.r =number of teeth on the excentric gear wheel
It will be understood that to obtain a large gear ratio the difference in the number of teeth should be small. But already at a difference of about 8 teeth there is risk of so-called tooth interference. See FIG. 12.
To obtain a large gear ratio, the number of teeth must be great so that the module (size of the tooth) will not be too small. Such gears are used as reduction gears in large ships' engines. Scaling down these gears to sizes suitable as servo gears is practically impossible since the module would be impossibly small.
Harmonic Drive is a type of gear which has managed to avoid the problem of the teeth interfering with each other while maintaining a high gear ratio. In this gear the means corresponding to the moving gear wheel comprise a special thin wall ball race which is pressed on to an elliptical inner former, this ball race then receiving an externally toothed steel ring. This now oval-shaped ring meshes with an internally toothed rigid steel ring, which is also stationary. Thus, in this known apparatus two diametrically opposed parts of the internally toothed ring are in engagement with the elliptical, elastically shaped means. As a result of the elliptical shape, the unmeshing teeth are kept away from the regions where they would otherwise interfere with the teeth of the stationary ring. For this known structure to function well and have small backlash, it is required that very close tolerances on the component parts are maintained, which results in that this gear becomes expensive to produce. Furthermore, both the moment of inertia and the frictional moment for the input shaft are large, which is a disadvantage, e.g. in connection with servo systems.
Cyclo Drive is another gear structure avoiding the interference problem by not utilising teeth with involute cutting, but a kind of cycloid tooth. The stationary internally toothed ring has this cycloid form, and pins axially fastened to a disc on the output shaft roll off the teeth one after the other. The radial forces in this structure are very high, and very tight tolerances on the components are required to obtain small backlash. The greatest disadvantage with this gear is that a superposed pulsation on the output shaft is obtained, i.e. the gear does not give a true angular transmission. The "Dojan" is a variant of this type of gear.
Conventional spur gear boxes comprise a plurality of reduction stages. The greater the total gear ratio desired, the greater the number of reduction stages. This gear box thus comprises a plurality of reduction stages connected in series. The total gear ratio is the product of the gear ratios in the participating reduction stages. In order that the gear box is not given a too large radial extension, gear ratios of more than 6:1 are seldom used in each stage. In practice, about four reduction stages are required to o
REFERENCES:
patent: 1770035 (1930-07-01), Heap et al.
patent: 3019705 (1962-02-01), Wilkinson
patent: 3037400 (1962-06-01), Sundt
patent: 3424036 (1969-01-01), Colgan
patent: 3546972 (1970-12-01), Morozumi
patent: 4402544 (1983-10-01), Bahnng
patent: 4512213 (1985-04-01), Newton
Herrmann Allan D.
Ta Khoi O.
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