Gear apparatus

Details Gear apparatus Gear apparatus

2001-09-18

2003-10-14

Marmor, Charles A. (Department: 3681)

Planetary gear transmission systems or components

Planetary gearing or element

Coaxial teeth around planet pinion engage axially spaced...

C475S341000, C475S342000, C475S904000

Reexamination Certificate

active

06632154

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a gear train, and relates particularly to the configuration of a gear train having a planetary gear train mechanism suitable for configuring a small speed reducer.
PRIOR ART
Speed reducers using a planetary gear train mechanism are widely used in the drive mechanism of various types of mechanical devices because they are generally compact and achieve a high gear reduction ratio. An example of this type of speed reducing apparatus is the compact speed reducer disclosed in Japanese Patent Laid-Open Publication (kokai) H2-31047. This compact speed reducer has a sun gear, a planetary gear meshing with the sun gear, a fixed internal tooth gear meshing with a first gear part of the planetary gear, and a movable internal tooth gear coaxial to the fixed internal tooth gear and meshing with a second gear part of the planetary gear. The first gear part meshing with the fixed internal tooth gear and the second gear part meshing with the movable gear part are disposed coaxially and mutually adjacent in the axial direction, and are described in kokai H2-31047 as having the same number of teeth. The first gear part and second gear part of the planetary gear, however, generally have different numbers of teeth.
The gear reduction ratio can be greatly increased in a gear train having this type of planetary gear train mechanism without incurring an increase in the size of the mechanism. For example, if in the above-cited mechanism e is the tooth count of the sun gear, z
1
is the tooth count of the first gear part of the planetary gear, z
2
is the tooth count of the second gear part of the planetary gear, I
1
is the tooth count of the fixed internal tooth gear, I
2
is the tooth count of the movable internal tooth gear, the sun gear is the input part, and the movable internal tooth gear is the output part, the gear reduction ratio can be written as
r={
1+(
I
1
/e
)}/{1−(
z
2
/z
1
)*(
I
1
/I
2
)}  (1)
As described in kokai H2-31047, if z
1
=z
2
, sun gear tooth count e=6, fixed internal tooth gear tooth count I
1
=60, and movable gear tooth count I
2
=61, the difference in the tooth counts of the fixed internal tooth gear and movable internal tooth gear is 1, and the gear reduction ratio is 671. If the other tooth counts remain the same and the tooth count I
2
of the movable internal tooth gear is 62, the tooth count difference is 2 and the gear reduction ratio is 341. If the other tooth counts remain the same and the tooth count I
2
of the movable internal tooth gear is 63, the tooth count difference is 3 and the gear reduction ratio is 231, and if the tooth count I
2
of the movable internal tooth gear is 64, the tooth count difference is 4 and the gear reduction ratio is 176.
However, if the difference between the tooth count of the fixed internal tooth gear and the tooth count of the movable internal tooth gear is 1 in a gear train using a planetary gear train mechanism in which the tooth counts z
1
of the first gear part and z
2
of the second gear part of the planetary gear are the same as described above, the number of planetary gears that can be assembled between the sun gear and the fixed internal tooth gear and movable internal tooth gear is n=1. As a result, the sun gear and planetary gear always mesh at only one place (one tooth each), the torque applied to the teeth becomes very high and the load on the gears becomes great, leading to problems of reduced durability and difficulties in the miniaturization of the gears due to the requirement for increased tooth strength. Practical implementation is therefore difficult particularly when size is reduced. In addition, when the tooth count difference between the fixed internal tooth gear and movable internal tooth gear is 2 to 4, the number n of planetary gears that can be provided is also 2 to 4, but the gear reduction ratio drops as the above tooth count difference increases. More specifically, using three planetary gears and a tooth count difference of 3 between the fixed internal tooth gear and movable internal tooth gear, the achievable gear reduction ratio of approximately 231 (the value shown in the previously described example) is the design limit. Furthermore, considering variation in tooth profile when the gear module is small, the gear reduction ratio is often only about half that or approximately 100 for safety in an actual, practical implementation. The gear reduction ratio of a gear train using a planetary gear train mechanism is therefore often normally approximately 100, and even in extreme configurations using gear forms with a small module it is still only possible to achieve a practical gear reduction ratio of less than 200.
On the other hand, if the constraint of having the planetary gears z
1
=z
2
is removed, the number of planetary gears n is not necessarily dependent upon the tooth count difference between the fixed internal tooth gear and the movable internal tooth gear. Furthermore, according to equation (1) it would seem that an infinitely high gear reduction ratio could be achieved. However, in order to actually construct a small gear mechanism, three conditions must be met: it must be possible to actually form gear forms with the gear module required by the size; it must be possible to achieve the required operating strength (rigidity) in the teeth of the gear forms; and in the positioning of the gears it must be possible to overcome the reduced backlash resulting from the miniaturization of the gear train when interlocking the gears to each other in the assembling the gear train.
The present invention solves the problems described above. An object of the invention is to achieve a configuration for a planetary gear train mechanism suitable for the miniaturization of a gear module that does not introduce problems to its operation or assembly, even when the gear forms of the gears are used in the construction of a small gear module, and thereby provide a gear train that can be made smaller than the prior art while providing a sufficient gear reduction ratio.
SUMMARY OF THE INVENTION
A gear train according to the present invention for resolving the above problems has the following items: a sun gear with a tooth count of e; n planetary gears (where n is a natural number of 2 or more) with each planetary gear having a first gear part with a tooth count of z
1
meshing with the sun gear and a second gear part with a tooth count of z
2
; a fixed internal tooth gear with a tooth count of I
1
meshing with the first gear part of the planetary gears; and a movable internal tooth gear with a tooth count of I
2
meshing with the second gear part of the planetary gears. The gear train according to the present invention is further characterized by having each of the tooth counts e, z
1
, z
2
, I
1
, and I
2
be a multiple of n.
With the present invention it is possible to configure a gear train that has high durability despite miniaturization and that can be reliably and easily assembled because the first gear parts of the n planetary gears between the sun gear and the fixed internal tooth gear can be assembled meshing at equal intervals around the axis, and because the second gear parts of the n planetary gears can be reliably assembled meshed in the same way with the movable internal tooth gear. Thus, a very well balanced condition can be achieved in the planetary gear mechanism even if the gear train is small (i.e. even if the gear module of the gear forms is small) because the phases, i.e. the alignment, of the gear forms at equidistant points dividing the circumference of each gear in n equal parts are mutually matched as a result of the tooth count e of the sun gear, the tooth count z
1
of the first gear parts of the planetary gears, the tooth count z
2
of the second gear parts of the planetary gears, the tooth count I
1
of the fixed internal tooth gear, and the tooth count I
2
of, the movable internal tooth gear all being a multiple of n (where n is a natural number of

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