Planet gear type torque transmission device

In a planet gear type torque transmission device, the internal teeth of an internal gear integrally formed by resin molding are arranged regularly for the ribs for coupling the inner periphery and the outer periphery of the gear. Thus, the mold can also be modified regularly so as to form the internal teeth with high accuracy.

BACKGROUND OF THE INVENTION 
The present invention relates to a planet gear type torque transmission 
device for use in a starter for vehicles or the like. 
Previously known planet gear type torque transmission devices are disclosed 
in e.g. Postexamined Japanese Utility Model Publication (Kokoku) 
Hei-2-7262 and Unexamined Japanese Utility Model Publication (Kokai) 
Hei-4-121469. 
First, respective components in FIGS. 9 to 13 for explaining the prior will 
be explained. Reference numeral 1 denotes an internal deceleration type 
starter including a planet gear type deceleration device 2 (described 
later) serving as a torque transmission device. Numeral 3 denotes a front 
machine frame externally fixed. Numeral 4 denotes an internal gear mounted 
on the inside 3a of the front machine frame 3. This internal gear is 
integrally formed by high-polymer synthetic resin) by molding. Numerals 5 
denote two concave portions which are provided symmetrically in the outer 
periphery 6 (described later) of the internal gear 4. The internal gear 4 
is set in the front machine frame when the concave portions 5 are fit in 
the convex portions 3b formed in the front machine frame 3. Numeral 6 
denotes the outer periphery of the thin internal gear. Numeral 7 denotes a 
rib serving as a pillar coupling portion for coupling the outer periphery 
6 and an inner periphery 13 (described later). Numeral 9 denotes a 
through-hole formed by the outer periphery 6, rib 7 and inter periphery 
13. A through bolt (not shown) penetrates through the through-hole. 
A flange portion 10 is provided at the right end of the internal gear 4 in 
FIG. 9. The inside of the flange portion 10 constitutes a boss portion 11. 
A sleeve bearing 12 is fit in the inside of the boss portion 11. Internal 
teeth 14 are successively provided at regular intervals in the inner 
periphery 13 of the internal gear 4. A planet gear 15, which abuts on the 
insides of the internal teeth 14, engages the internal teeth 14. The 
planet gear 15 has a through-hole 16. The planet gear 15 is supported by a 
supporting pin 17 fit in the through-hole 16 through the sleeve bearing 
18. Since the supporting pin 17 is fit in the output rotary shaft flange 
portion 20 integrally formed on the left end of an output rotary shaft 19 
in FIG. 9, the revolving force of the planet gear 15 is transmitted to the 
output rotary shaft 19. The output rotary shaft 19 is rotatably supported 
by the flange portion 10 through the sleeve bearing 12. 
The planet gear 15 also engages a spur gear 22 attached to the input rotary 
shaft 21 on the side opposite to the side where it engages the internal 
teeth 14. The input rotary shaft 21, which is the rotary shaft of a DC 
motor (not shown), provides a revolving force to the planet gear 15 
through the spur gear 22. The input rotary shaft 21 and output rotary 
shaft 19 are made rotatable by the sleeve bearing 23 and a rotary ball 24. 
Incidentally, numeral 25 denotes an iron for jointing the DC motor (not 
shown) to the front machine frame 3. In the arrangement described above, 
the planet gear deceleration device 2 is composed of the internal gear 2, 
planet gear 4 and spur gear 22. 
In operation, when the input rotary shaft 21 rotates by energization of the 
DC motor (not shown), this rotation is supplied to the planet gear 
deceleration device 2. Thus, the rotation with reduced rotary speed and 
enhanced rotary force will be transmitted from the planet gear 
deceleration device 2 to the output rotary shaft 19. 
In fabrication of the internal gear 4, a mold is used for molding 
high-polymer synthetic resin. When the mold has a shape not collected as 
indicated by a solid line in FIG. 12, in the fabrication process, thermal 
shrinkage of the high-polymer synthetic resin called "mold reduction" 
occurs. Particularly, the mold reduction occurs in the directions of 
arrows a at the ribs 7 in FIG. 12. As a result, the ribs 7 are distorted 
as indicated in one-dot chain line in FIG. 12 after molding. In order to 
obviate such an inconvenience, correction indicated by a solid line in 
FIG. 13 is previously added to the mold. After molding, the mold reduction 
leads to the internal gear 4 having the shape indicated by a solid line in 
FIG. 13. 
In the planet gear type torque transmission device as described above, in 
integrally forming the internal gear by resin molding, the mold reduction 
due to the shrinkage of the ribs occurs so that the internal teeth are 
deformed. This required the mold to be modified in forming the internal 
gear by resin molding. 
Further, since the ribs provided in the outer periphery of the internal 
gear are irregularly located with respect to the internal teeth, 
deformation of the internal teeth due to the mold reduction for the ribs 
occurs irregularly. This makes it impossible to perform regular correction 
for the internal teeth portion of the mold, thus making it difficult to 
increase the accuracy of the internal teeth. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a planet gear type torque 
transmission device in which internal teeth of an internal gear are 
provided with high accuracy owing to easiness of modification of a mold 
used for fabricating the internal gear. 
In order to attain the above object, in accordance with one aspect of the 
present invention, there is provided a planet gear type torque 
transmission device comprising a planet gear driven by a rotary shaft and 
an internal gear composed of an inner periphery provided with internal 
teeth engaged with the planet gear, an outer periphery and coupling 
portions for coupling the inner periphery and outer periphery, the 
internal gear being integrally formed by resin molding, wherein the 
coupling portions are located at fixed positions for the inner teeth. 
Thus, deformation of the internal teeth due to the shrinkage of the 
coupling portions in integrally forming the internal gear by resin molding 
is regular so that the mold can be modified regularly and so easily. 
In another aspect of the present invention, the internal teeth are arranged 
symmetrically with respect to the lines extended toward the radial 
direction of the internal gear from the joint points of the coupling 
portions and the inner periphery. Thus, deformation of the internal teeth 
due to the shrinkage of the coupling portions in integrally forming the 
internal gear by resin molding is symmetrical so that the mold can be 
modified symmetrically and so easily. 
In a still another aspect of the present invention, the tips of the 
internal teeth are arranged in the radial direction from the joint points 
of the coupling portions and the inner periphery. Thus, deformation of the 
internal teeth due to the shrinkage of the coupling portions in integrally 
forming the internal gear by resin molding is symmetrical on both sides of 
the teeth tips so that the mold can be modified symmetrically on both 
sides of the teeth tips and so easily. 
In a further aspect of the present invention, the bottoms of the inner 
teeth are arranged in the radial direction of the internal gear from the 
joint points of the coupling points and the inner periphery. Thus, 
deformation of the internal teeth due to the shrinkage of the coupling 
portions in integrally forming the internal gear by resin molding is 
symmetrical on both sides of the teeth bottoms so that the mold can be 
modified symmetrically on both sides of the teeth bottoms and so easily. 
In a still further aspect of the present invention, the coupling portions 
have gradients for the radial direction of the internal gear at the joint 
points of the coupling portions and the inner periphery. Thus, the 
direction of shrinkage of the coupling portions in integrally forming the 
internal gear by resin molding is inclined so that the internal teeth are 
deformed slightly and so their accuracy can be enhanced. 
In a further aspect of the present invention, each of the coupling portions 
is branched at the joint points of the coupling portions and inner 
periphery. Thus, the shrinkage force due to shrinkage of the coupling 
portions in integrally forming the internal gear by resin molding is 
dispersed so that the internal teeth are deformed slightly and so their 
accuracy can be enhanced. 
In a further aspect of the present invention, each of the coupling portions 
is branched in both sides of the radial direction of the internal gear at 
the joint points of the coupling portions and internal teeth. Thus, the 
shrinkage force due to shrinkage of the coupling portions in integrally 
forming the internal gear by resin molding is canceled in the peripheral 
direction so that the internal teeth are deformed slightly and so their 
accuracy can be enhanced. 
In a further aspect of the present invention, at least two bottoms of the 
internal teeth are arranged between the coupling portions. Thus, at least 
one internal tooth with less deformation due to the shrinkage of the 
coupling portions in integrally forming the internal gear by resin molding 
so that the internal teeth are deformed slightly and so their accuracy can 
be enhanced.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Embodiment 1 
FIG. 1 is a front view showing the internal gear according to the first 
embodiment of the present invention. FIG. 2 is a view for explaining 
deviation of the internal gear from the mold for molding it. 
In FIG. 1, reference numeral 30 generally denotes the tip of an internal 
tooth 14. Specifically, the internal teeth 14a, 14b, 14c, 14d and 14e have 
their tips 30a, 30b, 30c, 30d and 30e, respectively. Reference numeral 31 
denotes the extended line of the center line of a rib 7, represented by a 
dotted line in FIG. 2. Namely, the rib 7a has an extended line 31a of its 
center line, and the rib 7b has an extended line 31b of its center line. 
The ribs 7 are arranged at regular intervals W which is a width between 
the lines 31a and 31b. 
In FIG. 2, the solid line of the internal teeth 14 designate a mold shape 
32. The one-dot chain line designates the shape of the internal teeth 4 
formed by molding. In FIGS. 1 and 2, remaining reference numerals refer to 
like parts in FIGS. 9, 10 and 11 showing the prior art planet gear type 
transmission device. 
In the planet gear torque transmission device thus formed, as shown in FIG. 
2, the extended line 31 of the rib 7 passes through the predetermined 
position of the internal tooth 14 so that the rib 7 is arranged at a 
predetermined position for the internal tooth 14. 
Thus, the deformation produced in the internal teeth 14 when the internal 
gear 4 is integrally formed by resin molding is the repetition of the 
deformation of the internal tooth 14 over the entire inner periphery of 
the internal gear 4. For this reason, the mold can be modified regularly. 
Embodiment 2 
FIG. 3 is a front view showing the internal gear according to the second 
embodiment of the present invention. FIG. 4 is a view for explaining 
deviation of the internal gear from the mold for molding it. 
In FIG. 3, reference numeral 40 generally denotes the tooth bottom between 
internal teeth 14. Specifically, a tooth bottom 40a is located between 
internal teeth 14a and 14b; a tooth bottom 40b is located between internal 
teeth 14b and 14c; a tooth bottom 40c is located between internal teeth 
14c and 14d; and a tooth bottom 40d is located between internal teeth 14d 
and 14d. 
As seen from FIG. 4, since the extended line 31b passes through the tooth 
bottom 40c, the deformation between the internal teeth 14c and 14d due to 
"mold reduction" is symmetrical with respect to the extended line 31b. 
Thus, the mold can be modified symmetrically with respect to the extended 
line 31b. As a result, the mold can be modified symmetrically with respect 
to all the extended lines 31 over the entire inner periphery of the 
internal gear 4. 
Embodiment 3 
FIG. 5 is a front view showing the internal gear according to the third 
embodiment of the present invention. FIG. 6 is a view for explaining 
deviation of the internal gear from the mold for molding it. 
As seen from FIG. 6, since the extended line 31b passes through the tooth 
tip 30e, the deformation of the internal tooth 14d due to "mold reduction" 
is symmetrical with respect to the extended line 31b. Thus, the mold can 
be modified symmetrically with respect to the extended line 31b. As a 
result, the mold can be modified symmetrically with respect to all the 
extended lines 31 over the entire inner periphery of the internal gear 4. 
The internal tooth 14c between the extended lines 31a and 31b has a tooth 
bottom 40b between itself and the extended line 31a and a tooth bottom 40c 
between itself and the extended line 31b. For this reason, the internal 
tooth 14c is free in a certain degree from the influence by the mold 
reduction occurring in the ribs 7 and so its deformation is smaller than 
that in the internal teeth 14b and 14c. 
Embodiment 4 
FIG. 7 is a front view showing the internal gear according to the fourth 
embodiment of the present invention. FIG. 8 is a view for explaining 
deviation of the internal gear from the mold for molding it. 
In FIG. 8, reference numeral 50 denotes a rib as a coupling portion and 
reference numeral 51 denotes an extended line extended from the joint 
point 52 of the ribs 50 and inner periphery 13 toward the radial direction 
of the internal gear 4. A rib 50a joins a rib 50b in an outer periphery 6; 
the rib 50b joins a rib 50c at the joint point 52a and the rib 50c joins a 
rib 50d in the outer periphery 6. 
The rib 50 shrinks in the direction of an arrow a owing to the mold 
reduction. Thus, the shrinkage force due to mold reduction in the inner 
periphery 13 will be dispersed transversely by a degree inclined from the 
extended line 51a. Further, the shrinkage forces in ribs 50b and 50c due 
to mold reduction will be canceled each other in the peripheral direction. 
As a result, as seen from FIG. 8, less or no deviation of the internal 
teeth 14 from the mold will occur. 
The respective embodiments could be applied to the internal deceleration 
type starter, but may be applied to other transmission devices.