T-117
Besides the above three basic conditions, there can be an interference problem between
the internal gear C and the planet gears B. See SECTION 5 that discusses more about
this problem.
13.3.2 Speed Ratio of Planetary Gear System
In a planetary gear system, the speed ratio and the direction
of rotation would be changed according to which member is
fixed. Figures 13-6(a), 13-6(b) and 13-6(c) contain three typical
types of planetary gear mechanisms, depending upon which
member is locked.
(a) Planetary Type
In this type, the internal gear is fixed. The input is the sun
gear and the output is carrier D. The speed ratio is calculated
as in Table 13-1.
Table 13-1 Equations of Speed Ratio for a Planetary Type
za
–––
zc 1
Speed Ratio = ––––––––– = ––––––––
(13-11)
za zc
1 + ––– ––– + 1
zc za
Note that the direction of rotation of input and output axes
are the same.
Example: za = 16, zb = 16, zc = 48, then speed ratio = 1/4.
(b) Solar Type
In this type, the sun gear is fixed. The internal gear C is
the input, and carrier D axis is the output. The speed ratio is
calculated as in Table 13-2, on the following page.
Fig. 13-6(a)
Planetary Type
Planetary Gear
Mechanism
Sun Gear A
za
Planet Gear B
zb
Internal Gear C
zc
Carrier D
No.
Description
+1
za
+ –––
zc
za
1 + –––
zc
1
2
3
Rotate sun gear A once
while holding carrier
System is fixed as a
whole while rotating
+(za /zc)
Sum of 1 and 2
za
– –––
zb
za
+ –––
zc
za za
––– – –––
zc zb
za
– –––
zc
za
+ –––
zc
0
(fixed)
0
za
+ –––
zc
za
+ –––
zc
Fig. 13-6(b)
Solar Type
Planetary Gear
Mechanism
B
C (Fixed)
D
A (Fixed)
B
C
D
