13.3 Planetary Gear System
The basic form of a planetary gear system is shown in Figure 13-4. It consists of a Sun
Gear (A), Planet Gears (B), Internal Gear (C) and Carrier (D). The input and output axes of
a planetary gear system are on a same line. Usually, it uses two or more planet gears to
balance the load evenly. It is compact in space, but complex in structure. Planetary gear
systems need a high-quality manufacturing process. The load division between planet
gears, the interference of the internal gear, the balance and vibration of the rotating carrier,
and the hazard of jamming, etc. are inherent problems to be solved.
Figure 13-4 is a so called 2K-H type planetary gear system. The sun gear, internal
gear, and the carrier have a common axis.
13.3.1 Relationship Among The Gears In A Planetary Gear System
In order to determine the relationship among the numbers of
teeth of the sun gear A, (za), the planet gears B, (zb), and the internal gear C, (zc), and the number of planet gears, N, in the system, the
parameters must satisfy the following three conditions:
This is the condition necessary for the center distances of the
gears to match. Since the equation is true only for the standard gear
system, it is possible to vary the numbers of teeth by using profile
shifted gear designs.
To use profile shifted gears, it is necessary to match the center
distance between the sun A and planet B gears, ax1, and the center
distance between the planet B and internal C gears, ax2.
This is the condition necessary for placing planet gears evenly
spaced around the sun gear. If an uneven placement of planet gears
is desired, then Equation (13-8) must be satisfied.
θ = half the angle between adjacent planet gears
Condition No. 3:
Satisfying this condition insures that adjacent planet gears
can operate without interfering with each other. This is the condition
that must be met for standard gear design with equal placement
of planet gears. For other conditions, the system must satisfy the
dab = outside diameter of the planet gears
ax = center distance between the sun and planet gears
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.