Biomedical Engineering Reference
In-Depth Information
FIGURE 3-68
Gear configuration
for a planetary
gearhead.
The design of gears must consider strength under static and dynamic loads, quietness
and smoothness of operation, backlash, lubrication, temperature and heat dissipation, as
well as overall lifetime (Black and Adams, 1968). Most motor manufacturers provide a
wide range of gearheads designed for their motors, so it is seldom necessary to design
gearing systems from scratch.
Two common gearhead designs are spur and planetary. In general, spur gearheads are
simpler and less expensive than planetary units and perform well in low-torque applica-
tions. Torque capacity of spur types is limited because each gear in the train bears the entire
torsional load. In contrast, planetary gearheads share the load over multiple planet gears.
As shown in Figure 3-68, the input shaft drives a central sun gear that, in turn, drives the
planet gears. Each of the planet gears simultaneously delivers torque to a rotating carrier
plate (not shown) coupled to a geared output shaft.
The gear ratio,
N
, for a stationary ring gear is determined by the internal diameter of
the ring gear,
φ
r
(mm), and the outer diameter of the sun gear,
φ
s
(mm),
=
φ
r
N
φ
s
+
1
(3.64)
The formula holds if the respective diameters are replaced by the number of teeth in each
case.
It is obvious that a 2:1 ratio cannot be achieved as it would require that the diameter
of the sun and the ring gears be the same, and that is not possible. Typical ratios are from
3:1 to 7:1, as shown in Figure 3-69.
FIGURE 3-69
Size of sun gear
compared with ring
gear for different
planetary gear
ratios.
