Biomedical Engineering Reference
In-Depth Information
FIGURE 3-71
Stacked planetary
gearhead.
(a) Cutaway of
motor. (b) detail of
gearhead. (Courtesy
of MicroMo
Electronics.)
in Figure 3-71. Increasing the number of stages increases the reduction ratio and output
torque as well as overall length and also lowers the mechanical efficiency. A typical
single-stage spur gearhead is about 90% efficient, whereas a two-stage device is about
85% efficient. Planetary gearheads are slightly more efficient at approximately 97% and
94%, respectively, for one- and two-stage units. Obviously, the efficiency decreases further
as additional stages are included.
MicroMo Electronics offers a wide range of gearheads with diameters of from 6 mm
to 44 mm (to suit motor diameters). Reduction ratios are available from 3.7:1 to 23.104:1.
Maximum input speeds range from 3,500 rpm to 8,000 rpm, and continuous load torques
are available from 25 mNm to 16,000 mNm.
As can be seen in Table 3-9, planetary gearheads are superior to spurs on all counts
except mechanical noise and cost. Selection of the appropriate gearhead for a mechatronic
application should take all of these factors into account.
Belt drives are commonly used over longer center distances than those used for gears.
Conventional flat or V belts slip slightly so the driven speed is less than the ratio of pulley
diameters. However, there is no slip in toothed belts.
The maximum amount of power than can be transmitted by a belt drive is determined
by belt stretch or excessive slippage on the smaller pulley. This is in turn is determined by
the angle of contact, the belt tension, and the coefficient of friction between the belt and
the pulley.
TABLE 3-9
Comparison of planetary and spur gear characteristics
Factor
Planetary
Spur
Backlash
Low
High
Efficiency
High
Low
Load capacity
Good
Adequate
Operating speed
Fast
Slower
Mechanical noise
Noisy
Quiet
Size
Small
Large
Cost
High
Lower
