Biomedical Engineering Reference
In-Depth Information
FIGURE 3-70
Different
configurations for a
planetary gearhead.
Because the size of the gears determines their load carrying capacity, it is important
to maintain a good balance between the diameters. Figure 3-70 shows the gears in a 3:1,
4:1, and 10:1 system. For a 3:1 ratio, the sun gear is large and the planets are small. In
this case the planets have thin walls and therefore limit the space for bearings and carrier
pins and thus the load torque. The 4:1 ratio is well balanced with sun and planets about
the same size, while the 5:1 and 6:1 ratios (not shown) still yield fairly well-balanced gear
sizes. With higher ratios approaching 10:1, the small sun gear becomes a strong limiting
factor for the transferable torque.
Adding planets increases the torque transfer capacity of the arrangement. Figure 3-70
shows that with lower ratios additional planet gears can be used but that with higher ratios
such as 10:1 multiple gears would cause interference (Anthony, 2008).
The material from which the gears are built is also a consideration in regard to torque
capability. Sintered nickel steel is often used because the sintering process produces gears
that are able to run at closer tolerances. Additionally, because the material is porous,
these gears hold lubricant better than steel units do. However, cut-steel gears tend to
be more durable and are therefore a better choice for higher-torque applications. That
notwithstanding, good lubrication is important regardless of gear material, especially at
high speeds and loads. Planetary gearheads have an advantage because oil flying outward
from the sun gear is captured by the planet gears and carrier plate. Spur gearheads, on
the other hand, fling lubricant off and away from the gears. This is one reason planetary
gearheads have higher speed ratings.
Backlash is a measure of positional accuracy usually specified in arc-minutes. For
example, a typical spur gearhead has about 10 arc-min of backlash, whereas its planetary
counterpart may be rated better than half of that.
Reduction ratios for both spur and planetary gearheads range from near unity up to
several thousand to one. Spur gearheads, with a single geared input shaft coupled to a geared
output shaft (single stage), provide up to about a 6:1 reduction. As discussed previously,
planetary units can reach 10:1 in a single stage. For higher ratios and proportionally
greater output torque, multiple stages or gear sets are stacked together axially, as shown
