Page 120 - The Unofficial Guide to Lego Mindstorms Robots

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Figure 5-5.
Minerva's directional transmission has four outputs

The basic directional transmission designs I've described are quite simple. Try to use one of these to drive a robot, however,
and things get a little more complicated. Minerva, as you've seen, is a gear hog. The drivetrain alo ne sucks up all of the 8t
g ears, all of the 16t gears, and most of the 24t gears that come with RIS. If you decide directional transmissions are useful for
your robot, you may want to have some extra gears handy from other sets.

Pulleys

P ulleys are an interesting alternative to gears. A pulley is simply a slotted wheel, like the ones used with outdoor clotheslines.
The RIS comes with a handful of rubber bands that can be used to link pulleys together. This type of linkage is similar to using
gears to transmit motion from one shaft to another. By using pulleys of different sizes, you can achieve the same power and
speed tradeoffs as with gears. The only difference is that pulleys connected by a band turn in the same direction, while two
gears meshed together turn in opposite directions. If you twist the band around once, you can get the pulleys to move in
opposite directions, just as with gears. Howeve r, a twisted band will rub on itself, which may significantly reduce its life span.
P ulleys also give you the flexibility to transmit motion between two perpendicular shafts. Figure 5-6 shows three different
pulley arrangements.

One of the early designs of Minerva's drivetra in used pulleys to replace several gears. Although pulleys are useful for light-
d uty work, like the grabber arm, they don't work very well for drivetrains. Unless the band that connects two pulleys is very
tight, it's likely to slip if it's used to do heavy work, like moving an entire robot.