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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 alone sucks up all of the 8t gears, 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
Pulleys 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. However, a twisted band will rub on itself, which may significantly reduce its life span. Pulleys 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 drivetrain used pulleys to replace several gears. Although pulleys are useful for light-duty 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.
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