However, ratios between pulleys are less reliable than
ratios between gears because there is no solid connection
between driver and follower, just an elastic rubber band or
a string that can slip, extend, or retract under load, thus
altering the ratio. We can actually use this lack of a solid
connection to our advantage—for instance, such slippage
could prevent a motor from stalling. The diameter-based
calculation should, therefore, be considered just an approximate
value. The effective ratio depends on a number of factors,
including the torque transferred and the tension of the
element connecting the pulleys, and it varies rather than
staying at one fixed value.
Using pulleys with strings is the subject of the next
section. For now, we will focus on rubber bands. It’s perfectly
possible to use any kind of thin rubber band, but LEGO actually
has its own rubber bands, which work noticeably better.
The rubber bands found in Technic sets are made of a
high-quality silicone that rarely breaks and stays elastic for
years, and they have a round cross section that fits pulleys’
grooves better than the square cross section of ordinary
rubber bands.
The pulley-dedicated LEGO rubber bands come in five
sizes from a 2×2-stud band to a 7×7-stud band. Other than
size and color, the bands are identical, and each of them can
be stretched to a larger size, with the bigger bands able to
stretch more than the smaller ones. The various bands and
their most popular colors are shown in Figure 6-11.
The general behavior of two pulleys connected with a
rubber band is very similar to that of two gears connected
with a chain: The rubber band acts as a belt, keeping all the
pulleys inside it rotating in the same direction. Its shape can
be changed with idler pulleys, and it pulls pulleys together
when subjected to high torque. One band can also be used