This 24-tooth gear is white with dark grey in the center, and
it has the unique ability to harmlessly slip around the axle
if a sufficiently high torque is applied. It’s most often used
for end-to-end applications—that is, applications in which the motor can run only until it reaches a certain point. This
includes almost all steering mechanisms, in which the wheels
can be turned only within a limited angle, and the railroad
barrier mechanism, by which the barrier can be raised or
lowered only to a certain degree. In these types of mechanisms, the gear slips when that end point is reached so that
the motor can continue to run while the mechanism itself is
stopped. This gear is also used for the motorized winches in
official LEGO sets, where it ensures that the motor doesn’t
get damaged when the end of the winch’s string is reached.
Note that this gear slips under a very specific amount of
torque, and in most cases, you will want it to slip only under
extremely high torque (in other words, you’ll need to make
sure that the steering mechanism stops turning when the
end point is reached, not merely when a wheel meets an
obstacle). This can be achieved by adding some gearing down
between it and the mechanism it controls (see Figure 5-20).