1) The "gear ratio" should never exceed 7. Put another way,
you should never design an input speed that is 7 times
greater or lesser than the output speed. Put a final way,
for those who are new to gears, the size of the bigger gear
should never be more than 7 times greater than that of the
smaller one.
2) A good rule of thumb for chain dimensions is that the
distance between the sprockets' centers should be 30 to 50
times the chain pitch (distance between the rollers in the
links). The center distance should never in any case exceed
80 times the chain pitch. So the easiest way to start is by
determining how far apart the sprocket centers will be.
There's a relatively wide range of center distance that is
okay for an application, so go ahead and take a best guess
for the distance. For example, on my motorcycle I'm just
using the current center distance as a starting point.
3) In order to prevent the same links from always running
on the same sprocket teeth, try to choose sprockets with an
odd number of teeth.
4) In order to reduce premature stretching of the chain,
try to limit all sprockets to no more than 110 teeth.
5) Due to the physical construction of a chain, if
you want to avoid using a special connecting link (as
show in Fig. 3 in the preceding link) you must design
for an even number of links.
6) Optimum roller chain speed is around 12.5m/s. Silent
chains' optimum speed is a bit faster: 20m/s.
7) The true formula for length is
L=sqrt(4*C^2-(d2-d1)^2)+1/2*(d2*theta2+d1*theta1), where
theta1=pi-2*beta, theta2=pi+2*beta, and
beta=arcsin((d2-d1)/(2*C)). It can be approximated as is
done on the parent page. Of course, this length is an
absolute minimum, as it assumes perfect geometry, whereas
in reality the chain will sag.