Gears are probably the most efficient method of mechanical power
transmission we have today, and much of the reason for that is the shape
that gear teeth are ground to.
Unlike the triangular or rectangular teeth you see on comical depictions
of gear trains, gear teeth are actually ground to a shape called an
If you have a string wound around a cylinder, and you keep that string
tight as you unwind it from the cylinder, the shape of the curve the end
of that string makes is called an "involute". It's a shape similar to a
The unique advantage of involute gear teeth is that when two involute
gear teeth meet, they actually ROLL over each other. That is, the point
of contact between the two teeth is a ROLLING contact, not a sliding
And it's this rolling contact made possible by the involute shape of the
gear teeth that results in negligible loss of mechanical power to
friction, and therefore the excellent mechanical efficiency of gear
If gear teeth slid across each others surface, as is depicted in comical
gear trains with rectangular teeth or the "peg" shaped gear teeth you
might see on a TV show like "Gilligan's Island", then much of the
mechanical power would be lost to friction.
Watch the gear animation at the top of this web page:
'Involute gear - Wikipedia, the free encyclopedia'
The black dot is the point of contact between any two gear teeth, and
it's the involute shape of the gear teeth allows them to roll over each
other, thereby transmitting power with negligible friction losses.