Two
non-circular gears
with the same elliptical outline, each pivoting around
one focus and positioned at the proper angle, will turn
smoothly while maintaining contact at
all
times. Alternatively, they can be connected by a
link chain
or
timing belt,
or in the case of a bicycle the main
chainring may be elliptical, or an
ovoid
similar to an ellipse in form. Such elliptical gears may
be used in mechanical equipment to produce variable
angular speed or torque from a constant rotation of the
driving axle, or in the case of a bicycle to allow a
varying crank rotation speed with inversely varying
mechanical advantage.
Elliptical bicycle gears make it easier for the chain to
slide off the cog when changing gears.
An example gear application would
be a device that winds thread onto a conical bobbin on a
spinning machine. The bobbin would need to wind faster
when the thread is near the apex than when it is near
the base.