An ellipse is defined by a set of points in a plane, such that the sum of the distances from two fixed points - the foci - to any point in the set is a constant. This enables elliptical gears cut about their foci to run at a constant center distance. The velocity ratio resulting from elliptical gears varies between 1/K and K during one cycle of speed change, where K is the maximum gear ratio. Multilobe gears can be derived from unilobe gears. In appropriate situations, elliptical gears may be cascaded to obtain varying speed-change characteristics, giving an effective ratio of from 1 to K^2 . Bilobe gears (oval gears) have been used in flowmeters and pumps. Using precision elliptical bilobe gears, flowmeters can have good linearity over a wide range of flow rates and viscosities
