The variation of the outer diameter of a shaft surface when referenced to the true geometric centreline of the shaft. Often described as Out-of-roundness.


The ratio of the length of the radius vector through a point on the conic to the distance of the point from the directrix.


The eccentricity e of an ellipse can be computed by the formula:

where: a is the semimajor axis and b is the semiminor axis.

Ellipse Linear Eccentricity

The ratio of the distance between the center and focus of an ellipse to its semimajor axis.

See also: Eccentricity Ratio, Ellipse, Off Centre, Runout.

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Subjects: Mechanical Engineering