ovality

Ovality is a measure of how much a circular cross-section deviates from a perfect circle. In engineering terms, it describes the difference between the maximum and minimum diameters observed in a single cross-section. It is commonly expressed as either a length (Dmax minus Dmin) or as a percentage of the nominal diameter (ovality = (Dmax − Dmin) / Dmean × 100%). Ovality is a two‑dimensional metric and should not be confused with three‑dimensional roundness or cylindricity.

Ovality arises from manufacturing variances, wear, and deformation. Common sources include machining nonuniformity, thermal effects, bending stresses, mounting or clamping distortions, and surface wear during service. Parts with tight circularity requirements, such as shafts, journals, pistons, bearings, pipes, and precision tubes, are particularly sensitive to ovality.

Measurement of ovality typically involves cross-section analysis. Methods include contact-based probes or styluses and non-contact optical or laser techniques. Measurements are taken around the circumference to determine the extreme diameters, from which Dmax and Dmin are derived. In practice, ovality can be evaluated by comparing Dmax − Dmin to a specified tolerance or by expressing it as a percentage of the nominal diameter. Coordinate measuring machines, ring gauges, and roundness profilers are common tools, often used in conjunction with standard methods for roundness and related tolerances.

Ovality is closely related to, but distinct from, roundness (the deviation of the profile from a perfect circle) and from other form tolerances such as cylindricity. Understanding ovality helps ensure proper fit, seal integrity, and mechanical performance in cylindrical components.