Unbalance: Refers to the uneven distribution of mass in rotating parts of mechanical systems, resulting in vibration during rotation. The magnitude of vibration is directly proportional to the speed of rotation and the degree of unbalance. See residual unbalance.

Unbalanced seal: Seals with a balance ratio greater than 1, which is defined as the ratio of the closing area to the opening area of the seal faces.

Undercut or undercutting: The process of reducing the effective diameter of an impeller by trimming the vanes while keeping the shroud intact.

Underfile or underfiling: The manual grinding of metal from the impeller vanes, which alters the pump’s performance by reducing vane thickness and increasing the area of opening between adjacent vanes.

Unfilled carbon: Pure carbon without any impurities. Filled carbon contains additional components that may react with the process fluid if used in manufacturing seal faces.

Unfiltered vibration: Vibrations can occur at various frequencies. Unfiltered vibration encompasses the entire spectrum of possible vibrations, while filtered vibration refers to specific selected frequencies.

Upstaging: The process of adding impellers to a multi-stage pump that has previously been de-staged.

USCS: United States Customary System, a system of measurement commonly used in the United States.

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