Unbalance: Referred to with respect to rotating parts in mechanical systems. If these parts have an uneven mass distribution, this will cause vibration as the part rotates. The vibration is directly proportional to the speed of rotation and the amount of unbalance. See residual unbalance.
Unbalanced seal: Seals with a balance ratio > 1. The balance ratio is defined as the ration of the closing area to the opening area of the seal faces.
Undercut, or undercutting: Reducing the effective diameter of an impeller by trimming the vanes, while leaving the shroud intact.
Underfile, or underfiling: Changing the impeller vanes by hand grinding some metal off of them. This will change the pump’s performance by reducing the vane thickness and increasing the area of opening between adjacent vanes.
Unfilled carbon: Pure carbon, containing no impurities. Filled carbon has other components that may react with process fluid, if used to manufacture seal faces.
Unfiltered vibration: Vibrations can occur at many different frequencies. Unfiltered vibration refers to the entire spectrum of possible vibrations, while filtered vibration refers only to those specific frequencies selected.
Upstaging: The addition of impellers to a multi stage pump that has been previously de-staged.
U.S.C.S.: A system of measurements commonly used in the United States. Acronym that stands for: United States Customary System.