. Unbalance - the uneven mass distribution on a rotating element that will cause it to vibrate due to centrifugal force when the element is rotating. The magnitude of vibration increases proportionately with unbalance mass and rotational speed. Unbalance is the product of the unbalance mass multiplied by its distance from the axis of rotation, or shaft centerline, and is commonly measured in ounce-inches. See residual unbalance. Read more.
Unbalanced seal - a mechanical seal that is not hydraulically balanced to minimize the heat developed at its seal face.
Undercut, or undercutting - trimming the effective diameter of an impeller by cutting its vanes only, and leaving its shroud uncut.
Underfile, or underfiling - the removal, by hand-grinding, of some amount of metal on the impeller vanes, either on the inlet side, or outlet side, for the purpose of reducing the vane thickness and increasing the area opening between two adjacent vanes, to change the performance of a pump.
Unfiltered vibration - vibration data that include the spectrum of different frequencies under observation. By comparison, filtered vibration data are unique to a discreet, or specific, frequency under observation.
Upstaging - the addition of one or more impeller/s to previously de-staged multistage pump to increase its differential head. (See definition of de-staging.)