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The following definitions are intended to give basic understanding of the terms
commonly used in the pump industry. Some industry standards, or customer
specifications, may have specific meaning of certain terms that may be different
from their common usage - in such case the terms should be interpreted in the
context of the standards, or specifications, being referred to.
Tapered shaft - a shaft with tapered fit, as opposed to shaft with slip fit design.
Taper fit - a style or method of machining whereby the part is machined with a
This is typically done on a pump shaft (a tapered shaft) at the coupling and
impeller hub area. The taper is intended to provide a positive lock between the
mating parts and provide ease of disassembly when the parts are heated.
Test baseplate - A specially designed structure to support the pump during a
performance test in a test well.
Test curve - A graphical representation of the results of a performance test,
under controlled laboratory conditions, showing the capacity, head, efficiency,
horsepower, and NPSHR of a tested pump.
Throat area – see volute throat area.
Throat bushing -
Throttle bushing (or throttle sleeve) -
Tip speed -
T.I.R. - see Total Indicator Runout.
Torque - the measured ability to produce rotation or tuning motion (such as the
torque of a motor), or the ability to resist such rotation (such as the torque of a
Torsional critical speed - a resonant frequency associated with the torsional
or angular deflection of a rotor; not to be confused with lateral critical speed
which is associated with the lateral deflection of a rotor.
Torsional stress -
Total Indicator Runout (T.I.R.) - the runout (or being out of dimension) of a
surface as measured manually on a dial indicator. A measurement on a pump
part or component will indicate its out-of-squareness equal to the reading, or its
eccentricity equal to half the reading, on the dial indicator.
Trip speed - a preset speed at which an emergency over-speed control device
shuts down a pump. This is to prevent a variable speed pump from developing
excessive capacity or pressure, or to prevent the driver from being overloaded.
Two plane balancing - the correction, or reduction, of residual unbalance to
comply with specified or acceptable unbalance limit by removing (preferred
method) or adding weight in two correction planes. See single plane balancing.
Two-stage pump - a pump designed with two impellers that are mounted in
series. One impeller discharges the liquid into the next impeller and the pump
develops two-stage pressure. The two impellers are usually mounted on the
shaft back-to-back, or face-to-face for axial thrust balance.