Radial: Refers to an arrangement that is oriented along the radius of a circle, particularly in relation to a circular shaft.

Radial bearing: Most bearings are designed to support radial loads, although their ability to support axial loads varies depending on the specific type of bearing.

Radial flow pump: A type of centrifugal pump that is specifically designed to handle moderate head and medium flow rates, or high head and low flow rates.

Radial split pump: A pump configuration where the pump casing is split along a line that is perpendicular to the centerline of the shaft.

Radial vane pump: Also known as a partial emission pump, this pump is designed to handle corrosive chemicals at high head with low flow rates. It was developed during World War II for pumping fuel into German Ram jet engines.

Rated operating point: The specific point on a pump’s efficiency curve where the pump manufacturer guarantees its performance.

Reaction bonded: A manufacturing method used to produce silicon carbide by causing a reaction between porous carbon (graphite) and molten silicon.

Recessed impeller pump: Sometimes referred to as a vortex pump, this pump design reduces contact between the impeller and the process fluid by recessing the impeller. It is particularly suitable for handling slurries containing solids or fibers.

Recirculation: The phenomenon where the process fluid tends to circulate at both the suction and discharge sides of the pump at high and low flow rates in comparison to the pump’s best efficiency point (BEP) on the efficiency curve.

Relative density: Also known as specific gravity, it is the ratio of a liquid’s density to that of water measured at 4°C.

Renkin scale: A temperature scale that shares the same unit size as Fahrenheit, but with the zero point defined as absolute zero.

Repeller: An additional impeller used in a pump to reduce the pressure in the stuffing box.

Residual unbalance: The remaining amount of unbalance, measured in terms of mass, after a rotor or shaft has been balanced using a balancing machine. It is typically expressed in ounce-inches and can cause vibration during operation, with the magnitude depending on the level of unbalance and rotor speed.

Reynolds number: A dimensionless number that relates the ratio of inertial forces (velocity) to viscous forces in a fluid.

Rheopectic: Refers to a fluid whose viscosity increases with time.

Rigid rotor: A rotor that operates at or below 70% of its first critical speed and does not exhibit vibration.

Rigid shaft: A pump shaft that operates below its critical speed.

RMS: Root mean square, a statistical measure of the magnitude of variation in a given quantity.

Rockwell “C”: A hardness scale used to measure the hardness of materials.

Rotating seal: In mechanical seals, if the movable portion of the seal rotates with the shaft, it is referred to as a rotating seal.

Rotor: The assembly of all rotating components in a pump, including the shaft, impeller, wear rings, bearings, and other related parts.

Runout: A measure of deviation from true rotation when the rotating shaft does not rotate exactly along its centerline. It is typically expressed as twice the distance of this deviation.

Runner: A historical term for the impeller, which is rarely used in modern contexts.

Related Links

• Pump Glossary
• Pump info