A cantilever shaft pump generally utilizes rolling element bearings to support an overhung shaft that extends downward into a sump or tank. The bearings are mounted above the maximum liquid level so that they are never exposed to the pumped fluid. Because there are no submerged bearings, the pump handles solids without the need for external flush, or any sealing devices, to keep the pumped fluid out of the support column. Cantilever pumps may also be operated dry, or with loss of prime, without damage.
Cantilever pumps require no seals or packing when used on open sumps. When applied to closed tanks with hazardous materials, the rolling element bearings limit shaft run-out so that mechanical seals may be applied reliably.
With the exception of a few special cases, the shaft and bearing assembly of a cantilever pump is designed so as to operate below 1st lateral critical speed1. As the amount of shaft overhang increases, the shaft diameter is increased so as to provide sufficient stiffness to limit deflection and provide rotor stability. The design diameter of the shaft is a function of the length of the shaft overhang, the rotational speed, and the mass of the impeller. Typically, the practical maximum shaft diameter is about eight inches. Above this diameter, conflicts with bearing speed and load ratings start to occur. This diameter equates to a maximum overhung distance of somewhere between six and eight feet for most pumps.
The lowest speed at which the natural frequency of the shaft is synchronized with the operating speed. Critical speed is characterized by very high vibration levels. Designing below 1st critical speed provides for smooth operations under all ‘good practice’ operational conditions.
The maximum lift is very design and application dependent. On certain designs the maximum lift can be extended to about 20 feet. The trade-offs with increasing the maximum lift capability are with efficiency and axial thrust.
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