Mechanical End of Centrifugal Pump

The impeller is mounted on a shaft. The shaft is usually made of steel or stainless steel and is sized to support the impeller. Impellers have to be sized carefully. An undersized shaft can result in increased pump vibration, shorter bearing life, the potential for shaft breakage, and an overall reduced pump life.

Generally speaking the pump shaft is covered with a shaft sleeve. The shaft sleeve is a sleeve of metal, usually bronze or stainless steel, that is designed to either slide or thread onto the shaft. The shaft sleeve is used to position the impeller correctly on the shaft, and it also protects the shaft.

In order for the shaft to hold the impeller within the casing it must pass through the casing. The point where the shaft enters the casing is called the stuffing box and must be sealed. The most typical sealing mechanism is the mechanical seal.

Mechanical seals vary tremendously in design, performance, and cost. The simplest seal consists of just a few primary parts: a stationary face, a rotating face, a gland, and a spring. The rotating face is a ring of smooth hard material that is fastened to shaft sleeve, and the stationary face is a second ring of smooth hard material fastened to the casing. The gland bolts to the outside of the casing and the spring is placed under tension between the gland and the stationary seal face causing it to press against the rotating face. As the pump shaft rotates the rotating face will rotate against the stationary face. A small amount of the pumped liquid will make it’s way between the faces keeping them cool and lubricated. As long as the seal faces stay clean, smooth, and lubricated they will virtually eliminate leakage around the shaft.

The final part of the mechanical end is the bearing arrangement. Generally speaking centrifugal pumps are equipped with standard ball-type anti-friction bearings. These are the same bearings used in everything from electric motors, to roller skates, to automobiles, and they are lubricated by grease or oil. The pump shaft is supported and held in place by the bearings which have to be designed to handle all of the loads created by the rotation of the impeller, and sized to provide a reasonable service life. Bearing failures are one of the most common causes of pump downtime so designing Engineers and End-Users will often be particularly interested in the specifics of bearing design, and the pump professional will be well served to spend time learning the ins-and-outs of the bearing systems of the pumps in question.

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