Sleeve

FIGURE 69 A sleeve threaded onto a shaft with no external locknut and cartridge-type mechanical seals. When shaft sleeve nuts are used to retain the sleeves and impellers axially, they are usually manufactured with right- and left-hand threads. The friction of the pumpage and inadvertent contact with stationary parts or bushings will tend to tighten the nuts against the sleeve and impeller hub (rather than loosen them). Usually, the shaft sleeves utilize extended impeller keys to prevent rotation.

Some manufacturers favor the sleeve shown in Figure 69, in which the impeller end of the sleeve is threaded and screwed to a matching thread on the shaft. A key cannot be used with this type of sleeve, and right- and left-hand threads are substituted so that the frictional grip of the packing on the sleeve will tighten it against the impeller hub. As a safety precaution, the external shaft nuts and the sleeve itself use set screws for a locking device.

In pumps with overhung impellers, various types of sleeves are used. Most pumps use mechanical seals, and the shaft sleeve is usually a part of the mechanical seal package supplied by the seal manufacturer. Many mechanical seals are of the cartridge design, which is set and may be bench-tested for leakage prior to installation in the pump. (For a further discussion of mechanical seals, see Subsection 2.2.3.)

For overhung impeller pumps that utilize packing for sealing, the packing sleeves generally extend from the impeller hub through the seal chambers (or stuffing boxes) to protect the pump shaft from wear (see Figure 70). The sleeves are usually keyed to the shaft to prevent rotation. If a hook-type sleeve is used, the hook part of the sleeve is clamped between the impeller and a shaft shoulder to maintain the axial position of the sleeve. A hook-type sleeve used to be popular for overhung impeller pumps that operate at high temperatures because it is clamped at the impeller end and the rest of the sleeve is free to expand axially with temperature changes. But with the increased use of cartridge-type seals, the use of hook-type sleeves is diminishing.

In designs with a metal-to-metal joint between the sleeve and the impeller hub or shaft nut, a sealing device is required between the sleeve and the shaft to prevent leakage. Pumped liquid can leak into the clearance between the shaft and the sleeve when operating under a positive suction head and air can leak into the pump when operating under a negative suction head. This seal can be accomplished by means of an O-ring, as shown in Figure 71, or a flat gasket. For high temperature services, the sealing device must be either acceptable for the temperature to which it will be exposed, or it must be located outside the high temperature liquid environment. An alternative design used for some high-temperature process pumps is shown in Figure 72. In this arrangement, the contact surface of the hook-type sleeve and the shaft is ground at a 45-degree angle to form a metal-to-metal seal. That end of the sleeve is locked, but the other is free to expand with temperature changes.

When O-rings are used, any sealing surfaces must be properly finished to ensure a positive seal is achieved. All bores and changes in diameter over which O-rings must be passed should be properly radiused and chamfered to protect against damage during assembly. Guidelines for assembly dimensions and surface finish criteria are listed in O-ring manufacturers' catalogs.

Material for Packing Sleeves Packing sleeves are surrounded in the stuffing box by packing. The sleeve must be smooth so that it can turn without generating too much friction and heat. Thus, the sleeve materials must be capable of taking a very fine finish, preferably a polish. Cast-iron is therefore not suitable. A hard bronze is generally used for pumps handling clear water, but chrome or other stainless steels are sometimes pre-

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Survival Treasure

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