FIGURE 16 Metal disk coupling with flanged, tubular spacer (Thomas Coupling Div./Rexnord)

integral motor pumps not having a flexible coupling) and those that, for one or more reasons, have the prime mover located a substantial distance from the pump. These latter types of systems require a modification to the basic flexible coupling designs previously described.

Spacers One means of accommodating shaft separation in excess of the normal amount provided in a standard coupling is to employ a flanged tubular spacer between the two coupling halves (Figure 18). These components are lightweight and are commonly used with end-suction pumps, where it is possible to remove the pump impeller while the pump

FIGURE 17 Vertical double-engagement gear coupling (Koppers)

FIGURE 18 Tubular spacer coupling (Koppers).

FIGURE 18 Tubular spacer coupling (Koppers).

housing and piping, as well as the prime mover, remain in place (Figure 19). For highspeed drive systems, a lightweight spacer is usually the only practical means of achieving the goal. This spacer may be used to tune the system torsionally (as previously discussed) by varying the body diameter and wall thickness. Spacers can be manufactured in lengths up to several feet, but the cost generally restricts its usage to shorter lengths, except where no other means is suitable.

Floating Shafts Floating shaft couplings have a purpose similar to that of spacer couplings; namely, to connect two widely separated shafts. The basic difference is in the construction of the component. Whereas spacers are normally made with the body having an integrally formed flange to connect the two coupling halves, a floating shaft usually is made by attaching a flange to a piece of solid or tubular shafting by means of mechanical keys or by welding (Figure 20). This type of construction is generally less expensive than a one-piece spacer, especially when very large shaft separations must be spanned. Graphite composite center members are also being used. This material has the advantage of being much lighter and stiffer than standard metal components, allowing longer spans between bearing supports without adding mass.

FIGURE 19 Spacer coupling enables end-suction pump to be dismantled without moving piping, pump casing, or driver (Worthington Pump).
FIGURE 20 Section of floating shaft and couplings (Kop-Flex)

Floating shaft arrangements are widely used on horizontal pump applications of all types and are especially common on vertical pump applications, such as in water pumping and sewage treatment stations. In these latter applications, the pump may be submerged in a pit usually 30 to 50 ft (9 to 15 m) below ground level, whereas the motor (to prevent damage during flooding) is mounted at ground level. Settings as deep as 100 ft (30 m) have been constructed. Such systems require long floating shafts, which are generally made in sections and supported by line bearings at intermediate supports or floors.

Rigid Shafts Vertical centrifugal pumps can be designed to contain their own thrust and line bearings. These pumps employ a flexible coupling at the pump shaft. When vertical intermediate shafting is used, the shaft need be designed only to transmit torque. Some pumps are designed to contain only a single line bearing. These pumps require a rigid coupling at the pump shaft so axial thrust can be carried by the thrust bearing in the driver or gear located above. The constructions of a single oil-lubricated line-bearing pump is shown in Figure 109 of Subsection 2.2.1. In order to carry thrust, the drive shaft, if made in sections, must use rigid intermediate couplings and the coupling at the driver or gear must also be rigid. Intermediate bearings used with rigid shafting must be designed to provide only lateral support, thereby assuring that all the axial thrust is carried by the driver or gear. An intermediate oil-lubricated sleeve-type guide bearing is shown in Figure 108 of Subsection 2.2.1. Pump and intermediate shaft sleeve guide bearings can be either grease- or oil-lubricated, and they can also be antifriction if preferred.

A rigid intermediate shaft connected to a pump provided with a rigid coupling must be designed to carry its own weight, the axial thrust from the pump, and a bending load. Volute-type centrifugal pumps produce a radial reaction on the pump shaft that in turn is transmitted to the vertical shafting through the rigid pump coupling. Intermediate bearings then act as line bearings. The bearing supports must also be designed to resist this bending force. The pump shaft and bearing, intermediate shafts and line bearings, and the driver or gear shaft and bearings must be treated as a single shaft supported at each bearing when analyzing the critical speed of the entire rotor system. If the intermediate line bearing supports are assumed to be nodes in the critical speed calculation, these supports must be absolutely rigid. The supplier of the shafting should, however, confirm what flexibility will be allowed at these supports and give the design forces. The support for the guide bearings must also be designed not to have a natural frequency of vibration within the operating speed range of the pump.

Flexible Drive Shafts Universal joints with tubular shafting (Figure 21) can be substituted for flexible couplings whenever it is necessary to (a) eliminate the need for critical alignment, (b) provide wider latitude in placement of pump and driver, and (c) permit large amounts of relative motion between pump and driver. This type of shafting can be used horizontally as well as vertically to provide a short spacer arrangement or a large separation between pump and driver, such as that required for deep settings (Figure 22).

Flanges are furnished to fit pump and driver shafts to the universal joints, which are splined to allow movement of the shaft. An intermediate steady bearing is required at each joint, which must also support the weight of a section of shafting (Figure 23). Because of the spline, pump thrust cannot be taken by the driver, and it is necessary to use a combination pump thrust and line bearing. Because of the universal joint, the intermediate bearing or bearings take no radial load from the pumps and therefore act only as a steady bearing or bearings for the shaft.

The shaft must be selected to transmit the required torque and be of a length and diameter that will have a critical speed well removed from the operating speed range of the pump. The support for the guide bearing must not have a natural frequency of vibration within the speed range. Shafts of this type are often pre-engineered and stocked. Selection charts are available from the manufacturer to make a proper size and length selection. Standard tubular, flexible drive shafts have limited torque-carrying capacity.

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