APPLICATIONS The type of flexible coupling most suitable for a particular application depends upon a number of factors, including power, speed of rotation, shaft separation, amount of misalignment, quiet operation, torsional tuning, cost, and reliability. In the design of a system, it is the goal of the designer to use the least expensive coupling that will do the job. In low-cost systems, cost alone may be the most important criterion, and the least expensive coupling that transmits the rated power and accepts some small degree of misalignment is generally the choice, albeit at some sacrifice of reliability and durability. On the other hand, high-power, high-speed machinery generally represents a critical piece of equipment for a power station, sewage plant, or other vital process, and in these cases a coupling should be selected that will not compromise the overall reliability of the system.  FIGURE 18 Key driven permanent magnet coupling (MagnaDrive Corp.) Low-power pumps (up to about 200 hp, 150 kW) driven by electric motors can usually be coupled successfully by any of the couplings described here. Selection procedures vary from manufacturer to manufacturer, but generally the following data are required: power rating, speed, anticipated misalignment, type of pump (reciprocating, vane, centrifugal, and shaft diameter of the driver and driven side of the coupling). Pumps of the same power range are very often driven by reciprocating engines (diesel, gasoline, natural gas). This is quite common in remote areas, such as at pipeline pumping stations, or agricultural watering systems (Figure 19), where a source of electric power is not available or is expensive. Because reciprocating engines of the diesel type produce a cyclic torsional vibration, it is often necessary to perform a torsional vibration analysis of the drive system to ensure that the normal operating speed is well removed from a speed that may produce a torsional resonance. Such an analysis requires that the torsional stiffness of the coupling be known. It is quite often possible to tune the drive system to avoid operating at a resonant condition by selecting the proper coupling stiffness. The selection data required for a system of this type include rotating inertias and connecting shaft stiffness. Most coupling manufacturers will, however, assign a higher service factor to an application involving a reciprocating prime mover, to compensate for fatigue effects due to torque fluctuations, particularly if a torsional analysis is not completed. If a diesel engine is used, a torsional analysis should be performed, as oversize couplings may be so stiff as to raise the torsional resonance right into the operating or idle speed. In addition, the remote location of many engine-driven pumps indicates a special need to ensure a high degree of reliability of the system. Another commonly employed prime mover is a steam turbine. These machines, which range from about 100 hp (75 kW) to well over 50,000 hp (37,000 kW) for pump drives, operate very efficiently and economically, providing there is a source of steam available at the installation. Any flexible coupling employed on a machine driven by a steam turbine must be capable of accepting the thermal gradient at the turbine shaft and must also accommodate the axial growths of the turbine shaft as it warms up to operating speed.  FIGURE 19 Flywheel coupling for vertical pump (Lovejoy Inc) Steam turbines are generally high-speed machines (4,000 to as high as 10,000 to 15,000 rpm in some cases) and as such require a relatively high degree of system balance to avoid critical vibration. Gas turbines could also be the prime mover, but they are seldom found on pumps drives. Elastomer couplings have occasionally been applied successfully to steam turbine drives, but because of the high speeds involved, all-metal couplings are most commonly employed. Disc or diaphragm coupling designs that are light, flexible, and do not require lubrication are the most popular for this application. High-speed machinery requires that the weight of rotating components be minimized to decrease shaft deflections and hence increase the lateral critical speed of the system. The gear coupling is an efficient design for transmitting large amounts of power at high speeds and with minimum weight and with axial slide capability, but the requirement for a lubrication system makes the gear coupling less desirable, and it is no longer the most common choice. Where coupling weight and torsional stiffness are critical values to the overall system, special designs may be created that provide the specific values required for satisfactory system operation. Special designs can be disc, diaphragm or, in some cases, gear couplings. NONSPARKING AND GALVANIC CURRENT Some applications require a coupling that will not cause a spark or a coupling that will isolate a galvanic current. Usually these couplings use elastomeric flexing elements and require that both sides of the coupling be grounded. Coupling manufacturers have their own design features to make the couplings conform. Most nonsparking couplings have shorter maintenance review cycles and elastomer replacement cycles to assure the elastomer never wears out and allows a spark from metal to metal contact. The couplings could be fail safe or fusible-link type. Coupling manufacturers can offer ex-couplings for ATEX applications. BALANCE For higher speed, higher power applications, coupling balance (or potential unbalance) is an important factor to consider. Elastomeric couplings may have a considerable amount of potential unbalance because of their construction, and they don't lend themselves to balancing. The amount of inherent unbalance in metal couplings is controlled largely by the level of precision to which they are manufactured. When pumps operate at speeds exceeding four-pole motor speeds and low equipment vibration levels are critical to service life, elastomeric couplings may not be a good choice. Such requirements are important to certain pump types, like those required for API Standard 610.1 For more information on coupling manufacturing and balance classes and requirements, the reader is referred to the API and AGMA standards listed at the end of this section. LIMITED END FLOAT Many horizontal motor-driven pump systems utilize motors that are equipped with journal, or sleeve-type, bearings. These bearings are intended only to absorb the transient thrust created by the motor rotor during acceleration and deceleration. The coupling for this type of drive should be equipped with suitable provisions for limiting the axial float of the motor rotor to some fraction of its total float. This may be done by positioning the motor in the center of its axial travel and then employing a coupling having a total float that is less than the float of the motor. Any motor thrust is taken by the pump bearing. Gear coupling total float can be limited by inserting a button between shaft ends, as shown in Figure 20. This type of coupling prevents the motor rotor from ever contacting the thrust shoulders on the shaft bearings. Certain types of elastomer and disc couplings having inherent float-restricting characteristics provide centering without any additional modifications (Figures 21 and 22). VERTICAL OPERATIONS As previously noted, rigid couplings are commonly used on vertical-drive systems where the system characteristics warrant such a coupling. However, many vertical-drive systems require a flexible coupling to accommodate shaft misalignment. It is generally possible to use a nonlubricated coupling, such as one of the many elastomer designs, in a vertical position without modification, provided the shafts are supported in their own bearings and the coupling does not have to transmit a thrust force. Lubricated designs, such as the gear and spring-grid types, usually require some modification to make certain that lubricant is retained in both halves of the coupling. |
