To properly select couplings for use with a pump, a user must carefully study the pump, plus its application and maintenance requirements, then match the coupling to the application. Although this process takes time, it avoids many maintenance problems and helps the pump to provide long and satisfactory service life.

Pump application conditions affect both the type and size of shaft couplings required. When selecting a coupling type, the engineer must consider a number of factors, such as ease of pump maintenance, ease of replacing the coupling’s flexible elements, plant maintenance constraints, and the forces that the coupling imposes on bearings when misaligned.

When selecting a coupling size, the engineer must consider the power transmitted, operating speed, and uniformity of torque transmission. The last requirement, torque uniformity, is sometimes overlooked, leading to the selection of a coupling that is too small and will therefore cause frequent problems.

Coupling type

Before sizing a flexible coupling, a user should first decide what type of coupling is best for the particular pump and plant conditions. Table 1, which summarizes the features of typical coupling types, can be used as a guide.

The same pump and coupling combination can give satisfactory service in one plant and trouble in another. The reasons for this discrepancy are found in the following criteria for selecting a coupling type:
• Ease of flexible-element replacement.
• Maintenance constraints.
• Forces that couplings impose on pump bearings.

Flexible-element replacement. All flexible couplings require periodic maintenance, which means replacing either the entire coupling or just its flexible elements. The useful life of these metal or elastomeric elements depends greatly on the amount of misalignment, and on environmental conditions. For example, sunlight can cause deterioration of rubber elements, and polluted rain water can harm thin metallic elements.

For some couplings, Figure 1, the replacement of flexible elements requires moving the coupling hubs along the shaft or moving the pump (or motor) on its base. For others it may require neither.

One type of coupling has been developed especially for pump applications, Figure 2. With minor variations, it is available from a number of manufacturers. This type has two main advantages:
• Can be “dropped out” for easy replacement.
• Provides space between shafts, so that a pump shaft assembly can be removed without moving the motor.

In some cases, continued operation of a pump may be very important for safety or for the productivity of a plant. For example, failure of a coupling for a boiler-feed pump is more critical than for an irrigation pump. In such critical applications, a coupling that is easy to service generally requires less downtime and it runs longer. However, there are exceptions. A coupling that is easy to service may require frequent maintenance; and a coupling that is difficult to service may work without problems for many years.

Maintenance constraints. In some plants, coupling lubrication presents no problem. But, lubricated couplings have a basic requirement for lubrication that adversely affects operation in other plants: the machine must be stopped! Accordingly, pumps that must operate a long time without interruption make coupling lubrication very difficult. On the other hand, the advent of special coupling greases allows operation without relubrication for as long as three years.

One advantage of most nonlubricated couplings is that the condition of the flexible element can be determined while the machines are running. To accomplish this on-the-run inspection, the coupling must be visible (either through perforated or expanded metal coupling guards), and the inspector must have a strobe-light that can be synchronized with the coupling rotation.

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