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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Bearing forces. Coupling reaction forces caused by misalignment can vary greatly depending on the coupling type. A stiffer coupling, Table 2, imposes larger forces on the pump and motor bearings for a given amount of misalignment between shafts. And, these larger forces shorten pump and motor bearing life. This aspect of coupling selection is particularly important for pumps with lightweight rotors, where coupling forces can be much larger than the rotor’s weight. Here, the larger coupling forces can lift the rotor off the bearing, causing instability that leads to vibration.
For couplings with two flexible elements (called double-engagement couplings), reaction forces depend more on the distance between elements than on their stiffness. Examples of such couplings are the gear type and the disk pack.
Coupling size
The size of a coupling must be selected so that a satisfactory service life is obtained without paying a penalty in dimensions and price. Factors that determine coupling size are power and speed. Selection can also be made based on torque (or horsepower per hundreds of rpm), where the operating speed is smaller than the maximum rated speed of the size selected.
Torque (or hp/100 rpm) is determined from the pump demand, not the motor’s name plate rating. For example, if a pump rated at 18 hp is driven by a 20-hp motor, the pump rating (18 hp) is used in selecting the coupling size (Torque = Pump hp × 63,025/rpm, lb-in.).
Once the torque is determined, the influence of torque fluctuations must be assessed. Torque fluctuations are caused either by nonuniform pump requirements or nonuniform power delivery. For example, a centrifugal pump has an essentially uniform torque requirement, whereas a reciprocating pump requires a large torque when the piston is about half-way in its stroke, and practically no torque at either end of the stroke. Vanetype pumps also have torque fluctuations, but these fluctuations are much smaller than for reciprocating pumps.
Electric motors deliver an essentially uniform torque, whereas piston engines exhibit a rough torque delivery, especially when the number of cylinders is small.
These torque fluctuations cause coupling stresses, which require derating the coupling from the catalog values. This derating factor, known as the service factor, often causes the selection of a coupling three times larger than if only catalog ratings were used.
Service factors
In selecting couplings for applications where torque fluctuates cyclically, Figure 3, service factors are an important consideration. Here, couplings must be selected for the maximum torque that occurs during one cycle. As this value is seldom known, coupling manufacturers use previous experience to establish a ratio between the maximum and average torque. This ratio is called a service factor.
The maximum torque during each cycle should not be confused with the peak torque, a larger-than-normal torque that occurs only occasionally. Typically, peak torque occurs at pump start-up when pumps must deliver pressure and flow while accelerating their own mass and the mass of the column of liquid they process.
Peak torque is usually determined by the maximum torque that the driving machine can deliver. For example, induction motors can deliver about three times more torque at start-up than their normal rating.
To illustrate the use of a service factor, consider a reciprocating pump rated at 38 hp that is driven by an electric motor rated for 40 hp @1,800 rpm. Assuming that the coupling type has a service factor of 2, the coupling size should have a rating of 38 × 2/18 = 4.222 hp/100 rpm.
Note that in selecting the coupling size, the power consumed, by the pump (38 hp), and not the power from the driving machine, was used in the calculation.
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Service factors vary widely among couplings with elastomeric elements because of different damping characteristics for rubber and urethane elastomers. When these elastomers are subjected to continuous flexing, they absorb part of the energy transmitted through the coupling. The energy absorbed (damping) is transformed into heat, raising the coupling temperature, which softens and weakens the elastomer. The amount of heat absorbed depends on the amount of torque fluctuation, operating speed, and type of elastomer. Without the cooling provided by windage (caused by rotation), elastomeric elements can become very hot and their strength diminish (in one case the elements actually melted because the coupling guard did not permit any air circulation).
Generally, urethane has a larger damping coefficient than rubber, and thus it absorbs more energy under the same operating conditions. Because urethane has more damping, manufacturers recommend a larger service factor than those used for rubber couplings, Table 3.
Service factors are also affected when the torque generated by the driving machine is cyclic. This is the case with reciprocating engines. Here, the maximum torque transmitted through a coupling during one cycle depends on the torque variation of both the driving and driven machines. Most coupling manufacturers list two service factors, one for the driver and one for the driven machine. These factors are added to obtain the total service factor value.
As an example, a reciprocating pump with a service factor of 2 is driven by a four-cylinder gasoline engine with a service factor of 1.5. Here, the coupling selected should have a service factor of 2 + 1.5 = 3.5. Typical service factors for driving machines are shown in Table 4.
Some manufacturers recommend a service factor larger than 1 for even the best possible conditions. When this is so, the published coupling ratings can not be used at face value because they must always be reduced by applying a service factor. It is important to remember that:
• When the torque consumed is constant over time, the service factor for the driven machine is 1.
• When the torque generated is constant over time, the service factor for the driving machine is 0.
Service factor standardsTo help with coupling selection, manufacturers publish long lists of service factors. Because each manufacturer has its own particular list of service factors, the American Gear Manufacturers Association (AGMA) attempted to make them uniform by publishing a standard titled “Load Classification and Service Factors” (No. 514.01) in 1968. With the advent of elastomer couplings, however, it was found that various materials react differently to torque fluctuations, and that service factors must therefore, be a function of coupling material as well as application. AGMA’s standard has since been withdrawn; only manufacturers data should be used. |
Michael M. Calistrat is a consultant on power transmission design and failure analysis and owner of Michael Calistrat & Associates, Missouri City, Texas.