Coupling selection is one of the most critical parts when selecting a new pump. As with any other component on a pump, selecting the proper couplings for the application can have a positive or negative effect on pump life and performance. These different types of couplings are an important part of keeping pumping equipment running efficiently. In this article, we will discuss the various types of pump couplings.
What are the Different Types of Pump Couplings? There are three types: Rigid couplings, Elastic couplings, Magnetic couplings.
Rigid couplings
As the name suggests, rigid couplings connect the pump and motor shafts together very firmly, which allows them to transmit more power to the pump than a flexible coupling of the same size. Rigid couplings, however, cannot handle any shaft misalignment, have essentially zero tolerance for axial, angular or radial movement of the shaft, and cannot absorb any vibrations from pump operation or thermal expansion. Common rigid couplings include clamp couplings, flange couplings and sleeve/split half sleeve couplings.
They are commonly used for:
1) Those occasions that do not require precise alignment through the couplings, such as the shaft has been precisely positioned in perfect radial and angular alignment during installation
2) When the pump itself is not equipped with a thrust bearing and its axial thrust is borne by the motor, rigid couplings (such as many vertical axials flow pumps and mixed flow pumps, flange couplings) are required.
3) Cases that do not require alignment or light and medium loads (such as VS6 pumps composed of multiple shafts, connections between shafts and shafts inside the pump, sleeve couplings). Rigid couplings have a very high probability of failure if they are not truly centered, and there is no flexibility in this type of coupling. Rigid coupling's structure is relatively simple, the price is relatively cheap.
Rigid couplings:
ü Capable of withstanding huge thrust loads, ideal for heavy duty pumps
ü Reduce maintenance time
ü Improve the overall performance of the system
Elastic couplings
Unlike rigid couplings, elastic couplings (also known as flexible couplings) can accommodate some movements, such as vibration and thermal expansion, without damage. This makes them ideal for applications where there is some unavoidable misalignment between the drive shaft and the pump shaft. Since they absorb vibrations so well, they can also be used to reduce excessive operating noise. With these benefits, the design is relatively complex and relatively expensive. However, since elastic couplings protect shafts and bearings from damage and premature wear, they save maintenance costs in the long run. Below are four of the more common types of flexible couplings, each better suited for different applications.
Gear couplings are an older design that are no longer as popular as their counterparts, but can still be found in some larger pumps. Gear couplings have the advantage of being mechanically flexible, which means that there can be some degree of misalignment between the pump pump and motor shafts.
This type of coupling consists of a sleeve containing internal teeth that mesh with teeth on the outside of the hub. The fit of the teeth in a gear coupling allows for misalignment, but the degree to which angular misalignment is compensated depends on the profile of the gear teeth and the clearance between them. Due to their design, these torsionally rigid couplings can take some shock loads, but not a lot of them.
Gear coupling:
ü Suitable for applications requiring torsional stiffness
ü Suitable for high speed, high power and high torque, low speed applications
ü Lubrication is required
ü Periodic inspection is required (the pump must be turned off)
ü Requires a lot of maintenance and upkeep
ü Not suitable for high temperature applications, the maximum temperature limit is 350 °F (176.7 °C)
ü Prone to axial misalignment faults
ü No warning in case of failure
ü Life expectancy 3 to 5 years
Grid couplings enable flexibility and efficiency at high speeds or low speeds and high torques. This coupling consists of two grooved flanged hubs connected by conical steel springs in a grid pattern. Grid couplings are most likely to be found on large pumps with large motors and high torque applications.
Their misalignment capability is not as good as some common flexible couplings, so the alignment of the pump shaft is more critical. Replacing this type of coupling requires moving equipment, replacing and realigning. This type of pump coupling requires more maintenance time and production time than some common elastic couplings.
Grid coupling:
ü Transmission of high torque via spring set
ü Can run at up to 400 hp/100 rpm
ü Regular lubrication is required to avoid wear, which means shutting down the pump and removing the couplings.
ü Reduce vibration and cushion shock loads
Disc couplings are designed to be extremely uniform and allow high speed rotation. Pumps running at high RPM are good choices for these well-balanced, smooth-running disc couplings. This coupling design is complex and sensitive to misalignment and axial movement. They are ideal for medium to high torque applications in the marine, power generation, API and critical process industries. This type of coupling is best suited for applications where a certain amount of torsional stiffness is required while allowing for some misalignment.
The disc coupling can withstand higher torque than some ordinary elastic couplings, and of course the price is relatively expensive. Should these couplings require repair, they can be rebuilt on site without removing the hub. Rebuilds can often be accomplished with less than 20% replacement.
Disc Coupling:
ü Capable of operating at high temperatures up to 750 °F (398.9 °C)
ü No lubrication required
ü Requires minimal maintenance
ü Can be checked at runtime
ü Life expectancy is about 20 years
Elastomeric couplings are among the most commonly used couplings today. They are found on almost all small to medium pumps due to their quick and easy installation and ability to absorb all types of shocks, misalignments and end float. Shear jaw and bonded tire type couplings are examples of elastomeric types of pump couplings.
They consist of two metal hubs attached to the motor and pump shaft, with hard rubber between them. Depending on the distance, a spacer shaft (intermediate extension) design is sometimes required. Spacer shafts should be used if there is a large distance between the shaft ends.
Often, the only component that fails is the insert, which is relatively inexpensive to replace. Elastomer couplings are the best choice for many applications:
ü No need for maintenance and lubrication
ü Not suitable for high torque applications
ü Sensitive to chemicals and high temperature
For centrifugal pumps, the most common, simplest, and most used elastic couplings are metal diaphragm couplings and elastic pin couplings - on the basis of rigid flange couplings, metal diaphragms or the elastic pin becomes an elastic coupling.
Magnetic couplings
Magnetic couplings differ from rigid and elastic couplings because the motor is not a mechanical shaft, but is connected to the pump by magnets. Like all other couplings, this coupling is designed to transfer torque from one shaft to another. However, what makes magnetic couplings special is that they do not have a physical mechanical connection. This makes them suitable for fluid pumping applications, as connections can be made through thin barriers, which help keep the rotating channel sealed.
With magnetic coupling pumps, there is a small air gap between the magnets and the motor and pump. Since there is no physical contact, misalignment and thermal expansion are not issues. The absence of mechanical seals also eliminates the possibility of leakage, making mag drive pumps ideal for handling corrosive or hazardous fluids.
Magnetic couplings are only suitable for cleaning liquids as the presence of any particles can cause problems as they will stick to the magnets. They are also sensitive to extreme operating conditions; if the torque exceeds the capacity of the magnetic coupling, the magnetic element may disengage, the pump shaft will slip and cause a shutdown. Therefore, when selecting a magnetic coupling, it is necessary to ensure that it is consistent with the power of the system.
Some notes about magnetic couplings:
1) Can be used to transmit power to a centrifugal pump
2) Energy loss due to magnetic resistance at the couplings interface
3) Provides thermal isolation so no heat transfer occurs
4) No contact parts in the couplings
5) Has a built-in safety feature (in the event of a couplings overload, it will move to the next position and keep going)
6) When heavy loads or load spikes occur, the overload protection of the couplings can be dangerous
7) Due to the use of permanent magnets, no external power supply is required
8) Can handle only light torque loads and applications with slow starts or very low moments of inertia
9) Large diameter, allowing for relatively light torque loads
10) Moderate radial loads on support bearings
There are many factors to evaluate when selecting a coupling. In addition to application conditions, system designers and users should also consider other factors, including duty cycle, environmental conditions, maintenance level, total cost and expected lifetime. Familiarity with and availability of a given coupling may also influence your final decision.