Elastomeric Coupling Solutions for Pump Applications
While much emphasis is placed on the selection of motors and pumps, the importance of selecting the ideal coupling that connects these two expensive components is often underestimated.
Also, proper procedure for installation and alignment of the selected coupling is frequently overlooked. Faulty installation and/or uncontrolled misalignment can cause premature wear and potential failure of the coupling, as well as possible damage to the pump and/or motor.
A coupling’s basic function is to transmit power, accommodate misalignment and compensate for axial movement (end movement of shafts). Sometimes, a coupling is expected to absorb shock or vibration. Choosing the right coupling depends on four basic conditions of shaft misalignment or movement:
Parallel misalignment occurs when the two shafts do not share the same rotation axis. Their end faces may be parallel, but their center axes are laterally displaced in respect to each other.
Angular misalignment applies when shafts are neither coaxial nor parallel. The angle at which the shafts are misaligned may be symmetrical or asymmetrical.
End float occurs when either or both shafts display axial movement, moving in and out. A sleeve-bearing motor, for example, “floats” as the rotor hunts for the magnetic center of the winding. Temperature variation can also cause thermal expansion and variation in position of the shafts.
Torsional flexibility is the torsional movement in planes perpendicular to the shaft axis. Shock or vibration typically causes this. A torsionally flexible coupling absorbs and dampens these movements.1
To be considered flexible, a coupling must handle parallel and angular misalignment. Couplings with four-way flexibility accommodate both end float and torsional movement.1 The following overview of the various coupling types available reveals the pros and cons of each.
Couplings for High Speed, High Torque Applications
While elastomeric couplings are commonly used for numerous general industrial pump applications running up to 115 horsepower (hp)/100 revolutions per minute (rpm), elastomeric material limits are surpassed in applications that need transmission of higher torques and speeds.
Gear couplings are excellent performers in higher torque applications. However, these stiffer couplings do not provide much torsional dampening, which results in more vibration getting transmitted from the motor to the pump (Image 2). Grid couplings are torsionally flexible and provide some torsional dampening.
Disc couplings can rotate at high speeds, making them a good choice for pumps operating at a high rpm. However, these couplings only accommodate small amounts of axial misalignment. Flexing of a disc or metallic component beyond its yield point can cause fatigue, and extensive axial movement can cause failure .
Elastomeric Coupling Choices
There are three basic types of elastomeric couplings: tire-type, sleeve-type and jaw-type. Tire-type and sleeve-type couplings operate in shear (versus compression) to absorb shock and protect equipment. The elastomeric flexible member in these couplings can deform in shear, displacing under a load. The amount of torsional displacement or elastomer windup is a measure of the shock that can be absorbed. The element also compresses easily and can accommodate end float.
The torsional flexibility allows these two elastomeric shear couplings to dampen the amplitude of vibrations, isolating one shaft from the effects of the other.
One of the biggest benefits of tire-type and sleeve-type elastomeric shear couplings is that they are designed to fail under excessive shock loads and disconnect the pump from the motor to protect the pump in the event of a lockup or any other condition creating excessive shock.
The cost savings and ability to better analyze failures can be invaluable. Depending on brand and environment, elastomers used in sleeve-type couplings can degrade from excessive changes in temperature and chemical exposure.
Tire-type couplings are popular for use on continuous-duty American National Standards Institute (ANSI) pumps in varying applications that require minimal vibration. For example, these work well with applications that require variable flow rates. As flow rate changes, the chances of vibration increase throughout the pump and is harder on bearings. The tire-type coupling acts as a dampener and handles vibration more consistently. The big advantage to tire-type couplings is that they can be easily replaced without having to move the pump and motor.
Sleeve-type couplings are a good choice for use with many general-purpose pumps, including centrifugal pumps used with variable frequency drives. These torsionally soft couplings are ideal for moderate misalignment applications. Typical sleeve coupling applications include pit pumps, vertical end suction centrifugal pumps, vertical turbine pumps and some horizontal split case pumps.
Economical elastomer sleeve couplings typically have a misalignment capability of 1/4 to 1 degree, while moderately priced elastomer tire couplings typically have a misalignment capability of 2 to 4 degrees.
Jaw-type couplings use elastomeric material in compression and do not have the same amount of misalignment capacity or axial and torsional dampening capabilities as in shear couplings. In compression, the rubber element is squeezed to transmit torque, rather than twisted. These couplings are generally used on lower power applications where cost is a primary selection criteria. Jaw-type couplings can be found on a wide range of pumping applications including viscous material gear, screw, progressive cavity and lobe positive displacement pumps .
A big advantage to jaw couplings is their fail-safe design. Equipment will continue to drive if the elastomer fails.
With coupling selection, pump users need to beware of the “more is better” trap. Oversizing a coupling can result in reduced flexibility or misalignment compensation, and a coupling that is too large can put added stresses on the pump and motor being coupled.
Conversely, a coupling with too much misalignment capability may be too soft or yielding, which can cause vibration or an unbalanced condition in rotation.
After a pump/motor has been installed, it is always a good rule of thumb to recheck the alignment in about six months. The pump/motor base may settle and could throw the pump out of alignment. It is important when choosing the right elastomeric coupling to first check the surrounding environment where the pump/motor will be operating. Question if it will be exposed to extreme variations in temperature.
Also, understand what type of torque (service factor) is required to pump the material from point A to point B. Lastly, it is important to consider if the pump will be under any pipe strain that could cause it to fall out of alignment over time.