A flexible coupling is sometimes specified late in a pump project. Yet, the elastomeric material that links the hubs can determine whether motor bearings reach their expected service life or have an unnecessarily short life. Understanding how different elastomers behave under load, temperature and chemical exposure enables engineers to pick a coupling that actively defends motor and pump reliability, rather than merely transmitting torque.
Thermoset vs. Thermoplastic: Two Roads, Many Compounds
Elastomers fall into two broad camps: thermoset and thermoplastic.
- Thermosets experience cross-linking during the curing process, making the final product permanent or “set.” They offer a combination of flexibility and high shock damping. Common thermosets include natural rubber, nitrile, neoprene and ethylene propylene diene monomer (EPDM).
- Thermoplastic materials use a similar curing process; however, they do not experience cross-linking and can be melted down and remolded. These elastomers offer excellent power density and, as a result, higher stiffness. Typical examples of thermoplastics are polyurethane, thermoplastic polyester elastomer (TPEE) and some specialty nylons.
Misalignment Loads: Align First, Then Let the Coupling Work
Best practice is still laser alignment at installation; however, thermal growth, pipe strain and baseplate settling increase the likelihood that some misalignment will occur over time. When shafts are offset, the coupling’s bending stiffness converts angular error into a reaction force that is transmitted straight into the nearest bearings. Laboratory tests on tire-style couplings show that a polyurethane element can generate, on average, 2.55x the reaction force of a geometrically similar natural-rubber element under the same 4 degrees of angular deflection. The extra load multiplies at the bearing centerline, accelerating the fatigue of either the pump or motor bearings.
An example from a 5-horsepower (hp) centrifugal pump that is coupled to a motor with a split-tire style coupling illustrates the point. With 4 degrees of misalignment, the inboard pump bearing sees its reactionary load increase by 1.52x when a soft natural-rubber coupling is used and 3.88x when using a stiffer polyurethane version of the same coupling style. L10 bearing life calculations predict roughly a 16-fold bearing life advantage for the softer element under those conditions. The message is not “skip alignment” but rather that the elastomer is the last line of defense when the real-world shifts machinery out of specification.
Damping, Conductivity & Safety Margins
Natural rubber’s low glass-transition temperature and internal damping help quench torsional oscillations that can excite impeller vane-pass frequencies or motor system resonant frequencies. Polyurethane offers higher torsional stiffness, which can be helpful where positioning accuracy matters (e.g., metering pumps, positive-displacement units) but can transmit more vibration downstream.
Static electricity is another issue to consider when selecting a coupling. Natural rubber compounds are inherently conductive enough to bleed charge across the coupling, providing a redundant ground path. Most polyurethane grades are orders of magnitude more resistive, potentially allowing static to arc through bearings or mechanical seals.
Other elastomers in the toolbox
- Nitrile (NBR): Good oil resistance and moderate damping, upper temperature approximately 212 F
- Neoprene (CR): Balanced chemical resistance, ozone tolerance; slightly higher stiffness than nitrile
- EPDM: Excellent hot-water and steam resistance, poor with hydrocarbons
- TPEEs: Wide temperature span and high torque capacity, but stiffness approaches that of rigid metallic designs
Each material family can be tuned with fillers and plasticizers, so published ranges vary. Always consult compound-specific data for critical applications.
A practical checklist:
- Define the environment first: Temperature spikes and chemical splash often eliminate half the material list before torque is even calculated.
- Check misalignment reaction force, not just angular “allowance”: Two couplings may both list 4 degrees of capability yet differ 2x or more in bearing load.
- Balance torsional stiffness with system dynamics: Softer elements damp vibration and protect seals; stiffer ones control positioning.
- Consider static-conductive paths: Where variable frequency drive (VFD) grounding is questionable, a conductive elastomer can prevent electrical pitting of bearings.
- Plan for maintenance access: Radially split tire-style couplings allow for removal without moving hubs, minimizing mean-time-to-repair and preserving shaft alignment.
Elastomer selection is not a footnote to coupling size charts; it is a design decision that ripples through bearing life, vibration behavior and total ownership cost. For most low- to medium-power centrifugal pump sets, a soft thermoset rubber remains the best all-around guardian of motor and pump health, provided the temperature and chemicals are compatible. Harder thermoplastic urethanes and TPEEs earn their place where torque density or high positioning accuracy outweighs the penalty of higher reaction loads.
Choosing wisely and aligning carefully allows the coupling to fade into the background, while the pump focuses on what truly matters: the process fluid.
