Evaluate elastomeric and metallic couplings for an application.
by Vance Chavis & Stefanie Casimir
September 13, 2019

A municipality will use various applications throughout the wastewater treatment process—from headworks, to primary and secondary treatment, to sludge processing and general pumping. In order to operate, each process requires a power source. How users choose to connect those applications can vary based on preference and operating conditions.

Selecting the right coupling type for an application can increase equipment performance and reliability, reduce maintenance costs and minimize downtime.

equipment lifeImage 1. The material used can extend the equipment life in tough applications such as wastewater. (Images courtesy of ABB)

What Are Couplings & How Are They Used?

Couplings are designed to connect two shafts and transmit torque in a rotary motion from one piece of equipment to another. Based on the wide variety of styles, couplings can perform secondary functions such as accommodating misalignment between the driving and driven shafts, vibration dampening, absorption of shock loads, allowing for soft start applications, and protecting equipment from torque overload. Some variations offer spacers for applications where equipment cannot be moved.

Couplings are categorized in two groups usually referred to as elastomeric and metallic, and further categorized as flexible and rigid.

All elastomeric-style couplings are flexible to some degree. This coupling style typically uses a rubber, urethane or other composite material to transmit torque. The advantage of elastomeric-style couplings is that they can handle high vibration and shock load dampening. This makes elastomeric couplings popular in pumping, compressor and agitator applications where high vibration is common. Their misalignment capabilities make them viable for applications that are not capable of precision alignment. They are designed to align easily and require no lubrication for a maintenance-free design. They also tend to be less costly.

pump skidImage 2. Example of a pump skid, which has a motor (driver), coupling and pump (driven equipment) commonly found in wastewater treatment facilities.

Elastomeric-style couplings can be categorized into two types: compression or shear. Couplings that operate in compression are also a fail-safe design, where even though the element should fail, the hubs come into contact and can continue to transmit torque (example: jaw couplings). Couplings that operate in shear will not transmit torque once the element fails.

Elastomeric Coupling Material: Urethane vs. Rubber

One concern in a wastewater treatment environment is water. There can be a lot of water ingress and exposure that affects how a coupling operates. Elastomeric-style couplings are commonly found in many wastewater applications. Understanding the material used can extend the equipment life in these tough applications. Urethane, for instance, tends to absorb water while natural rubber repels it.

This impacts performance of coupling elements that are used in a coupling assembly.

coupling stylesImage 3. The breakdown of elastomeric and metallic coupling styles
elastomeric styleImage 4. Elastomeric-style couplings use rubber, urethane, Hytrel or other nonmetallic components.

Hysteresis is a disadvantage found in urethane and rubber couplings. Hysteresis in urethane occurs when the material absorbs torsional vibration from high-frequency pulsations. In dampening these vibrations, it converts the energy to heat. If the frequency is high enough, the urethane cannot dissipate the heat fast enough, and the urethane temperature continues to rise until it melts. Hysteresis also occurs in rubber, but due to its chemical makeup, higher levels of conductivity and temperature capabilities, it is more efficient at dissipating heat. When the effects of vibration are high enough, however, rubber will begin to locally revert toward its initial natural properties prior to vulcanization. The result is a soft and/or tacky rubber material.1

Other considerations in material could include the hub and shaft material being used; some may specify stainless steel to minimize rusting and corrosion. This will make it easier to replace or conduct maintenance on couplings.