All Couplings Are Not Created Equal

Flexible shaft couplings connect rotating machines, transferring power from a motor or engine to a driven machine such as a gearbox, pump or generator, within a drive train system. In this case, the coupling was installed on a vertical disc aerator for an effluent reservoir.

One of the primary functions of a flexible shaft coupling is to compensate for the inevitable misalignment within any coaxially coupled shaft arrangement. If the misalignment isn’t dealt with then noise, vibration, power loss, or bearing, seal and component wear, can result. In the worst cases, shafts can even fail through fatigue.

A coupling should also act as a “fuse” within the drive train system – protecting connected machinery from mechanical overloads, which could otherwise cause damage ranging from costly to catastrophic.
That’s why it’s important for a coupling supplier to carefully consider all the application parameters, and to have a thorough understanding of the end-user’s needs, before the coupling choice is made.

Too much choice?

There are two very different types of flexible shaft couplings. 

Mechanical Flexing shafts – such as Gear or Grid designs – permit movement or flexing within the coupling assembly, by incorporating increased clearances within the components. Material Flexing designs permit movement by incorporating a pliable, flexible element into the design. 

There are also two types of Material Flexing couplings: Elastomeric and Metallic. 

Mechanical Flexing designs tend to be torsionally stiffer and extremely robust, and are often deployed for larger shaft diameter installations. Material Flexing designs are torsionally softer, which can introduce a damping mechanism into the drive train system, to absorb moderate vibration and shock loads. 

The importance of choosing the correct option was highlighted when ERIKS were approached by the customer from the water treatment plant, who had been experiencing repeated flexible coupling failures. 

Right coupling, wrong place

The size of the customer’s installation means that the machine rotor has to be split into sections, connected by flexible couplings. This allows a single drive source to be employed for each rotor assembly. 

The original installation incorporated a coupling design known as a toothed or gear elastomer in shear. This is essentially an elastomeric element held between two drive hubs, with the torque transferred by
a series of mating teeth. Replacement of the element – the most likely component to wear – simply requires sliding the hubs along the shafts to release it. 

However, in this particular operating environment, that wasn’t simple at all.

The coupling is situated between the rotor sections in an exposed location, prone to the build-up of contaminating environmental residue. This collects on exposed areas of the shafts, and prevents the coupling hubs from sliding along them for maintenance. Additionally, the manufacturer had left the
coupling element mostly exposed, in an attempt to make inspection easier. But all it achieved was the need for additional regular cleaning of the toothed profile.

All these complications arose from a basic lack of understanding of the operating environment. The coupling design wasn’t at fault. It was simply the wrong coupling for the application.

Quicker, easier, safer

The Wrapflex is another elastomer in shear design, but with a “replace-in-place” wraparound element feature, which improves on the design originally installed. 

Because the hubs don’t need to be moved to replace the elastomeric element, it’s now a quick and effortless maintenance procedure. In addition, the element is covered and secured in place by a retaining ring, which largely protects it from contaminants.

The location of the coupling – in a cramped space, precariously close to the waterline of the effluent reservoir – made technicians’ safety an important consideration. 

Guard-rails and other safety features are in place, but tackling time-consuming maintenance operations in these conditions is obviously potentially hazardous. So reducing the maintenance time, thanks to the
flexible wrap element, will also reduce the risks involved and optimise safety. 

Which makes choosing the right coupling straightforward after all. It’s the Rexnord Wrapflex.