How to Optimize Coupling Performance

Couplings are critical components in many industrial machines and machine systems. But, as these machine systems become more advanced, they put even greater strain on couplings. That’s why operators need to understand how to properly maintain these components to ensure they perform at their best.

This two-part tip series will discuss how couplings work, common coupling operating challenges, and lubrication best practices to maximize productivity.

First, what is a coupling?

A coupling is a mechanical device that connects two pieces of rotating machinery, allowing for power transmission and a certain amount of shaft deflection between the driven and driving machine.

In general, there are two major classifications of couplings: rigid and flexible. Rigid couplings are comprised of two aligned shafts as they allow for no misalignment, while flexible couplings accommodate some misalignment between the driving and driven shafts.

Making things a bit more complicated is the fact that flexible couplings can also be further divided into two classes; lubricated and non-lubricated.

A non-lubricated flexible coupling uses plastic, rubber or metallic hubs between the mating surfaces to absorb misalignment. A lubricated flexible coupling uses sliding motion between two contacting surfaces to handle misalignment.

The two most common types of lubricated flexible couplings are gear couplings and grid couplings – shown below. We will focus the rest of this tip series on these two types.

How do these couplings work?

Gear couplings operate by engaging the hub’s external teeth with the sleeve’s internal teeth. The teeth mesh at the outer circumference of the coupling inner cavity. Clearances and profile between the internal and external teeth allow the teeth to slide and pivot in relation to one another while transmitting torque smoothly. Lubrication is required to protect against this sliding motion.

Grid couplings operate in a manner similar to gear couplings, but they employ a spring steel grid pre-formed to snake back-and-forth between two shaft mounted hubs, nesting in slots formed around the external circumference of each hub. The coupling flexes by allowing the grid to slide along the slots in the hubs. These couplings require lubrication to protect the sliding surfaces from wear.

Misalignment and wear – two common operating challenges

A coupling designed to accommodate some level of misalignment, which occurs when two shafts are not in line with each other.

There are three types of misalignment:

  • Offset: shafts are parallel but not in line.
  • Angular: shafts not centered but parallel.
  • Axial: where the shafts move towards each other during operation due to float or expansion.

Gear couplings can handle misalignment of about 0.01-0.02 inch in parallel and 2 degrees in angular. Grid couplings may tolerate parallel misalignment up to 0.30 inch and angular misalignment of about ¼ degree.

If the coupling is offset and or angularly misaligned, sliding motion will be introduced, causing the face of the mating surface to wear away. This is because the external shaft gear teeth will slide across internal sleeve teeth as the coupling makes a revolution and then back again as it completes a full rotation. Sliding wear can be destructive often leading to premature failure of the coupling.

Our next post in this two-part series will focus on lubrication best practices. In the meantime, if you have any questions or comments, leave a note in the section below!