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:
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!
Eduardo - coupling inspections can be very interestng. You need to examine all factors affecting the coupling. 1. Current greasing procedures. 2. Environmental conditions. Note any leakage around the coupling gasket or o-rings. 3. Disassemlbe the coupling and look at the mating parts. Check for sliding wear. Compare to a new coupling if one is available. 4. Note the grease in the coupling, is there enough, etc. Good luck and post pictures when you get them.
Arturo - From your question, there appears to be an issue with a gear coupling in a steel mill where the Kopflex representative blames the abrasive moly in EP 111. First - I do not know if EP 111 contains abrasive moly. The forms of moly has changed over the years and now many types are dissolved and not dispersed. Check with your XOM representative to see the current form of moly in EP 111. Now for the coupling issue - it is easy for the OEM to blame the grease as it shifts the burden away from his coupling. I have seen gear couplings fail and I never considered abrasive moly the cause. I did consider lack of lubrication and excessive sliding forces. The only way to tell for sure is to post pictures of the failed coupling so that we may do a proper failure analysis.
Soon i will be conducting an inspection of Couplings in water pumps of a mine where it was recommended to use the Mobil Grease XTC, until now has reached to increase the useful life of the Couplings 3 times more than normal with respect to the grease used previously in these elements, to carry out this inspection, temperature readings, inspections Visuals and even review the lineup, Rick Russo, do you have any recommendation to perform my inspection in a better way?
Hi Rick, we arose a claim with the Mobilux EP 111 used in spindles brand Kopflex inside a hot mill as they showed wear and therefore a failure. Consultando con el usuario, nos comento lo siguiente de acuerdo a un correo del proveedor: "in the steel mills the construction of the spindles and couplings are different. Te coupling are usually carbon to alloy with no surface treatment whereas the gear spindles are surface hardened, induction, nitrided or carburized. Gear spindles generally work well with special grease with moly-disulfide additives. These additives can present problems when used in the gear couplings. The softer teeth can prematurely wear due to ths additive.
Of course, after this comment the provider recommended to the Customer the grease Syn-Tech of Kopflex.
Thanks for the clarification Rick Russo, about this doubt i had with the selection of Couplings for axes, greetings.
While the diameter of the shafts may be a consideration, the primary criteria to select a coupling would be the type of start (hard or soft), the amount of deflection needed and the load to be transmitted.
I have a question. Rick Russo. Can a coupling be selected according to the hole diameters? Thank you in advance for your answer. Yours sincerely,