Speaking from 30,000 feet, power transmission couplings are devices used to connect two shafts together, transmitting system torque from one shaft to the other. (When one shaft spins, the coupling's job is to make the other shaft spin.) Within the huge array of coupling solutions, couplings can be broadly broken down into two primary types: rigid (which we will briefly touch on) and flexible (which we will then dive into).

In contrast to rigid couplings, flexible couplings (the focus of this article) have an integrated flexing element or design component that allows for some degree of misalignment handling & management. Within the vast flexible coupling world, couplings can further be broken into two major sub-groups: elastomeric couplings and metallic couplings.
Elastomeric (flexible) couplings - Elastomeric couplings are couplings that include a flexing rubber or plastic element to both accommodate misalignment and dampen system vibrations. Within the elastomeric coupling subset, there are many style and designs. This post will focus in on and provide a quick cliff notes overview of the 3 major elastomeric flexible coupling product types: compression loaded, shear loaded, and torsional.
(Note: Lovejoy's "The Coupling Handbook" takes a much deeper dive into everything this blog post covers, and the Mechanical Power Transmission Association also has an excellent 7 page PDF document, title Elastomeric Coupling Primer, that would make for great follow-on reading as well.)





Selecting the proper torsional coupling is not a trivial task (generally involves a formal torsional vibration analysis), and it is highly recommended that you consult with a coupling manufacturer's staff prior to making your own product selection. (Lovejoy offers multiple types of torsional couplings, and can be interchangeable with specific other manufacturers.)
Metallic (flexible) couplings - Metallic couplings are different from elastomeric couplings in that they do not employee elastomeric (soft) materials to provide coupling flexibility & dampening. The breadth of metallic coupling offerings is massive (covered in depth in The Coupling Handbook), and this post will focus in on and provide a quick cliff notes overview of the 2 major metallic flexible coupling product types: lubricated and non-lubricated.
Lubricated metallic couplings achieve flexibility through loose fitting parts rolling or sliding against one another, while non-lubricated metallic couplings achieve flexibility through a flexing or bending of a metal component itself. Lubricated couplings are generally less expensive, but do require periodic maintenance/more lubrication, and will eventually "wear out". Non-lubricated are generally more expensive, require minimal maintenance, and categorizes as having theoretical "infinite life" (no metal on metal wearing parts).
(Note: While the Lovejoy brand is near synonymous with the elastomeric coupling market, the company has been a major player in the metallic coupling industry for several decades. Many people are surprised to learn that Lovejoy's metallic coupling sales are on par with Lovejoy's elastomeric coupling sales, and that their knowledge of the products and applications is so great.)
Lubricated Couplings: The three major types of lubricated metallic couplings are: gear, grid, and chain. The primary form of failure for these type couplings is wear (metal on metal contact), meaning torque peaks/overloads as well as poor or improper lubrication/grease maintenance will shorten the coupling's life.


In contrast to gear and grid, chain couplings are somewhat of a dirty step child. In full disclosure, Lovejoy does not manufacture chain couplings... so we may be a bit biased... but, generically speaking, chain couplings are found and used on unsophisticated applications (i.e. - makeshift farming equipment). Chain couplings are known for being relatively rugged and very low cost. A chain coupling consists of two sprocket hubs with a single double roller chain connecting the two hubs. These couplings are relatively easy to install, maintain, and rough align.
Non-Lubricated Couplings: Popular non-lubricated metallic flexible couplings include the disc (or disk), diaphragm, link, spiral wound, bellows, and beam coupling types. All six have a theoretical infinite life (meaning they have no metal on metal wearing parts), assuming the flexing or load carrying methods stay within the mechanical endurance limits of the flexing metal material. (Overload on these type couplings, be it continuous torque or cyclic misalignment forces, will result in fatigue failure.)
These couplings have been historically complex to understand and evaluate... as significant stress analysis (finite element analysis) must be conducted to flush out performance characteristics (inclusive of taking torque load, misalignment, temperature, and varying system speeds into consideration). Non-lubricated couplings generally have a higher upfront cost, relative to traditional lubricated couplings, but can offer long term "total cost of ownership" savings opportunities.

One drawback of disc couplings is that they are generally less tolerant of misalignment (queue smiles from the shaft laser alignment product sales folks). Without diving too deep into disc couplings, disc packs can be circular (call it version 1.0), flat sided (version 2.0), or scalloped (version 3.0) on the outer dimension... with each revision offering improved performance. (Circular disc packs acts as a beam... stressing the extreme edges, flat sided packs avoid the curved disc pack drawbacks, and scallop disc packs both avoid the curved disc pack drawbacks and provide more flexibility/misalignment handling capability. The additional capability can be attributed to the disc pack's reduced cross-section... which requires less force to flex, translating to lower reactionary loads on the system's adjacent bearings.) All three disc pack styles are readily available on the market (though Lovejoy only sells the scalloped version).
Diaphragm couplings were originally introduced to service very high speed, high horsepower applications in the petrochemical industry... and has since progressed to other extreme applications such as helicopter drives. Diaphragm couplings handle misalignment through use of a flexing metal plate (or series of flexing metal plates in parallel). The metal plate(s) is loaded in shear, with torque being introduced at the outside diameter of the coupling and then transferred on to the inside diameter. (The process reversed at the opposite flex point.)
Diaphragm couplings are known for their large outside diameters, and, generally, very high cost. Diaphragm couplings are generally sold as custom solutions, and there are a wide variety of options to consider... so those seriously in the market for this coupling should spend a considerable amount of time speaking with manufacturers' application engineering staffs. (Note: Lovejoy does not sell diaphragm couplings, though Lovejoy does sell API 610 or 671 disc couplings that can sometime compete for the same application.)



