Disc Couplings - Common Terminology (Part 2)

Design Basics - Common Terminology

DI "Drop-Out" Type Disc Coupling

The diagram of an industrial DI Type "drop-out" style disc coupling at right highlights a number of useful terms common in describing a disc couplings.

(Note that because this coupling has two disc packs, it is equipped to handle parallel in additional to angular misalignment. It is for this reason that many disc coupling designs make use of two disc packs, and not just one.)

"Hubs" are the metal element designed to attach to the two shafts of a power transmission system.

"BSE" stands for "Between Shaft Ends" and describes the specified distance between to two shafts that the coupling is design to connect.

"PW" describes the space allocated for the each disc pack.

The "Spacer Assembly" is called out specifically in this design as the whole center section of the coupling, including the two disc packs, is designed to be able to be removed from an installation with requiring the coupling hubs to be moved or removed. A drop-out spacer assembly section, as shown in the image above, is an additional feature not common to all spacer disc couplings. An example of a spacer disc coupling without a drop-out assembly section would be Lovejoy's SX Type.

In the case of Lovejoy's DI Type disc coupling, "Guard Rings" are included and hubs are piloted to ensure proper centering of the drop in spacer assembly. This piloting serves as an anti-flail feature and aids in the couplings ability to meet API 610 requirements.

SX Type Disc Coupling with SLD

While disc couplings commonly use interference fit hub-to-shaft connections, another hub-to-shaft connection feature available for disc couplings are shaft locking devices ("SLDs").  As shown in the photograph at right and the installed photograph below, shaft locking devices allow pressure to be exerted on a hub to connect the hub firmly to a shaft for torque transmission. The advantage of using shaft locking devices is that users can avoid interference fits that require significant maintenance time to remove. The disadvantage of shaft locking devices is the initial upfront cost. Additionally, watching tolerances and understanding the ratings of a shaft locking device are also critical as the shaft locking device may have ratings below the coupling and or system requirements. 

To learn more about disc couplings, please check out Part 1 of this blog series... or Lovejoy's disc coupling product pages & catalogs, the non-lubricated section of The Coupling Handbook, and/or subscribe to this blog as there will be several more blog posts on disc couplings in the immediate future.

Disc Couplings – General Overview (Part 1)

A Single 6-Bolt Disc Pack SU Type Disc Coupling

Disc couplings are a popular coupling design that continues to pick up steam within the power transmission industry in large part because they can transmit a significant amount of torque, while also being minimal maintenance. Unlike lubricated gear and grid couplings, disc pack couplings require no lubrication and periodic re-lubrication. This makes them ideal for applications that are hard to reach or get to... which, given increased maintenance department budget constraints... is broadly appealing across countless high torque applications and industries.

Unitized Disc Pack Profile

Disc packs can be visually inspected without taking apart the coupling (a major time savings not true with gear or grid couplings) and have "theoretical infinite life" in that they have no wearing metal-on-metal misalignment handling flex points (as is the case with lubricated gear and grid couplings). Rather, misalignment handling occurs through the flexing of unitized disc packs connecting two flanges together. Should a disc pack wear out (generally due to fatigue brought on by excessive misalignment), bolts and the unitized disc pack can be generally removed and replaced without having to move the installed coupling hubs.

Furthermore (yes.. there are a lot of positives with this coupling type), generally being all metal (with no seals or gaskets) gives disc couplings the ability to withstand harsh environments.. And, lastly, to give one more potential major benefit (based on the application) over their lubricated brethren, disc couplings have torsional rigidity with no backlash.

Limitations of disc couplings include no vibration dampening capability (a strength of elastomeric & grid couplings), and limited misalignment handling capability... particularly in regards to parrallel misalignment. (For a disc coupling to handle parallel misalignment, two disc packs must be used.) Additional considerations include speed limitations if not balanced (though these can be overcome as evidence by widespread disc coupling usage in API 610 and API 671 applications), and possible anti-flail concerns (though, again, anti-flail safety features can be added to many designs).

Design Basics  - Disc Packs

Six Bolt Unitized Disc Pack

Disc couplings and their disc packs are generally referred to by the number of bolts they have. Common bolt configurations include 4 bolt, 6 bolt, & 8 bolt. The reason bolt configurations are even is that torque is only carried in tension across every other leg of a disc pack (while the remaining legs will be in compression). In a 4 bolt design, 2 legs will be carrying the torque. In a 6 bolt, 3 legs... and, in an 8 bolt, 4 legs.

The trade-off between the number of bolts is torque versus misalignment handling capability. More bolts (and more legs sharing the load) can carry higher torque loads, but this increase also decreases the flexibility of the disc pack and ability of the pack to handle increased misalignment.

Eight Bolt Unitized Disc Pack

Shown at right are both six bolt and eight bolt designs. In both cases, the unitized disc pack has a scalloped design. This scalloped design is a design enhancement over earlier disc pack designs, which had circular or straight sides. By eliminating the excess material around the outside diameter of disc pack from earlier designs, the scallop eliminates a prior source of peak stress within the disc pack and allows the disc pack to accommodate more misalignment. (Note: All of Lovejoy's disc couplings leverage an advanced scalloped design.)

Disc packs are generally made of thin layers of stainless steel (though materials can vary) that have been "unitized" into a single easy to handle/install/replace unit by use of bushings. Some early and current designs styles still make use of non-unitized layers, which can be a bit trickier to deal with from a maintenance and handling perspective. (Note: All of Lovejoy's disc couplings leverage a unitized design.)    

As illustrated in the diagram of the 6 bolt disc pack below, fasteners are then used to connect the disc pack bushings to the coupling flanges. For each bushing location, the disc pack will be connected to only one of the two coupling flanges and the flange it is connected to will switch back and forth as you circle around the disc pack in either direction. 

Disc Pack Bushings Fasten to Alternating Flanges

To learn more about disc couplings, please go to Part 2 of this blog series... or check out Lovejoy's disc coupling product pages & catalogs, the non-lubricated section of The Coupling Handbook, and/or subscribe to this blog as there will be several more blog posts on disc couplings in the immediate future.

Proud to be "Mobile Friendly"!

It may have taken us a bit longer that we had hoped, but we are still very excited to share that our Lovejoy website (www.lovejoy-inc.com) is now "mobile friendly" for all our smart phone users out there!

Unlike this blog, which was started long after "mobile friendly" became a hot topic... and was "mobile friendly" since inception, the Lovejoy website was built on a platform long before mobile friendly was of primary concern).

As such, recoding the site to be "mobile friendly" was both labor and cost intensive... but we recognized the value being mobile friendly was for our dedicated users base (especially those out in the field looking to access a key dimension or installation guideline) and committed the necessary resources.

If you happen to be viewing this blog on a mobile device, we certainly invite you to head over to our main website to have a look around. If and when you do, please leave us a note about what you liked and/or what you would like still improved. (Your satisfaction is our #1 goal!)

Frankenstein Couplings - Unique Configurations, Materials, and other Oddities

With Halloween just over the horizon, we thought it would be worthwhile do write a blog post on something Lovejoy is particularly well renowned for: Frankenstein couplings!

What exactly is a Frankenstein coupling? It's a one-off, highly customized or unique coupling that most coupling manufacturers run away from.

Fortunately, given Lovejoy's tremendous depth of product knowledge, experience, and local manufacturing capabilities... such couplings are standard practice for us, and these are opportunities that we readily welcome.

In fact, our reputation is such that, while they might not like to openly admit it... other well-respected marketplace players have been known to send such business our way.

So... to borrow the lyrics of Ghostbusters:

If there's somethin' strange in your neighborhood, who ya gonna call? (Lovejoy Application Engineers!)

If it's somethin' weird and it won't look good, who ya gonna call? (Lovejoy Application Engineers!)

From the most exotic materials (Inconel, Specialty Stainless Steels, Composites, etc.) & head-scratching unique configurations...to our tremendously deep lineup of "standard" off-the-shelf solutions, Lovejoy's team of Application Engineers and Specialist are on standby to help you and your organization succeed.

We look forward to being a growing resource for you, and wish you nothing short of the happiest Halloween ever!