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.

Coal Fired Power Plant Pulverizer Gear Coupling Failure

The Challenge

A plant manager, from a leading coal-fired plant located in the Midwestern United States, approached us with a question about a gear coupling failure in its coal pulverizer drives. 

Power Generation Industry background

More than 50 million consumers and business owners from Ohio to New York and Canada would agree that a consistent power supply is not only vital to the overall economy, but also for the comfort and stability of daily life. The major power outage that struck the U.S. and Canada in mid-August of 2003 cost New Yorkers alone, an estimated $500 million. Although experts point to a dated power grid as the main cause of the recent outage, every detail of the power generation system, including the  smallest component, plays a critical role in maintaining the electric power plant.

Equipment and Components

Depending on the size, a single plant can have more than 25 coal pulverizers. Coal pulverizing allows for less emissions and more efficient use of the fuel. The coal burns to create the steam that turns the generators and provides electric power. Large electric motors use a worm gear arrangement to drive the coal pulverizers and a blower that sends the pulverized coal to the furnaces. The shafts of the motor must be mechanically connected to the worm gear and blower. The worm gear is a relatively small, yet vital component. If it fails, the pulverizers and blowers can no longer operate.

Dissecting the problem

Gear couplings allow for axial movement of the shafts, which increases the wear on the gears, sleeve bearings and other components. This wear on the components would cause them to fail prematurely.

The loss of production due to downtime was becoming an issue. Bearing replacement costs were in the thousands of dollars per bearing.

Preventative maintenance costs associated with gear couplings were also becoming a burden due to time spent on inspection and lubrication to keep them operative. The power plant’s rigorous preventative maintenance program required regular disassembly of the gear couplings for inspection, cleaning and re-lubrication of the gear teeth. In addition, assessment of the condition of the couplings was a judgement call unless extensive and time-consuming measurements were performed. Coupling replacements also require heavy machinery to move the motors.

Old technology, old solution

The existing gear coupling design was typical of the technology available when the plant was built in the early 1970’s. Where gear couplings did not limit axial movement of the motor shaft, sleeve bearings were used. This old technology configuration increased wear and shortened life spans of both the motor sleeve bearings and the gear couplings.

In other words, increased sliding velocity of the gear teeth in the gear coupling creates a lot of heat and wear, just picture yourself rubbing two sticks together. The motor moves a bit, the gear on the coupling moves a bit, and this motion could be repeated up to a million times over the course of the day, contributing to premature failure.

Contemplating a similar solution

Install gear couplings with limited end float; however, its maintenance nightmare will continue because the gear couplings are difficult to assess for wear and the inspection and replacement of the gear coupling still required removing the motors. The gear couplings would still have to be disassembled, inspected, cleaned, and re-greased every time; definitely not an effective solution.

New Technology, new solution

Install a Disc Coupling. Unlike gear couplings, the Lovejoy SX Disc Coupling design is inherently balanced. There is less vibration than the gear coupling as it wears. The disc coupling performs a limited end float function without special design considerations for the sleeve bearings. Maintenance-free operation is achieve because the Lovejoy coupling has no moving parts and requires no lubrication. This translates into a long coupling life.

Actual Pulverizer Solution

Preventative maintenance and inspections of the disc coupling’s flexible elements was now possible using a simple strobe light and while the coal pulverizer is running. Hence, there is no need for an expensive shutdown and disassembly. Replacement of the disc packs became a breeze without moving any machinery or disturbing the original alignment.

Bonus features

Lovejoy was able to provide a heavier duty coupling than the gear type in the same space as before. Nothing had to be moved or modified for installation. Additionally the component cost was comparable to those of a gear coupling. The Lovejoy’s design allows for more capacity and misalignment capability than any other disc coupling available today.

In addition, disc coupling signals the maintenance staff when it needs replacement – you can actually see it with your own eyes without any disassembly. In contrast, in gear coupling is more of judgment call, unless you take it completely off the machine and measure it extensively.  With the Lovejoy disc coupling, the outer leaf of the disc will crack before the entire disc pack fails, signaling to the maintenance staff it needs replacing.

Happy conclusion

The coal pulverizer is critical equipment, if it is not blowing coal, it is not generating power; needless to say, would you rather have your pulverizer blow coal or blow off your money?. The disc coupling solution reduced the maintenance nightmare, and the plant has been experiencing success for more than a year with the original installation. Plant management approved the proposal to convert all gear couplings on 24 coal pulverizers to Lovejoy disc couplings, based upon the proven benefits and demonstrated cost savings.

Increase coupling performance while reducing time, money and enviromental waste

The paper industry continues to be a large, lucrative industry in the world today, rolling out hundreds of thousands of tons of paper each year from individual plants.  The industry incorporates quite a few technologies in the production of paper.  Each of these areas is critical to maintaining a clean and smooth paper production process.

The pain

One pain area for many pulp and paper companies has always been the paper mill pump house.   A pump house is a centralized location where the electrical motors and pumps are located. Most of the fluids used in paper production are pumped through this central location.

Generally, for of ease of maintenance, the motors and pumps are spaced apart to allow easier access to pump seals, pumps and motor bearings, etc.  It is a common practice for the paper mill to shut down for a few days each year for annual inspections of all the equipment, including connecting couplings.  If needed, the equipment or couplings are serviced and/or replaced during this shutdown. For many years, the couplings used to connect the motors and pumps have been floating shaft gear couplings.

Some of the concerns voiced by the maintenance teams at these paper mills when servicing gear couplings are: 

The messiness and waste: When the couplings are disassembled for maintenance reasons, there is usually a large amount of grease to contend with, sometimes gallons.  This grease in not easy to handle and there is always the eventual issue of how to dispose of the used grease. 

Difficulties with hubs and sleeves inspection: The hubs and sleeves need to be cleaned to inspect the gear teeth.  If the lubrication has started to breakdown, it can be difficult to get residue out from between the gear teeth in the coupling, especially the sleeve where access can be difficult.  Depending on wear, the seals may need to be replaced.  It is common for the hubs and sleeves to be replaced if the amount of wear exceeds normal levels. 

Downtime to re-install couplings: When the coupling is reinstalled, new grease needs to be loaded into the coupling(s) and alignment checked.  This process has been known to take anywhere from an hour to as long as a day to complete.  If components need to be replaced, this service could run into days (plural). 

Moist environment: Another important is that these pump houses are usually very wet environments and rust is often a serious issue that can lead to premature failures when using the floating shaft gear couplings.

A versatile solution

To assist the paper industry with resolving this maintenance concern, a versatile disc coupling (such as the Lovejoy DIR Coupling) is used nowadays to replace the floating shaft assemblies in the existing gear couplings.  This versatile disc coupling can have a fixed floating shaft length, or when used with an external shaft locking device, it can vary in length (up to a couple of inches). These couplings can be designed to accommodate the high torque capacities of the gear couplings.

Unique features – easy access, easy inspection and quick replacement

These versatile disc couplings are popular because of the ease of access to the flexible member of the coupling, or the stainless steel disc packs, and the ability to inspect the disc packs without disassembly of the coupling.  The disc packs are clearly visible in disc couplings and can be inspected for wear, misalignment issues, or breakage with the coupling either stationary, or in operation. 

With the use of a safety shield and strobe light, you can inspect the coupling without shutting down the equipment.  If one of the disc packs show signs of wear or breakage, it can be easily removed by undoing the bolts that hold the disc pack in place, slide in a new disc pack, and replace the bolts.  Since the disc coupling does not require grease, there is no issue with disposal and replacement of grease when performing inspections.  For concerns with buildups of rust in humid or wet environments, Electroless Nickel plating is an available to mitigate this rust problems with the couplings.

Lasting benefits

Essentially, the time (hours or days) it took to inspect and service a coupling is decreased to a fraction of that time.  This huge saving makes up for the difference in the cost of the technology.
How about lasting effects to the environment?  Since the disc coupling does not require grease, there is no issue with grease waste; hence, there is not any need for disposal or replacement. This action in itself will help cut down environmental contamination footprint. 

A good case to demonstrate both benefits is a naval project completed not too long ago: maintaining 15,000 surface ship pump applications. Sailors spent 29 Sailor-days per year per pump repairing and replacing mechanical seals, couplings, and bearings.  Also, the yearly grease waste output was about 3 pounds per pump.   Let’s do some math: 3x15,000 = 45,000 pounds of waste or about 22 tons a year … that is a lot of oozing grease.  Once these versatile couplings were installed, time was cut down to a savings of 1700 sailor years and waste footprint was reduce considerably.

Supplier Relationships - The Power of a Compliment

While we often hear how important relationships are in business, it's not often you run across a story or situation quite like one we had a few weeks back... a gentleman named Mike set in a question to the CouplingAnswers.com feedback form regarding his Lovejoy inventory. After reading and answering his question, we couldn't help but get excited about the types of projects he supported (including projects with SkyTrac and Hilltrac, who are both growing players in the mountain transportation industry).
 

After our team's initial blog response, Mike quickly informed us that he had been relying on Lovejoy as one of his trusted suppliers since 1998... and was a particular fan of Lovejoy's SU disc coupling. During his 17 year span of working with Lovejoy, he shared that he had worked under several company banners... but that the one thing that had remained constant was his Lovejoy representative, Bella. Mike then went on to describe Bella as "a competent sweet professional lady"... and asked that we "give her (his) best".

• Could Mike have paid Bella a better complement? Not likely. 
• Did it physically cost him or his company anything? No. 
• Did this news get back to Bella... and is she likely to bend over backward (even further) to do absolutely everything she can to help him out if/when a future favor is ever needed? Most certainly. 

While all too often we view buying decisions as one-time transactions, Mike's behavior served as a refreshing reminder that partnerships and supply lines can be greatly strengthened simply by treating others with respect and lavishing praise if and when appropriate.

Lovejoy is incredibly lucky to serve such amazing clients like Mike, and is equally blessed to have loyal and compassionate employees like Bella... who can step up and exceed customer expectations. Thank you Mike... and best wishes with whatever you choose to conquer in life!

Note: Lovejoy does not recommend using Lovejoy products in any condition where human welfare might be put in jeopardy should there be a failure inclusive of drive train, steering mechanisms, or people moving devices. Couplings, by design, are created to fail under a number of circumstances including but not limited to peak torque, excessive misalignment, environmental conditions, and general fatigue. For any further questions, please consult a Lovejoy application specialist and/or see Lovejoy's purchase terms and conditions.

API 671 Coupling Types - Disc, RM Disc, & Diaphragm

Multiple options exist when an API 671 (ISO 10441) coupling is required for an application: Disc, RM Disc and Diaphragm.  Each coupling type integrates features to meet the needs of a particular segment within the marketplace.

Disc Couplings

This coupling type is commonly used when API 671 adherence is required for auxiliary equipment such as a pump system.  A Disc type coupling provides a good balance between capability and cost as the coupling type is based on upgrading the standard API 610 design to meet the API 671 requirements.  While providing a high torque capacity which permits use in smaller turbine driven systems, this coupling type is limited in regards to maximum permitted rotational speed.

RM Disc

Specifically designed for API 671 applications, the Reduced Moment (RM) Disc coupling is used in applications that a standard disc coupling is unable to be specified (typically due to a rotational speed requirement).  More cost effective than the Diaphragm type, the RM Disc coupling utilizes similar features such as a larger diameter thin walled spacer while retaining the disc packs element as seen in standard Disc Couplings.  The RM Disc Coupling further integrates the guard rings and hubs into a single unit to reduce mass & inertia allowing the coupling to withstand higher rotational speeds.  A recent application example is a 47,000 HP turbine driven axial compressor system that had a trip speed of 6,600 rpm.

Diaphragm

Replacing the disc pack element with a contoured continuous disc, the Diaphragm Type Coupling offers increased torque and rotational speed capacity at the expense of misalignment capability.  As the contoured disc is complex to manufacture, the Diaphragm type is one of the highest cost API 671 couplings available.  This limits the usage of Diaphragm couplings to critical applications, such as the primary drive of a high powered mechanical drive turbine system.

Recommended Follow-On Reading: To learn more about API671 couplings, we recommend the following CouplingAnswers.com article: API 671 Coupling Standard.

Scalloped Disc Pack Couplings - Ideal for Paper Mill Applications

Today's paper mills have been targeted with aggressive up-time requirements to keeping them running as efficiently and profitably for their shareholders as possible. Once such technology that has quickly crept into paper mills over the past few years has been the flexible disc coupling. 

Whereas gear couplings have long been used, and require system shutdown for coupling inspection and re-lubrication, disc couplings (paired with a steel mesh coupling cover and strobe light) can be inspected while running. Furthermore, they require no lubrication and have a theoretical infinite life. 

A stumbling block that Lovejoy has seen with disc coupling installations at various mills is that they have selected a disc coupling with misalignment limitations far below what the given system actually sees. Under such circumstances, the disc coupling will quickly fail and the operator will question why they even bothered trying to switch. 

One of Lovejoy's veteran sales personnel documented the case of a specific east coast US paper mill going through a disc coupling every ~90 days in the dryer section of the machine. The failures caused production stops of an entire shift, with economic consequences that were certainly being felt by upper management. 

Broadly speaking, paper machines are comprised of two main areas (a wet area and a drying area), and can have somewhere in the range of 100 to 125 processing pumps supporting a single machine. 

The Right Solution

Advanced scalloped disc pack coupling designs (such as Lovejoy's offering) offer considerably more misalignment capability relative to traditional early iterations of the disc coupling, and are generally well suited for many paper mill applications. 

Even when utilizing scalloped disc pack designs, it remains critical that installers understand and work actively to mitigate the inherent misalignment in a given system. Disc couplings are beautiful, in that, when well aligned, they have a theoretical infinite life (no wearing parts) and can last 10 years or more. 

And regarding the competitor disc coupling that had been failing rapidly above, the Lovejoy sales rep reported back to Lovejoy that, after 3 years, the new Lovejoy disc coupling was still running well. That's certainly dollar signs and peace of mind to the supervisors of the plant in question! 

To learn more about flexible scalloped disc pack couplings, please visit Lovejoy's product page and/or download their PDF product catalog. 

How sensitive are Grid Couplings to misalignment?

Grid couplings are very sensitive to misalignment. They are designed to handle almost no parallel shaft misalignment, and only minimal angular misalignment (as called out in the Lovejoy grid coupling rating chart and the coupling pre-selection guide below).

While misalignment must be considered and proactively addresses when using a grid coupling (see Why Lovejoy Offers Shaft Alignment Dial Indicator Kits), there are several major benefits to using a grid coupling. The biggest is that it is an all metallic flexible coupling design that can transmit significant torque in a small footprint (high power density) while also (unlike metallic gear & disc couplings) providing system vibration dampening capability. Grid couplings are a well proven technology, and are readily available from stock from a few leading coupling brands (inclusive of Lovejoy). 

Lovejoy Grid Coupling Misalignment Capacity 

(Standard and Spacer)

	Max Installation		Max Operational		Nominal
	Misalignment (in)		Misalignment (in)		Gap (in)
Size	Parallel	Angular (X-Y)	Parallel	Angular (X-Y)	+/-10%
1020	0.006	0.002	0.012	0.010	0.118
1030	0.006	0.003	0.012	0.011	0.118
1040	0.006	0.003	0.012	0.013	0.118
1050	0.008	0.004	0.016	0.015	0.118
1060	0.008	0.004	0.016	0.018	0.118
1070	0.008	0.005	0.016	0.020	0.118
1080	0.008	0.006	0.016	0.024	0.118
1090	0.001	0.007	0.016	0.028	0.118
1100	0.010	0.008	0.020	0.032	0.177
1110	0.010	0.009	0.020	0.035	0.177
1120	0.011	0.010	0.022	0.040	0.236
1130	0.011	0.012	0.022	0.047	0.236
1140	0.011	0.013	0.022	0.053	0.236

While the power density of a grid coupling is enviable, one final drawback to consider when selecting a grid (or gear) coupling is the fact that it requires lubrication (grease). Unlike elastomeric flexible couplings (which generally also provide system vibration dampening capability at lower power density levels), this means your maintenance team will need to periodically re-grease the coupling through the lubrication ports on the coupling's cover and be careful to avoid and/or properly address any grease leakage environmental concerns.

Alternatives to grid couplings include disc couplings (see Why to Switch from Grid to Disc Couplings), which avoid lubrication concerns but do not offer the grid coupling's level of vibration dampening, and jaw in-shear couplings (see Jaw In-Shear Couplings - A Straight Forward Value Add) which avoids lubrication but has a lower power density.