API 671 Coupling Cold vs Hot Installed Length

A common issue with API 671 couplings is the potential variation in length from the as installed (cold) to the steady state operating condition (hot).  This variation in coupling length can be of sufficient deviation to require the coupling to operate outside of the allowable axial displacement permitted.  

In order to compensate for machinery movement due to thermal and dynamic transformations, the coupling can be designed with the intention to have shims inserted.  The shims are specifically designed to allow insertion at a bolted joint within the coupling eliminating the need to remove the coupling hub(s) from the equipment (as the hubs are installed with an interference fit). This allows tuning of the installed length in small increments and gives a straightforward method of altering the installed length as the equipment ages.

Note: If shimming the coupling causes the coupling to operate outside of the manufactures recommended limits, it is advised that the coupling manufacturer (which we certainly hope is Lovejoy) be contacted as an engineering analysis might be required to ensure that the coupling will have no operational issues.

API 671 Coupling Guarding Requirements

Guarding requirements for API 671 couplings are similar to all coupling guards as they must meet all applicable codes and regulations (such as OSHA 1910.219).  However, additional requirements are included in the API 671 Standard.  A few key features are:

• The ability to inspect the coupling without requiring any additional work beyond removing the guarding.
• Capability to withstand a 200 pound static load.
• A minimum of 1 inch radial clearance between the guard inner wall and coupling maximum outer diameter.
• Anti-swirl baffles installed as required to diminish the effects of windage.
• Guards that are directly installed on the machine base or bed plate should be removable as a single piece.
• No guarding should be constructed of woven wire, only expanded metal, perforated plate or plate is permitted.

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.

API610, API671 Marketplace – Obtaining Approved Vendor Status

Guest Post: Jim Jones, Lovejoy Canada

Utilizing the standards of several worldwide associations ensures that the manufactured & supplied equipment will be consistent in quality for the end user performance criteria, regardless of where the products are sourced.

The American Petroleum Institute (API) and International Standards Organization (ISO) are two of the most respected standard associations globally and are especially in force in the North American oil & gas industry.

Our flexible coupling customers rely on these API  & ISO organizations technical committees to research and advise on conditions that may affect the high performance requirements of rotating equipment.

End users will either use these API / ISO standards for their equipment or enhance them to ensure they receive quality manufactured equipment that performs to the intended field conditions.

In the oil & gas industries and especially in the refining marketplace compliance to the API610, API671, API676 or ISO 10441 standards is required for a majority of flexible coupling applications. The AGMA 9000 Class 9 is also a base for coupling design and supply under the API standards.

The coupling compliance and approval process requires a review of either the applicable API Standards, the customer Standards or enhanced Standards incorporating both API and Customer requirements.

The approval process involves detailed review of the specific application criteria; examples would be material, tolerances, component design, speed considerations and balance for various flexible coupling rotating equipment installations.

One must understand why the customer has specified either the API/ ISO standards, the API / ISO package in conjunction with client’s specifications or client’s standalone requirements. 

By understanding our customer’s position the suppliers will have a positive effect on their submission for the  technical and supply approval process.

For more information on API couplings, please see the API 610 Standard - Coupling Highlights and API 671 Coupling Standard blog posts. 


About the Author: Jim Jones has several decade power transmission technical sales expertise. To tap into Jim's rich industry knowledge and experience, you can get to him through the Lovejoy webpage and/or LinkedIn. To find a highly qualified Lovejoy representative or distributor in your area please see Lovejoy's Find a Sales Representative or Find a Distributor tool. 

API 671 Coupling Standard

API 671 couplings are typically specified in critical applications that operate for extended periods of time, are often non-spared, have high torque and/or speed requirements.  

Metallic flexible-element couplings are the primary type permitted for API 671 (ISO 10441) applications.  Only disc couplings and diaphragm couplings can be used in API 671 applications.  The application speed & torque are the primary determining factors as to which type of metallic coupling is best suited for the application.  Gear and quill shaft type couplings are permitted in the API 671 standard but are not common. 

While the coupling meets the API 610 requirements; additional steps such as component serialization, precision balance, record retention of 20+ years and an assembly print containing application information are just some of the additional requirements needed to meet the API 671 specification.  This is to ensure that a high quality coupling was provided for a critical application and that spare components can be produced decades into the future. 

One feature of the API 671 specification are the various methods allowed to achieve and measure the residual imbalance.

• Method 1 only requires the major components to undergo balance verification but is limited to applications at or below 1,800 rpm.
• Method 2 is the standard for applications operating above 1,800 rpm.  While the major components are balanced as with Method 1, the coupling is fully assembled and the balance of the entire coupling is verified prior to shipment. 
• Method 3 is similar to Method 1 where the major components are individually balanced.  The coupling is then fully assembled and the coupling is balanced as a single entity (where Method 2 only checked the balance of the coupling assembly). One limiting item is that method 3 balancing removes the ability to interchange coupling components (with the exception of disc pack units) as the coupling assembly was balanced as a cohesive unit.

Recommended Follow-On Reading: To learn about API610 Couplings, we recommend: API 610 Standard - Coupling Highlights

API 610 Standard - Coupling Highlights

The API 610 standard (with ISO 13709 as the Identical European standard) was created by the American Petroleum Institute to outline regulations regarding centrifugal pumps for petroleum, petrochemical and natural gas applications.  The standard covers multiple components (housing castings, bearings, mechanical shaft seals, etc.) within the pump system in addition to the coupling connection.

For the coupling connection the standard defaults to an all-metal flexible element spacer type coupling, commonly known as a Disc Coupling.  In addition, the Disc coupling is required to be designed to retain the spacer if a flexible element (disc pack) fractures thereby preventing the spacer from impacting the coupling guarding and have a minimum balance of class 9 according to the AGMA 9000 standard. 

The standard allows other couplings to be used (with Gear coupling being the leading alternate) but still requires the coupling to have a balance of Class 9 to obtain the API 610 certification.

An important item to consider is that couplings operating above 3,800 rpm the API 610 standard states the coupling must be balanced to meet the API 671 (ISO 10441) standard.  While this will not provide a coupling that meets all the API 671 requirements, it will deliver the proper coupling balance needed for the application.

Active Coupling Bore and Keyway Standards - What Are They & Where Can I Find Them?

The first thing to know about coupling bore and keyway standards is that there are quite a few currently active and available. Given your application and the origin of the product/shaft in question, you should be able to quickly determine what standard applies to you. (If not, call an application engineer at a coupling company you trust... and they will quickly point you in the right direction.) 

Active Standards include: 

In North America...

• ANSI/AGMA 9002-B04 - Bores and Keyways for Flexible Couplings (Inch Series)
• ANSI/AMGA 9112-A04 - Bores and Keyways for Flexible Couplings (Metric Series)
• ANSI/AGMA 9003-B08 - Flexible Couplings – Keyless Fits  
• ANSI/AGMA 9103-B08 - Flexible Couplings – Keyless Fits (Metric Edition)

• API STD 610 - Centrifugal Pumps for Petroleum, Petrochemical and Natural Gas Industries
• API STD 671 - Special Purpose Couplings for Petroleum, Chemical and Gas Industry Services 

• AIA/NAS NA0139 - Keyway and Key Slot Dimensions Metric (an Aerospace standard)

 

In Germany... 

• DIN 6885-1 - Drive Type Fastenings without Taper Action; Parallel Keys, Keyways, Deep Pattern
• DIN 7172 - Tolerances and limit deviations for sizes above 3150 mm up to 10000 mm; principles, standard tolerances and limit deviations (ISO 286 Addendum for larger sizes)

 

In Japan...

• JIS B 1301:1996 - Keys and their corresponding keyways

 

And Globally (ISO)...

• ISO 286-2:2010 - Geometrical product specifications (GPS) -- ISO code system for tolerances on linear sizes (Part 1: Basis of tolerances, deviations and fits & Part 2: Tables of standard tolerance classes and limit deviations for holes and shafts)
• ISO 3912:1977 - Woodruff keys and keyways
• ISO 3117:1977 - Tangential keys and keyways

In addition to these active standards, the Mechanical Power Transmission Association's excellent (and free) 18 page "Frequently Asked Questions on Metric Bores and Keyways in Couplings" PDF can be used to discover a number of no longer active (expired or withdrawn) bore and keyway standards. (Note: Unfortunately, the MPTA document does not provide a route to secure these documents... but at least they can confirm the standard you seek did in fact exist at one time.) 

To broadly and briefly touch on bore and keyway standards, the North American (inch/imperial) standards tend to be more straight forward with limited variation (derived off the shaft)... while the metric options tend to be much more diverse (and were largely derived off the bore hole and were fragmented by various national approaches). Case in point, the metric standard (ISO 286), calls out over 100 different shaft and bore tolerances! Fortunately, great strides have been made to popularize a much smaller subset of these metric possibilities...  and, today, many coupling manufacturers default to using just a few metric tolerances as their standard (with Js9 keyways, and H7 clearance fit or P7 interference tolerances for the bore being a very popular default).
  
And as a final (also free) reference: Lovejoy's Engineering Data PDF catalog, which covers both inch and metric shaft tolerances in great detail on pages ED-10 through ED-20. 

(Bonus Hint: Lovejoy's Engineering Data catalog does comply with and specifically reference ANSI/AGMA 9002-B04 and ANSI/AMGA 9112-A04...and does use Js9 keyways as well as both H7 & P7 as default metric tolerances... so, if your need is narrow in scope, you might possibly consider saving a few bucks by pulling the data you seek directly from Lovejoy.)