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.