The coupling between a driver and driven machine is one of the most overlooked components in a rotating equipment train, yet a coupling failure on an unspared centrifugal compressor or critical process pump can shut down an entire unit. API 671 (Special-purpose Couplings for Petroleum, Chemical, and Gas Industry Services, 5th Edition, August 2020) exists precisely for these high-consequence services — large, high-speed, continuously operating machines that are often unspared and critical to the continued operation of the installation. Alongside the coupling itself, the baseplate that supports the equipment train is governed by API 686 (Machinery Installation and Installation Design), and together these two documents define much of what determines whether an equipment package stays aligned and vibration-free for years or starts drifting within months.
What API 671 covers — and what it deliberately leaves out
API 671 specifies requirements for couplings that transmit power between the rotating shafts of two machines in special-purpose applications in the petroleum, petrochemical, and gas industries. The standard covers the design, materials of construction, manufacturing quality, inspection, and testing of the coupling itself — but it deliberately does not define criteria for selecting which coupling type to use on a given application. That selection is the purchaser’s responsibility, based on the machinery train’s torsional characteristics, speed, misalignment conditions, and transient loading.
The couplings covered are designed to accommodate three types of shaft misalignment without imposing unacceptable mechanical loading on the coupled machines: parallel (lateral) offset, angular misalignment, and axial displacement. Clutch, hydraulic, eddy-current, rigid, radial spline, chain, and bellows couplings are explicitly excluded from the standard’s scope.
The four coupling types under API 671
Metallic flexible element (disc / diaphragm)
The default choice under API 671 unless the purchaser specifies otherwise. Disc and diaphragm couplings transmit torque through the elastic bending of thin metallic membranes — no lubrication required, zero backlash, and a predictable failure mode. They accommodate angular and axial misalignment directly; parallel offset is handled by using a double-engagement (spacer) configuration with two flex points. Confirm the design factor and fatigue-life basis against the specified edition and actual loading.
Gear couplings
Not normally recommended for new applications under API 671. The standard permits gear couplings only where large axial displacements cannot be practically accommodated with disc or diaphragm elements, or where the coupling diameter is restricted and the gear coupling’s higher power density is the only type that fits. Gear couplings require regular lubrication, will wear over time, and need an agreed wear and maintenance basis for the application.
Quill shaft couplings
A thin, torsionally flexible shaft that connects the driver and driven machine through a hub arrangement, often used on the low-speed side of a gear train or between a motor and a speed-increasing gearbox. Quill shaft couplings require assessment of the shaft, hubs and torsional behavior for the specified drive train.
Torsional damping and resilient couplings
Elastomeric or spring-type elements that add torsional flexibility and damping to the equipment train, typically on the low-speed side. Their sizing must address steady-state and transient torque; they may be used in combination with a metallic flexible element coupling elsewhere in the train to accommodate axial movement and reduce overhung mass. Confirm peak capacity and the applicable design factors with the manufacturer.
Service factors and minimum design life
API 671 ties service factors directly to coupling type, reflecting the different fatigue and wear characteristics of each design. The purchaser specifies the operating conditions — steady-state torque, maximum driver power, rated torque, transient peak torque, trip condition torque, and continuous cyclic loading — and the coupling manufacturer sizes the element to meet or exceed the applicable service factor across all of these conditions.
| Coupling type | Min. service factor | Min. design life | Lubrication |
|---|---|---|---|
| Metallic flexible element | Per approved specification | Per fatigue assessment | Typically none for flexible elements |
| Gear | Per approved specification | Per wear assessment | Manufacturer-specified lubrication |
| Quill shaft | Per approved specification | As specified | Confirm the complete assembly |
| Torsional damping / resilient | Per torsional assessment | As specified | Depends on construction |
DBSE and spacer length
Distance Between Shaft Ends (DBSE) is a critical dimension on any API 671 coupling datasheet. It is measured from the extreme end of one shaft (including any threaded end) to the extreme end of the next shaft, or in the case of integral flanges, between the mating faces. A spacer coupling with adequate DBSE allows the coupling element to be removed for maintenance without moving the driver or driven machine — a practical requirement on pumps and compressors where alignment is time-consuming and where any disturbance risks introducing soft foot or piping strain. The DBSE must be specified on the datasheet and matched to the equipment layout drawing, and the coupling manufacturer’s rated torque and misalignment capacity must be verified at the specified DBSE, since these change with spacer length.
Baseplate design under API 686
A coupling transmits torque between shafts, but the baseplate transmits every other force — dead weight, live load, piping strain, thermal expansion, and the dynamic loads from unbalanced rotor forces — into the concrete foundation. API 686 (Machinery Installation and Installation Design) sets the requirements for baseplate and soleplate design, grouting, anchor bolts, and leveling, and provides installation guidance for machinery projects in petroleum and gas service.
The most important principles are:
- Specify the approved mounting arrangement and identify the boundaries between machinery package, support structure and foundation.
- Baseplates must have sufficient strength and rigidity to transfer all applied forces to the foundation through anchor bolts, not through grout adhesion alone.
- Confirm the required mounting surface dimensions, flatness, coplanarity and the measurement method against the approved specification.
- Review baseplate details and stress-concentration concerns with the machinery and structural designers.
- Define the approved leveling and alignment provisions, including the permitted adjustment methods and installation procedure.
- Select grout and its installation requirements using the approved design and grout manufacturer instructions.
- Establish foundation dimensions and dynamic behavior through the applicable engineering design; do not rely on a universal equipment-to-foundation mass ratio.
What changes between pump and compressor installations
API 610 centrifugal pumps typically arrive as a complete, factory-assembled package — pump, driver, coupling, and baseplate assembled and aligned at the manufacturer’s shop. The coupling type is usually specified by the pump manufacturer to match the baseplate layout and DBSE, and the purchaser confirms the selection on the datasheet. Centrifugal compressors, by contrast, are more likely to be assembled on-site, with the coupling selection and DBSE determined by the layout engineer and the torsional analysis of the complete equipment train. The coupling manufacturer receives the torsional analysis results — torque at every operating condition, speeds, and transient events — and sizes the coupling accordingly. For pump packages, the API 610 standard explicitly references API 671 for special-purpose couplings, and API 686 governs the foundation and installation.
Inspection and testing
API 671 requires the coupling manufacturer to provide a manufacturer’s data report covering materials, dimensions, and test results. The standard requires a visual and dimensional inspection, verification of material certifications, and — for metallic flexible element couplings — confirmation that the flex elements meet the specified fatigue life at the rated conditions. Torsional analysis of the complete equipment train is not performed by the coupling manufacturer; it is a system-level study that the purchaser or engineer provides, and the coupling is sized to meet the resulting requirements. IOGP S-700, the supplementary specification published by the International Association of Oil and Gas Producers, adds additional requirements for procurement, including data sheets, quality requirements, and information requirements that overlay API 671 for projects that adopt the JIP33 framework.
What to check on a coupling or baseplate RFQ
A coupling datasheet is deceptively simple — one page of fields — but an incomplete datasheet leads to either an oversized coupling (wasted money and overhung mass) or an undersized one (premature fatigue failure). The baseplate specification is equally important, because a baseplate that cannot hold flatness after grouting will undermine the best coupling and alignment in the plant.
- Specify the coupling type explicitly on the datasheet (metallic flexible element, gear, quill shaft, or torsional damping/resilient) — do not leave it to the manufacturer to guess
- Provide the complete torsional analysis, including steady-state, rated, transient peak, trip, and continuous cyclic torque at all operating speeds
- State the DBSE and verify it matches the equipment layout drawing and allows coupling element removal without moving the machinery
- Confirm the specified baseplate flatness tolerance, support condition and inspection method before accepting the dimensional report.
- Specify the coupling guard arrangement and confirm it meets the safety requirements of the site and applicable local regulations
- Include the API 671 edition (5th Edition, August 2020) and any supplementary specification (e.g. IOGP S-700) on the datasheet so the manufacturer quotes to the correct requirements
- For baseplates, agree grout shoulder geometry and expansion joint details with the responsible designer and grout manufacturer.
Sourcing special-purpose couplings or complete rotating equipment packages? Browse our pumps & rotating equipment category, or send us your coupling datasheet and Oillinko will circulate it to potential manufacturers, checking every returned offer against the API 671 edition, torsional requirements, and baseplate specifications you need confirmed before the order is placed.
Frequently asked questions
What is API 671 and what does it cover?
API 671 (Special-purpose Couplings for Petroleum, Chemical, and Gas Industry Services), 5th Edition, August 2020, specifies requirements for couplings used to transmit power between the rotating shafts of two machines in special-purpose applications. It covers design, materials of construction, manufacturing quality, inspection, and testing of gear, metallic flexible element, quill shaft, and torsionally resilient/damping couplings.
Which coupling type does API 671 default to?
Unless the purchaser specifies otherwise, the coupling shall be a metallic flexible element coupling (disc or diaphragm type). Gear couplings are not normally recommended for new applications and are only used when large axial displacements or space restrictions make a metallic flexible element impractical.
What service factors does API 671 require?
The applicable factor depends on coupling construction, loading and the specified edition. Ask the manufacturer to state the continuous and transient torque basis, selected factor and compliance with the project specification. Do not use an isolated factor as a complete sizing calculation.
What does API 686 require for rotating equipment baseplates?
Confirm the applicable API 686 edition and purchaser requirements for mounting, alignment, grout, anchorage and installation. The responsible structural and machinery engineers must approve the arrangement and tolerances for the actual package; this article does not provide a universal foundation design.
