Special-purpose Couplings for

Petroleum, Chemical, and Gas Industry

Services
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API STANDARD 671

FIFTH EDITION, AUGUST 2020

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 Special Notes

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 Foreword

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 Contents
Page

1 Scope............................................................................................................................................................. 1

2 Normative References................................................................................................................................... 1

3 Terms, Definitions, Acronyms, and Abbreviations.......................................................................................... 2

3.1 Terms and Definitions.................................................................................................................................... 2
3.2 Acronyms and Abbreviations......................................................................................................................... 8

4 Requirements................................................................................................................................................ 9
4.1 Units of Measure........................................................................................................................................... 9
4.2 Statutory Requirements................................................................................................................................. 9

5 Coupling Selection......................................................................................................................................... 9

6 Coupling Design.......................................................................................................................................... 11

7 Coupling Ratings......................................................................................................................................... 15

8 Coupling Requirements............................................................................................................................... 16
8.1 Metallic Flexible-element Couplings............................................................................................................ 16
8.2 Machining.................................................................................................................................................... 17
8.3 Spacer......................................................................................................................................................... 17
8.4 Hub Type...................................................................................................................................................... 17
8.5 Integral Flanges........................................................................................................................................... 17
8.6 Hubs............................................................................................................................................................ 17
8.7 Idling Adapter (Solo Plate) .......................................................................................................................... 19
8.8 Moment Simulator........................................................................................................................................ 20
8.9 Component Fit Tolerances and Potential Unbalance Calculations.............................................................. 20
8.10 Fasteners (Including Studs)......................................................................................................................... 21
8.11 Electrical Insulation ..................................................................................................................................... 21
8.12 Dynamics..................................................................................................................................................... 21

9 Balance........................................................................................................................................................ 22
9.1 General........................................................................................................................................................ 22
9.2 Balancing Methods...................................................................................................................................... 23
9.3 Balance Criteria........................................................................................................................................... 25
9.4 Trim Balance Holes...................................................................................................................................... 28

10 Materials...................................................................................................................................................... 29

11 Accessories................................................................................................................................................. 29
11.1 Coupling Guards.......................................................................................................................................... 29
11.2 Special Tools................................................................................................................................................ 29
11.3 Torque Measuring System(s)....................................................................................................................... 30

12 Manufacturing Quality, Inspection, Testing, and Preparation for Shipment................................................. 31

12.1 Manufacturing Quality.................................................................................................................................. 31
12.2 Inspection and Testing................................................................................................................................. 31
12.3 Inspection.................................................................................................................................................... 32
12.4 Testing......................................................................................................................................................... 33
12.5 Preparation for Shipment............................................................................................................................. 33

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 Contents
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13 Vendor’s Data.............................................................................................................................................. 33
13.1 General........................................................................................................................................................ 33

Annex A (informative) Coupling Datasheets............................................................................................................. 34

Annex B (normative) Torsional Damping Couplings and Resilient Couplings.......................................................... 41

Annex C (normative) Gear Couplings...................................................................................................................... 44

Annex D (normative) Quill-shaft Couplings.............................................................................................................. 46

Annex E (informative) Factors for Couplings............................................................................................................ 47

Annex F (informative) Examples of Misalignments.................................................................................................. 52

Annex G (informative) Example of the Determination of Potential Unbalance......................................................... 53

Annex H (normative) Coupling Guards.................................................................................................................... 57

Annex I (informative) Coupling Tapers..................................................................................................................... 61

Annex J (informative) Inspection of Taper Bore Hubs with CMM Method................................................................ 62

Annex K (normative) Procedure for Residual Unbalance Check............................................................................. 64

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Annex L (normative) Torque Measuring System...................................................................................................... 70

Annex M (informative) Inspector’s Checklist............................................................................................................ 73

Annex N (informative) Purchaser’s Checklist........................................................................................................... 79

Annex O (informative) Contract Documents and Engineering Design Data............................................................. 85

Bibliography.............................................................................................................................................................. 91

Figures

1 Typical Coupling Selection Process............................................................................................................. 10

2 Component Balance Procedure................................................................................................................... 23
3 Procedure for Component Balance with Assembly Check Balance............................................................ 24
4 Procedure for Component Balance with Assembly Balance........................................................................ 25
E.1 Typical Fatigue Diagram.............................................................................................................................. 49
E.2 Example of Coupling Arrangement.............................................................................................................. 50
F.1 Parallel Offset.............................................................................................................................................. 52
F.2 Angular Misalignment.................................................................................................................................. 52
F.3 Lateral Offset with Angular Misalignment in the Same Plane and in the Same Direction............................ 52
F.4 Lateral Offset with Angular Misalignment in the Same Plane but in the Opposite Direction........................ 52
G.1 Typical Flexible Element Coupling............................................................................................................... 53
J.1 Axial Probe Locations.................................................................................................................................. 63
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 Contents
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J.2 Radial Probe Locations................................................................................................................................ 63

K.1 Residual Unbalance Worksheet Example................................................................................................... 67
K.2 Sample Residual Unbalance Worksheet for Left Plane (SI)........................................................................ 68
K.3 Sample Residual Unbalance Worksheet for Right Plane (USC)................................................................. 69

Tables

1 Summary of Balancing Methods.................................................................................................................. 22

G.1 Contributory Unbalance............................................................................................................................... 55
I.1 Axial Hub Advance Required to Achieve 25 µm (1 mil) of Bore Dilation...................................................... 61
K.1 Trial Weight Multiplier vs Nmc....................................................................................................................... 65
M.1 Inspector’s Checklist.................................................................................................................................... 73
N.1 Purchaser's Checklist.................................................................................................................................. 79
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 Introduction

Users of this standard should be aware that further or differing requirements may be needed for individual applications.
This standard is not intended to inhibit a vendor from offering, or the purchaser from accepting alternative equipment
or engineering solutions for the individual application. This may be particularly appropriate where there is innovative
or developing technology. Where an alternative is offered, the vendor should identify any variations from this standard
and provide details.

This standard requires the purchaser to specify certain details and features.

A bullet (●) at the beginning of a paragraph indicates that either a decision is required or further information is to be
provided by the purchaser. This information should be indicated on the datasheet(s), typical examples of which are
included as Annex A; otherwise it should be stated in the quotation request or in the order.

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 Special-purpose Couplings for Petroleum, Chemical, and Gas Industry
ServicesAPI STANDARD 671FIFTH EDITION, AUGUST 2020
1 Scope

This standard specifies the requirements for couplings for the transmission of power between the rotating shafts
of two machines in special-purpose applications in the petroleum, petrochemical, and natural gas industries.
Such applications are typically in large and/or high-speed machines, in services that can be required to operate
continuously for extended periods, are often unspared and are critical to the continued operation of the installation.
By agreement, it can be used for other applications or services.

Couplings covered by this standard are designed to accommodate parallel (or lateral) offset, angular misalignment,
and axial displacement of the shafts without imposing unacceptable mechanical loading on the coupled machines.
It is applicable to gear, metallic flexible element, quill-shaft and torsionally resilient type couplings. Torsional
damping and resilient type couplings are detailed in Annex B, gear-type couplings are detailed in Annex C, and
quill-shaft type couplings are detailed in Annex D.
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This standard covers the design, materials of construction, manufacturing quality, inspection, and testing of
special-purpose couplings.

This standard does not define criteria for the selection of coupling types for specific applications.

This standard is not applicable to other types of couplings, such as clutch, hydraulic, eddy-current, rigid, radial
spline, chain, and bellows types.

2 Normative References

The following referenced documents are indispensable for the application of this document. For dated references,
only the edition cited applies. For undated references, the latest edition of the referenced document (including
any amendments) applies.

ANSI/AGMA 9000,1 Flexible Couplings—Potential Unbalance Classification

ANSI/AGMA 9002-C14, Bores and Keyways for Flexible Couplings (Inch Series)

ANSI/AGMA 9003-B08, Flexible Couplings—Keyless Fits (Inch Series)

ANSI/AGMA 9004-B08, Flexible Couplings—Mass Elastic Properties and other Characteristics (Inch Series)

ANSI/AGMA 9103-B08, Flexible Couplings—Keyless Fits (Metric Series)

ANSI/AGMA 9104-A06, Flexible Couplings—Mass Elastic Properties and other Characteristics (Metric Series)

ANSI/AGMA 9112-B15, Bores and Keyways for Flexible Couplings (Metric Series)

ANSI/ASME B1.1,2 Unified inch screw threads, UN and UNR thread form

ANSI B11.19-2010, Performance Requirements for Safeguarding

BS EN ISO 80079-36:2016, Explosive atmospheres. Non-electrical equipment for explosive atmospheres. Basic
method and requirements

1
American Gear Manufacturers Association, 500 Montgomery Street, Suite 350, Alexandria, Virginia 22314-1560, www.
agma.org.
2
ASME International, Three Park Avenue, New York, New York 10016-5990, www.asme.org.
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 2 API Standard 671

ISO 262, ISO general-purpose metric screw threads—Selected sizes for screws, bolts and nuts

ISO 286-2, Geometrical product specifications (GPS)—ISO code system for tolerances on linear sizes—Part 2:
Tables of standard tolerance classes and limit deviations for holes and shafts

3 Terms, Definitions, Acronyms, and Abbreviations

3.1 Terms and Definitions

For the purposes of this document, the following terms and definitions apply.

3.1.1
AFD
adjustable frequency drive
A device that allows speed adjustment accomplished by electrical frequency variation in an AC motor.

3.1.2
angular misalignment
(double-engagement couplings)
Two minor angles between the extension of each machine centerline and the centerline of the structure joining
the two flexible elements.

3.1.3
angular misalignment
(single-engagement couplings)
Minor angle between the extensions of two machine-shaft centerlines.

NOTE If the shaft centerlines do not intersect, a single-engagement coupling is not appropriate.

3.1.4
assembly balance
Procedure in which a completely assembled coupling is balanced as a unit.

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3.1.5
assembly balance check
Procedure in which an assembled coupling is placed on a balancing machine and the residual unbalance is
measured.

NOTE An assembly balance check can be performed on a component balanced coupling, or on an assembly-balanced
coupling.

3.1.6
axial displacement
Change in the relative axial position of the adjacent shaft ends of two coupled machines, usually caused by
thermal expansion.

3.1.7
component balance
Procedure for achieving coupling balance in which the components or factory assembled subassemblies are
balanced separately before assembly of the coupling.

3.1.8
continuous torque rating
Coupling manufacturer’s declared maximum steady state torque that the coupling is capable of transmitting
continuously for unlimited periods.

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