Unified Inch Screw Threads (UN, UNR, and UNJ Thread Forms)

AN A MERICAN NA TIONAL S T AND ARD

ASME B1.1-2019

[Revision of ASME B1.1-2003 (R2018)]

ASME B1.1-2019

[Revision of ASME B1.1-2003 (R2018)]

Unified Inch Screw

Threads (UN, UNR, and UNJ Thread Forms)

AN AMERICAN NATIONAL STANDARD

Two Park Avenue • New York, NY • 10016 USA

Date of Issuance: June 30, 2020

The next edition of this Standard is scheduled for publication in 2024. This Standard will become effective 1 year after the Date of Issuance.

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CONTENTS

Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v

Committee Roster . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii

Correspondence With the B1 Committee . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . viii

1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

2 Screw Thread Profile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

3 Screw Thread Series . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

4 Screw Thread Classes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

5 Screw Thread Allowance and Tolerance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

6 Screw Thread Designation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106

1. Dimensional Accommodation of Coating or Plating for 60-Deg Threads . . . . . . . . . . . . . 110

2. Limits of Size for Standard (UN, UNR, and UNJ) and Special (UNS, UNRS, and UNJS) Series of Threads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115

9 Thread Form Tolerances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117

10 Formulas and Nomenclature for Thread Form . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142

11 Tables of Basic Dimensions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142

Nonmandatory Appendices

FOREWORD

ASME B1.1, Unified Inch Screw Threads, is an integrated system of threads for fastening purposes in mechanisms and structures. Its outstanding characteristic is its general interchangeability of threads, achieved through the standardiza- tion of thread form, diameter-pitch combinations, and limits of size.

This Standard is the outgrowth of and supersedes previous editions that were published as ASME B1-1924, ASME B1.1- 1935, ASME B1.1-1949, ASME B1.1-1960, ASME B1.1-1974, ASME B1.1-1982, ASME B1.1-1989, and ASME B1.1-2003.

The achievements represented by ASME B1.1 in development, standardization, and unification are the result of the cooperation and coordination of many organizations, including The American Society of Mechanical Engineers (ASME), SAE International (formerly Society of Automotive Engineers), National Institute of Science and Technology (formerly National Bureau of Standards), Committee B1, the former National Screw Thread Commission, the former Interdepart- mental Screw Thread Committee, British Standards Institution, CSA Group (formerly Canadian Standards Association), and American National Standards Institute (ANSI).

This Standard has its basis in the work done more than a century ago by William Sellers in the United States and Sir Joseph Whitworth in Great Britain. Through the intervening years, there have been many developments and revisions, culminating in the Unified Thread Standard approved and adopted for use by all inch-using countries.

The unification of screw thread standards meets the need for interchangeability among the billions of fasteners made in different countries and used in the complex equipment of modern technology. Unification is equally important for the international trade in mechanisms of all kinds and the servicing of transportation equipment that moves from country to country. Unification is therefore not only highly advantageous but also essential.

Complete unification of certain thread series and six tolerance classes in sizes 1∕4 in. and larger was achieved with the

signing of an accord in Washington, D. C. on November 18, 1948. Since that time, unification has extended to smaller sizes. Developed by Technical Committee No. 1 of the International Organization for Standardization (ISO), the unified inch standard that was adopted as ISO 5864 is parallel to the ISO metric screw thread system. Both systems have a common basic profile. The standard was subject to Quadripartite Standardization Agreement (QSTAG) 247 in the ABCA Army

Standardization Program of America, Britain, Canada, and Australia.

Throughout this history, special attention has been given to the practical aspects of thread standardization, and many details of ASME B1.1 result from studies and tests based on real-world use. For example, users communicated the need for free assembly in high-production industries and the desirability of providing for threads that require a coating. The tolerance classes 2A and 2B were developed to meet these two major requirements as well as to provide a general standard for externally and internally threaded fasteners. Thread symbols and nomenclature are now consistent with ASME B1.7. Thread acceptability now follows ASME B1.3.

In 1992, ASME B1.30 implemented eight-place decimal and rounding rules that are mandatory for all new editions and future revisions of ASME B1 documents. To comply with this decision, the 2003 edition, ASME B1.1-2003

1. revised some of the values in Table 2 and created Table E-1 of Nonmandatory Appendix E, which identifies and lists the revised dimensions from Table 2 in the ASME B1.1-1989 edition. The majority of the dimensional changes are within

   ±0.0001 in. As stated in para. 8.2.1, the values in this former Table 2, now Tables 2A and 2B, and Table E-1 should be considered acceptable until a future revision of this Standard makes the values in Tables 2A and 2B the only acceptable values.

2. moved Table 3B, which provides calculated values for various UNS (unified specials), to Nonmandatory Appendix

   D. The ASME B1 Committee strongly urges users to adopt the standard thread sizes in Tables 2A and 2B instead of those listed in Table D-1.

3. moved Tables 31 through 40, which include some values that differ from those derived by use of the formulas in paras. 5 and 8, to Nonmandatory Appendix D and renamed these Tables D-2 through D-11. (All future special threads should be based on calculations only.)

4. eliminated all references to thread engagement from this Standard. Past changes in the thread form designation of the “basic” thread height from 0.7500H to 0.62500H confused the calculation of percent of thread engagement.

5. included the definition of “functional diameter” and added the term to Table 2 in the same column as “pitch diameter,” since both characteristics have the same limits of size.

6. explained in greater depth the effects of coating on threads (see section 7).

   Changes to this 2019 edition include the splitting of ASME B1.1-2003 Table 2, which contained values for both internal and external threads for UN and UNR only, into two tables, Table 2A: Limits of Size for Standard Series External Threads (UN, UNR, and UNJ), and Table 2B: Limits of Size for Standard Series Internal Threads (UN, UNR, and UNJ). The metric translation of this Standard was removed, as were Tables D-2 through D-11 (formerly Tables 31 through 40 in ASME B1.1- 1989).

   Finally, the UNJ thread profile, formerly defined in ASME B1.15, was added to this Standard. Following the U.S. Depart- ment of Defense (DoD) approval of SAE AS8879C-2003, ASME B1 Subcommittee 15 recognized it would become the standard used by the aerospace industry for this thread form. As a result, Subcommittee 15 recommended that the technical information from ASME B1.15 be included in ASME B1.1 for non-aerospace applications.

   The UNJ thread form having the enlarged root radius in the external thread was introduced to minimize size and weight in parts for applications requiring high-fatigue strength under high working-stress levels, as in aerospace applications. It is also appropriate for designs in commercial products where stresses are critical. To meet these requirements, the UNJ external thread root radius is designed to be between 0.15011107P and 0.18042196P and the minor diameter of the mating internal thread is increased to ensure the necessary clearance.

   This Standard includes Classes 2A and 2B UNJ screw threads. Either Class 2A or Class 3A UNJ threads are appropriate for commercial applications commensurate with the fatigue and stress levels required.

   The UNJ thread form is the UN thread form modified to 0.562500H, which allows the 0.18042196P maximum root radius in the external thread. The first known U.S. standard of similar thread form was SAE AS-82, published in March 1942, which describes a modified American National thread form to 75% h basic thread depth and specifies 0.10800P to 0.1800P root radius in the external thread. This thread was symbolized NR, National Round, and was developed for aircraft engine applications.

   Tension fatigue testing of aircraft fasteners in 1942 demonstrated the importance of the external thread root contour in the fatigue life of a screw thread rolled after heat treatment. Fatigue testing isolated the elements of good external thread root design. The root should be radiused, not sharp. Theoretically, it should be a continuous circular arc, blending smoothly with the thread flanks. The radius should be as large as possible within the allowable design form. The root contour should also be smooth throughout and free of any imperfections, tool marks, or other minor notches. Recognizing the need for improved 160,000 psi tensile strength bolts, the DoD published MIL-B-7838A, the bolt procurement specification for aircraft applications based on the unified thread form of 0.62500H, in April 1952, thus acknowledging a larger external root radius requires a shallower internal thread depth to clear the flank tangency

   point.

   The root radius of the external thread was increased to 0.15011107P minimum and 0.18042196P maximum for the 180,000 psi and higher tensile strength bolts. This external thread form was developed in 1955 by the aerospace fastener industry and was known as the “Hi R” thread form.

   Through coordinated effort with the SAE E-25 Engine and Propeller Standard Utility Parts Committee and the Aero- space Industries Association National Aerospace Standards Committee (NASC), the DoD developed and published in September 1960 the thread specification MIL-S-8879, which features the “Hi R” thread root radius in the external thread and the internal thread modified to 0.562500H basic. In aircraft gas turbine engines, the high-temperature threaded fasteners exhibited better elevated temperature performance using MIL-S-8879 UNJ thread root radius, as the stress- rupture life of bolts was greatly improved.

   The UNJ thread form has been adopted by the aerospace industry as the all-purpose thread standard, except for electrical hardware and thread sizes 0.1380 and smaller, which may use the UN thread form.

   The UNJ profile as defined in this Standard is similar to SAE AS8879C-2003 (superseding MIL-S-8879C) and equivalent to ISO 3161:1977 for thread Classes 3A and 3B. British Standards Institution BS 4084:1978, including Amendment 1, is technically identical to ISO 3161:1977, except for Appendix A, which provides information for a 20-UNJ constant pitch series for diameters through 3 in.

   ASME B1.1-2019 was approved by the American National Standards Institute (ANSI) on August 26, 2019.

   
   

   ASME B1 COMMITTEE

   Standardization and Unification of Screw Threads

   
   

   (The following is the roster of the Committee at the time of approval of this Standard.)

   
   

   STANDARDS COMMITTEE OFFICERS

   1. L. Barrows, Chair

D. S. George, Vice Chair

D. Papert, Secretary

STANDARDS COMMITTEE PERSONNEL

A. L. Barrows, Swanson Tool Manufacturing, Inc.

1. Bly, Vermont Thread Gage

2. Borowski, Greenslade & Co., Inc.

H. J. Cox, Frank Cox Metrology, Ltd.

G. A. Cuccio, Capitol Manufacturing Co.

R. Dodge, Pennoyer Dodge Co.

D. Everett, National Institute of Standards and Technology

J. O. Gehret III, Gehret Gage, LLC

D. S. George, Michigan Metal Coating

J. R. Gervasi, Kerr Lakeside, Inc.

P. Holahan, Fastenal Co.

L. C. Johnson, Johnson Gage Co.

D. D. Katz, Precision Fittings

D. R. Maisch, PMC Lone Star

D. Miskinis, Consultant

D. Papert, The American Society of Mechanical Engineers

J. R. Popovic, Cleveland Specialty Inspection Services, Inc.

M. W. Rose, Glastonbury Southern Gage

P. Larouche, Alternate, Johnson Gage Co.

R. J. Hukari, Contributing Member, SPS Technologies

R. P. Knittel, Contributing Member, Consultant

1. R. Oas, Contributing Member, Seaway Bolt & Specials Corp.

2. Schwartz, Contributing Member, Consultant

B. F. Sheffler, Contributing Member, Consultant

D. Skierski, Contributing Member, Sterling Gage & Calibration, LLC

R. D. Strong, Contributing Member, Lear Corp.

C. J. Wilson, Contributing Member, Consultant

SUBCOMMITTEE 1 — UNIFIED SCREW THREADS

A. L. Barrows, Chair, Swanson Tool Manufacturing, Inc.

D. Miskinis, Vice Chair, Consultant

1. Bly, Vermont Thread Gage

2. Borowski, Greenslade & Co., Inc.

R. Dodge, Pennoyer Dodge Co.

D. S. George, Michigan Metal Coating

J. R. Gervasi, Kerr Lakeside, Inc.

P. Holahan, Fastenal Co.

L. C. Johnson, Johnson Gage Co.

P. Larouche, Johnson Gage Co.

M. Oliver, M. Oliver Consulting

S. Brahimi, Contributing Member, Industrial Fasteners Institute

M. Cox, Contributing Member, Consultant

R. J. Hukari, Contributing Member, SPS Technologies

J. C. Jennings, Contributing Member, Naval Surface Warfare Center, Philadelphia Division

X. Li, Contributing Member, China Productivity Center for Machinery Industry

E. Schwartz, Contributing Member, Consultant

B. F. Sheffler, Contributing Member, Consultant

R. D. Strong, Contributing Member, Lear Corp.

C. J. Wilson, Contributing Member, Consultant

CORRESPONDENCE WITH THE B1 COMMITTEE

General. ASME Standards are developed and maintained with the intent to represent the consensus of concerned interests. As such, users of this Standard may interact with the Committee by requesting interpretations, proposing revisions or a case, and attending Committee meetings. Correspondence should be addressed to: