Unified Thread Standard
Encyclopedia
The Unified Thread Standard (UTS) defines a standard
thread form and series—along with allowances, tolerances, and designations—for screw thread
s commonly used in the United States
and Canada
. It has the same 60° profile as the ISO metric screw thread
used in the rest of the world, but the characteristic dimensions of each UTS thread (outer diameter and pitch) were chosen as an inch
fraction rather than a round millimeter value. The UTS is currently controlled by ASME
/ANSI
in the United States.
Each thread in the series is characterized by its major diameter Dmaj and its pitch, P. UTS threads consist of a symmetric V-shaped thread. In the plane of the thread axis, the flanks of the V have an angle
of 60° to each other. The outermost 0.125 and the innermost 0.25 of the height H of the V-shape are cut off from the profile.
The pitch P is the distance between thread peaks. For UTS threads, which are single-start threads, it is equal to the lead
, the axial distance that the screw advances during a 360° rotation. UTS threads do not usually use the pitch parameter; instead a parameter known as threads per inch (TPI) is used, which is the reciprocal
of the pitch.
The relationship between the height H and the pitch P is found using the following equation:
In an external (male) thread (e.g., on a bolt), the major diameter Dmaj and the minor diameter Dmin define maximum dimensions of the thread. This means that the external thread must end flat at Dmaj, but can be rounded out below the minor diameter Dmin. Conversely, in an internal (female) thread (e.g., in a nut), the major and minor diameters are minimum dimensions, therefore the thread profile must end flat at Dmin but may be rounded out beyond Dmaj.
The minor diameter Dmin and effective pitch diameter Dp are derived from the major diameter and pitch as
This number pair is optionally followed by the letters UNC, UNF or UNEF if the diameter-pitch combination is from the “coarse”, “fine” or “extra fine” series, and may also be followed by a tolerance class.
Example: 6-32 UNC 2B (major diameter: 0.1380 inch, pitch: 32 tpi)
* 1-12 was formerly a widespread standard, but 1-14 is current UNF.
The following formula is used to calculate the major diameter of a numbered screw greater than or equal to 0: Major diameter = Screw # × 0.013" + 0.060". For example, a number 10 calculates as: #10 × 0.013" + 0.060" = 0.190" major diameter.
A Unified Miniature screw thread series is defined in ANSI standard B1.10, for fasteners of 0.3 to 1.4 millimetres (0.0118 to 0.0551 inch) diameter. These sizes are intended for watch
es, instruments, and miniature mechanisms and are interchangeable with threads made to ISO Standard 68. These screw sizes are denoted by multiple zeroes, i.e. #000. The formula for number sizes smaller than size #0 is given by Major diameter = 0.060" - Zero size × 0.013", with the zero size being the number of zeroes after the first. So a #00 screw is .047" dia, #000 is .034" dia, etc.
The number series of machine screws once included more odd numbers and went up to #16 or more. Standardization efforts in the late 19th and the early part of the 20th century reduced the range of sizes considerably. Now, it is less common to see machine screws larger than #14, or odd number sizes other than #1, #3 and #5. Even though #14 and #16 screws are still available, they are not as common as sizes #0 through #12.
Sometimes "special" diameter and pitch combinations (UNS) are used, for example a 0.619 in (1.6 cm) major diameter with 20 threads per inch. UNS threads are rarely used for bolts, but rather on nuts, tapped holes, and threaded OD's. Because of this UNS taps are readily available. Most UNS threads have more threads per inch than the correlating UNF or UNEF standard, therefore they are often the strongest thread available. Because of this they are often used in applications where high stresses are encountered, such as machine tool spindles
or automotive spindles
.
Currently this gaging UTS is controlled by:
These standards provide essential specifications and dimensions for the gages used on Unified inch screw threads (UN, UNR, UNJ thread form) on externally and internally threaded products. It also covers the specifications and dimensions for the thread gages and measuring equipment. The basic purpose and use of each gage are also described. It also establishes the criteria for screw thread acceptance when a gaging system is used.
used with assembled parts.
Classes 1A, 2A, 3A apply to external threads; Classes 1B, 2B, 3B apply to internal threads.
Class 1 threads are loosely fitting threads intended for ease of assembly or use in a dirty environment.
Class 2 threads are the most common. They are designed to maximize strength considering typical machine shop capability and machine practice.
Class 3 threads are used for closer tolerances.
Thread class refers to the acceptable range of pitch diameter for any given thread. The pitch diameter is indicated as Dp in the figure shown above. There are several methods that are used to measure the pitch diameter. The most common method used in production is by way of a Go/no go gauge.
Standardization
Standardization is the process of developing and implementing technical standards.The goals of standardization can be to help with independence of single suppliers , compatibility, interoperability, safety, repeatability, or quality....
thread form and series—along with allowances, tolerances, and designations—for screw thread
Screw thread
A screw thread, often shortened to thread, is a helical structure used to convert between rotational and linear movement or force. A screw thread is a ridge wrapped around a cylinder or cone in the form of a helix, with the former being called a straight thread and the latter called a tapered thread...
s commonly used in the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
and Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
. It has the same 60° profile as the ISO metric screw thread
ISO metric screw thread
The ISO metric screw threads are the world-wide most commonly used type of general-purpose screw thread. They were one of the first international standards agreed when the International Organization for Standardization was set up in 1947.-Basic profile:...
used in the rest of the world, but the characteristic dimensions of each UTS thread (outer diameter and pitch) were chosen as an inch
Inch
An inch is the name of a unit of length in a number of different systems, including Imperial units, and United States customary units. There are 36 inches in a yard and 12 inches in a foot...
fraction rather than a round millimeter value. The UTS is currently controlled by ASME
American Society of Mechanical Engineers
The American Society of Mechanical Engineers is a professional body, specifically an engineering society, focused on mechanical engineering....
/ANSI
American National Standards Institute
The American National Standards Institute is a private non-profit organization that oversees the development of voluntary consensus standards for products, services, processes, systems, and personnel in the United States. The organization also coordinates U.S. standards with international...
in the United States.
Origins
- See the "History of standardization" section of the screw thread article.
Basic profile
Thread angle
The thread angle of a screw is the angle between the threads. This is a defining factor for the shape of a screw thread. Standard values include:...
of 60° to each other. The outermost 0.125 and the innermost 0.25 of the height H of the V-shape are cut off from the profile.
The pitch P is the distance between thread peaks. For UTS threads, which are single-start threads, it is equal to the lead
Lead (engineering)
Lead is the axial advance of a helix or screw during one complete turn The lead for a screw thread is the axial travel for a single revolution....
, the axial distance that the screw advances during a 360° rotation. UTS threads do not usually use the pitch parameter; instead a parameter known as threads per inch (TPI) is used, which is the reciprocal
Multiplicative inverse
In mathematics, a multiplicative inverse or reciprocal for a number x, denoted by 1/x or x−1, is a number which when multiplied by x yields the multiplicative identity, 1. The multiplicative inverse of a fraction a/b is b/a. For the multiplicative inverse of a real number, divide 1 by the...
of the pitch.
The relationship between the height H and the pitch P is found using the following equation:
In an external (male) thread (e.g., on a bolt), the major diameter Dmaj and the minor diameter Dmin define maximum dimensions of the thread. This means that the external thread must end flat at Dmaj, but can be rounded out below the minor diameter Dmin. Conversely, in an internal (female) thread (e.g., in a nut), the major and minor diameters are minimum dimensions, therefore the thread profile must end flat at Dmin but may be rounded out beyond Dmaj.
The minor diameter Dmin and effective pitch diameter Dp are derived from the major diameter and pitch as
Designation
The standard designation for a UTS thread is a number indicating the nominal (major) diameter of the thread, followed by the pitch measured in threads per inch. For diameters smaller than ¼ inch, the diameter is indicated by an integer number defined in the standard, for all other diameters, the inch figure is given.This number pair is optionally followed by the letters UNC, UNF or UNEF if the diameter-pitch combination is from the “coarse”, “fine” or “extra fine” series, and may also be followed by a tolerance class.
Example: 6-32 UNC 2B (major diameter: 0.1380 inch, pitch: 32 tpi)
| Major diameter [in (mm)] |
Threads per inch | Tap drill size | ||||
|---|---|---|---|---|---|---|
| Coarse (UNC) | Fine (UNF) | Extra fine (UNEF) | Coarse | Fine | Extra fine | |
| #0 = 0.06 (1.524 mm) | – | 80 | 3/64 in | |||
| #1 = 0.073 (1.8542 mm) | 64 | 72 | #53 | #53 | ||
| #2 = 0.086 (2.1844 mm) | 56 | 64 | #50 | #50 | ||
| #3 = 0.099 (2.5146 mm) | 48 | 56 | #47 | #45 | ||
| #4 = 0.112 (2.8448 mm) | 40 | 48 | #43 | #42 | ||
| #5 = 0.125 (3.175 mm) | 40 | 44 | #38 | #37 | ||
| #6 = 0.138 (3.5052 mm) | 32 | 40 | #36 | #33 | ||
| #8 = 0.164 (4.1656 mm) | 32 | 36 | #29 | #29 | ||
| #10 = 0.19 (4.826 mm) | 24 | 32 | #25 | #21 | ||
| #12 = 0.216 (5.4864 mm) | 24 | 28 | 32 | #16 | #14 | |
| 1/4 in | 20 | 28 | 32 | #7 | #3 | |
| 5/16 in | 18 | 24 | 32 | F | I | |
| 3/8 in | 16 | 24 | 32 | 5/16 in | Q | |
| 7/16 in | 14 | 20 | 28 | U | 25/64 in | |
| 1/2 in | 13 | 20 | 28 | 27/64 in | 29/64 in | |
| 9/16 in | 12 | 18 | 24 | 31/64 in | 33/64 in | |
| 5/8 in | 11 | 18 | 24 | 17/32 in | 37/64 in | |
| 3/4 in | 10 | 16 | 20 | 21/32 in | 11/16 in | |
| 7/8 in | 9 | 14 | 20 | 49/64 in | 13/16 in | |
| 1 (25.4 mm) | 8 | 14* | 20 | 7/8 in | 59/64 in |
The following formula is used to calculate the major diameter of a numbered screw greater than or equal to 0: Major diameter = Screw # × 0.013" + 0.060". For example, a number 10 calculates as: #10 × 0.013" + 0.060" = 0.190" major diameter.
A Unified Miniature screw thread series is defined in ANSI standard B1.10, for fasteners of 0.3 to 1.4 millimetres (0.0118 to 0.0551 inch) diameter. These sizes are intended for watch
Watch
A watch is a small timepiece, typically worn either on the wrist or attached on a chain and carried in a pocket, with wristwatches being the most common type of watch used today. They evolved in the 17th century from spring powered clocks, which appeared in the 15th century. The first watches were...
es, instruments, and miniature mechanisms and are interchangeable with threads made to ISO Standard 68. These screw sizes are denoted by multiple zeroes, i.e. #000. The formula for number sizes smaller than size #0 is given by Major diameter = 0.060" - Zero size × 0.013", with the zero size being the number of zeroes after the first. So a #00 screw is .047" dia, #000 is .034" dia, etc.
The number series of machine screws once included more odd numbers and went up to #16 or more. Standardization efforts in the late 19th and the early part of the 20th century reduced the range of sizes considerably. Now, it is less common to see machine screws larger than #14, or odd number sizes other than #1, #3 and #5. Even though #14 and #16 screws are still available, they are not as common as sizes #0 through #12.
Sometimes "special" diameter and pitch combinations (UNS) are used, for example a 0.619 in (1.6 cm) major diameter with 20 threads per inch. UNS threads are rarely used for bolts, but rather on nuts, tapped holes, and threaded OD's. Because of this UNS taps are readily available. Most UNS threads have more threads per inch than the correlating UNF or UNEF standard, therefore they are often the strongest thread available. Because of this they are often used in applications where high stresses are encountered, such as machine tool spindles
Spindle (tool)
In machine tools, a spindle is a rotating axis of the machine, which often has a shaft at its heart. The shaft itself is called a spindle, but also, in shop-floor practice, the word often is used metonymically to refer to the entire rotary unit, including not only the shaft itself, but its bearings...
or automotive spindles
Spindle (automobile)
In an automobile, the spindle is a part of the suspension system that carries the hub for the wheel and attaches to the upper and lower control arms. The spindle is referred to as an Upright in UK-built vehicles and in areas dominated by these cars like Australia, New Zealand etc.-Design:There are...
.
Gaging
A screw thread gaging system comprises a list of screw thread characteristics that must be inspected to establish the dimensional acceptability of the screw threads on a threaded product and the gage(s) which shall be used when inspecting those characteristics.Currently this gaging UTS is controlled by:
- ASME/ANSI B1.2-1983 Gages And Gaging For Unified Inch Screw Threads
- ASME/ANSI B1.3-2007 Screw Thread Gaging Systems for Acceptability: Inch and Metric Screw Threads (UN, UNR, UNJ, M, and MJ)
These standards provide essential specifications and dimensions for the gages used on Unified inch screw threads (UN, UNR, UNJ thread form) on externally and internally threaded products. It also covers the specifications and dimensions for the thread gages and measuring equipment. The basic purpose and use of each gage are also described. It also establishes the criteria for screw thread acceptance when a gaging system is used.
Tolerance classes
A classification system exists for ease of manufacture and interchangeability of fabricated threaded items. Most (but certainly not all) threaded items are made to a classification standard called the Unified Screw Thread Standard Series. This system is analogous to the fitsEngineering fit
Fit refers to the mating of two mechanical components. Manufactured parts are very frequently required to mate with one another. They may be designed to slide freely against one another or they may be designed to bind together to form a single unit...
used with assembled parts.
Classes 1A, 2A, 3A apply to external threads; Classes 1B, 2B, 3B apply to internal threads.
Class 1 threads are loosely fitting threads intended for ease of assembly or use in a dirty environment.
Class 2 threads are the most common. They are designed to maximize strength considering typical machine shop capability and machine practice.
Class 3 threads are used for closer tolerances.
Thread class refers to the acceptable range of pitch diameter for any given thread. The pitch diameter is indicated as Dp in the figure shown above. There are several methods that are used to measure the pitch diameter. The most common method used in production is by way of a Go/no go gauge.
Related standards
- ASME/ANSI B1.1 – 2003 Unified Inch Screw Threads, UN & UNR Thread Form
- ASME/ANSI B1.10M – 2004 Unified Miniature Screw Threads
- ASME/ANSI B1.15 – 1995 Unified Inch Screw Threads, UNJ Thread Form
See also
- British Association screw threadsBritish Association screw threadsBritish Association or BA screw threads are a largely obsolete set of small screw threads, the largest being 0BA at 6 mm diameter. They were, and to some extent still are, used for miniature instruments and modelling....
- British standard pipe threadBritish standard pipe threadThe British Standard Pipe thread is a family of standard screw thread types that has been adopted internationally for interconnecting and sealing pipe ends by mating an external with an internal thread.-Types:...
- British Standard WhitworthBritish Standard WhitworthBritish Standard Whitworth is one of a number of imperial unit based screw thread standards which use the same bolt heads and nut hexagonal sizes, the others being British Standard Fine thread and British Standard Cycle...
- ISO metric screw threadISO metric screw threadThe ISO metric screw threads are the world-wide most commonly used type of general-purpose screw thread. They were one of the first international standards agreed when the International Organization for Standardization was set up in 1947.-Basic profile:...
- List of screw drives
- National pipe threadNational pipe threadNational Pipe Thread Tapered Thread is a U.S. standard for tapered threads used on threaded pipes and fittings. The taper rate for all NPT threads is measured by the change of diameter over distance. The angle between the taper and the center axis of the pipe is 1° 47′ 24″...
- Nut (hardware)Nut (hardware)A nut is a type of hardware fastener with a threaded hole. Nuts are almost always used opposite a mating bolt to fasten a stack of parts together. The two partners are kept together by a combination of their threads' friction, a slight stretch of the bolt, and compression of the parts...
- William SellersWilliam SellersWilliam Sellers was a mechanical engineer, manufacturer, businessperson, and inventor who filed more than 90 patents, most notably the design for the United States standard screw thread...
- United States Standard threadUnited States Standard threadUnited States Standard thread , also known as Sellers Standard thread, Franklin Institute thread and American Standard thread, is a standard for inch based threaded fasteners and washers....
External links
- Unified Coarse standard and drill sizes
- Unified Fine standard and drill sizes
- Unified Coarse/Fine diameters and tap drill sizes (US units)
- Unified Coarse/Fine tap drill sizes (US units)
- Imperial Metric fastening size conversion charts
- International Thread Standards
- Conversion chart Whitworth/BSF/AF and metric
- Spanner Jaw Sizes Additional information and spanner jaw size table.
- Unified Screw Threads Series