Unified Thread Standard

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The Unified Thread Standard (UTS) defines a standard thread form and series—along with allowances, tolerances, and designations—for screw threads 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.

1 Origins
2 Basic profile
3 Designation
4 Preferred sizes
5 Gauging
6 Tolerance classes
7 References
8 See also
9 External links

[edit] Origins

See the "History of standardization" section of the screw thread article.

[edit] Basic profile

The pitch, P, is given by the distance between adjacent threads, and is related to TPI (teeth per inch or threads per inch) by:

$P=1/\mbox{TPI}\,$

The height is related to the pitch by the sine of the angle of 60°:

$H= P\sin{(60)} = \frac{\sqrt{3}}{2}P=0.866025 \times P\,$

The minor, major and effective pitch diameters (D_min, D_maj and D_p) are given by simple addition of the height. Generally, the major thread diameter is the given dimension.

$D_{min} = D_{maj} - 2\cdot\frac{5}{8} H = D_{maj} - 1.082532 \times P\,$

$D_{p} = D_{maj} - 2\cdot\frac{3}{8} H = D_{maj} - 0.649519 \times P \,$

[edit] 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)

[edit] Preferred sizes

major diameter [in]	pitch [1/in]			major diameter [mm]
major diameter [in]	coarse UNC	fine UNF	extra fine UNEF	major diameter [mm]
#0 = 0.0600	–	80		1.5240
#1 = 0.0730	64	72		1.8542
#2 = 0.0860	56	64		2.1844
#3 = 0.0990	48	56		2.5146
#4 = 0.1120	40	48		2.8448
#5 = 0.1250	40	44		3.1750
#6 = 0.1380	32	40		3.5052
#8 = 0.1640	32	36		4.1656
#10 = 0.1900	24	32		4.8260
#12 = 0.2160	24	28	32	5.4864
1/4	20	28	32	6.3500
5/16	18	24	32	7.9375
3/8	16	24	32	9.5250
7/16	14	20	28	11.1125
1/2	13	20	28	12.7000
9/16	12	18	24	14.2875
5/8	11	18	24	15.8750
3/4	10	16	20	19.0500
7/8	9	14	20	22.2250
1	8	12	20	25.4000

Note : sometimes "special" thread profiles ( UNS) are used e.g on a 0.619 inch (15.723mm) major dia -a pitch (1/20 inch ) i.e. 20 teeth per inch for a stronger assembly fit of external and internal threads.These may be design considerations in aerospace,defence and automobile applications.

[edit] Gauging

A screw thread gauging 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 gauge(s) which shall be used when inspecting those characteristics.

Currently this gauging UTS is controlled by:

ASME/ANSI B1.2-1983 Gauges And Gauging For Unified Inch Screw Threads
ASME/ANSI B1.3M-1992 Screw Thread Gauging Systems Dimensional Acceptability Inch Metric Screw Threads

These Standards provide essential specifications and dimensions for the gauges 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 gauges and measuring equipment. The basic purpose and use of each gauge are also described. It also establishes the criteria for screw thread acceptance when a gauging system is used.

[edit] 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 fits 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-NoGo gauge.