Unified Thread Standard

From Wikipedia, the free encyclopedia

Jump to: navigation, search

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.

Contents

[edit] Origins

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

[edit] Basic profile

The basic profile of all UTS threads is the same as that of all ISO metric screw threads, only the commonly used values for Dmaj and P differ between the two standards.

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 1/8 and the innermost 1/4 of the height H of the V-shape are cut off from the profile.

The pitch P is the distance a screw advances during a 360° rotation. It is related to a TPI (teeth per inch or threads per inch) figure by:

P=\frac1{\mathrm{TPI}}

The height H and the pitch P are related through the 60° angle via

H = \sin(60^\circ)\cdot P = \frac{\sqrt 3}2\cdot P = 0.866025404 \times P

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

D_\mathrm{min} = D_\mathrm{maj} - 2\cdot\frac58\cdot H = D_\mathrm{maj} - \frac{ 5 {\sqrt 3}}{8}\cdot P = D_\mathrm{maj} - 1.082532 \times P
D_\mathrm{p} = D_\mathrm{maj} - 2\cdot\frac38\cdot H = D_\mathrm{maj} - \frac{ 3 {\sqrt 3}}{8}\cdot P = D_\mathrm{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]		 threads per inch
 major
diameter
[mm]		 tap drill size
coarse
UNC		 fine
UNF		 extra fine
UNEF		 coarse
UNC		 fine
UNF		 extra fine
UNEF
#0 = 0.0600 – 80 1.5240 3/64 in
#1 = 0.0730 64 72 1.8542 #53 #53
#2 = 0.0860 56 64 2.1844 #50 #50
#3 = 0.0990 48 56 2.5146 #47 #45
#4 = 0.1120 40 48 2.8448 #43 #42
#5 = 0.1250 40 44 3.1750 #38 #37
#6 = 0.1380 32 40 3.5052 #36 #33
#8 = 0.1640 32 36 4.1656 #29 #29
#10 = 0.1900 24 32 4.8260 #25 #21
#12 = 0.2160 24 28 32 5.4864 #16 #14
1/4 20 28 32 6.3500 #7 #3
5/16 18 24 32 7.9375 F I
3/8 16 24 32 9.5250 5/16 in Q
7/16 14 20 28 11.1125 U 25/64 in
1/2 13 20 28 12.7000 27/64 in 29/64 in
9/16 12 18 24 14.2875 31/64 in 33/64 in
5/8 11 18 24 15.8750 17/32 in 37/64 in
3/4 10 16 20 19.0500 21/32 in 11/16 in
7/8 9 14 20 22.2250 49/64 in 13/16 in
1 8 12 20 25.4000 7/8 in 59/64 in

Formula for number screw sizes: Diameter = Screw Size # × 0.013" + 0.060"
Example: #10 × 0.013" + 0.060" = 0.190" diameter

Sometimes "special" thread profiles (UNS) are used, for example a 0.619 inch (15.723 mm) major diameter with 20 teeth per inch. This gives a stronger assembly fit of external and internal threads. These may be design considerations in aerospace, defense 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.

[edit] References

  • 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

[edit] See also

[edit] External links

Retrieved from "http://en.wikipedia.org/wiki/Unified_Thread_Standard"
Personal tools
Languages