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Standards Craftech Industries, 2013 8 Dock Street, Hudson, NY 12534 Definitions and Terminology Website: www.craftechind.com Blog: info.craftechind.com/blog E-mail: [email protected] Phone: 518-828-5001 or 800-833-5130 Fax: 518-828-9468 Acme Threads The term “Acme thead” refers to the original and most widely used trapezoidal thread form. The Acme thread was developed in the latter half of the 19th century for use in power screws. It was developed as an alternative to the square thread. It is easier to cut than a square thread. It also wears better and is stronger than a comparably sized square thread. The Acme thread form has a 29° thread angle.

Acme Thread Classes Acme threaded classes are 2G, 3G, 4G, and 5G, respectively. 2G is normally employed and is the kind which is widely available commercially. Classes 3G, 4G, and 5G have progressively tighter tolerances. For example, a 5G screw usually requires special fitting and is used only where a permanent seal is required.

Acme Thread Designations Acme thread screws are designated by the major diameter, number of threads per inch, the number of threads, (if it is not a single thread screws), the name of the series (acme), its class, and the letters LH, if it is not right-handed. For example, 1/4-16 acme -2G is a screw having a 1/4 inch diameter and 16 threads per inch. Its class is 2G and it is a conventional right-handed screw. Alternately, they may be designated using pitch and lead, for example, a 2-1/2-.5P-.6L-ACME-4G would indicate a right handed screw, single threaded, with a 2.5 inch major diameter, a .5 inch pitch and a lead of 0.6 inches.

American Standard Pipe Thread (NPT) In pipe threading, the major diameter gets bigger as the thread moves into the pipe so that as it tightens it creates a tight seal that prevents leakage. Some standard pipe fittings are made with a straight thread and use an o-ring or crush washer to seal fluids in.

Chamfer The chamfer is the end of the screw which first engages the nut. It is usually ground at a different angle than the remaining threads. For example, in the unified series the chamfer is ground at 45 degrees while the succeeding threads are milled at a 30 degree angle.

Gender Matched pairs of threads can be described as male and female. For example, a screw has male threads while the hole it screws into has female threads.

2 Hose Coupling Thread There are three classes of hose coupling threads. They are NH, NPSH, and NHR. The hose coupling threads illustrated in Craftech’s catalog are standard NH threads. The NPSH thread is designed to be used with a gasket. NHR threads are considered formed threads such as those used in garden hoses. For further information, see ANSI B2.4-1966, American National Standard Hose Coupling Threads.

Lead The lead is distance that the nut moves parallel to the screws major axis when the nut is given one complete revolution.

LH LH is used as the designation for a left-hand screw. A screw is right handed by default therefore a screw with no designation is a right hand screw.

Major Diameter The basic major diameter is the largest diameter on the thread of a screw.

Metric International Organization for Standardization (ISO) Threads This is the system adopted under British Standard B.S. 6343. It is also the metric standard which is most widely used in the U.S. There are three main classes of fit, which are designated using a number of sub-classes. The sub-classes are termed tolerance zones and fundamental deviations. Tolerance specifications are given by numbers, and fundamental deviations are given by letters. The letters are capitalized to designate an internal thread, while external threads are indicated by lower case. The major classes for external threads are designated as close, medium, and free. All are specified for a particular engagement length. Of the three classes, the medium class is the one most commonly used. The ISO fit between a bolt and a nut is unique, in that the internal crest is flat, while the external root is rounded.

Metric ISO Designations ISO threads are designated by the letter “M”, followed by the major diameter size in millimeters, the pitch in millimeters, and the combined class number. For example, a M7 x 1-5H indicates a nut of 7 millimeters major diameter, which has a pitch of 1 and is close fitting. A slash between two last numbers, e.g. 7H/7g, indicates a composite fit between two mating parts. In practice, three common, combined fits are often specified. 5H/4h indicates a close fit, 6H/6g is a medium fit, and 7H/8g marks a free fit.

3 Metric Threads Système Internationale (SI) This system was adopted by the International Congress for the standardization of screw thread in 1898, in Zurich, Switzerland. The threads are similar to those specified by the American Standard. The major difference between the two standards is that the thread depth is greater. Standard practice mandates flat crests and rounded roots. Depth, d, should be between .0735 and .6855 of the pitch length, where d is the distance from the base of the rounded root to the flat crest. The root radius, r, is between .0633 and .054 of the pitch length. The flat crest is specified as .125 times the pitch. A clearance between the root and its mating crest is specified as being .054 times the pitch. Screws are classified according to major diameter size as given in millimeters. While the SI standard is not as common as the ISO Standard, commonly employed in Britain and the U.S., it does form the basis for those used currently by some European countries.

Minor Diameter The minor diameter is the screw thread’s smallest diameter. It is equal to two times the minor radius.

Multiple Threads “Multiple threads” is the term employed to describe the practice of cutting two or more threads side by side. Consequentially, in such cases, the lead will be two, three, or more times the pitch.

NPSC Designates American National Standard straight pipe for couplings.

NPSM Designates American National Standard straight pipe thread for free-fitting mechanical joints.

Pitch The pitch is the distance between two adjacent threads, with reference to the center line of the screw’s shaft. It is equal to the inverse of the number of threads per inch.

Pitch Cone The pitch cone’s diameter is the mean of each succeeding groove and crest diameter, such that a cone is formed whose projection equals the taper angles of the tapered screw. In other words, the pitch cone is the imaginary cone in a bevel gear that rolls without slipping on a pitch surface of another gear. It is the tapered screw’s analogue to the pitch cylinder (see below).

4 Pitch Cylinder The pitch cylinder is the same as the pitch cone in concept, differing only in shapes. It describes a cylinder whose diameter is equal to that of the mean, averaged using the crest and the groove diameters.

Pitch Diameter thread angle a = minor diameter b = pitch diameter The pitch diameter is the pitch cylinder’s pitch c = major diameter diameter.

Stub Acme Threads Stub acme threads are similar to standard acme threads, except that the thread height is more a b c truncated. For example, whereas basic thread height for the acme is .5 times the pitch, the corresponding height for the stub acme is .3 times the pitch.

Tensile Stress Area The tensile stress area is computed using the mean of the major (or pitch) diameter and the minor diameter. For metals, the general rule is that threaded rod will yield the same tensile stress as unthreaded rod of this mean diameter. For plastics, this information is not always directly applicable in terms of design criteria.

Thread Angle The thread angle of a screw is the angle between the threads. Standard values include:

Thread Standard Thread Angle Most V-threads (including ISO, NPT, and UTS) 60° Whitworth threads 55° Pipe Thread, British Standard Pipe Thread 55° Knuckle/Round thread 30° Acme thread 29° Metric trapezoidal threads 30° Buttress threads 45° German buttress threads 30° Square threads 0° Steel conduit thread aka Panzergewinde 80° Löwenherz thread 53° 8’ Bodmer thread 50°

5 Unified Thread Standard (UTS) The Unified Thread standard 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 millimeter value. The UTS is currently controlled by the ASME/ANSI in the United States.

United States Standard Thread (USS Thread) The United States Standard thread system was the predecessor to the Unified Thread Standard. Also known as Sellers Standard thread, Franklin Institute thread, and American Standard thread, USS thread is no longer supported and has been incorporated into the Unified Thread Standard. It was similar to the UTS in almost all respects. The major difference between the two is that the American Standard did not always account for the working tolerances needed to produce a non-binding part. See ANSI B1.1-1949)

UN This is one of two root and crest configurations allowed by the unified thread series specifications. In this series, rounding of root and crest is optional. Unified threads are specified by three parameters. These are: nominal diameter, the number of threads per inch, and the thread series. Thus, a 1/4x.25x20 screw has a size designation of 1/4, a 0.25 inch major diameter, and 20 threads per inch. For screws less than 1/4 inch in diameter, sizes are specified by numbers, which do not correspond to their major diameters.

8UN, 12UN, 16UN A constant-pitch UN series specified for large-diameter threads.

UTF Tolerance Classes Thread class refers to the acceptable range of pitch diameter for any given thread. Classes 1A, 2A, and 3A apply to external threads. Classes 1B, 2B, and 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, the most common, are designed to maximize strength considering typical machine shop capability and machine practice. Class 3 threads are used for closer tolerances.

6 UTF Thread Designations UTF threads are generally designated by the major diameter, number of threads per inch, series initials, class, the initial indicating whether it is an external or internal thread, and the designation LH, if it is not right-handed. For example, a 1/4-20 UNC-2B-LH, would indicate an internally threaded, left- handed fastener with a major diameter of .25 inch, of the UN class.

UNC One of three thread series specified by UTS and UNF standards. This is the most commonly used thread series and is used where good holding in softer materials is desired. By using UNC threads, maximum resistance to stripping of the internal thread is obtained.

UNF This is the 2nd of the UTF threads. Threads are finer than UNC. It is often specified where tensile strength is preferred to holding power, or where the screw’s engagement length is short.

UNEF This is the finest of the three thread series specified by the UTF.

UNR UNR screws are identical to UTF standard screws except that rounding of root and crests is mandated and must be of a specified radius. The usual radius is R=.14434 times the pitch. All other UTF specs apply.

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