understanding thread - how read thread for national unified threads (unc, unf, unef, un)
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Written by: johnsinit • Edited by: Lamar Stonecypher Published Sep 29, 2009 • Related Guides: ISO | UN
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A Guide to Thread Standards – How to Interpret American ThreadDesignation
How to read thread specifications? What does 2-56 UNF- 2B mean? What is the difference between theDIN and the UN standards? What does "LH" stands for in thread designation? Read this helpful article andget the answer to these questions.
Introduction
As was already covered in the first part of the thread standard series, there are numerous thread standardsused in different countries. ISO threads (designated according to DIN standard) are used in Europe andhave metric dimensions, as also those of UK (BSP standards). The UNC-UNF threads are dimensioned ininches and are used in American-developed systems. There are also more exotic threads – like UNJF andNPT (used in pipe systems). Every thread is designated according to certain rules – and in this article wewill provide an overview of these rules.
American Standard Thread Designations
Major American standards use the following code: DD-PP-SS-XA/B-(LH)
DD is the major diameter of the thread. Thus 3/8-inch thread will have a 3/8 standing at the left most of thethread specification. The dimensions usually come at margins of an inch (1/2, ¼, 1/8 ,1/16 etc). It must benoted that threads with a diameter smaller than ¼ have been given special numbers, from 0 to 10.
PP stands for number of threads per inch. Thus, -20 means that there are 20 threads in each inch along thethread.
SS is the thread standard. Mostly, it is defined by the diameter and number of coils, but sometimes there areseveral standards (such as UNC and UNJC) that use the same numbers. Thus, this should not be omitted –also helping to prevent errors. If you will write ¼-30 –UNF beside your thread, anyone looking at this willnote that something is wrong, as Unified National Fine standards defines 28 threads per inch.
X is the level of closing between the male and female thread, called class. 1 is the most loose thread,meaning there is less contact between threads and more clearance. 2 is the most common class and 3 isthe most precise fit, used in accurate applications.
A or B is used to specify whether it is an external or internal thread. External thread is designated A andinternal – B.
LH is used to specify the unusual left hand – used sometimes for locking against rotational force or otherspecial applications.
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Typical designations examples
So, after reading the above paragraph, you should know that:
2-56 UNF 2A means it is a #2 thread (sometimes called also 0.086, as this is 0.086 inch) with 56 threadsper inch, manufactured, dimensioned and checked according to Unified National Fine Standard and this is anormal external thread.
¼ -20 – UNC -3B –LH means this is a ¼ inch diameter thread with 20 thread per inch, manufactured inaccordance with Unified National Coarse Standard. The thread is internal and precise and also left handed.
7/8 – 20 – UNEF -2B stands for internal normal precision thread with a diameter of 7/8 inch and 20 threadsper inch, which is Unified National Extra Fine Standard.
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Written by: johnsinit • Edited by: Lamar Stonecypher Published Sep 18, 2009 • Related Guides: ISO | UN
Leave a commentHome > Engineering > Mechanical Engineering > Machine Design
Overview of Thread Standards – Part One
There are many types of thread standards depending on the country and purposes. You have probablyheard about ISO, UNF, and UNC. But there are more thread standards present – some used for specificpurposes and some used in certain countries. Familiarize yourself with the topic by reading below.
Introduction
There are many threads used in different applications.
The standard ISO, UNC, UNF and other, the extra fine UNEF threads, used in delicate mechanisms, theNPT threads used in fittings, British BSF, and BSP.
Here is a list of some most common threads:
International Organization for Standardization (ISO)
National Coarse (NC USA)
Unified National Coarse (UNC USA)
National Fine (NF USA)
Unified National Fine (UNF USA)
Unified National Extra Fine (UNEF USA),
Unified National 8-, 12- and 16 pitch series (UN USA),
Special Threads of American National Form (UNS USA),
National Taper Pipe 1:16 (NPT USA),
National Taper Pipe Dryseal 1:16 (NPTF USA),
National Standard Straight Pipe (NPS USA),
National Standard Straight Pipe for free fitting mechanical (NPSM USA)
National Standard Internal Straight Pipe Dryseal (NPSF USA)
British Standard Withworth Coarse (BSW)
British Standard Fine (BSF)
British Standard Pipe (BSP)
British Standard Pipe Taper (BSPT)
Similar yet Different
While most American and European international standard threads use a thread angle of 60 degrees, varioustypes of thread standards available in Great Britain have a thread angle of 55 degree.
Sometimes the specifications and standards are complementary or supplementary. For example UNC, theUnified Coarse Thread is very much comparable with the ISO metric standard, and those can sometimes beused together.
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Thread Data
The data for each standard thread should contain the following features:
Incline angle – is the angle formed by the two adjacent flanks.
Pitch – is the distance from one thread groove to the next.
Pitch diameter – It is defined as the measured length between the pitch points taken in thegroove between the threads. This is an important measure of the quality. There are also major andminor diameters that can be calculated from the pitch diameter and the incline angle.
Tolerance – is the limit of the acceptable deviation from the intended design to the manufacturedthreads. This is very important for testing purposes done in various ways. One of the importantmethods to assure the quality standards is gauging inspection. This is the physical inspection ofthe thread for the features using a device with an established standard size. The result of thegauging inspection will be a pass/fail or go-no go condition.
Depth of a thread - is the length of the space measured between the base and the top. This isthe distance from the crest to the base of the root.
The thread name is usually a combination of the above data. Understanding thread specifications will becovered in the next article.
Thread Standards
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Written by: johnsinit • Edited by: Lamar Stonecypher Published Apr 1, 2009
Leave a commentHome > Engineering > Mechanical Engineering > Machine Design
Guide to Fasteners - Part I - Overview of Bolts and Screws
Bolts and screws are commonly used fasteners to keep the machine parts together. There are many differenttypes of bolts and screws – varying types of heads, threads and more. Different versions are useddepending on the application requirement.
Introduction
Many people know that without fasteners like screws, bolts and nuts, no industry can survive. These are thebasic hardware parts required by any machines, whether simple household items to complex high-endmachines in industries. It is many times essential to use high quality nuts, bolts and screws. From smallfurniture crafting to oil sector industries the most essential hardware items include these fasteners – boltedjoint is the most trustworthy part connection – moreover, it usually gives an option of dismantling andreinstalling. Anyway, the generalization and systematization process and the most modern technologicaladvancements have changed the field of fasteners for the best.
Materials
One of primary feature used by most of the machines is threaded joint as part of the design. The bolted jointcan hold severe loads and the parts can be separated and reconnected repeatedly. Today, these fastenersare available in different materials like stainless steal, copper, brass and alloy steels. With the advancementof the technologies even porcelain and glasses are used as screws for many applications including canningjars and electrical insulators.
Bolt vs Screw
Though many times confused by amateurs, bolts and screws are different types of fasteners. Bolts areheaded fasteners with external threads suitable for non-tapered nut. They require a nut (or some otherthread) as a counter - to lock the joint. The bolts are made with uniform threads satisfying the standardspecifications such as UNJ, UNR, MJ etc. and are used in most industrial applications – secured with aspring-washer or a locktite. To sum it up, bolts are designed to use as a fastener placed through alreadydrilled holes in the machine parts and these requires a nut to tighten it from the other end.
Screws are headed externally threaded types of fasteners without meeting the standard specifications of thebolts. They are shafts with provision at one end to turn the screw and have a helical thread on its surface,which is capable of piercing strongly into the surface. They do not require a nut and are used mostly inhome-based application – such a connecting a picture to the wall. It is actually the simplest example ofconverting torque into linear force.
Head types
Depending up on the design of the heads the screws/bolts are named differently. Pan head is a low discwith chamfered outer edge. Hexagon head bolts are generally used for bigger loads. Socket head is acylinder with (usually) Hex socket (Allen) opening – those bolts usually have the smallest head. Flat head orcountersunk screws are conically shaped with tapering inner face and flat outer face. It will allow the screwto sink into the material – providing “flat surface”. There are also many special types of bolts with differentheads including oval or raised head, bugle head, cheese head. Even square head screws are present – aswell as there are headless screws and bolts.
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Bolt and Screw types
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