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'''United States Standard thread''' ('''USS thread'''), also known as '''Sellers Standard thread''',<ref name="adt">{{Cite book | last = Adt | first = Howard | title = Screw Thread Production to Close Limits | page = 7 | url = http://books.google.com/books?id=WL9CAAAAIAAJ&pg=PA7 | accessdate = 2009-01-16}}</ref> '''Franklin Institute thread'''<ref name="adt"/> and '''American Standard thread''',<ref>{{cite web | title = History of screw threads | url = http://www.sizes.com/tools/thread_history.htm | accessdate = 2011-04-24}}</ref> is a standard for inch based threaded [[fastener]]s and [[washer (hardware)|washer]]s.

The USS standard is no longer supported.  It, together with the [[SAE International|SAE]] fastener standard, was incorporated into the [[Unified Thread Standard]].  However, the term, ''USS'', continues to be used occasionally today to describe inch based threaded fasteners with a coarse thread pitch and inch based washers that are a little bit larger than the corresponding SAE washer.  The Unified Thread Standard uses the term UNC (Unified National Coarse) to describe a fastener that previously would have been designated USS and the Unified Thread Standard uses the term UNF (Unified National Fine) to describe a fastener that would have previously been designated SAE.

==Thread form==
The thread form is defined by flats at the tip and root of the thread form. This flat length is defined as the pitch divided by eight. The thread depth, which is from flat to flat is 0.6495 times the pitch.<ref>{{Cite book | last = American Railway Master Mechanics' Association | title = Report of proceedings of the annual convention of the American Railway Master Mechanics' Association | publisher = American Railway Master Mechanics' Association | volume = 49 | year = 1916 | page = 71 | url = http://books.google.com/books?id=YKYR9N5cbUQC&pg=PA71}}</ref> For {{frac|1|4}} inch screws and larger, the pitch is defined as:

:<math>P = 0.24 \sqrt {D + 0.625} - 0.175</math>

where P is the pitch and D is the diameter of the rough stock.<ref name="adt"/>

==History and use==
[[William Sellers]] originally developed the USS thread, and set forth many of its details in his paper, "A System of Screw Threads and Nuts", presented in April 1864 to the [[Franklin Institute]].  In 1898, the standard for metric threaded fasteners was established.<ref>{{citation | last = Oberg | first = Erik | last2 = Jones | first2 = Franklin D. | last3 = McCauley | first3 = Christopher J. | last4 = Heald | first4 = Ricardo M. | title = [[Machinery's Handbook]] | edition = 27th | year = 2004 | page = 1726 | publisher = [[Industrial Press]] | isbn = 978-0-8311-2700-8 | postscript =.}}</ref> The metric standard used the same thread geometry as the USS standard but differed in that the dimensions and pitch were based on metric units.  In 1906, the [[Association of Licensed Automobile Manufacturers|A.L.A.M]]  developed what would be the [[Society of Automotive Engineers|SAE]] thread standard for threaded fasteners based on the USS standard but with a finer thread pitch.<ref>{{Cite book
  | last = 
  | first = 
  | title = The Society of Automobile Engineers 1912 Transactions Part II, pages 43-44
  | publisher = 
  | url = http://books.google.com/books?id=nl1kAAAAMAAJ&pg=PA43&lpg=PA43&dq=alam+sae+threads&source=web&ots=gBTCULeSDs&sig=3xlLXOa5WIW-ARpLw66uui11Pvk&hl=en&sa=X&oi=book_result&resnum=4&ct=result#PPA43,M1
  | accessdate = 2009-01-17}}</ref><ref>{{Cite book
  | last = 
  | first = 
  | title = Browning's Industrial Magazine, page 590
  | publisher = The Browning Press
  | url = http://books.google.com/books?id=fisAAAAAMAAJ&pg=PA590&dq=a.l.a.m.+screw&ei=zAmASbGjMZHKMs3M-LwE&client=firefox-a#PPA590,M1
  | accessdate = 2009-01-27}}</ref>

A Unified Thread Standard UNC thread is mechanically interchangeable with a USS thread of the same diameter.  However, there are tolerance and other differences between a thread compliant with the USS thread and a Unified Thread Standard UNC thread.<ref>{{cite book|last=Oberg|title=Machinery's Handbook 23rd edition|year=1988|page=1479}}</ref> The Unified Thread Standard for quarter inch and larger threaded fasteners was adopted on November 18, 1948,<ref>{{cite web
  | last = National Bureau of Standards
  | first = 
  | title = Screw-Thread Standards for Federal Services, pages 10-12
  | publisher = 
  | url = http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA279290&Location=U2&doc=GetTRDoc.pdf
  | accessdate = 2009-01-16}}</ref> and was subsequently adopted for smaller thread series.

==See also==
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* [[British Standard Whitworth]]
*[[Screw thread#History of standardization|"History of standardization" section of the screw thread article]]
*[[ISO metric screw thread]]
{{multicol-break}}
*[[Screw]]
*[[Screw thread]]
{{multicol-end}}

==References==
{{reflist}}

==External links==
*[http://anniversary.asme.org/2005landmarks3.shtml The United States Standard Screw Threads (1864)]
*[http://openwetware.org/images/4/4c/Sellers.1864.pdf Text of Seller's presentation to the Franklin Institute, April 21, 1864]
*[http://www.jaredzone.info/2011/01/metric-threaded-fasteners.html AMERICAN NATIONAL STANDARD METRIC SCREWS]

[[Category:Screws]]
[[Category:Mechanical standards]]