View source for Trapezoidal thread form

{{Short description|Screw thread profiles with trapezoidal outlines}}
{{more footnotes|date=April 2015}}
[[File:Trapetsgänga.png|thumb|right|[[International System of Units|Metric]] trapezoid thread, TR-40×7.]]
[[File:acme thread.jpg|thumb|A [[gender of connectors and fasteners|male]] Acme thread]]
'''Trapezoidal thread forms''' are [[screw thread#Form|screw thread profiles]] with [[trapezoid]]al outlines. They are the most common forms used for [[leadscrew]]s (power screws). They offer high strength and ease of manufacture. They are typically found where large loads are required, as in a [[Vise (tool)|vise]] or the leadscrew of a [[lathe (metal)|lathe]].<ref>{{harvnb|Bhandari|2007|pp=202–204}}</ref> Standardized variations include [[screw thread#Lead, pitch, and starts|multiple-start threads]], [[screw thread#Gender|left-hand threads]], and self-centering threads (which are less likely to bind under lateral forces).

The original trapezoidal thread form, and still probably the one most commonly encountered worldwide, with a 29° thread angle, is the '''Acme thread form''' ({{IPAc-en|ˈ|æ|k|m|iː}} {{respell|AK|mee}}). The Acme thread was developed in 1894 as a profile well suited to power screws that has various advantages over the [[square thread form|square thread]],<ref group=note>See:
*  Simpson, John and Proffitt, Michael, ed.s, "acme", ''Oxford English Dictionary: Additions Series'', vol. 3 (Oxford, England:  Oxford University Press, 1997), [https://books.google.com/books?id=gUGcAQAAQBAJ&pg=RA2-PR1 p. 10.]
*  The Acme thread was proposed by Albert Man Powell, then president of the Powell Planer Co. of Worcester, Massachusetts.  See:
**  Powell, A.M. (24 January 1895) [https://babel.hathitrust.org/cgi/pt?id=osu.32435063039879;view=1up;seq=52;size=150 "The proposed new screw thread,"] ''American Machinist'', '''18''' (4) :  66.
**  Powell, A.M. (24 January 1895) [https://babel.hathitrust.org/cgi/pt?id=osu.32435063039879;view=1up;seq=55;size=150 "A new screw thread,"] ''American Machinist'', '''18''' (4) :  69–71.
**  For a brief biography of Albert Man Powell (with photograph), see:  Rice, Franklin P., ''The Worcester of Eighteen Hundred and Ninety-Eight:  Fifty Years a City'' (Worcester, Massachusetts:  F.S. Blanchard & Co.,1899),[https://archive.org/stream/worcesterofeight00rice#page/150/mode/2up pp. 150–151.]
*  The name "Acme thread" was proposed by A. W. [Albert Ward] Handy (1845 October 7 (Bristol, Rhode Island) – 1915 August 27 (Malden, Massachusetts)), who was then a sales representative of the Acme Machinery Co. of Cleveland, Ohio, which made various machine tools.  See:  (Editorial staff) (3 January 1895) [https://babel.hathitrust.org/cgi/pt?id=osu.32435063039879;view=1up;seq=13;size=150 "A proposed new standard screw thread,"] ''American Machinist'', '''18''' (1) :  1–2.</ref> which had been the form of choice until then. It is easier to cut with either [[Threading (manufacturing)#Single-point threading|single-point threading]] or [[tap and die|die]] than the square thread is (because the latter's shape requires [[tool bit]] or die tooth geometry that is poorly suited to cutting). It wears better than a square thread (because the wear can be compensated for) and is stronger than a comparably sized square thread. It allows smoother engagement of the [[Split nut|half nuts]] on a lathe leadscrew than a square thread.<ref name="Oberg1908p30">{{Harvnb|Oberg|1908}}, [https://books.google.com/books?id=nplKAAAAMAAJ&pg=PA29 p. 30].</ref><ref name="Jones1964pp176-177">{{Harvnb|Jones|1964}}, [https://books.google.com/books?id=DFP7cLe3VIUC&pg=PA176 pp. 176–177].</ref> It is one of the strongest symmetric thread profiles; however, for loads in only one direction, such as vises, the asymmetric [[buttress thread]] profile can bear greater loads.

The '''trapezoidal metric thread form''' is similar to the Acme thread form, except the thread angle is 30°.<ref name="bhandari204">{{harvnb|Bhandari|2007|p=204}}</ref><ref name="trap">[http://www.gewinde-normen.de/en/trapezoidal-coarse-thread.html Trapezoidal coarse thread]</ref><ref name="trap1">[http://www.gewinde-normen.de/en/trapezoidal-fine-thread.html Trapezoidal fine thread]</ref> It is codified by [[Deutsches Institut für Normung|DIN]] 103.<ref name="Green1996p1703">{{Harvnb|Machinery's Handbook|1996|p=1703}}</ref> While metric screw threads are more prevalent worldwide than imperial threads for ''triangular'' thread forms, the imperially sized ''Acme'' threads predominate in the trapezoidal thread form.

==Acme thread characteristics==

[[File:Acme thread.svg|thumb|250px|Basic Acme thread profile]]

The Acme thread form has a 29° [[thread angle]] with a thread height half of the [[screw thread#Lead, pitch, and starts|pitch]]; the apex (or ''crest'') and valley (or ''root'') are flat.  This shape is easier to machine (faster cutting, longer tool life) than a square thread.  The tooth shape also has a wider base which means it is stronger (thus, the screw can carry a greater load) than a similarly sized square thread.  This thread form also allows for the use of a [[split nut]], which can compensate for nut wear.<ref>{{harvnb|Bhandari|2007|pp=203–204}}</ref>

The line of General Purpose (GP) Acme threads (ASME/ANSI B1.5-1997) are not designed to sustain external radial loads and both the nut and bolt are, ideally, independently supported (the nut by a linear guide and the screw by shaft bearings).  This is due to the need to avoid "wedging" of the thread flanks when subjected to radial loads, which would contribute substantially to friction forces and thread wear.  However, there is a Centralizing Acme-thread standard (also specified in ASME/ANSI B1.5-1997) which caters to applications where the threads are not radially supported, where the roots and crests of opposing threads are designed to come into contact before the flanks do under radial loads.  This adds the requirement that the sum of the allowances (clearances) and tolerances on the [[Screw thread#Design|major diameters]] of nut and bolt be less than the sum of the allowances on the [[Screw thread#Design|pitch diameters]] (PD).  The drawback is that for a given amount of end play (axial clearance due solely to PD clearances), closer tolerances and a cleaner work environment are necessitated in the application of a Centralizing Acme thread.

Compared to square threads, disadvantages of the Acme thread form are lower efficiency due to higher friction and some radial load on the nut (angular offset from square).<ref name="bhandari204" />

When created before 1895, Acme screw threads were intended to replace square threads and a variety of threads of other forms used chiefly for the purpose of traversing on machines, tools, etc.  Acme screw threads are now extensively used for a variety of purposes.  Long-length Acme threads are used for controlled movements on machine tools, testing machines, jacks, aircraft flaps, and conveyors.  Short-length threads are used on valve stems, hose connectors, bonnets on pressure cylinders, steering mechanisms, and camera lens movement.<ref>{{Citation | last = American Society of Mechanical Engineers | author-link = American Society of Mechanical Engineers | title = ASME B1.5 - 1997 Acme Screw Threads | publisher = ASME Press | year = 1997 | isbn = 0-7918-2482-9 | postscript =.}}</ref>

The thread form shown in the figure (Basic ACME thread profile) is called "basic".  The actual thread heights on both the internal (nut) and external (bolt) threads differ from {{sfrac|''P''|2}} by ''allowances'' (or ''clearances''):
* A minimum root-crest clearance of {{cvt|0.01|in}} (diametral) between opposing threads with 10 tpi (threads-per-inch) or fewer, and {{cvt|0.005|in}} for finer pitches.  (This is also true for the [[Screw thread#Design|minor diameters]] of the Centralizing Acme thread, though not its major diameters, where the allowance is made less than the PD allowance.)
* A PD allowance, which makes the PD smaller than "basic" in the case of the GP and external Centralizing Acme threads, but greater in the case of the internal Centralizing Acme thread.

The net effect is that the minimum thread heights are greater than "basic" for internal and external GP threads and for external Centralizing threads, and the maximum height for internal Centralizing Acme threads is shorter than "basic".  The maximum diameter (within tolerance) at the crest of the external threads (called the ''max. major diameter of external thread'') is that of the basic thread form and equals the "nominal diameter", ''D'', stated in the screw's designation.  The minimum diameter (within tolerance) at the crest of the internal thread (called the ''min. minor diameter of internal thread'') is that of the basic thread form and equals the nominal diameter minus twice the basic thread height (i.e. ''D''&nbsp;−&nbsp;''P'').

There is also a "Stub Acme" thread standard, identical in all respects to the one just described ''except'' for the height of the basic thread being 0.3''P''.

{| class="wikitable"
|+ Standard Acme thread pitches for diameters in [[Imperial and US customary measurement systems|Imperial and US customary units]]<ref>{{harvnb|Shigley|Mischke|Budynas|2003|p=400}}</ref>
|-
! Nominal <br/>diameter (in) !! Thread <br/>pitch (in) !! Thread <br/>density (in{{sup|−1}})
|-
| {{frac|1|4}} || {{frac|1|16}} || 16
|-
| {{frac|5|16}} || {{frac|1|14}} || 14
|-
| {{frac|3|8}} || {{frac|1|12}} || 12
|-
| {{frac|1|2}} || {{frac|1|10}} || 10
|-
| {{frac|5|8}} || {{frac|1|8}} || 8
|-
| {{frac|3|4}}, {{frac|7|8}} || {{frac|1|6}} || 6
|-
| 1, {{frac|1|1|4}} || {{frac|1|5}} || 5
|-
| {{frac|1|1|2}}, {{frac|1|3|4}}, 2 || {{frac|1|4}} || 4
|-
| {{frac|2|1|2}} || {{frac|1|3}} || 3
|-
| 3 || {{frac|1|2}}|| 2
|}

== Metric trapezoidal thread characteristics ==
[[File:Not Go Ring.JPG|thumb|right|A [[thread pitch gauge]] with metric Tr{{nnbsp}}30 threads (30&nbsp;mm diameter, 6&nbsp;mm [[thread pitch|pitch]], tolerance class 7e).]]
In case of the trapezoidal thread form the angle is 30° instead of 29°.<ref name="trap" /><ref name="trap1" /> All dimensions are in millimeters.<ref name="trap" /><ref name="trap1" />

Trapezoidal threads are defined as follows by [[International Organization for Standardization|ISO]] standards:

: Tr{{nnbsp}}60×9

where ''Tr'' designates a trapezoidal thread, ''60'' is the nominal diameter in millimeters, and ''9'' is the pitch in millimeters. When there is no suffix it is a single start thread. If there is a suffix then the value after the multiplication sign is the [[lead (engineering)|lead]] and the value in the parentheses is the pitch. For example:

: Tr{{nnbsp}}60×18(P9)LH

would denote two starts, as the lead divided by the pitch is two. The "LH" denotes a left hand thread.<ref name="bhandari205">{{harvnb|Bhandari|2007|p=205}}</ref>
{| class="wikitable"
|+ Standard trapezoidal thread pitches for [[Metric system|metric]] diameters<ref name="bhandari205" />
|-
! Nominal <br/>diameter (mm) 
! Thread <br/>pitch (mm) 
|-
| 10 || 2 
|-
| 12 || 3 
|-
| 14, 16 || 4
|-
| 24, 28 || 5 
|-
| 32, 36 || 6 
|-
| 40, 44 || 7 
|-
| 48, 52 || 8 
|-
| 60     || 9 
|-
| 70, 80 || 10 
|-
| 90, 100 || 12 
|}

==Other trapezoidal threads==
For maintaining [[air conditioning]] systems using [[R134a]] gas, a non standard "ACME" thread is specified for gas canisters.<!-- comment: this is strictly not in accordance ASME/ANSI B1.5-1988 since the standard dictates 10 tpi, so the word ACME may be misleading --><ref>{{cite web|url=https://www.epa.gov/mvac/unique-fittings-label-colors-mvac-refrigerants|title=Unique Fittings & Label Colors for MVAC Refrigerants|date=7 January 2016|publisher=[[United States Environmental Protection Agency]] |access-date=10 July 2017 }}</ref>
{| class="wikitable"
|+ Thread pitch for R134a gas canisters
|-
! Nominal <br/>diameter (in) !! Thread <br/>pitch (in) !! Thread <br/>density (in{{sup|−1}})
|-
| {{frac|1|2}} || {{frac|1|16}} || 16
|}

==See also==
* [[Buttress thread]]
* [[Leadscrew]]
* [[Ball screw]]

==Notes==
{{Reflist|group=note}}

==References==
{{Reflist|30em}}

==Bibliography==
*{{Citation | last = Bhandari | first = V B | title = Design of Machine Elements | publisher = Tata McGraw-Hill | year = 2007 | url = https://books.google.com/books?id=f5Eit2FZe_cC | isbn = 978-0-07-061141-2}}.
* {{Citation |last=Flather |first=John Joseph |year=1895 |title=Rope-driving: a treatise on the transmission of power by means of fibrous ropes |edition= |publisher=J. Wiley & Sons |location=New York |url=https://books.google.com/books?id=hQ9IAAAAIAAJ&pg=PR1 |postscript=. |lccn=06034155}} 
* {{MachinerysHandbook25e}}
* {{Citation | last = Jones | first = Franklin D. | year = 1964 | title = Machine shop training course | edition = 5th | volume = 1 | publisher = Industrial Press | location = New York | isbn = 978-0-8311-1039-0 | oclc = 661244 | postscript =. }}
* {{Citation | last = Oberg | first = Erik | year = 1908 | title = Handbook of small tools: comprising threading tools, taps, dies, cutters, drills, and reamers, together with a complete treatise on screw-thread systems | publisher = John Wiley & Sons | location = New York | url = https://books.google.com/books?id=nplKAAAAMAAJ&pg=PA29 | postscript =. ''Co-edition, 1908, Chapman & Hall, London.''}}
*{{Citation | last1 = Shigley | first1 = Joseph E. | last2 = Mischke | first2 = Charles R. | last3 = Budynas | first3 = Richard Gordon | title = Mechanical Engineering Design | publisher = McGraw Hill | year = 2003 | edition = 7th | url = https://books.google.com/books?id=j8xscqTxWUgC | isbn = 978-0-07-252036-1}}.

== External links ==
* [https://web.archive.org/web/20170318005519/http://www.diracdelta.co.uk:80/science/source/a/c/acme%20thread/source.html Standard Acme threads]
* [http://www.bornemann-gewindetechnik.de/wordpress/wp-content/uploads/2016/04/Trapezoidal-thread-EN.pdf Nominal dimension for trapezoidal thread profiles]
* [http://www.gewinde-normen.de/en/trapezoidal-coarse-thread.html Trapezoidal coarse thread]
* [http://www.gewinde-normen.de/en/trapezoidal-fine-thread.html Trapezoidal fine thread]
* [http://www.engineersedge.com/hardware/acme-stub-thread.htm Stub ACME Thread Sizes and Data Chart]
* [https://www.asme.org/products/codes-standards/b15-1997-acme-screw-threads ASME: Acme Screw Threads B1.5-1997]
* [https://www.technobg.com/en/trapezoidal-screw-threads-din-103  Manufacturer of trapezoidal screws]
* [https://www.threadspecification.com/tr/  Metric Trapezoidal Threads]
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