![acme thread](https://upload.wikimedia.org/wikipedia/commons/6/64/Acme_thread.jpg)
Trapezoidal thread forms are
screw thread profiles with
trapezoid
A quadrilateral with at least one pair of parallel sides is called a trapezoid () in American and Canadian English. In British and other forms of English, it is called a trapezium ().
A trapezoid is necessarily a Convex polygon, convex quadri ...
al outlines. They are the most common forms used for
leadscrew
A leadscrew (or lead screw), also known as a power screw or translation screw,Bhandari, p. 202. is a screw (simple machine), screw used as a linkage (mechanical), linkage in a machine, to translate Rotation, turning motion into linear motion. ...
s (power screws). They offer high strength and ease of manufacture. They are typically found where large loads are required, as in a
vise
A vise or vice (British English) is a mechanical apparatus used to secure an object to allow work to be performed on it. Vises have two parallel jaws, one fixed and the other movable, threaded in and out by a screw and lever.
A vise grip is no ...
or the leadscrew of a
lathe
A lathe () is a machine tool that rotates a workpiece about an axis of rotation to perform various operations such as cutting, sanding, knurling, drilling, deformation, facing, and turning, with tools that are applied to the workpiece to c ...
. Standardized variations include
multiple-start threads,
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 ( ). The Acme thread was developed in 1894 as a profile well suited to power screws that has various advantages over the
square thread
The square thread form is a common screw thread profile, used in high load applications such as leadscrews and jackscrews. It gets its name from the square cross-section of the thread.Bhandari, p. 203. It is the lowest friction and most effi ...
,
[See:
* Simpson, John and Proffitt, Michael, ed.s, "acme", ''Oxford English Dictionary: Additions Series'', vol. 3 (Oxford, England: Oxford University Press, 1997)]
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
"The proposed new screw thread,"
''American Machinist'', 18 (4) : 66.
** Powell, A.M. (24 January 1895
"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
pp. 150–151.
* The name "Acme thread" was proposed by A. W. lbert WardHandy (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
"A proposed new standard screw thread,"
''American Machinist'', 18 (1) : 1–2. which had been the form of choice until then. It is easier to cut with either
single-point threading or
die
Die, as a verb, refers to death, the cessation of life.
Die may also refer to:
Games
* Die, singular of dice, small throwable objects used for producing random numbers
Manufacturing
* Die (integrated circuit), a rectangular piece of a semicondu ...
than the square thread is (because the latter's shape requires
tool bit
A tool bit is a non-rotary cutting tool used in metal lathes, shapers, and planers. Such cutters are also often referred to by the set-phrase name of single-point cutting tool, as distinguished from other cutting tools such as a saw or water jet ...
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
half nuts on a lathe leadscrew than a square thread.
p. 30
pp. 176–177
It is one of the strongest symmetric thread profiles; however, for loads in only one direction, such as vises, the asymmetric
buttress thread
Buttress thread forms, also known as sawtooth thread forms or breech-lock thread forms. are screw thread profiles with an asymmetric shape, having one square face and the other slanted. They are most commonly used for leadscrews where the load i ...
profile can bear greater loads.
The trapezoidal metric thread form is similar to the Acme thread form, except the thread angle is 30°.
[Trapezoidal coarse thread](_blank)
/ref>
/ref> It is codified by DIN 103.[.] 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
The Acme thread form has a 29° thread angle
The thread angle of a screw is the included angle between the thread flanks, measured in a plane containing the thread axis.. This is a defining factor for the shape of a screw thread
A screw thread, often shortened to thread, is a helical stru ...
with a thread height half of the 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.
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 major diameters of nut and bolt be less than the sum of the allowances on the 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).
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.
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 by ''allowances'' (or ''clearances''):
* A minimum root-crest clearance of (diametral) between opposing threads with 10 tpi (threads-per-inch) or fewer, and for finer pitches. (This is also true for the 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'' − ''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''.
Metric trapezoidal thread characteristics
In case of the trapezoidal thread form the angle is 30° instead of 29°. All dimensions are in millimeters.
Trapezoidal threads are defined as follows by ISO
ISO is the most common abbreviation for the International Organization for Standardization.
ISO or Iso may also refer to: Business and finance
* Iso (supermarket), a chain of Danish supermarkets incorporated into the SuperBest chain in 2007
* Iso ...
standards:
: Tr60×9
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
Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cu ...
and the value in the parentheses is the pitch. For example:
: Tr60×18(P9)LH
would denote two starts, as the lead divided by the pitch is two. The "LH" denotes a left hand thread.
Other trapezoidal threads
For maintaining air conditioning
Air conditioning, often abbreviated as A/C or AC, is the process of removing heat from an enclosed space to achieve a more comfortable interior environment (sometimes referred to as 'comfort cooling') and in some cases also strictly controlling ...
systems using R134a
1,1,1,2-Tetrafluoroethane (also known as norflurane (INN), R-134a, Freon 134a, Forane 134a, Genetron 134a, Green Gas, Florasol 134a, Suva 134a, or HFC-134a) is a hydrofluorocarbon (HFC) and haloalkane refrigerant with thermodynamic properties ...
gas, a non standard "ACME" thread is specified for gas canisters.
See also
* Buttress thread
Buttress thread forms, also known as sawtooth thread forms or breech-lock thread forms. are screw thread profiles with an asymmetric shape, having one square face and the other slanted. They are most commonly used for leadscrews where the load i ...
* Leadscrew
A leadscrew (or lead screw), also known as a power screw or translation screw,Bhandari, p. 202. is a screw (simple machine), screw used as a linkage (mechanical), linkage in a machine, to translate Rotation, turning motion into linear motion. ...
* Ball screw
A ball screw (or ballscrew) is a mechanical linear actuator that translates rotational motion to linear motion with little friction. A threaded shaft provides a helical raceway for ball bearings which act as a precision screw. As well as bein ...
Notes
References
Bibliography
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External links
Standard Acme threads
Nominal dimension for trapezoidal thread profiles
ASME: Acme Screw Threads B1.5-1997
Manufacturer of trapezoidal screws
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Screws
Thread standards
Threading (manufacturing)