ANSI Large-Diameter-Screw Thread Standards for Dimensions

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The American National Standards Institute (ANSI) issues standards for large-diameter-screw threads, including their dimensions and naming convention. These standards are managed jointly with the American Society of Mechanical Engineers (ASME). ANSI standards include allowable body diameter, head diameter, spline socket size and thread length. ANSI has issued standards for screws based on their type and design and whether they are in metric or American units. ANSI standard family B18 defines the acceptable dimensions for ANSI screws.

Screw Designation Standards

  • According to "Manufacturing Processes and Materials" by George Schrader, Ahmad Elshennawy, and Lawrence Doyle, "Screws are commonly designated by the following sequence of information: (1) nominal size or basic diameter, (2) threads per mm (in), (3) product length, mm (in), (4) product name, (5) material and (6) protective finish." Screws are specified as the nominal size (x optional shank length) -- number of threads per inch -- thread pitch family -- class allowance. That sequence can be followed by the hand and engagement lengths. ANSI B17.M R92 is the Nomenclature Definitions and Letter Symbols for Screw Threads.

Unified Thread Standard

  • The Unified Thread Series was developed to create a single American standard for threaded fasteners. This is now called the American Standard for screws. ANSI standard B1.2, Gages And Gaging For Unified Inch Screw Threads, defines unified thread standards in American units. ANSI B1.3M, Screw Thread Gaging Systems Dimensional Acceptability Inch Metric Screw Threads, defines the American standard unified thread fasteners built in metric units. Both of these standards are shared with the ASME.

ANSI Screw Standards

  • ANSI B18.3 is the standard for hexagon and spline socket flat countersunk-head cap screws. ANSI B18.6.2 is the ANSI standard for slotted flat countersunk-head cap screws. ANSI B18.11 applies to miniature screws. ANSI B1.8 defines stub acme screw threads. B18.1.2 is used for rivets a half inch and larger in diameter. According to "Industrial Maintenance" by Michael Brumbach and Jeffrey Clade, "The American National Standards Institute adopted a screw thread with an International Standards Organization (ISO) profile (ISO 68 metric thread)." This standard profile has a 60-degree angle at the thread root.

Related Standards

  • ANSI Y14.6 is the screw thread representation standard. This standard defines how screws are to be drawn on blueprints. ANSI Y14.6aM describes how metric screws are drawn and dimensioned. ANSI B16.4 is the standard for cast-iron threaded fittings on pipes. ANSI B16.3 is the standard for malleable iron threaded fittings for pipes.

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