Technical Information
Threaded components are the primary means of assembly for interchangeable connections. Thread requirements are simple: assembly and assembly integrity. The requirements for dimensional screw thread performance are described in specifications and standards such as AS8879, MIL-S-8879, MIL-S-7742, MIL-S-1222, FED-STD-H28 ISO, and the ASME B1 series of documents. In addition to specifying design profiles with tabulated dimensions and tolerances, the standards also introduce terms and definitions essential to understanding the screw thread. Among the key concepts are:

  • PITCH DIAMETER , the diameter of the imaginary cylinder passing through the thread in a manner as to equalize the width of thread ridge and groove, defines the Minimum Material Limit of External and Internal threads. As Pitch Diameter is not affected by Lead or Flank Angle variation, it is the primary datum of the screw thread for isolating size, form and profile variation. The design reference for all mathematical calculations for tensile/static/shear strength, elongation, and fatigue, the inspection of Pitch Diameter is critical to torque/preload performance. According to ASME B1.3M and the National Institute of Standards and Technology, GO and NOT GO Thread Ring or Plug Gages do not inspect Pitch Diameter Size.

  • FUNCTIONAL SIZE , which defines the Maximum Material Limit External and Internal threads, includes variation in Size, Angle, Lead (including Uniformity of Helix), Taper, and Roundness. This cumulative effect of all profile variation creates an illusion of material where none exists. At best, the inspection of Functional Size only assures assembly, not dimensional integrity of assembly. While GO and NOT GO Thread Ring or Plug Gages are often utilized for the inspection of Functional Diameter, many SPC/SQA programs as well as some acceptance standards and specifications require more comprehensive inspection. Reliance solely on Functional Diameter thread inspection, known as SYSTEM 21 THREAD INSPECTION , is not recommended by FED-STD-H28 Section 20 for applications when threads require specific mechanical strength properties. System 21 (Plug and Ring) acceptance methods do not assure dimensional conformance with the material limits specified in ASME B1.1, MIL-S-8879, MIL-S-7742, MIL-S-1222 and FED STD H28. NIST Position, March 1994.

    The sequential inspection of both Pitch Diameter Size and Functional Size is known as SYSTEM 22 THREAD INSPECTION in Military, Government , Commercial standards and specifications. In fact, System 22 is the minimum level of thread inspection in documents such as AS8879, MIL-S-8879, MIL-S-7742 and MIL-S-1222. Internally-generated engineering documents of many industrial corporations also invoke System 22 of the dimensional acceptance of threaded products. Both System 21 and System 22 also require the inspection of Major Diameter Size and Minor Diameter Size as applicable.

    A quantitative measure of the entire effect of thread form and profile error, the DIFFERENTIAL BETWEEN THE FUNCTIONAL AND PITCH DIAMETER SIZE is the key to both efficient production and ultimate performance. Control of the differential will minimize process error, optimize initial setup and production, and assure maximum flank-to-flank in any threaded connection.
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