HELICAL COIL SCREW THREAD INSERTS HELICAL COIL SCREW THREAD INSERTS Introduction Machinery's Handbook, 31st Edition
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The ANSI/ASME B18.29.2M standard delineates the dimensional, mechanical, and performance data for the metric series helical coil screw thread insert and threaded hole into which it is installed. Appendices that describe insert selection, STI (screw thread insert) taps, insert installation, and removal tooling are also included. Helical coil inserts are screw thread bushings coiled from wire of diamond-shape cross section. Inserts are screwed into STI-tapped holes to form nominal size internal threads. Inserts are installed by torquing through a diametral tang. This tang is notched for removal after installation. In the free state, they are larger in diameter than the tapped hole into which they are installed. In the assembly operation, the torque applied to the tang reduces the diameter of the leading coil and permits it to enter the tapped thread. The remaining coils are reduced in diameter as they, in turn, are screwed into the tapped hole. When the torque or rotation is stopped, the coils expand with a spring-like action anchoring the insert in place against the tapped hole. Dimensions.— Dimensions in this standard are in millimeters and apply before any coat ing. Symbols specifying geometric characteristics are in accordance with ASME Y14.5. Tolerance Classes 4H5H and 5H.— Because helical coil inserts are flexible, the class of fit of the final assembly is a function of the size of the tapped hole. Helical coil STI taps are available for both tolerance class 4H5H (or class 4H6H) and class 5H tapped holes. Toler ance class 5H tapped holes provide maximum production tolerances but result in lower locking torques when screw-locking inserts are used. The higher and more consistent torques given in Table 5 are met by the screw-locking inserts when assembled and tested in tolerance class 4H5H (or class 4H6H) tapped holes. Compatibility.— Assembled helical coil inserts will mate properly with items that have M Profile external threads in accordance with ANSI/ASME B1.13M. Also, due to the radius on the crest of the insert at the minor diameter, the assembled insert will mate with MJ Profile externally threaded parts with controlled radius root threads per ANSI/ASME B1.21M. Types of Inserts.— Free-running inserts provides a smooth, hard, and free-running thread. Screw-locking inserts provides a resilient locking thread produced by a series of chords on one or more of the insert coils. STI-Tapped Hole.— The tapped hole into which the insert is installed shall be in accor dance with ANSI/ASME B1.13M, except that diameters are larger to accommodate the wire cross section of the insert (See Fig. 1). Dimensions of the STI-tapped hole are shown in Table 1 and are calculated per General Note (c) to Table 1. Screw Thread Designation for Tapped Hole: The drawing note for the STI-threaded hole per Table 1 to accept the helical coil insert shall be in accordance with the following: Example 1: MS × 1.25-5H STI; 23.5 T per ANSI/ASME B18.29.2M. Designation for a Helical Coil Insert: Helical coil inserts shall be designated by the fol lowing data, in the sequence shown: a) product name; b) designation of the standard; c) nominal diameter and thread pitch (4) nominal length; and d) insert type (free-running or screw-locking). Example 2: Helical Coil insert, ANSI/ASME B18.29.2M, M8 × 1.25 × 12.0 free-running. Helical Coil insert, ANSI/ASME B18.29.2M, M5 × 0.8 × 7.5 screw-locking. The recommended B18 part number (PIN) code system for helical coil inserts is included in ANSI/ASME B18.24. This system may be used by user needing definitive part-numbering.
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