Machinery's Handbook, 31st Edition
British Unified Threads 1997 Limits of Size: Formulas used to determine limits of size are given in Table 4 ; the limits of size are given in Table 5. The diagram on page 1996 illustrates the limits of size and Table 6 gives values for the minimum flat at the root of the external thread shown on the diagram. Classes of Threads: The standard establishes one class of thread with zero allowance on all diameters. When coatings of a measurable thickness are required, they should be included within the maximum material limits of the threads since these limits apply to both coated and uncoated threads. Hole Sizes for Tapping: Suggested hole sizes are given in the Tapping Section. Unified Screw Threads of UNJ Basic Profile British Standard UNJ Threads.— This British Standard BS 4084: 1978 arises from a re - quest originating from within the British aircraft industry and is based upon specifica tions for Unified screw threads and American military standard MIL-S-8879. These UNJ threads, having an enlarged root radius, were introduced for applications requiring high fatigue strength where working stress levels are high, in order to minimize size and weight, as in aircraft engines, airframes, missiles, space vehicles and similar designs where size and weight are critical. To meet these requirements the root radius of external Unified threads is controlled between appreciably enlarged limits, the minor diameter of the mating internal threads being appropriately increased to insure the neces sary clearance. The requirement for high strength is further met by restricting the toler ances for UNJ threads to the highest classes, Classes 3A and 3B, of Unified screw threads. The standard, not described further here, contains both a coarse and a fine pitch series of threads. BS 4084: 1978 is technically identical to ISO 3161-1977 except for Appendix A. ASME Unified Inch Screw Threads, UNJ Form.— The ANSI/ASME B1.15-1995 standard is similar to Military Specification MIL-S-8879, and equivalent to ISO 3161-1977 for thread Classes 3A and 3B. Basic profile dimensions are given in Table 1, page 1998. The ANSI/ASME B1.15-1995 standard establishes the basic profile for the UNJ thread form, specifies a system of designation, lists the standard series of diameter-pitch combinations for diameters from 0.060 to 6.00 inches, and specifies limiting dimensions and tolerances. It specifies the characteristics of the UNJ inch series of threads having 0.15011 P to 0.18042 P designated radius at the root of the external thread, and also having the minor diameter of the external and internal threads increased above the ANSI/ASME B1.1 UN and UNR thread forms to accommodate the external thread maximum root radius. UNJ threads are similar to UN threads except for a large radius in the root, or minor diameter, of the external thread. The radius eliminates sharp corners in the minor diameter of the bolt to increase the stripping strength. The fillets or radius in sharp corners increases strength at stress points where cracking or failure may occur due to change in temperature, heavy loads, or vibration. Other dimensions are the same as the UN thread. Because the radius on the external thread increases the minor diameter of the bolt, the internal thread, or nut, is modified accordingly to permit assembly. The minor diameter of the internal thread is enlarged to clear the radius. This is the only change to the internal thread. All other dimensions are the same as standard Unified threads. Different types of tap drill sizes are required to produce UNJ thread. All tooling for external threads, thread rolls, and chasers must be made to produce a radius at the minor diameter. All runout or incomplete threads shall have a radius also. Thread conforming to the ANSI/ASME B1.1 UN profile and the UNJ profile are not interchangeable because of possible interference between the UNJ external thread minor diameter and the UN internal thread minor diameter. However, the UNJ internal thread will assemble with the UN external thread.
Copyright 2020, Industrial Press, Inc.
ebooks.industrialpress.com
Made with FlippingBook - Share PDF online