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1836 CENTRALIZING ACME SCREW THREADS Table 7. Formulas for Finding Basic Dimensions of Centralizing Acme Screw Threads Pitch = P = 1 ÷ No. threads per inch, n: Basic thread height h = 0.5P Basic thread thickness t = 0.5P Basic flat at crest F cn = 0.3707P + 0.259 × (minor. diameter allowance on internal threads) (internal thread) Basic flat at crest F cs = 0.3707P − 0.259 × (pitch diameter allowance on external thread) (external thread) Frn = 0.3707P − 0.259 × (major dia. allowance on internal thread) ;; ;; F rs = 0.3707P − 0.259 × (minor dia. allowance on external thread — pitch dia. allowance on external thread) 0.0945P Max 0.0707P Min ;;;;; 45° 0.067P Max 0.050P Min ;;r rN 1 rN = 0.06P Max ;;;; ;;;; ;;;; Detail of chamfer Detail of fillet ;;;;;;;; ;;;;;; ;;;; ;; One-half major dia. allowance (nut) P 4 Internal Thread (Nut) ;; ;;;; ;;;;;; ;;;;;;;; ;;;;;;;; ;;;;;; ;;;;; ;;;;;; Symbols: P = pitch h = basic thread height Machinery's Handbook 27th Edition 4 ;;;;;; Max major dia. of nut Min major dia. of nut Nominal (basic) major dia. (D) Max major dia. of screw Min major dia. of screw Min depth of engagement h h – 0.05P Max pitch dia. of screw Min pitch dia. of screw One half minor dia. ;;;;; External Thread (Screw) Basic pitch dia. 1/2 pitch dia. allowance Max minor dia. of nut Min minor dia. of nut Basic minor dia. Max minor dia. of screw Min minor dia. of screw Min pitch dia. of nut Max pitch dia. of nut Fig. 4. Disposition of Allowances, Tolerances, and Crest Clearances for Centralizing Single-Start Acme Threads—Classes 2C, 3C, and 4C est number of items in order to reduce to a minimum the inventory of both tools and gages. This series of diameters and associated pitches is given in Table 9. Basic Diameters: The maximum major diameter of the external thread is basic and is the nominal major diameter for all classes. The minimum pitch diameter of the internal thread is basic for all classes and is equal to the basic major diameter D minus the basic height of thread, h. The minimum minor diameter of the internal thread for all classes is 0.1P above basic. Length of Engagement: The tolerances specified in this Standard are applicable to lengths of engagement not exceeding twice the nominal major diameter. Pitch Diameter Allowances: Allowances applied to the pitch diameter of the external thread for all classes are given in Table 10. Major and Minor Diameter Allowances: A minimum diametral clearance is provided at the minor diameter of all external threads by establishing the maximum minor diameter 0.020 inch below the basic minor diameter for 10 threads per inch and coarser, and 0.010 inch for finer pitches and by establishing the minimum minor diameter of the internal thread 0.1P greater than the basic minor diameter. r S allowance (screw) r S = 0.1P max Detail of optional fillet Copyright 2004, Industrial Press, Inc., New York, NY P 0.05P One-half minor dia. allowance (nut)
Machinery's Handbook 27th Edition CENTRALIZING ACME SCREW THREADS 1837 Table 8a. American National Standard Centralizing Acme Single-Start Screw Threads — Formulas for Determining Diameters ASME/ANSI B1.5-1988 D = Nominal Size or Diameter in Inches P = Pitch = 1 ÷ Number of Threads per Inch No. Classes 2C, 3C, and 4C External Threads (Screws) 1 Major Dia., Max = D (Basic). 2 Major Dia., Min = D minus tolerance from Table 12, columns 7, 8, or 10. 3 Pitch Dia., Max = Int. Pitch Dia., Min (Formula 9) minus allowance from the appropriate Class 2C, 3C, or 4C column of Table 10. 4 Pitch Dia., Min = Ext. Pitch Dia., Max (Formula 3) minus tolerance from Table 11. 5 Minor Dia., Max = D minus P minus allowance from Table 12, column 3. 6 Minor Dia., Min = Ext. Minor Dia., Max (Formula 5) minus 1.5 × Pitch Dia. tolerance from Table 11. Classes 2C, 3C, and 4C Internal Threads (Nuts) 7 Major Dia., Min = D plus allowance from Table 12, column 4. 8 Major Dia., Max = Int. Major Dia., Min (Formula 7) plus tolerance from Table 12, columns 7, 9, or 11. 9 Pitch Dia., Min = D minus P/2 (Basic). 10 Pitch Dia., Max = Int. Pitch Dia., Min (Formula 9) plus tolerance from Table 11. 11 Minor Dia., Min = D minus 0.9P. 12 Minor Dia., Max = Int. Minor Dia., Min (Formula 11) plus tolerance from Table 12, column 6. A minimum diametral clearance at the major diameter is obtained by establishing the minimum major diameter of the internal thread 0.001 D above the basic major diameter. These allowances are shown in Table 12. Major and Minor Diameter Tolerances: The tolerances on the major and minor diameters of the external and internal threads are listed in Table 12 and are based upon the formulas given in the column headings. An increase of 10 per cent in the allowance is recommended for each inch or fraction thereof that the length of engagement exceeds two diameters. For information on gages for Centralizing Acme threads the Standard ASME/ANSI B1.5 should be consulted. Pitch Diameter Tolerances: Pitch diameter tolerances for Classes 2C, 3C and 4C for various practicable combinations of diameter and pitch are given in Table 11. The ratios of the pitch diameter tolerances of Classes 2C, 3C, and 4C are 3.0, 1.4, and 1, respectively. Application of Tolerances: The tolerances specified are such as to insure interchangeability and maintain a high grade of product. The tolerances on the diameters of internal threads are plus, being applied from the minimum sizes to above the minimum sizes. The tolerances on the diameters of external threads are minus, being applied from the maximum sizes to below the maximum sizes. The pitch diameter tolerances for an external or internal thread of a given class are the same Limiting Dimensions: Limiting dimensions for Centralizing Acme threads in the preferred series of diameters and pitches are given in Tables 8b and 8c. These limits are based on the formulas in Table 8a. For combinations of pitch and diameter other than those in the preferred series the formulas in Tables 8b and 8c and the data in the tables referred to therein make it possible to readily determine the limiting dimension required. Copyright 2004, Industrial Press, Inc., New York, NY
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