Machinery's Handbook, 31st Edition
Centralizing Acme Screw Threads Table 7. Formulas for Finding Basic Dimensions of Centralizing Acme Screw Threads
2065
Pitch = P = 1 ÷ No. threads per inch, n :
Basic thread height h = 0.5 P
Basic thread thickness t = 0.5 P Basic flat at crest F cn = 0.3707 P + 0.259 × (minor. diameter allowance on internal threads) (internal thread) Basic flat at crest F cs = 0.3707 P − 0.259 × (pitch diameter allowance on external thread) (external thread) F rn = 0.3707 P − 0.259 × (major dia. allowance on internal thread) F rs = 0.3707 P − 0.259 × (minor dia. allowance on external thread—pitch dia. allowance on external thread)
0.067 P Max 0.050 P Min
0.0945 P Max 0.0707 P Min
Internal Thread (Nut)
r N
r 1
r N = 0.06 P Max
45 °
Detail of chamfer
Detail of fillet
P 4
P 4
Symbols: P = pitch h = basic thread height
One half minor dia. allowance (screw)
r S
r S = 0.1P max
External Thread (Screw)
Detail of optional fillet
Fig. 4. Disposition of Allowances, Tolerances, and Crest Clearances for Centralizing Single-Start Acme Threads—Classes 2C, 3C, and 4C
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 0.1 P 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 0.1 P greater than the basic minor diameter. A minimum diametral clearance at the major diameter is obtained by establishing the minimum major diameter of the internal thread . D 0001 above the basic major diameter. These allowances are shown in Table 12.
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