threads - ~1 II ,8 ()I I Threads II II Threads, either...

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~1 II ,8 ()I I Threads II II Threads, either internal or external, serve three basic functions: (1) to fasten or hold members together, (2)to adjust one part relative to another, and (3) to transmit power. To meet each of these requirements, special thread forms (profiles) have been developed. To better understand threads and thread notes, it is helpful to be aware of thread terminology, Figure 8.1 illustrates the following terms: Crest: the top surface that joins the two sides or faces of a thread. Root: the bottom surface of two adjacent sides or faces of a thread. Major diameter: the largest diameter of a straight screw thread. Minor diameter: the smallest diameter of a straight screw thread. II r II II FACE OR FLANK MINOR DIAMETER THREAD ANGLE L PITCH MAJOR DIAMETER FIGURE 8.1 Thread terminology. 112
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Pitch: the distance from one point on a screw thread to a corresponding point on the adja- cent thread measured parallel to the axis. Lead: the distance a thread moves axially in one revolution (not shown on Figure 8.1). Thread angle: the included angle between the faces or sides of the thread. Face: the surface that connects the crest and the root. Form: the profile of the thread. Thread Forms In the United States the Unified National (UN) thread form (ANSI), adopted in 1957,is accepted as a standard for the vast majority of applica- tions. This form (see Figure 8.2A) was also the standard in Great Britain and Canada prior to their adoption of the metric system. The Unified thread is a modification of the 1935 American National thread form. Because the American National and Unified have essentially the same profile (with several minor exceptions) and be- cause of the interchangeability of these two thread forms, the Unified thread has been accept- ed in the United States. The Sharp V thread (Figure 8.2B) is used in vernier height gages, micrometers, vernier calip- ers, etc. Because calibration readings are obtained from the movement of the threaded portions, a fine thread is necessary or desirable. The Sharp V is also used for applications where holding power is a prime requirement. In those situations where power or motion must be transmitted via a thread, the Acme, Stub Acme, Modified, or Brown Sharpe Worm thread are used (refer to Figures 8.2C and D).All three of these threads are stronger than a V-shaped thread because almost all the forces are transmitted parallel to the screw's axis. Since the develop- ment of the Acme thread, the Square thread is not used extensively. The Buttress thread (Figure 8.2E) is used to transmit power in one direction. Note the profile of this thread. The trapezoidal shape produces an extremely strong and efficient thread. The But- tress form is used in gun breeches, jacks, lead screws, and so on. The
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This note was uploaded on 02/12/2010 for the course MAE 19010 taught by Professor Norton during the Spring '07 term at UC Irvine.

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threads - ~1 II ,8 ()I I Threads II II Threads, either...

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