ASSEMBLY ELEMENTS: THREADED FASTENERS
This section deals with threaded fasteners that can be classified as "assembly elements". These elements may be used for aligning one part to another or may be used to transmit motion or force. The use of threaded fasteners is so common that they receive the most attention.
SCREW THREAD TERMS
Screw thread can be defined as a ridge of uniform section in the form of a helix on the external or internal surface of a cylinder or hole. Figure 1 shows the primary terms useful in defining screw threads. These terms are:
1. External thread (screw): A thread on the external surface of a cylinder.
2. Internal thread (nut): A thread on the inernal surface of a cylinder.
3. Right-hand thread: A thread that when viewed axially winds in a clockwise and receding direction. Threads are always right-hand unless otherwise specified.
4. Left-hand thread: A thread that when viewed axially winds in a counterclockwise and receding direction. All left-hand threads are designated LH.
5. Major diameter (nominal diameter), D: The largest diameter of a screw thread
6. Minor diameter (tap drill diameter): The smallest diameter of a screw thread.
7. Pitch diameter: The diameter of an imaginary cylinder, the surface of which cuts the thread forms where the width of the thread and groove are equal.
Figure 1. Screw-thread terminology
8. Crest: The edge or surface that joins the sides of a thread and is farthest from the cylinder or cone from which the thread projects.
9. Root: The edge or surface that joins the sides of adjacent thread forms and coincides with the cylinder or cone from which the thread projects.
10. Depth of thread: The distance between crest and root measurednormal to the axis.
11. Pitch, p: The distance between corresponding points on adjacent thread forms measured parallel to the axis.
12. Threads per inch, n: The reciprocal of the pitch and the value specified to govern the size of the thread form. ( n = 1/p )
13. Lead, L: The distance a threaded part moves axially, with respect to a fixed mating part, in one complete revolution. See Figure 2.
Figure 2. Single and Multiple threads
14. Single thread: A thread having the thread form produced on only one helix of the cylinder. On a single thread , the lead and pitch are equivalent. Threads are always single unless otherwise specified. ( L = p )
15. Multiple thread: A thread combination having the same form produced on two or more helices of the cylinder. For a multiple thread , the lead is an integral multiple of the pitch; i.e. on a double thread, lead is twice the pitch ( L = 2*p ) ; on a triple thread, lead is three times the pitch ( L = 3*p ). A multiple thread permits a more rapid advance without a coarser (larger) thread form.
16. Form: The profile of the thread. See Figure 3 for various forms.
Figure 3. Thread profiles
THREAD SPECIFICATION AND SYMBOLS
3/4-16UNF-2A-LH, DOUBLE, 1 3/4 LG
M20 x 1.5 - 4g6g, 45 LG
Figure 4. Tools for creating threads
Figure 5. Simplified thread representation
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