ASSEMBLY ELEMENTS: THREADED FASTENERS

SCREW THREADS


INTRODUCTION

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

ANSI UNIFIED THREAD (UN/UNR) SPECIFICATION:
3/4-16UNF-2A-LH, DOUBLE, 1 3/4 LG

3/4: The major (nominal) diameter of the thread. Appendix 10 in textbook lists various diamaters available.

16UNF: 16 indicates number of threads per inch (n). UNF (unified fine); UNEF (uni. extra fine); UNC (uni. coarse)

2A: indicates the class of fit (1A, 2A, 3A : increasing precision order), A stands for external threads and B for internal threds.

LH: indicates left hand thread. When a thread is right-hand no notation is made in the thread specification.

DOUBLE: indicates factor of multiple thread.(L = 2*p). When a thread is single no notation is made in the thread specification.

1 3/4 LG: Length of thread depends on designer, but given as in even increments and depends on major diameter. May be given as a dimension or as an addendum, 1 3/4 LG. DP (deep) would be used for threaded holes.

Examples:

3/4-16UNC-2A
3/4-10UNC-2B-1 3/4 LG

 

METRIC THREAD SPECIFICATION:
M20 x 1.5 - 4g6g, 45 LG

M20: The major (nominal) diameter of the thread. Appendix 13 in textbook lists metric screw thread series.

1.5: is the pitch of the thread.

4g6g: is the tolerance class. It is a combination of tolerance grade and tolerance position. It indicates the allowance and tolerance for the pitch and major diameters of external threads, and pitch and minor diameters of internal threads. Tolerance grade (e.g. 4 close tolerance, 6 medium 8 coarce tolerance); Tolerance position (e.g. g,h for external threads; G,H for internal threads ; h,H stands for zero allowance)

45 LG: Length of thread is the choice of designer.

M20 x 1.5, M16 x 2 are some examples. Such notations are sometimes called basic designation.

 

SYMBOLS FOR STRAIGHT THREADS:

Threads are generated on external and internal surfaces and figure 4 illustrates how threads are generated.

Figure 4. Tools for creating threads

 

A true representation of a thread is not needed on a working drawing. Symbols are used instead. Three types of symbols are in use: detailed, schematic and simplified. We will prefer simplified symbolic representation during our studies. Figure 5 presents the simplified thread symbols. As a common practice metric and UN/UNR threads are drawn in the same manner.

Figure 5. Simplified thread representation

 

TAPPED HOLE SPECIFICATIONS:

An internal thread in thick material can be threaded only if a properly sized hole is first made. For every major diameter there is a specific drill size to precede the tapping operation. Proportions for tapped holes is given in figure 6 and detailed depths for drilling and tapping holes is given in table 1. For metric threads use 4*p instead of 4/n.

Figure 6. Proportions for tapped holes

A: min. entrance length C: thread depth (A+B) F: depth of hole (C+E)
B: thread clearance E: unthreaded hole length  

 

Table 1. Detailed depths (p. 515)

 

Appendix 11: Tap Dril Sizes for Specific Unified Inch Screw Threads

Appendix 14: Tap Dril Sizes for Specific Metric Screw Threads

 

Example: Calculate detailed depths for 1-8UNC-2B thread on cast iron part and draw using simplified representation.

Nominal dia., D = 1

# of threads per inch, n = 8

Using Appendix 11; minor diameter for nominal dia. of 1 and n=8 is 7/8.

from table 1, for C.I., A=1 1/2*1 = 1 1/2 , B=4/8=1/2, C=A+B= 2 ,

E=4/8= 1/2, F= C+E = 2 1/2

 

Example: Show M10 x 1.5 internal thread on C.I. part according to minimum requirements.

Nominal dia., D = 10mm

pitch, p= 2.5mm

Using Appendix 14; minor diameter for nominal dia. of 10 and p=1.5 is 8.50mm.

from table 1, for C.I., A=1 1/2*10 = 15mm , B=4*1.5=6mm, C=A+B= 21mm ,

E=4*1.5= 6mm, F= C+E = 27mm

Draw and specify this thread on 40mmx20mmx35mm C.I part on CADKEY as a class exercise.

 

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