Sumber:
http://www.me.metu.edu.tr/me114/threaded_fasteners.htm
ASSEMBLY ELEMENTS: THREADED FASTENERS
INTRODUCTION
Most engineering products are composed of separate parts held together by some means
of fastening. Threaded fasteners provide an advantage over permanent methods, in that
they are easily disassambled.
The most common types of threaded fasteners are: (In textbook, p.517, Figure 21)
Bolt
Stud
Cap Screw
Machine Screw
Setscrew
BOLTS
A bolt, having an integral head on one end and a thread on the other end, is passed
through clearance holes in two parts and draws them together by means of a nut screwed
on the threaded end.
Two major groups of bolts have been standardized: Round-head bolts and wrench-head
bolts (machine bolts).
Round-head bolts are used as through fasteners with a nut, usually square. Appendix 17
in textbook shows various head forms.
Wrench-head bolts have two standard head forms: square and hexagonal. Appendix 16 in
textbook can be referred for these forms.
STUDS
A stud is a rod threaded on each end. The fastener passes through a clearance hole in one
piece and screws permanently into a tapped hole in the other. A nut then draws the parts
together. (p.518, Figure 22)
CAP SCREWS
A cap screw passes through a clearance hole in one piece and screws into a tapped hole in
the other. The head draws the parts together as the screw enters the tapped hole. Several
types of heads are shown in p. 518 in Figure 23.
American standard cap screws
MACHINE SCREWS
A machine screw is a small fastener used in the same manner as bolt or a cap screw.
Several standardized head shapes are available. (p. 519, Figure 24&25)
American
standard machine screw heads
SETSCREWS
A setscrew screws into a tapped hole in an outer part and bears with its point against an
inner part, usually a shaft. These are made of hardened steel and hold two parts in relative
position. p.519, Fig. 26&27.
American standard setscrew heads and points
NUTS
Nuts are used with bolts and studs. These are available in two wrench-type styles: square
and hexagonal. Together with plain form usually associated with bolts, several specialpurpose styles are also available. p. 519, Fig. 28.
American standard nuts
FASTENER TERMS
1. Nominal Diameter: The basic major diameter of the thread.
2. Width across Flats-W: The distance separating parallel sides of the square or
hexagonal head or nut, corresponding with the nominal size of the wrench.
3. Tops of Boltheads and Nuts: The tops of heads and nuts are flat with chamfer to
remove the sharp corners. The angle of chamfer with the top surface is 25° for the square
form and 30° for the hexagonal form; both are drawn at 30°. The diameter of the top
surface is equal to the width across flats.
4. Washer Face: The washer face is a circular boss turned or otherwise formed on the
bearing surface of a bolthead or nut to make a smooth surface.
5. Fastener Length: The distance from the bearing surface to the point.
6. Regular Series: Regular boltheads and nuts are for general use.
7. Heavy Series: Boltheads and nuts in this series are for use where greater bearing
surface is necessary. They are larger in over-all dimensions.
8. Thick Nuts: These are higher than regular nuts.
9. Unfaced bolts and nuts: are not machined on any surface except the threads. The
bearing surface is plain.
10. Semifinished Boltheads and Nuts: These have a smooth bearing surface machined or
formed at right angles to the axis.
11. Finished Bolts and Nuts: Dimensional tolerances are tighter.
DRAWING FASTENERS
Drawing hexagonal and square boltheads is explained in the textbook in p. 522 & 523. If
an accurate drawing of the fastener is not essential, the W (width across flats), H (height
of head) , T ( thickness of nut) dimensions of hexagonal and square heads and nuts may
be obtained as follows. The resulting values will be quite close to the actual dimensions.
for regular series: W = 1 1/2 D , H = 2/3 D , T = 7/8 D
for heavy series: W = 1 1/2 D + 1/8 in , H = 3/4 D , T = D
Minimum thread length to be used in drawings can be calculated as follows:
l = 2 D + 1/4 in. for L =< 6 in.
l = 2 D + 1/2 in. for L > 6 in.
Using bolt length increments in Appendix 18 of the textbook, standard bolt lengths
should be chosen.
For Metric bolts,
for drawing use, the thread length may be calculated using the below formula.
l = 2 D + 6 mm for L =< 150 mm
l = 2 D + 12 mm for L > 150 mm
The following bolt length increments should be used for metric bolts:
1 mm --- 3< L <6 mm
2 mm --- 8< L <16 mm
5 mm --- 20< L <90 mm
10 mm --- 100< L <200 mm
20 mm --- 220< L <300 mm
Appendix 11: Tap Dril Sizes for Specific Unified Inch Screw Threads
Appendix 14: Tap Dril Sizes for Specific Metric Screw Threads
EXERCISE
Draw the following specified fasteners.
A : 1 - 8UNC - 2A (Bolt&Nut)
B: 3/4 - 16UNF - 2A (Cap Screw)
C: 5/8 - 18 UNF - 2A (Stud)
D: M12 x 1.25 (Cap Screw)
Answers:
A : 1 - 8UNC - 2A (Bolt&Nut)
From Appendix 11 tap drill diameter for D=1 and n=8 is 7/8 in
Clearance of 1/16 in. should be used from both sides.
Min bolt length is : = 1 1/2 + 4 + T ----- Nut thickness: T = 7/8 D
= 1 1/2 + 4 + 7/8 * 1 = 6 3/8
Refer to Appendix 18 for increments: for D = 1 , increment is 1 in. for 6-12 in. lengths
So, Minimum bolt length, L = 7 in.
Now, calculate the min. thread length:
for L > 6; l = 2 D + 1/2
Therefore, l = 2 * 1 + 1/2 = 2 1/2 in.
B : 3/4 - 16UNF - 2A (Cap Screw)
From Appendix 11 tap drill diameter for D=3/4 and n=16 is 11/16 in.
Recall min. entrance length can be calculated using Table 1 in p. 515. For Cast Iron A = 1
1/2 D;
A = 1 1/2 * 3/4 = 9/8
Therefore min. cap screw length is : = (1 1/2 - 1/4) + 9/8 = 2 3/8
Refer to Appendix 18 for increments: for D = 3/4, increment is 1/4 in. for 1-6 in. lengths
So, Minimum cap screw length, L = 2 1/2 in.
Now, calculate the min. thread length:
for L <= 6 ; l = 2 D + 1/4
Therefore, l = 2 * 3/4 + 1/4 = 1 3/4 in.
Considering allowances on tapped hole;
Thread clearance; B = 4/n = 4/16 in.
Unthreaded hole length; E = 4/n = 4/16 in.
C : 5/8 - 18UNF - 2A (Stud)
From Appendix 11 tap drill diameter for D=5/8 and n=18 is 37/64 in.
Recall min. entrance length can be calculated using Table 1 in p. 515. For Cast Iron A = 1
1/2 D;
A = 1 1/2 * 5/8 = 15/16 in.
T = 7/8 D = 7/8 * 5/8 = 35/64 in.
Stud length is : = (1 1/2 + 1/8) + 15/16 + 35/64 = 3 7/64 in.
Referring Appendix 18, overall Stud length, L = 3 1/4 in.
Considering allowances on tapped hole;
Thread clearance; B = 4/n = 4/18 in.
Unthreaded hole length; E = 4/n = 4/18 in.
D : M12 x 1.25 (Cap Screw)
From Appendix 14 tap drill diameter for D=12 mm and p=1.25 is 10.75 mm
width across flats, W = 1 1/2 * 12 = 18 mm
height of head, H = 2/3 * 12 = 8 mm
Apply 1 mm clearance from both sides. M 12 ==> 14 Drill
Min. entrance length for Cast Iron A = 1 1/2 D;
A = 1 1/2 * 12= 18 mm
Min. cap screw length: = (30-2) + 18 = 46 mm
Recall increments; 5 mm for 20<L<90
Therefore, L = 50 mm
Thread length: = 2 D + 6 = 2*12 + 6 = 30 mm
Allowances on tapped hole;
Thread clearance; B = 4/n = 4*p = 4 * 1.25 = 5 mm
Unthreaded hole length; E = 4*p = 4 * 1.25 = 5 mm