1. CHAPTER 12 THREADS, FASTENERS AND SPRING

3. OBJECTIVES 1. Define and label the parts of a screw thread.
2. Identify various screw thread forms.
3. Draw detailed, schematic, and simplified threads.
4. Define typical thread specifications.
5. Identify various fasteners and describe their use.
6. Draw various screw head types.
7. Draw springs.

4. UNDERSTANDING THREADS AND FASTENERS
The three basic applications of Screw threads are as follows:
1. To hold parts together…
2. To provide for adjustment between parts…
3. To transmit power…
Thread Used for Attachment.
(Courtesy of Arthur S. Aubry/Getty
Images, Inc.-Photodisc.)

5. External and Internal Threads
External Thread

6. Major Diameter: The largest diameter of the thread Minor Diameter: The smallest diameter of the thread Pitch: Distance between two threads (mm) or no of threads per inch.

7. Pitch Diameter
The diameter of an imaginary cylinder passing through the threads to make equal the widths of the threads and the widths of the spaces cut by the cylinder.
Lead
The distance a screw thread advances axially in one turn.
Angle of thread
The angle included between the sides of the thread measured in a plane through the axis of the screw
Axis of screw: The longitudinal center line through the screw.

8. Crest: The top surface joining the two sides of the threads.
Root: The bottom surface joining the sides of two adjacent threads.
Side: The surface of the thread that connects the crest with the root.
Depth of thread: The distance between the crest and the root of the thread measured normal to the axis.
Form of thread: The cross section of thread cut by a plane containing the axis.

9. Screw Thread Forms

10. 13.3 SCREW THREADS FORM
Sharp V threads called United States Standard threads. Used on brass pipe works.
American National Threads replaces the Sharp V for general use.
Unified thread is the standard agreed upon by the USA, Canada, Britain in 1948 and has replaced American National form.
Square Thread are to transmission of power.

11. 13.3 SCREW THREADS FORM (Continue)
ACME Thread replaces the Square threads since it is difficult to manufacture.
Whitworth thread (British standard) has been replaced by the Unified thread.
Knuckle thread is used in electrical bulb and other electrical applications.
Buttress Thread is designed to transmit power in one direction.

12. Thread Pitch

13. Right-hand and Left-hand Threads

14. Single and Multiple Threads
A single thread, as the name implies, is composed of one ridge, and the lead is therefore equal to the pitch.
Multiple threads are used wherever quick motion, but not great power, is desired, as on ballpoint pens, toothpaste caps, valve stems, and so on.

15. Thread Symbol Three method of representing thread on drawing.
Schematic, use for smaller diameter, about 25mm (1 inch)
Detailed, for diameter more than 25 mm (1 inch)
Simplified, commonly used.

16. Three Methods for Drawing Thread
Detailed
Schematic
Simplified

19. Metric Threads fit
Two class of fit, one for general purpose, 6H for internal threads and 6g for external threads, therefore default class of fit is 6H/6g if not specified.
Second class for closer fit, 6H for internal threads and 5g6g for external threads.

20. Unified threads fit
A represents for external threads and B represents for internal threads.
Number 1, 2 and 3 represents the class of fit.
2A/2B is general class (equivalent to 6H/6g)
3A/3B for close fit.
1A/1B for Rapid assembly.

21. Thread Notes
ASME/ANSI Y “Screw Thread Representations” is a standard for representing, specifying, and dimensioning screw threads on drawings
These same notes or symbols are used in correspondence, in records, and in specifications for parts, taps, dies, tools, and gages

22. Thread Notes

23. Thread Notes

25. Clearance Holes A simple hole without thread used to insert a bolt.
There is clearance between clearance hole and the bolt or screw.

26. Tapped Holes A hole with internal thread is called Tapped hole
The bottom of a drilled hole formed by the point of a twist drill is cone shaped
The thread length is the length of full or perfect threads
The tap drill depth does not include the cone point of the drill

27. Tapped Holes

28. Bolts, Studs, and Screws

29. Bolt: Through bolt, head on one end and passes through clearance holes and nut on other side.

31. Specifications
In specifying bolts, the following information must be covered in order:
Nominal size of bolt body
Thread specification or thread note
Length of bolt
Finish of bolt
Style of head
Name

32. 13.27: Specification for Bolts and Nuts
UNC-2A x 2.50 HEXAGON CAP SCREW
M8 X HEX CAP SCR
5/8-11 UNC-2B SQURE NUT
M8 X 1.25 HEX NUT

33. Locknuts and Locking Devices

34. Cap Screw: Similar to a bolt, has greater length, use without a nut
Cap Screw: Similar to a bolt, has greater length, use without a nut. Other member has thread and acts as a nut.

35. Standard Cap Screws

36. MACHINE SCREW:
is similar to the slotted head cap screws but in general is smaller. Can be used with or without nut.
SET SCREW:
is a screw with or without a head that is screwed through one member and whose special point is forced against another member to prevent relative motion between the two parts.

37. Standard Machine Screws

38. Standard Set Screws

39. SET SCREWS – POINT STYLE

40. Standard Wood Screws

41. Miscellaneous Fasteners

42. Keys
Keys are used to prevent movement between shafts and wheels, couplings, cranks, and similar machine parts attached to or supported by shafts

43. Keys
Typical specifications for keys are…

44. A key seat in a shaft and a key way in the hub or surrounding part.
¼ x 1 ½ SQ KEY
No. 204 WOODRUFF KEY
¼ x 1/16 x 1 ½ FLAT KEY
No. 10 P & W KEY

45. Machine Pins
Machine pins include taper pins, straight pins, dowel pins, clevis pins, and cotter pins

46. Rivets
Rivets are permanent fasteners used to hold sheet metal or rolled steel together

47. Rivets

49. Springs
A spring is a mechanical device designed to store energy when deflected and to return the equivalent amount of energy when released
Springs are commonly made of spring steel, which may be hard-drawn wire, or oil-tempered wire. Other materials used for compression springs include stainless steel, beryllium copper, and phosphor bronze. Urethane plastic is used in applications where conventional springs would be affected by corrosion, vibration, or acoustic or magnetic forces.
Springs are classified as:
Helical springs
Flat springs

51. Helical Springs Helical springs have three types: Compression springs
Extension springs
Torsion springs

52. Helical Springs

53. Compression Springs

54. Extension Springs

55. Torsion Springs

56. Flat Springs