CH11: THREADS, FASTENERS AND SPRINGS Mechanical Engineering Department MECHANICAL DRAWING 1 MECHANICAL DRAWING Chapter 11: Threads Fasteners and Springs

CH11: THREADS, FASTENERS AND SPRINGS

Mechanical Engineering Department

MECHANICAL DRAWING

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MECHANICAL DRAWING

Chapter 11: Threads Fasteners and Springs

MECHANICAL DRAWING

Chapter 11: Threads Fasteners and Springs



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Objectives of threads and fastenersObjectives of threads and fasteners

3. To transmit power

1. To hold parts together (Assembly).

2. To provide for adjustment between

parts (Drum).



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Chapter ObjectivesChapter 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. Draw various screw head types.

6. Draw springs



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Screw Thread Nomenclature Screw Thread Nomenclature

Screw thread:

External thread: a thread on the outside of a member (shaft).

Internal thread: a thread on the inside of a member (hole).

Major diameter: the largest diameter of the screw thread.

Minor diameter: the smallest diameter of the screw thread.

Pitch: the distance from a point on a screw thread to the corresponding point on the next thread measured parallel to the axis (US units = 1/ number of threads per inch).

Pitch diameter: the diameter of an imaginary cylinder passing through the threads where the widths of the threads and the width of the spaces are equal



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Screw Thread Nomenclature Screw Thread Nomenclature

Lead: the distance that the screw advances in one turn.

Angle of a thread: the angle between the sides of the thread.

Crest: the outer surface of the thread

Root: the inner surface of the thread.

Side:

Axis of the screw:

From of the thread: the cross section of thread cut by a plane containing the axis

Series of thread: the standard number of threads per inch for various diameters.



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Important notes:Important notes:

Thread Form: International standard form (SI metric). Most widely used. United States forms (English units). USA, Great Britain and Canada.

Standardization of screw thread is very important



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Screw Thread FormsScrew Thread Forms

a) Sharp V thread: high friction because of full thread face.

b) American national: flatten roots and crests. General use. Replace by the following thread.

c) Unified thread (N): US standard thread (replaces the previous thread). Root is rounded, crest (rounded/flat).

Old standard US threads: (still included in the new standard - 11 series) (UN or N); general. (UNC or NC); course thread series. (UNF or NF); fine thread series. (UNS or NS); special series.

●) Unified extra fine series (UNEF)

Different number of threads per inch for

various standard diameters



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d) Metric thread (M): Standard (international screw thread fasteners ISO). Normally flat crest and root, could be rounded in external thread. Similar to the unified thread (lower depth).

a) Course thread (general purpose)

b) Fine thread

e) Square thread; power transmission (difficult to fabricate, and other problems). ACME thread is more used.

f) ACME thread: largely replaces the square thread.

●) Worm standard thread; similar to the ACME thread but deeper.



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g) Whitworth thread: replaced by the unified thread (angle=55)

h) Knuckle thread: usually rolled from sheet metal. (Ex. Electrical bulbs and sockets, bottle tops, …etc).

i) Buttress thread: transmits power in one direction only. High strength requirements.

ASME standards == American Society of

Mechanical Engineers.



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Thread Pitch, Right and Left-Hand Threads & Single and Multiple ThreadsThread Pitch, Right and Left-Hand Threads & Single and Multiple Threads

Thread Pitch:

Right and Left-Hand Threads

Single and Multiple Threads



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Drawing a ThreadDrawing a Thread



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Drawing a ThreadDrawing a Thread

Simplified and Schematic Internal Thread symbols



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Tables for Screw threads American and MetricTables for Screw threads American and Metric



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Detailed representation: External Metric, Unified, & American National Threads (DRAWING)Detailed representation: External Metric, Unified, & American National Threads (DRAWING)



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Simplified External and Internal Threads (DRAWING)Simplified External and Internal Threads (DRAWING)



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Schematic External and Internal Threads (DRAWING) Schematic External and Internal Threads (DRAWING)



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Thread Fits Thread Fits

American National Thread FitsA ≡ external thread

B ≡ internal thread Class 1 fit: Clearance between mating parts is essential for rapid assembly, (1A,

1B). Class 2 fit: High quality of commercial thread product, (2A, 2B). Class 3 fit: Very high quality of commercial thread product (high cost), (3A, 3B)

Metric and Unified Thread Fits

g ≡ external thread

H ≡ internal thread General-purpose application: (6H for internal, 6g for external) ≡ (2A, 2B). Closer fit: (6H for internal, 5g6g for external)



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Thread Notes Thread Notes



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Thread Notes Thread Notes



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Detailed representation: Internal Metric, Unified, & American National Threads (DRAWING)Detailed representation: Internal Metric, Unified, & American National Threads (DRAWING)

Note that slope line is in the opposite direction of the external thread as you see internal threads!



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Detailed representation: Internal Metric, Unified, & American National Threads (DRAWING)Detailed representation: Internal Metric, Unified, & American National Threads (DRAWING)

Note that slope line is in the opposite direction of the external thread as you see internal threads!



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Detailed External Square ThreadDetailed External Square Thread



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Detailed External Square ThreadDetailed External Square Thread

Use the same table for square and acme threads



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Detailed Internal Square ThreadDetailed Internal Square Thread

Lines from crest to crest

Lines from root to screw axis



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Detailed External ACME ThreadDetailed External ACME Thread



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Phantom Lines and Threads in Assembly Phantom Lines and Threads in Assembly

Phantom Lines For long threads use

phantom (hidden) lines

Threads in Assembly Stud, bolt heads, and

nuts are not sectioned External and internal

threads are shown in

V-shapes



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Bolts, Studs, and Screws Bolts, Studs, and Screws

Greater thread lengthOne of the parts is threaded (no nut)

Threaded at both ends



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Standard Bolts and Nuts Standard Bolts and Nuts

• Inch and metric sizes

• chamfered at 15-30o

• Regular Heads & Bolts

(W = 1½D, H = ⅔D, T =⅞ D)

• Heavy Heads & bolts

(structure)

(W = 1½D + ⅛" (3mm)

H = ⅔D, T = D)

See Table in Appendix 20for standard bolts and nuts dimensions (no standard for bolt lengths)

• Inch sizes only

• Chamfered at 30o

(W = 1½D, H = ⅔D, T =⅞ D)

Thread Length:

Screws up to 6" length = 2D+¼" (6mm)

Screws over 6" length = 2D+½" (12mm)



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Standard Bolts and NutsStandard Bolts and Nuts

Across Corners Across Flats



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Hexagonal Bolts, Cap Screws, and Nuts (DRAWING)Hexagonal Bolts, Cap Screws, and Nuts (DRAWING)



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Square Bolts, Cap Screws, and Nuts (DRAWING)Square Bolts, Cap Screws, and Nuts (DRAWING)



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Standard Cap Screws Standard Cap Screws

All screws except (e) can be drawn directly as in figures.

For exact dimensions see table (A21)



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Standard Cap Screws Standard Cap Screws



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Standard Machine Screws (Smaller, Thread all the length)Standard Machine Screws (Smaller, Thread all the length)

All screws can be drawn directly as in figures.

For exact dimensions see table (A21)



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Keys, Pins, and Rivets (Keys)Keys, Pins, and Rivets (Keys)



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Keys, Pins, and Rivets (Keys)Keys, Pins, and Rivets (Keys)

Square or Flat Keys Gib Head Key

Pratt & Whitney KeyWoodruff Keys and Key Slot Cutter



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Keys, Pins, and Rivets (Keys Drawing)Keys, Pins, and Rivets (Keys Drawing)

Square and Flat Keys



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Gib Head Keys



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Pratt and Whitney Keys



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Woodruff keys



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Woodruff key solt



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Keys, Pins, and Rivets (Machine Pins)Keys, Pins, and Rivets (Machine Pins)

Taper Pin:

Used to fasten collars to shaft

Collars

See table A31 for standard diameters for given lengths



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Keys, Pins, and Rivets (Rivets)Keys, Pins, and Rivets (Rivets)

Usage: Permanent fastening used to hold sheet metal or rolled steel together. Made of wrought iron, carbon steel, copper, or other metals. Hole diameter is slightly larger than the diameter of the rivet. Rivet diameter d = 1.2 to 1.4 t: thickness of the sheet A riveting machine is used to drive the rivet and forms the head on the

plain end. This causes the rivet to swell and fill the hole tightly.

t t



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Keys, Pins, and Rivets (Rivets)Keys, Pins, and Rivets (Rivets)

Riveted Joints :



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Springs (Types)Springs (Types)



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Springs (Representation/Drawing)Springs (Representation/Drawing)

Schematic:



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Springs (Representation/Drawing)Springs (Representation/Drawing)

Detailed:

Documents

CH11: THREADS, FASTENERS AND SPRINGS Mechanical Engineering Department MECHANICAL DRAWING 1 MECHANICAL DRAWING Chapter 11: Threads Fasteners and Springs