threads, fasteners, and springs chapter 11. 2 technical drawing 13 th edition giesecke, mitchell,...

Threads, Fasteners, and Springs

Chapter 11

2Technical Drawing 13th EditionGiesecke, Mitchell, Spencer, Hill Dygdon, Novak, Lockhart

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Objectives

• Define and label the parts of a screw thread

• Identify various screw thread forms• Draw detailed, schematic, and

simplified threads• Define typical thread specifications

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Objectives (cont.)

• Identify various fasteners and describe their use

• Draw various screw head types• Draw springs

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Understanding Threads and Fasteners• There are three basic applications

for screw threads:• To hold parts together• To provide for adjustment between

parts• To transmit power

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Screw Thread Terms

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Screw Thread Terms

• Screw thread – a ridge of uniform section in the form of a helix on the external or internal surface of a cylinder

• External thread – a thread on the outside of a member

• Internal thread – a thread on the inside of a member

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Screw Thread Terms

• Major diameter – the largest diameter of a screw thread

• Minor diameter – the smallest diameter of a screw thread

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Screw Thread Terms

• Pitch – the distance from a point on a screw thread to a corresponding point on the next thread measured parallel to the axis• Pitch (P) equals 1 divided by the

number of threads per inch

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Screw Thread Terms

• Pitch diameter – the diameter of an imaginary cylinder passing through threads where the widths of the threads and the widths of the spaces would be equal

• Lead – the distance a screw thread advances axially in one turn

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Screw Thread Terms

• Angle of thread – the angle between the sides of the thread measured in a plane through the axis of the screw

• Crest – the top surface joining the two sides of a thread

• Root – the bottom surface joining the sides of two adjacent threads

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Screw Thread Terms

• Side – the surface of the thread that connects the crest with the root

• Axis of screw – the longitudinal centerline through the screw

• Depth of thread – the distance between the crest and the root of the thread measured normal to the axis

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Screw Thread Terms

• Form of thread – the cross section of thread cut by a plane containing the axis

• Series of thread – standard number of threads per inch for various diameters

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Screw Thread Forms

• Typical thread forms include:• Sharp-V thread (60 degrees)• American national thread• Unified thread• Unified extra fine thread series• Metric thread• Square thread• Acme thread

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Screw Thread Forms

• Typical thread forms (cont.)• Standard worm thread • Whitworth thread• Knuckle thread• Buttress thread

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Screw Thread Forms

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Thread Pitch

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Right-hand and Left-hand Threads

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Single and Multiple Threads• Single threads are composed of

one ridge and the lead is equal to the pitch

• Multiple threads are composed of two or more ridges running side by side

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Single and Multiple Threads

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Thread Fits

• ANSI has established 3 classes of fit:• Class 1 – when clearance between

mating parts is essential• Class 2 – high quality for the bulk of

interchangeable screw thread work• Class 3 – exceptionally high quality

recommended only when high cost of precision is warranted

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Thread Fits

• Some specialized metric thread applications are specified by:• Tolerance grade• Tolerance positions• Class• Length of engagement

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Methods for Drawing Thread• There are three methods of

representing screw threads on drawings:• Schematic• Simplified• Detailed

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Methods for Drawing Thread

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Methods for Drawing Thread

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Methods for Drawing Thread

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Thread Notes

• ASME/ANSI Y 14.6-2001 “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

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Thread Notes

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Thread Notes

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External Thread Symbols

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Internal Thread Symbols

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Detailed Representation

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Detailed Representation

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Use of Phantom Lines

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Threads in Assembly

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Pipe Thread Representation

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Bolts, Studs, and Screws

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Types of Screw Heads

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Tapped Holes

• 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

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Tapped Holes

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Standard Bolts and Nuts

• Standard bolts and nuts have characteristics determined by:• Bolt types• Finish• Proportions• Threads• Thread lengths • Bolt lengths

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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

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Locknuts and Locking Devices

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Standard Cap Screws

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Standard Machine Screws

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Standard Set Screws

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Standard Wood Screws

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Miscellaneous Fasteners

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Keys

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

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Keys

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Machine Pins

• Machine pins include taper pins, straight pins, dowel pins, clevis pins, and cotter pins

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Rivets

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

52Technical Drawing 13th EditionGiesecke, Mitchell, Spencer, Hill Dygdon, Novak, Lockhart

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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 classified as:• Helical springs• Flat springs

53Technical Drawing 13th EditionGiesecke, Mitchell, Spencer, Hill Dygdon, Novak, Lockhart

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Helical Springs

• Helical springs have three types:• Compression springs• Extension springs• Torsion springs

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Helical Springs

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Compression Springs

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Extension Springs

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Torsion Springs

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Flat Springs