chapter 13: fasteners and springs

8/14/2019 Chapter 13: Fasteners and Springs

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Chapter 13: Fasteners and

Springs


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Unified Thread SeriesStandardization of screw threads- 1949: United States,

United Kingdom & Canada

Purpose Need for interchangeability of

screw thread fasteners Revised in 1974 for Metric application


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Screw Thread Terminology

Axis Body

Chamfer

Classes of Threads Crest

Depth of Thread

Die

Fit Included Angle

Lead

Left-hand Thread Major Diameter

Minor Diameter

Pitch


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Screw Thread Terminology

(continued) Pitch Diameter

Right-hand Thread

Root Tap

Tap Drill

Thread

Thread Form

Thread Series Threads per Inch


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Thread-Cutting Tools

Tap Machine tool to form internal threads

Die- Machine tool to form external threads

Lathe Internal and external threads can be

cut on a lathe


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Thread Forms

Unified Threads

American National

Threads

Sharp-V Thread Metric Thread

Whitworth Threads

Square Threads

Acme Threads

Buttress Threads

Dardelet Threads

Rolled Threads American National

Standard Taper Pipe

Threads


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Thread Representations

Detailed Thread Representations Notcommonly used / too time consuming

Schematic Representations

Simplified Representations Most commonly

used


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Demonstrate How to Draw

the Different Types ofThread Representations

Detailed

Schematic

Simplified


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Thread NotesA thread note is always written in the same order.

Metric Thread note example

M 10 X 1.5-6H

Unified & American National Threads

-13UNC-2A


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Thread Notes (continued)

Other thread form note examples

Acme Threads 5/8 8 ACME 2G

American National Standard Taper Pipe

thread note example - 14 NPT


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Threaded Fasteners

Bolts and Nuts

Machine Screws

Cap Screws

Set Screws


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Threaded Holes to Receive a

Fastener Through Holes

Blind Holes


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Most Common Method of

Producing Internal Threads is

With Taps

ASME/ANSI B94.9 covers designs of ten

different types of taps, furnished in taper,

plug, or bottoming chamfers.


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Three Physical Possibilities to

Consider when Designing External

Threaded Features1. Apply thread to a straight rod sized to the major

diameter of the thread.

2. Apply the thread to a feature diameter that isreduced in size from its adjacent feature, creatinga shoulder.

3. Apply the thread to a feature diameter that is

larger in size than the adjacent feature.


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Three Major Functions of Pipe

Threads

1) An assembly that creates a pressure-tight

joint, either with a sealer or without a sealer

2) An assembly with a free/loose fittingmechanical joint that is not pressure-tight

3) An assembly with a rigid mechanical joint

that is not pressure-tight


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Considerations for

Manufacturing Taper PipeThreads Pilot holes are round

Minimal waviness on produced thread, bothinternal and external (watch tool alignment)

Full crests for at least the first three threads,

internal


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Other Types of Fasteners

Lag Screws -designed to attach metal towood or wood to wood

Self-Tapping Screws - designed for use in

situations where the mating thread is createdby the fastener

Thread Inserts used by being screwed intoa threaded hole to form a mating internal

thread for a threaded fastener


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(continued)

Self-Clinching Fasteners any threaded device

that displaces the material around a mounting hole

when pressed into a properly sized drilled or

punched hole

Self-Clinching Nuts feature thread strengths

greater than those of mild screws and commonly

used wherever strong internal threads are needed


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(continued)

Self-Clinching Spacers & Standoffs usedwhere it is necessary to space or stack

components away from a panel. Self-Clinching Studs externally threaded

self-clinching fasteners that are used wherethe attachment must be positioned beforebeing fastened.


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Demonstrate the Following

Drawing Applications Various types of screw heads

Using templates to draw fasteners

Using CADD to draw fasteners

Drawing nuts, washers, dowel pins, taper & otherpins

Drawing retaining rings, keys, keyways & keyseats,and rivets


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Springs

A mechanical device often in the form of a helical coil, thatyields by expansion or contraction to pressure, force or stress

applied

Springs are made to return to their normal form.

Springs are designed to store energy for the purposeof pushing and pulling machine parts.

Spring material may have to withstand high

operating temperatures & high stresses under

repeated loading.


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Five Criteria for Effective Use of

Springs in Machine Design1. Material

2. Application

3. Functional Stresses

4. Use

5. Tolerances


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Common Material Used in

Spring Technology

High-carbon spring steels

Alloy spring steels

Stainless spring steels

Music wire

Oil-tempered steel

Copper-based alloys

Nickel-based alloys


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Types of Springs

Compression spring

Extension spring

Flat spring Torsion spring


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Spring Terminology

Ends Helix direction

Free length

Compression length

Solid height

Loading extension


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Spring Representations

Detailed

Schematic

Simplified


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Spring Specifications Include:

Outside or inside diameter Wire gage

Kind of material

Type of ends Surface finish

Free & compressed length

Number of coils


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Demonstrate the Following

Drawing Techniques forSprings

Detailed representations

Schematic representations

Simplified representations

chapter 13: fasteners and springs

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