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Spring Design

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Materials testing specific to helical compression springs is available, however ... Helical Extension Springs. Similar in most ways to compression springs ... – PowerPoint PPT presentation

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Title: Spring Design


1
Spring Design
  • Wayne Book
  • ME3180
  • Norton Machine Design

2
Basic Spring Behavior
3
Some Types of Springs
4
Some Types of Springs (2)
5
Some Types of Springs (3)
6
Some Types of Springs (4)
7
Spring Materials WiresProperties of interest
  • High strength
  • High yield
  • Modulus may be low for energy storage
  • Cost
  • Temperature resistance (e.g. valve springs)
  • Corrosion resistance

8
Common Materials (partial list for wires)
9
The Curious Behavior as Diameter Decreases
10
Helical Compression Springs
  • Lengths
  • Free
  • Assembled
  • Solid or shut height
  • Working deflection

11
End Treatment
  • End details affect active coils
  • Plain
  • Ground
  • Squared
  • Ground

12
Spring Definitions and Calculations
  • Spring index C
  • 4ltClt12
  • Active turns Na
  • Spring deflection results from torsion under load
  • Spring stress including
  • Torsion
  • Direct shear
  • Stress concentration Kc or Wahl factor Kw

13
Stress Distribution
14
The thing about residual stresses
  • Setting, also called removing the set consists
    of compressing the spring past yield as part of
    manufacture
  • The residual stresses remain in the inner surface
    of the wire
  • This reduces static stress during following
    cycles
  • Should not be used in tension
  • Shot peening can also be used primarily for
    fatigue strength

15
Buckling and Surge of Compression Springs
  • A problem for long skinny springs
  • Rod inside is sometimes used to eliminate, but
    wear and friction result
  • Depends on ratio Lf /D
  • Surge is another problem that occurs with
    excitiation near the natural frequency

16
Properties of Spring Materials--Yield
  • Yield strength for static loading
  • Depends on set
  • Before set removed use Wahl factor
  • After set removed no stress concentration used

17
Properties of Spring Materials--Fatigue
  • Fatigue Strength
  • Torsion is relevant loading- could use von Mises
    stress
  • Materials testing specific to helical compression
    springs is available, however
  • Correct for temp., reliability, environment

18
Properties of Spring Materials-- Endurance
  • Endurance Strength (steels) unlimited cycles
  • For high ultimate strengths, endurance limits max
    out at 45 kpsi (unpeened) and 67.5 kpsi (peened)
  • Small wires have high ultimate strength
  • Tests have been done specific to spring wire
  • Temperature may require compensation
  • Corrosion
  • Reliability

19
S-N and Modified Goodman Diagram
20
Designing Springs
Requirements
Design Choices
  • Functionality
  • Stiffness
  • Lengths
  • Diameter
  • Forces
  • Reliable operation
  • Static factor of safety
  • Fatigue factor of safety
  • Buckling and surge
  • Manufacturability
  • Index C
  • Material
  • Wire and coil diameter
  • Number of turns
  • End treatment and constraint
  • Set and shot peen

Constraints (other)
  • Bend radius

21
Helical Extension Springs
  • Similar in most ways to compression springs
  • Usually wound to be closed coil at zero force
  • Thus a preload is required to stretch any, i.e.
    yk(F-Fi )
  • Spring hook is a source of failure in bending and
    torsion
  • No set is used
  • One coil not considered active

22
End Hook Stresses
Bending stress
Torsional stress
23
Torsional Springs
  • The wire in a torsional spring is primarily in
    bending
  • Spring constant is rotary Mk?
  • Loading should act to wind up coil
  • Design process resembles compression springs

24
Compression Spring Design--Fatigue
  • Static design also important but details left to
    example
  • Data available for springs with loading from zero
    to some compresion value
  • Application often has preload how to use?
  • First construct (or find) S-N curve
  • Next construct Mod-Goodman chart
  • Apply load line for given preload and design
    stress
  • Find factor of safety to failure point
  • Details discussed via MathCad solution

25
Goodman Chart for Example 13-4
26
Homework 10
  • Due 11/28/01
  • In text 13-1, 13-13 (Must see web page for
    starting values in design and problem statement
    corrections)
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