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)