Spring Properties
Typical properties of
common spring materials:
|
Material
|
Young’s modulus
(GPa)
|
Modulus of rigidity
(GPa)
|
Density
(kg/cm3)
|
Maximum service
temperature (oC)
|
|
Music wire
Hard drawn wire
Oil tempered
Valve spring
|
207
|
79.3
|
7890
|
120
150
150
150
|
Preferred spring wire diameters
(mm):
|
0.10 |
2.00 |
|
0.12 |
2.20 |
|
0.16 |
2.50 |
|
0.20 |
2.80 |
|
0.25 |
3.00 |
|
0.30 |
3.50 |
|
0.35 |
4.00 |
|
0.40 |
4.50 |
|
0.45
|
5.00 |
|
0.50 |
5.50 |
|
0.55 |
6.00 |
|
0.60 |
6.50 |
|
0.65 |
7.00 |
|
0.70
|
8.00 |
|
0.80 |
9.00 |
|
0.90
|
10.00 |
|
1.00
|
11.00 |
|
1.10 |
12.00 |
|
1.20 |
13.00 |
|
1.40 |
14.00 |
|
1.60 |
15.00 |
|
1.80 |
16.00 |
Spring natural frequencies:
Springs can be vibrate both
laterally and longitudinally when excited near their natural frequencies. If a
helical spring fixed at one end, is given a sufficiently rapid compression at
the other, the end coil will be pushed against its neighbor before the
remaining coils have time to respond to the displacement. This phenomenon is
known as spring surge and causes very high stresses in the spring, which are
approximately equal to those when the spring is compressed to its solid length.
The natural frequency, fn
of spring surge depends on the
boundary conditions.
For fixed-fixed case:
where:
fn
= natural frequency (Hz);
k = spring rate
(N/m);
m = mass (kg).
The mass of helical spring is
equal to the product of density and volume, so for N coils in spring this is
given by
Substitution for the mass and
spring rate in the above equation
for steel spring with a
modulus of rigidity of 79.3 GPa and density 7860 kg/m3
Where:
d = wire diameter
(m);
D = coil diameter
(m);
N = number of
coils.
In order to avoid surge, the
spring should not be cycled at a frequency close to its natural frequency. The
natural frequency of spring surge should usually be made higher than the highest
significant harmonic of motion involved, which is typically about the
thirteenth. So the natural frequency should be at least 13 times the forcing
frequency of the load on the spring in order to avoid resonance.
Spring Creep:
Springs are subjected to
creep under load. This is sometimes evident in old cars where the sustained
weight on the suspension springs over the years has caused a permanent
shortening the spring’s overall length and the car body ground clearance is
reduced. This shortening by creep is known as set. Set is directly related to
yield stress.
Spring prestressing:
Prestressing, also known as
presetting, of a spring can be used to improve a spring ability to withstand
stress, increase its load-carrying capacity and fatigue resistance.
Prestressing takes place after the spring has been coiled, stress relieved and
ground. It involves compressing the spring to its solid length or a fixed
position that is greater than its maximum working length. This process is
repeated a number of times typically no less than three. During prestressing
the spring’s dimensions will alter.