Spring Properties

Spring Properties

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):

 0.1 2 0.12 2.2 0.16 2.5 0.2 2.8 0.25 3 0.3 3.5 0.35 4 0.4 4.5 0.45 5 0.5 5.5 0.55 6 0.6 6.5 0.65 7 0.7 8 0.8 9 0.9 10 1 11 1.1 12 1.2 13 1.4 14 1.6 15 1.8 16

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.