How to define an industrial spring? First, it’s important to consider what its principal functions are, namely either to store energy or release it. Next, establish the optimum choice of spring to meet the needs – coil, tension, compression or wafer, for example – and then the appropriate design and choice of material and manufacturing process.
To underline the importance of these factors, take just two applications at opposing ends of the scale. A small compression spring used in a microswitch assembly will be expected to perform each and every time in an operational life that will run into tens of thousands of cycles – and more. At the other end of the scale, a spring might be stored under compression for long periods before it is expected to perform just the once, but in a life or death situation – a spring used in ejector seat mechanisms.
Goss Springs, based in Epping, has been designing and manufacturing industrial springs for over 65 years. “Material choice is crucial” says Nick Goss, “generally the more expensive the material, the longer the life and better performance.”
The type of application will, of course, determine life expectancy, performance requirement and the type of material used. A typical application for a compression spring in an engine valve might involve 8,000 cycles per minute and the best material to guarantee this sort of performance is chrome silicon.
“The extreme environment of the offshore industry means that Inconel or Phynox is usually the material of choice,” Goss adds. “Inconel alloys are oxidation- and corrosion-resistant materials well suited for service in extreme environments. When heated, Inconel forms a thick, stable, oxide layer protecting the surface from further attack.”
Two considerations should be borne in mind here. On the one hand, it is true that in higher temperature applications springs can relax over time as loss in spring load occurs. By the same token, in some applications the spring may be inoperative for long periods of time and it is essential that storage is in reasonable ambient temperature conditions. Equally, the selection of a larger wire diameter can reduce undesirable stress or using a lower final load will relieve unwanted stress in the spring.
SHOCK TACTICS
Shock loading occurs when the load is suddenly increased or accelerated. The speed at which the load is dropped onto a spring can cause damage to the spring and so, if this can be minimised, the life of the spring will be increased. A process known as ‘shot preening’, whereby the surface of the spring is ‘dimpled’, helps the spring become more resistant to stress. Another process, pre-stressing the spring, increases the elastic limit in the torsion and thus makes the spring stronger and less subject to stress.
If the natural frequency of the spring matches the frequency of the operating speed, it will resonate, leading to vibration, which ultimately may make the spring break. So, it is important to ensure that the operating frequency of the spring is significantly lower than the natural frequency of the spring.
