The firt step to correctly calculate a compression spring is to know the working environment and, specially, the following parameters: temperature, exposure to corrosive agents, number of expected cycles and frequency.
This factors, in fact, greatly influence the choice of the correct material, for the followig reasons:
Not all materials can work properly at all temperatures. For example, EN 10270-1 wire is suitable for a working temperature between -20 and +80°C, AISI 302 stainless steel has a range between -200 and +250°C, for higher temperatures needed nickel superalloys such as Inconel or Nimonic.
EXPOSURE TO CORROSIVE AGENTS
If the working environment is is very aggressive (for example in contact with sea water, acids, methane…) the surface protections are no longer sufficient and therefore a material capable or withstanding this environment is needed.
NUMBER OF EXPECTED CYCLES AND FREQUENCY
If a spring has to survive millions of work cycles, this must be know in the design phase: a spring that works well in a static way could have a low fatigue life. Also in this case, there are materials more suitable than others for long durations ( EN 10270-2)…
Frequency is also important: doing 10 cycles per minute is different from doing 10 per second or 10 per hour. If the frequency is known, a resonance analysis can be done.
Once the material has been selected, we can consider the space available for the spring and ask ourselves whether a housing hole or a pin on witch the spring must slide has already been considered in the design of the device.
If so, we need to know this data so that we can comply with it where possible.
WORKING POINTS AND STROKE OF THE COMPRESSION SPRINGS
We will also need to know the expected work points and therefore the stroke that the spring must make. Length L1 is with the spring unloaded, mounted and slightly compressed, bearing in mind that the free spring will be a little longer than this measurement.
The length L2 is that of the spring at its maximum load and will be calculated so that the fully compressed lenght is less than L2.
The stroke of the spring will be given by L1-L2 and, alternatively, only L2 can be indicated with the desired elastic constant. L2 must be the lowest height that the spring can reach, even occasionally: if you leave the elastic range even once, the spring will no longer return to free height and will be irreparably damaged.
THE AVAILABLE SPACE AND THE LOADS OF THE COMPRESSION SPRINGS
It may not be possible to comply with all the dimensions provided, because the required loads are often incompatible with the available spaces: in this case we will calculate the spring which, respecting the spaces, is able to supply the maximum load and the one which, respecting the required loads, has the smallest footprint.