To Know the life

Please advise:

• what sort of spring?
• in what application?

Is a Flapper spring

I will use it as check valve

Although the other comments are ok there is an other to make. You can today simulate on a computer the behaviour of the spring and determine the stress level the material will reach in use. Looking at the roughnesses in the regions where you find the highest tensions it is possible to estimate with a quite large uncertainty the number of cycles. Todays soft allows such approaches and is based on tests as suggested by other comments. The probability of failure you get from the computations can indicate if you really need a test and how large should be the sample.

I already used this approach for some other types of springs with quite good results.

The only problem is that for low number of cycles it is more difficult to obtain a reliable result only from computations if high stress levels are reached.

Basically, no. You'll need to cycle it until it fail.

Pineapple

Basically, no. You'll need to cycle it until it fail.

Not only that, you will need to do that to at least 6 "identical" springs to get any kind of meaningful statistical data (mean life and standard deviation). The more samples, the more meaningful your data will be. If you only test one, you may not know if it failed earlier, or was some kind of "perfectly made" spring that had an extremely long life and you could not expect the same life from your average spring. Then again, if your operating conditions vary at all (loading, frequency, environment, etc.) you would want to repeat the testing for every combination at the extremes, maximum and minimum. Interactions between conditions may not give you the same answer as simply testing each maximum effect and minimum effect grouping separately.

If you have a lot of variables, you can reduce your sample size by applying Taguchi Methods, also known as Design Of Experiments (DOE), in setting up your test and analysis.

On the other hand, if it is a standard stock spring, the manufacturer may have some data or examples to support an estimated life for the spring or for similar ones, for a given set of conditions, which may be enough to suit your purposes.

Hi STL,

What about a spring that stays compressed for a long time. Does that cause it to lose some of its strength? I'm thinking about something like the spring in a handgun, or rifle, magazine. I usually remove cartridges periodically from a spare magazine and "let the spring rest for awhile". Wonder if that really does any good or improves the life of the spring?

-John

Any material placed under a load for a long period of time is subject to a phenomenon known as "creep". "Creep" is the tendency for individual atoms or molecules, or even groups of them to move and displace the next set, displacing the next set, and so on like dominoes. However, "creep", as the term implies is usually a very slow-acting phenomenon and often not noticeable, unlike rupture, which happens immediately. However, creep can eventually lead to rupture or some other mode of distortion, usually a fracture failure in brittle materials or a strain failure in ductile ones.

That being said however, in practical terms, with a good, well-tempered spring material, a static load, as in keeping the spring under normal compression, within its elastic limits, will not likely give any noticeably less life than periodically unloading it. When initially used, or if elastic limits are exceeded, springs can take a "set" or permanent displacement, but this is usually already accounted for in the design. Rubber and some plastic materials in particular are prone to taking sets over the long term, but most metal springs (steel, stainless, phosphor bronze, beryllium copper, etc.) do not, at least not to any appreciable degree.

So unless that spare magazine has a plastic spring, I would not worry about letting the spring "rest awhile". On the other hand, springs used in a hot environment, like an oven or "under the hood", i.e. near or mounted on the engine of an automobile or its exhaust system can suffer from continuously loaded condition during heat cycling, speeding up "creep". If temperatures are extreme enough, the spring could even anneal and lose all its stored energy very quickly, with the compression force going to zero or a totally relaxed state.

Thnaks STL,

I appreciate that.