Strain “for Dummies” and How It Relates to Load Cell Overload

What do we mean when we use the term “strain” as in “strain gauge”?

Strain results in a physical dimensional difference from loading a material.  When the last time you played with a slinky? Do you remember stretching it too much? You became downhearted as your nice new slinky wouldn’t retain its original shape.  As you stretch that slinky, the metal is getting thinner along the length that is stretched out (the small cross section of the metal).  Stretch it within its elastic range (the range in which it operates as a spring), it will come back to its original shape.  Stretch it too much (beyond its elastic range) and the slinky yields (bends).  Not only does it not go back to its original shape, but it also doesn’t quite work correctly anymore.  It doesn’t have its original “springyness” anymore.

Load cells operate on the same principle.  Operate them within their elastic range (maximum capacity) and they are like a nice new slinky.  Overload them, and they are like that slinky that is perfect out on the ends, but has that dumb wrinkle in the middle.

The slinky and anything else in this world including load cells, will never (yes, I know never say never, but in this case there are enough decimal places, as in infinite, to say never) return to its exact original shape.  This is called hysteresis.  If I deform material, even if it is well within its elastic range, the molecules are shifting, heat is occurring and everything on the atomic structure is just changing.  That is why you can’t flex a slinky forever without it eventually breaking, even if you only stretch it out a couple of inches.  It will break, eventually.  Every time you let it go back to its unloaded state, it has changed a little bit.  In the case of load cells, the strain gage and the electronics are not able to typically resolve this change when loading and unloading loads less than the maximum rated capacity.  For example, if you load 10 lbs on a 1000 lb load cell, you will read zero before you put on the 10 lb weight and then zero when you take it off.  However, if you could measure the displacement with enough decimal places, .000000000000 etc, you would eventually see a change.  The strain gage and electronics cannot see it though, which is actually very nice for all of us.

Click here for more technical information about strain and deformation.

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