pocket springs in series

dumdumheretx

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Jun 19, 2020
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Hope this is the right sub forum.
I was trying to understand in simple terms what happens if you place a layer of pocket springs on top of an identical layer as used in a mattress. I have tried searching the answer but the formulas are beyond me trying to relate to the real world what will happen. I understand the springs would be considered in series. My understanding is that a force applied from above will be transferred equally to the two springs below. Does this mean is say a 10 inch high spring compressed for the same weight 3 inches, that same weight now means the compression of amount you will sink will be doubled to 6 inches? The logic in my head says surely the force must still result in a 3 inches of compression shared at 1.5 inches for each spring as the springs can now be considered a single spring? but when i look at calculations this appears to be what happens when they are in parallel rather than series?
Can anyone help me apply the maths of springs to what i would observe in a mattress when doubling the spring layers with springs stacked. The only other thibng i seemed to gleam from the maths is that the now 20 inch spring would have more side to side movement or in other words be more flimsy due to increased deflection?
 

Cubist

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Oct 29, 2019
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My understanding is that a force applied from above will be transferred equally to the two springs below.
Yes, if we ignore all the adjacent springs in the mattress (which will influence the result especially as the deflection distance increases)

Does this mean is say a 10 inch high spring compressed for the same weight 3 inches, that same weight now means the compression of amount you will sink will be doubled to 6 inches?
If the spring obeys Hooke's law, and does not become distorted by any extreme deflection, then this would be true. EDIT - actually I think it would also be true for springs that don't obey Hooke's law!

Many springs don't obey Hooke's law, they have a profile (often seen physically as smaller diameter towards the middle, or one end, like a cone shape). This means that the deflection amount becomes progressively less for each increase in load. This type of spring can be great for upholstery because a light child still gets some deflection and a heavier adult does too (the spring is less likely to "bottom out" or sink too much because the spring pushes back much harder for each increase in load)

The logic in my head says surely the force must still result in a 3 inches of compression shared at 1.5 inches for each spring as the springs can now be considered a single spring?
It can be considered as a single spring, but the "spring constant" of the double spring will be twice the spring constant of each (identical) single spring.

Perhaps you could perform an experiment with a couple of elastic bands to help convince yourself! Obviously stretching them with a weight rather than compressing them. Remember to measure the extension ( loaded length - unloaded length ) rather than just comparing the loaded lengths.

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On a general note about buying mattresses, if that is what you are thinking of. Rather than considering the maths I recommend that you lie on the mattress in the store for an embarrassingly long time before you decide to buy! You'll be spending a long time on the mattress at night, so it is well worth giving it a REALLY good test before committing yourself!
 
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dumdumheretx

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thank you, that was very helpful.
 
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