How beds work: what they do, why, and how?
February 7, 2018
The body and the bed
In previous instalments, we have looked at various elements of the mechanics of a mattress. We have used an array of objects in our discussions which at first glance have nothing to do with beds or bodies. But the knife, the squash, the orange, the table, the sponge, the sardines, and the napkin, have illustrated the fundamentals.
We understand how a deforming surface (like a sponge) spreads contact over a larger area, so reducing pressure. We also know that too soft a sponge will ‘bottom out’, causing point contact (and high pressure) with the surface beneath.
We observed that an object can also be supported in a different way: using ‘hammock’ tension, like on the napkin. We saw that on some shapes (like an orange) this will spread the pressure evenly, wheras on other shapes (like the can of sardines) this will give us concentrated pressure points.
What is a body?
Every body some combination of ‘orange’ and ‘sardine can’. In some ways, and in some places, we are rounded like an orange. In other ways, and in other places, we have bony prominences – ‘corners’ – like a sardine can. Even those of us who are relatively well-upholstered with fat and other soft tissue have these bony prominences lurking within. Mechanically speaking, the thing about soft tissues is that they are soft. Under load, they tend to squish out of the way, and weight is actually taken on the bony ‘corners’.
Looking at the diagram below, we are seeing inside somebody about to lie on a mattress in a semi-recumbent (sitting up at 45 degrees) position, as you might if you were reading in bed.
So far, so good. The pelvic bones are well covered by the soft tissues of the buttocks, and it looks like we are going to present a nice round ‘orange’ to the bed.
However, look what happens when the body weight arrives (in this case made worse by the semi-sitting position, which puts some of the weight of the head and torso onto the buttocks. )
The soft tissues squish out of the way under load, and the pelvic bones effectively come into contact with the bed. We now have the ‘corners’ of the sardine can, and we can impress people no end by calling them the ischial tuberosities (the two ‘loops’ in the middle) and the great trochanters (the broadest part of the hips). On a hammock-like bed, the ‘napkin’ effect will give us real problems with pressure distribution. It will look something like this:
No problem, you may be thinking at this point. I have absolutely no intention to install a hammock in my bedroom. I don’t even have the palm trees. So the disagreeable situation pictured above will not befall my buttocks. Which brings us to the next question:
What is a bed?
Every bed is some combination of ‘sponge’ and ‘napkin’. Of course every mattress in the store has some kind of ‘sponge’ filling – something to allow the deformation of the bulk of the mattress to accommodate the shape of the body. This may consist of any or all of spring bases, foams, polyester fibre wadding, deformable gel, air or water bladders, and even yielding wooden slats in the base. Some beds will deform a little, and some beds will deform a lot, but they all obviously have some degree of this ‘sponge’ element.
Less obvious is that, to a greater or lesser extent, they all also have the ‘napkin’ or hammock element. Consider for example a simple (and not uncommon) construction consisting of a block of foam covered with a woven fabric cover. To some extent, the shape of the body will be accommodated by the yielding of the foam. However, as the foam yields, and the body sinks in, the woven fabric cover is placed under tension. It becomes a ‘napkin’/hammock. In fact, this ‘napkin’ mode of supporting the body weight does not apply only to the cover. As the body indents into the foam, tension is also created in the top of the foam, as it has to stretch sideways to take on the ‘dent’ shape of the body sinking into it. In fact, the softer the mattress, and therefore the further the body sinks into it, the more pronounced the ‘napkin’ tension becomes.
So the bed becomes a complex sponge/napkin ‘system’, always supporting the body by a combination of the two elements.
There are ways to mitigate the napkin effect, if it is less desirable for the body type. One way is to provide stretchiness in the cover material. Covers made of knitted materials will tend to allow more stretch without generating tension. Another is to eliminate any non-stretchy upper padding layers such as carded fibre.
There are even ways to deal with the tensile effects in the main padding material. Looking at the diagram below of the block of foam in the simple mattress, the arrows show the direction of tension that would occur in the upper surface of the foam, if we put our weight on the middle of the mattress.
Now imagine we made lots of vertical cuts into the top of the foam, like in the diagram below.
These vertical cuts mean that there is no continuity in the surface of the foam, so no tension can be sustained across the surface. With an appropriate stretch cover, and carefully chosen upper padding, this geometry will greatly reduce the ‘napkin’ effect. Pocket springs (if the pockets are not stitched together at the top surface) have exactly the same purpose.
When choosing a bed, the game becomes recognising that your body is part ‘orange’ and part ‘sardine can’, and recognising that the mattress is part ‘sponge’ and part ‘napkin’. Your aim is to find the right mix of those two elements for your body. In the next instalment, we will look at some simple drills to try in the bed store, in order to find that mix.