# Another pressure question

If I had a 1" wide elastic strip and I applied 20# tension on this strip while wrapping it around a 2" wide 1/2" thick composition of wood I was gluing together how many PSI’s would I be applying to the stack of wood. The wraps would have a 1/2" gap between each wrap. Also if I double wrapped with each twist would I double the PSI?

The wrap is 1 inch from edge to edge, and stretched across a 2" wide slab of wood. So the area under the strap is 2 square inches. Total pulling force is 40 pounds. So it should be an average of 20 psi under the strap. This would assume that the face of the wood stack has a semi-circular profile, which would result in a relatively even distribution of strap force against the surface of the wood. If the top of your wood stack is flat with sharp edges, then most of the strap’s force will be taken up at those edges, and very little applied at points midway between those edges.

Not clear what you mean by “1/2-cin gap between each wrap.” If there is a 1/2" of overlap of each wrap over each previous wrap, then the compression zones overlap: there is no area that doesn’t have two layers of strap squeezing it, so now it goes up to 40 psi. OTOH, if there is a 1/2" of bare wood between each wrap of the strap, then you only have 2/3 coverage by the strap, so the average pressure over a substantial length of wood will be 13.3 psi.

Multiple wraps in the same zone may be additive, provided that each successive wrap doesn’t compress the wood so much that it relieves the compression that was provided by prior wraps. This was a problem mentioned in Mother Earth, Motherboard:

Funny you posted that, I rememeber having fishing reels implode when I cranked them in under too much pressure.

You addressed the problem with the shape of the material, when I am using bamboo laminates I have a crown that does help apply pressure evenly, when I am using boards I have a flat surface and this is what I am concerned about. I was thinking of eliminating the stretch rubber alltogether and using a flat very dense foam on the top layer and then wrapping with a small strong rope. Do you think this would improve the pressure distrubution?

It certainly should help, if the flat layer of foam compresses at its edges so that the rope takes a more rounded path.

If this is a repetitive thing over time, would not halved dowel rods be a better idea?

Thats something I have considered but the shapes are very curvy, not conducive to anything rigid.