Another pressure question

HoneyBadgerDC · May 22, 2014, 4:41pm

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?

Machine_Elf · May 22, 2014, 5:31pm

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:

Neal Stephenson:

At the time, depths were sounded by heaving a lead-weighted rope over the side of the ship and letting it pay out until it hit bottom. So far, so easy, but hauling thousands of meters of soggy rope, plus a lead weight, back onto the ship required the efforts of several sailors and took a long time. The US Navy ameliorated the problem by rigging it so that the weight could be detached and simply discarded on the bottom, but this only replaced one problem with another one in that a separate weight had to be carried for each sounding. Either way, the job was a mess and could be done only rarely. This probably explains why ships were constantly running aground in those days, leading to a relentless, ongoing massacre of crew and passengers compared to which today’s problem of bombs and airliners is like a Sunday stroll through Disney World.

In keeping with his general practice of using subtlety where moronic brute force had failed, Kelvin replaced the soggy rope with a piano wire, which in turn enabled him to replace the heavy weight with a much smaller one. This idea might seem obvious to us now, but it was apparently quite the brainstorm. The tension in the wire was so light that a single sailor could reel it in by turning a spoked wooden wheel.

The first time Kelvin tried this, the wheel began to groan after a while and finally imploded. Dental hygienists, or people who floss the way they do (using extravagantly long pieces of floss and wrapping the used part around a fingertip) will already know why. The first turn of floss exerts only light pressure on the finger, but the second turn doubles it, and so on, until, as you are coming to the end of the process, your fingertip has turned a gangrenous purple. In the same way, the tension on Kelvin’s piano wire, though small enough to be managed by one man, became enormous after a few hundred turns. No reasonable wheel could endure such stress.

HoneyBadgerDC · May 22, 2014, 5:39pm

Machine_Elf:

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?

Machine_Elf · May 22, 2014, 6:06pm

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

GusNSpot · May 22, 2014, 8:44pm

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

HoneyBadgerDC · May 22, 2014, 9:23pm

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