10' Hourglass

I can tell you they don't use sand in hour glasses as it abrades the surface and makes the hourglass unreadable. The stuff is pulverised sea shells, very fine and not abrasive. Your case if acrylic may be more susceptible.

Actually, what were using is Ballotini. Supposed to be more round, & not as angular as regular sand. But.ya i agree on the wear / scratching on the acrylic. I was pushing for scratch resistant plexi, but the higher up, didn't want the extra cost,Whatever! LOL

I don't know if you can have it done on such a large piece, but I have used heat treated acrylic, and the surface is very hard. The surface of heat treated acrylic is also very resistant to solvent type adheisives.

Can you post a sketch/diagram of the thing ? I'd be more interested in how much of a 'kthunk' the 150lb of sand makes each time the hourglass is rotated. Why not put a temporary plug on the neck and a temporary seal on the, as yet, unsealed end ?

DON'T SAY KTHUNK! LOL I've got enough worries right now. The wheel is going to take 15 seconds to rotate, So I think the sand will shift gradually enough. My biggest concern on that level is that at 1/4 rotation of the wheel, all of that 150lbs of sand , causing the 3 4" Drive rollers to possibly slip due to all of the weight being off center. I've got a cad drawing of it but i'm not so computer literate,so im not sure how to go about posting it?

In the reply box you will see what looks like a camera in the bar (8 icons to the left of the smiley). Click on that and it will give you options to post an image.

It may well change the flow rate a bit, given your configuration. The other issue to be aware of is bridging especially in a sealed system.. In grain silos and other applications where material flows through a nozzle, it is not unusual for the grains to interlock in such a way that material builds up across the hole preventing flow.

The other hazard is static electricity. Flowing solid material through acrylic is asking for static electricity build up. Have you designed a grounding system?

This article talks about bridging.

http://www.jenike.com/pages/education/papers/solve-probs-retrofit.pdf

I'm guessing that grain silos and similar 'lock-up' when left un-operative with a stack of <whatever> resting on the throat or valve. In mining, rock often gets transferred down chutes but can lock-up if left there for a prolonged period. A stick of dynamite would be a bit drastic for an hourglass, but I don't think he'll get the problem since flow is always continuous.

Depends on the material.. In polymer resin silos bridging happens all the time, and I have seen it in cheap small hourglasses as well.

it only moves every fifteen minutes, when stationary it would be subject to bridging.

I didn't know it had a name ! Now I think about it, I've seen cheap hour glasses do it too. There must be a link between neck-size, particle-size , and angle of friction. It's probably impossible to be 100% certain that 'bridging' will never occur with a particulate media.

Just a thought but have you considered using something else besides sand? I was also thinking that since you are going with amonster size glass, why not go with larger particles. In the pharma industry we use glass or polymer beads for some products. Another possibility to consider that has larger size but lower mass and mean density is pingpong balls or the polypropelene equivalent that are smaller used in some toys. You may be able to wheedle a corporate sponsor for such material to be used. Foam balls would be heavier by also a higher surface friction. Not sure if bridging would be an issue with the fiam rubber though. As for the unbalancing issue - the simple solution would be to put most of the mass into the frame such that the net weight change from the shufting sands would be negligable.

NO GO on that issue,Dealing with DESIGNERS,need I say more?

My question is, that right now with the "mock-up", the top of the hourglass is open. When we fill the finished case it will be sealed around the perimeter with silicone. By making the case air tight, is that gunna change the flow rate of the sand as far as air displacement.

The inhibited air flow between the top and bottom globes of the sealed hourglass probably will slow down the flow of the sand. To remedy this, drill a 1 inch diameter hole at the vertical midpoint of each globe (assuming the sand does not reach that high). The holes could be covered with cloth to prevent airborne sand escaping. Or if cloth still allows fine dust to escape, you could hermetically seal the hourglass by connecting the two holes via a short length of plastic tubing (maybe have a whistle inside the tubing for sound effects).

I don't think air-flow is a problem, but here's another possibility - a tube of the right length that passes through the neck, allowing sand to slip through the annulus created. With a lot of experimentation, it might be possible to create something that uses two different coloured/density oils (?). Maybe even a modification of this ;

I don't think air-flow is a problem, but here's another possibility - a tube of the right length that passes through the neck, allowing sand to slip through the annulus created.

Good idea. By running the venting tube through the neck, no holes need be drilled through the sides of the globes. This would both simplify the design and make it look more elegant. On the other hand, the hourglass neck would need a larger diameter to accommodate the vent tube, which could cause a problem if the project already commits to a predetermined neck size. I don't know from experience whether or not a giant hourglass really needs a vent tube, but it seems reasonable since as the bottom globe fills, the displaced air must go somewhere. Without a vent tube, the displaced air must instead travel up through a pile of sand to move into the top globe -- we might expect packed sand to provide some resistance to air flow.