Suspended shelf construction

I'm building a suspended shelf which will hold a hefty (14kg) video projector. There's a picture of a previous, less substantial, version at

I'm not likely to find something of the right size ready-made, so is such a thing DIY-able? I have a suspicion that machine tooling and perhaps assembly under considerable pressure, would be required. But perhaps I'm wrong.

Alternatively, any other suggestions for a suitable approach will be welcome. Thanks.

Bert

The ikea tables @ £12 are made this way (honeycomb cardboard inner) - your picture shows an upside down table

Thanks very much for that, Alan. Annoyingly - but pretty much inevitably - the top of the small version is just a tad too small and the large one much too big. I could extend the width of the small one by fixing timber (or whatever) additions to the sides, though they would probably be tricky to blend in seamlessly. Do you think that the alternative - cutting down the large one - would severely compromise its strength?

Bert

Ha! It's just occurred to me that if by good chance the legs are the same width as the depth of the top they could be ideal for increasing the width.

I replaced a UPVC door panel recently. Two sheets of thin UPV on the outside, inner was 4mm mdf and foam. I'm not sure that there would be much weight saving but the wholw cobstruction appears to be stiff.

Example

Two sheets of corrugated metal sheeting, welded together at right angles?

Or the metal shelves with triangular reinforcement along the edges, like these

Owain

Hi bert. This is a classic engineering problem that many people do not understand, because they lack the basic concepts.

What are you trying to prevent here? flexing? Or breaking? They are NOT the same thing., A car spring flexes and thereby does not break, an eggshell does not flex, but thereby breaks... I will assume what you want to do is avoid sag. You want to construct stiffness, and you have noticed that a composite construction can be very stiff..well so it can, up to a point. The stiffness is a function of the elasticity of the surface material times the thickness of the composite. What is in between is largely immaterial - many trusses are mostly open space. BUT beware, at a given load what may happen is that the material in between becomes unstable and collapses, and the structure is compromised.

So what you want is something like a typical POS cheap door - two slabs of hardboard separated by a cardboard honeycomb. Stiff, but not amazingly strong. One filler material I used recently was in fact a small sheet of Kingspan insulation - the polyisocyanurate foam is extremely stiff and if you plated a slab of that with a decent veneered ply the whole thing would be pretty strong and look pretty good. Foaming gorilla glue sticks it really well if you remove the foil. High density polystyrene foam is another option. Or if you enjoy wasting your life on carpentry, simply cut and glue thin battens in a honeycomb or eggbox like structure onto one piece of ply and glue another sheet over the top. Use of wood like this will be no stiffer but will be significantly stronger.

Salient points: Composite structures are stiff and light, but they are thick. Thicker is stiffer. Lower elasticity surfaces are stiffer., Carbon fibre anyone? Ultimate strength is governed by tensile strength of lower surface (or compressive strength of upper, but this is usually far greater) unless the filler compresses and the whole thing buckles. So compressive strength of filler may become the limiting factor on load bearing composites. Foam plastic is extremely light and has amazingly good compressive strength but a built up egg crate or honeycomb of wood may be even better.

Me? Id use plastic foam with thin (3mm) plywood skin and try and get as near an inch thick as aesthetics allow. Foaming gorilla glue to stick it together

I'd be more worried about the shelf tearing way from the wall.

It is all eminently DIY able.

I assume the projector and therefore the shelf is relatively small. Thin plywood top with edge on batten around the edge underneath covered by another piece of ply on the bottom to make it look good and add a little bit more rigidity.

I think you're right. I do like the Ikea table notion, but on reflection, extending it would probably not be a good idea: since the uprights holding the shelf need to be close to the edges, I have a vision of the centre part, complete with £7000 worth of projector, plummeting floorwards while the extended sides remain firmly attached to the ceiling.

Neat ideas, so thanks, but a touch too industrial I think, as well as the first one probably being engineering overkill.

I was going to ask whether the projector is valuable ...

I suspect that, even if insured against accidental damage, the insurers would be reluctant to pay out.

Can you use the rigidity of the projector's casing to help?

Oh, and what's wrong with the one in the photo?

PIR foam is quite resilient. I insulated over the top of my mancave concrete floor as there was no insulation beneath the concrete. I laid 75mm thick PIR boards on the concrete and was able to walk across the unfloored insulation without any surface distortion. By the time it was floored with

I'm inclined to agree.

I don't think this is exactly what you mean, but there are ceiling mounts which use a central column attached to a flat plate which bolts to the projector's underside. I think they're very inelegant and, fairly or not, they don't fill me with very much confidence. An example:

The projector in the photo is considerably smaller and lighter than my new one, and that 15mm shelf strikes me as just a touch too thin. Also, I can't now remember how I fixed the threaded rods inside the purely cosmetic tubing into the shelf so that nothing showed on the underside.

That actually is a solution that hadnt occurred to me - use laminate as the skins and PIR foam as the core.

should work pretty well and look OK.

Without the link on your previous message I was thinking about 'suspended shelf' being the kind where the wall fixings go inside the shelf. Now we have the picture I don't see what messing about with composites will get you, since I don't think weight is a big deal. If you make the shelf strong enough so a child can hang off it, it'll be fine to handle 'only' a 14kg projector. For other thing, you don't want it to break in case there's an unexpected sideways load, like somebody bumping into it.

It sounds like the picture has the right idea: some sturdy steel posts, a sturdy piece of timber sheet, and the last part is the fixings - I'd bolt the posts to the sheet so you aren't concerned about screws tearing out. Then the last thing is the fixings into the ceiling joists - some long screws will likely be enough for that.

If you're worried about stiffness, eg for alignment reasons, then by all means line the support with something stiffer like foamboard, but don't rely on the foam to afford you structural strength - it's vulnerable to screws tearing out.

If you need more rigidity of the assembly, consider triangular cross braces to prevent it racking if a sideways force is applied.

Theo

"previous, less substantial, version"

Was the performance of the previous version acceptable ?

Does the projector have an air intake on the bottom ? Is there a need to avoid plugging any holes on the bottom ? (That would be a reason to avoid soft or spongy materials as a way of damping out vibration.)

You could try a Google on "projector ceiling mount". I'm not particularly impressed by the things, but that would be the commercial offering. There might not be any ready-made thing on the ceiling, to bolt it to.

On TV sets, we have the concept of "VESA plates" and standard mounting hole patterns. Do you know whether the projector is fitted with such standard holes on top and bottom (the name would not involve VESA, and VESA is just an example of a standard) ? I can't see these ceiling mounts working, unless it is common for projectors to have some sort of standard hole pattern.

One of the purposes of a ceiling mount, is to have an adjustment for pitch, like if the projector needs to be pointed downwards a bit. If you do a fixed support with four legs, it's a lot stronger, but then you can't adjust the pitch the same way. Some projectors, you can tilt them up on a table. But tilting below the horizon might be different.

Sometimes, mounting kits would be offered on the same web page as the projector product. Even if the mounting kits are expensive, at least you have a design to look at.

And the platform, is only as stable as the ceiling when you use a ceiling mount. If the ceiling is shaky, you'll need a different kind of hardware.

An example, is a SteadyCam mount. These are used in portable movie making, for avoiding a lot of cameraman artifacts from the filming. There are real SteadyCams, and fake ones. The fake ones being cheaper. They provide a kind of inertial damping.

One of my favourite SteadyCam films, is a filler the TV stations used at night. One of the shots, is done with a camera operator riding down an escalator in a mall and using a SteadyCam mount strapped to his body. And the camera is like it is "floating" and is ridiculously smooth. As far as I know, those have a limited mass rating, which would be a reason there would not be one suited to your project. Still, it's a neat gadget. If you need something to "float", that's a good toy to start with.

Paul

But there's no suspension involved with one of those; I've always known that type as a floating shelf.

It's an important factor if you're working alone and have to offer the thing up to the ceiling and secure it without an extra pair of hands.

Happily this will be well above head-bumping height and it's very unlikely that any other sort of collision will occur.

Actually, they're just cosmetic: hollow tubes concealing threaded rods invisibly fixed (but I don't remember how) into the bottom shelf.

On that earlier model, ensuring that the rods were immovably fixed at the bottom, tightening the nuts at the top and making sure that the ends of the tubes were cut dead square did give the whole thing a pleasing rigidity.

Another thing I've forgotten is what size threaded rods I used: presumably, the heftier they are the more rigid the construction will be, though I could be completely wrong and something like M8 would be fine.

Thanks for your thoughts.

Bert

I have successfully cut down egg box style internal doors in the past. You just need to cut a well fitting bit of timber to glue back into the cut edge to restore the strength.

Plain flat face light weight doors are a cheap way of getting a large board of that type:

Two sheets of 6mm ply, glued to either side of some expanded polystyrene or PIR foam insulating board. The edges inset with a fillet of timber.