acrylic legs for heavy dining table?

[Roger Benton]

Hello everyone, welcome to my first post.

I'm doing a large dining table with a solid slab top,
36"wide by 92" long, extending to 140" long.
Top will weigh around 250-300 lbs all told.
The table is attached to a column at one end,
the other end is supported by a single leg centered on the width of the top, about 30" in from the end.
That leg will slide out with the top to extend the table.
Client wants the leg to be as unobtrusive as possible
so I was thinking of using acrylic.
I'm not sure about the load bearing characteristics
of acrylic as compared to wood, especially considering the lateral movement.
I want the leg to be as small as possible, maybe 3"x3".
My concern is eliminating flex in the leg during movement,
and long term stability.
Does anyone have experience using acrylic furniture parts?

[Jamie Buxton]

In the long term, acrylic gets brittle, and eventually it shatters. That's not a good behavior for what you're doing.

You're going to have to put a wheel on the bottom of the leg, aren't you? Dragging a 300 lb table is difficult for the dragger, and will surely scratch the floor. So if you're going to have a wheel, it is going to be rather visible if you stick it on the end of a transparent leg anyhow. That is, I'd use a metal leg. It doesn't need to be 3"x3". It could be more like 1"x1", if it is well-braced at the top. The braces would be near the table's top, and mostly hidden from view.

[Roger Benton]

the legs would be acrylic, but the feet are walnut and steel, housing 1.5" dia. X 2" wide urethane rollers.

my original plan called for cold rolled steel legs, but I was asked for other options.
(what the designer really wants is a magical floating table.)

Does anyone know of a clear plastic material to use as an alternative to acrylic?

[Jamie Buxton]

Polycarbonate (one tradename is Lexan) is more shatter-resistant than acrylic.

If your designer is really determined to have a transparent leg, you might consider glass. In thick enough chunks, it is very strong. And the strength doesn't change with age.

[Roger Benton]

glass may just be the way to go... someone else suggested the same thing.
thanks a lot for the input.
once tis beast is done i'll try to get some photos up.

[Jamie Buxton]

You might talk to folks who make frameless glass shower enclosure. They work with heavy glass -- 1/2" thick and more -- and have methods of fastening it together. You can poke around the web site of C R Lawrence for a first look at the hardware they use.

You mentioned making the leg a simple square cross section -- 3"x3" or so. If you do that, there's a high stress on it at the top, where it gets fastened to the top. If instead you make the top of the leg wider, there's less stress on it. For instance, you could make the leg out of three triangular pieces of 1/2" glass. One would be maybe 20" wide at the top and 2" wide at the bottom. The other two would be 10" wide at the top and 1" wide at the bottom. After you fasten them together you get a thing that's 20"x20" wide at the top, and 2"x2" wide at the bottom.

[Earl Kelly]

Roger,

Have you considered the deflection of the table top on the end with a single leg? Even an 8/4 slab of that length and with just one point of support could flex. And maybe more than the client would be comfortable with.

If you can get away with a single steel leg securely welded to a bracket that attaches across the table top, I believe that would be stable enough.

Myself, I would try to talk the client into a lexan or acrylic panel, instead of a single leg.

Earl

[Tony Bilello]

my main consideration would be on how to attach the leg. This is going to be an awkward situation with the top being so heavy. It wont take much of a bump to have the table self destruct. Have you considered any way to make the table lighter? You can get cardboard or aluminum honeycomb and sandwich it between 2 layers of plywood and veneer the top. You can get very rigid and lightweight designs that way. Once you solve that problem it might be easier to select the leg material.

[Roger Benton]

thanks for the suggestions so far.

The single leg theory is pretty much shot- there's just too much table to for one leg. My original though was a big base centered under the table which would split in two as the table was extended, as in the sketchup model below left.
Now I'm toying with a structural glass design, two panes running most of the length of the table, about 12" apart at top tapering into each other at the bottom and meeting in a foot which would house the rollers for extension (sketchup model below right). The glass base could be two parts split in half with the outer half moving with the table top.
Or, the whole thing could slide out with the top.
Jamie, I don't know if that is the same general shape you were thinking of, but the from the end view you see the triangle, from the side, you see a square.
The aprons and framework for the slides are steel, with an outer apron sliding away from the column with the top to reveal an inner apron. I'm not too worried about stability of the top/aprons, my bigger concern is coming up with a design/materials for the legs that are as low profile and unobtrusive as possible but that can handle supporting the weight while sliding back and forth.
Tony, I hear ya! When this project first came up I was excited to make a table with a big slab top; at this point I'm cursing the weight of this thing every day. But, that's what they want. We'll see.

Thanks again for the input fellas, when the design is firmed up I will be certain to upload some photos of this project.

-Roger

[Jamie Buxton]

Wow. That is going to be one massive engineering job. If it happens, photos definitely will be required.


Have you considered motorizing it? Moving a 400 lb table, even if it is on good wheels, is daunting. On top of that, if the customer is willing to pay for this cool design, wouldn't it be extra whizzy if he could just push a button to have the table slide out?

[Roger Benton]

This is a quick sketchup model of the final design.

Jamie, Thanks for the CR Lawrence reference, it's proving helpful.

Looking into hydraulic pistons like on an suv's tailgate to help move the thing back and forth.

[Earl Kelly]

Roger,

Be careful with the width of the base. Remember this table top is only 36" wide and with someone sitting on each side, their feet may go under the table 12" or more.

Earl

[Jamie Buxton]

I'm not sure the gas struts from a tailgate will help you. The struts basically hold up the weight of the tailgate. I don't think they'd help move your table horizontally.

For horizontal movement, you might look into the systems intended to roll driveway gates horizontally. They're designed for horizontal motion. They have the weight capacity you need. They have safety interlocks so they stop if they hit something. Google with "gate operator" to see sites like http://gatemotors.net/