I need some suggestions on making a wooden shelf slidable

I have a 6 level wooden shelf system. I need to make one of the shelves slidable through addition of shelf on top of an existing shelf as shown here:
http://imageshack.us/a/img191/8757/woodenshelves.jpg
This should be a smooth action back and forth. It will need to be able to hold up to 40 lbs, but this load will be at the far end (shelving side) and will never extend past the existing shelves once the shelf is slid.
What's the best/ cheapest way to do what I want here?
Thanks, Tom
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On Tue, 02 Apr 2013 05:33:25 -0400, Thomas Test

Not clear on how you'll move the shelf. If you don't slide it out more than half way, it won't topple. But leverage is at work if somebody gets clumsy, even with 40 lbs on the other end. The top shelf will be cut narrower than the shelf it sits on, so it can get past the side rails. You can use pegs on the upper shelf to stop it when the pegs hit the rails so you never extend past a bit less than halfway Nails/screws would do. Depends on how much you care about looks. For "smooth action" all you need is wax on the friction surfaces. If you don't want it getting cocked as you slide it - more smooth action - you can attach cleats to lower shelf to contain the upper and guide it. Keep the cleats low so you use the pegs. For more support when the upper shelf is extended, here's 2 options. 1. A stick to the floor to support the outer end. You want to notch the end of the shelf on the bottom to secure a stick cut to length. 2. A dovetail spline (or 2) glued into a rabbet cut on the bottom of the upper shelf, and a dovetail cut into the lower shelf. That would eliminate the cleats mentioned above. Takes some router work and bits. That would be strong, but no better than a stiff stick. And you'd have an open dovetail exposed on the lower shelf to catch dirt unless you covered it with something when the shelf was extended. There's some ideas. Drawer slide hardware is probably a no-starter with that kind of shelving unit. Personally I'd go with cleats, stops, and a stick, but I'd never want to do what you're doing anyway.
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Tom,
Why must the sliding shelf be on top of another shelf? Good quality drawer slides would handle 40 lbs. Cheap track guides might work on top of a stationary shelf but 40 lbs may create a lot of friction. I think guides would fail quickly. I favor slides with good ball-bearings.
Dave M.
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David L. Martel wrote:

Yep. I, too, would vote for the sliding shelf to hang from the stationary shelf.
The stationary shelf must, of course, be sufficiently anchored to the uprights, then who cares how much weight you put on the slider.
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