I have a somewhat uncomfortable relationship with my TV. I don't have cable, and only use the TV to watch movies a few times a month. In fact, until this year I'd never even owned a television!
Also, due to the layout of my living room, I don't really have a good place to install a TV. I want it centered between my stereo speakers, not blocking a window or doorway, opposite the couch, and close enough to see. In my living room, the only way to accomplish this is to prop the TV on the coffee table when I want to watch it.
As a result, the idea of a hinged-top coffee table with the TV hidden underneath came to mind quickly. However, I wanted the screen to be at a comfortable height when viewing. And, if I was going to build something, I wanted it to be interesting.
I had a vague memory of an old black-and-white movie where a boxing glove pops out of a vehicle on a scissors mechanism. It turns out I was remembering the Buster Keaton movie "Cops", where the glove is used as a turn signal.
I love the whimsy of the spindly scissors arrangement, and since I had never done four-bar or other linkage design, I thought this 2D problem would be a good one to start with.
I also researched other scissors arrangements. For example, I
downloaded the patent for the original Lay-Z-Boy recliner, modeled it in
SolidWorks, and studied the motion created by that mechanism.
For my application, the mechanism needed to do several things:
To build all of these design requirements into SolidWorks, I created a layout sketch with three copies of the same mechanism: fully collapsed, fully extended, and free (for checking the motion by dragging). In this way, I could simultaneously enforce all of my design constraints upon the system while changing my remaining degrees of freedom -- the lengths of the various links, and the positions and types of pivots.
Because of the real-time solving of SolidWorks sketches, I quickly learned a few basic facts about four-link scissors mechanisms:
Through careful tuning of the lengths of the links, the pivot location, and the height of the slider, I was able to create a mechanism which met all of my requirements.
One of the drawbacks designing a mechanism that looks like it couldn't possibly hold a 26-pound television and a table-top is that...well, it might not be able to. The potential problems fall into three categories:
In addition, there is also the concern that if the scissors are used on both sides of the screen as I'm planning, the two may not stay in perfect sync while the screen is being moved, thus causing it to twist in an undesirable way. However, by connecting the two bottom pivots with a torsionally rigid beam, I'm hoping to mitigate this as well.
Currently, I'm considering two construction methods for the mechanism itself. The first is the use of carbon-fiber tubes with machined and anodized aluminum joints bonded to them. The tubes are available commercially and our milling machine is perfect for small high-precision parts like the ends. The second option is to machine the links entirely of aluminum, using FEA to optimize the shapes for minimum weight.
The only other major construction decision is the tabletop itself. It needs to be as lightweight as possible, while still being attractive. So I'm currently exploring both wood veneers and enamels on a fiberglass panel.
In keeping with my "simple-with-a-twist" sensibility, a friend and colleague at SolidWorks (formerly of IDEO) has done a design study of what the table might look like, which appears at the top of this page.