Re: Old Pharts Duck Under
One of my other pastimes is volunteering as an engineering mentor for a high school robotics team, so I have a fair amount of experience with these extrusions. They are aluminum and they are actually made by several sources 80/20 being one of the originals, Bosch makes a metric version, and several other smaller companies. This is used in industry for making quick and often one off structures like product testing rigs, robotics system, and portable industrial jigs and other structures.
Someone had previously posted about steel being a better choice and stronger than the aluminum. These extruded products are plenty strong especially for building a layout duck under. We have used this product to build a mechanism to lift our 140 pound competition robot over 8' in the air repeatedly.
Aluminum pros - lighter than steel, will not rust, easy to drill and cut (we use an 80 tooth carbide circular saw blade in a chop saw for most cuts), and these extrusions are self - squaring/aligning which helps with getting a smooth running mechanism.
Aluminum cons - pricier than steel or wood construction.
You also mentioned the nylon guide blocks. We've used those too and they are plenty strong for a duck under. If you'd prefer roller bearings, you can purchase V groove bearing that will run in the extrusion tracks as shown below. These are made by a few manufacturers including Bishop Wisecarver.
If you are curious about these types of products, I would recommend looking to some of the competition robotics suppliers out there such as:
http://www.revrobotics.com/rev-25-1238/ The picture above is from the Rev Robotics linear motion products using the extrusion and V groove bearing.
https://www.vexrobotics.com/linear-motion.html Vex has linear motion products that use simple aluminum gussets holding flangeless bearings that roll on aluminum box tubing either 1"x1" or 1"x2" - 0.125" wall thickness which can be found at some Home Depots and Lowes stores.
http://www.andymark.com/product-p/am-3844.htm Andymark has a similar product to the Vex product - gussets holding bearing running on box tubing.
Model railroaders are a resourceful bunch. I'm sure they can adapt one of these products into a bullet-proof duck under.