Sunday, April 29, 2012
If you have followed this blog for any length of time, you have witnessed version after version of bogies and track. I started this blog with the naïve hope that I could recruit a bunch of mechanical engineers to create an open source PRT design. Such a consensual design would have had a credibility that was especially lacking back in those dark days before Heathrow and Masdar. Well, lacking the participation I had hoped for, progress has been pretty slow, although, in my own defense, I’ve set the bar pretty high, not just in terms of performance capabilities, but in terms of simplicity, cost and practicality. That’s a bit like trying to mix oil and water.
Anyway, folks, get out the Champaign and the party hats, because I am pretty sure the search is basically over, although I’m still studying tight-quarters switching, and a few other details. Let me reiterate, for new readers, what differentiates these designs. The challenge is to go faster, climb steeper, turn tighter, be less expensive, and therefore be more versatile and practical than other transportation alternatives. I seek a fully 3D solution instead of a system that requires raised stations or long ramps. The pictures here show only the bare essentials. Most structure, including the passenger compartment and the track’s support means have been left out for clarity.
Here are the highlights of the changes. First, I have replaced the round running guides (A) with square tubing tilted to make a diamond profile. This is almost as easy to bend as round pipe, but offers flat running surfaces that won’t wear grooves into the guide wheels. I have inverted the drive wheel flanges (B) to be convex instead of concave. This results in a single point of angular contact that will be very prone to wear, but for the fact is that these flanges only need to come into contact for specific, short term duties, most of them at very low speeds. For switching they are employed for only a few dozen revolutions. The steering guide wheels (C) and what were called “hold-down” wheels in previous iterations have been combined. Now, when ascending or descending very steep or even vertical runs of track, both of these wheels can be raised to serve the hold-down function. The new geometry makes their previous “anti-rock” function unneeded. The running guide wheels (D) and steering guide wheels (C) must now, though, all move independently, instead of in pairs. Here is how switching would generally work in four pictures.
1. The first of the four is the ordinary running configuration. The drive wheel flanges, although close, do not actually make contact, so all wear is on the larger softer, smoother, and quieter hard rubber wheel surfaces.
2. Several steps have taken place here. First, a steering guide wheel is raised. Next the pairs of diamond guides “taper” into a position where contact can be made. (They start spread out.) Meanwhile, the drive wheel guides make contact with the plastic wheel flanges. This will probably be accomplished by raising the guide wheel slightly on the side that isn’t being lowered. This will pull the bogie out of center, clamping the track between the flange and the guide wheel.
3. With the prior steps taken, the track sides can begin to diverge. Note that one side of the bogie can now be unsupported. This solves the situation often referred to as the “frog problem.” (The “frog” is the little piece of track that, in suspended designs, has no means of support.)4. The fourth pic shows the resumption of support under both sides of the bogie. Any “frog” would be cantilevered from this point, as the thin wall separating the two tracks is the only means of its support. At this point, the top guides can be discontinued and the drive wheel flanges can cease contact, to stop wear. From here the tracks can diverge into their respective directions. I have included a couple of additional screenshots, below. As for me, well, you’ll find me throwing out a couple of years’ worth of now obsolete designs. Oh, sweet victory!
Posted by Dan at 9:32 PM