Sunday, June 13, 2010
I want to go back to the beginning in my discussion of PRT track, specifically track for hanging style PRT, for the benefit of the majority of readers who have not followed the ongoing development of the corresponding SMART (Standardized Modular Automatic Rail Transport) specification.
The most fundamental concept that needs to be understood is that of switchless steering. Unlike a railroad, which requires a track segment to be mechanically moved to provide switching, PRT designers have generally agreed that this can be more easily done by the cars themselves by means of engagable/disengagable guide wheels. These require a vertical surface to engage upon. Since the main wheels (or even a Maglev system) require a horizontal running surface, the two, in combination, look something like this.
It should be noted that only one half of this book-matched arrangement is theoretically needed, except for places where tracks are merging and diverging, though leaving the redundant half in has advantages (load distribution, simplicity of symmetry) which I believe out-weigh the disadvantages. This picture,
from post 54, illustrates the concept.
The second concept is the combining the functions of multiple wheels into one. A look at a roller coaster wheel setup shows how multiple wheels can hold a vehicle tightly to a track. A simpler approach, however, can be achieved by the use of flanges, as on a train wheel, or dished in or out wheel or track profiles. The angle of contact, though, in such systems, should be as close to a right angle as possible or the flange will tend to wear out, since it introduces a point of friction to the system. A quick look at my older posts will show various examples of radii on both wheels and track.
A third concept is that plate that keeps appearing in my track designs, pictured below.
Whenever a tube-like structure is bent, it tends to flatten out in the process. If a section of track is spanning a great distance and is bearing a great deal of weight, the downward forces will tend to want to flatten, then fold the track under that weight. Stopping this flattening arrests this tendency and strengthens the track. In the case of an encased track with a slot in the bottom, the initial flattening would cause the sides to fold inward or outward, thus narrowing or widening the slot. These plates, placed periodically, prevent this. In the second picture you can see how such bracing can be connected by long plate steel to make a complete box beam. Personally I find this design ugly and worry about single wall designs for PRT because of noise, condensation, and uneven expansion.
A fourth concept is modularity. Almost all discussions on PRT touch on the possibility of movement of goods as well as people, particularly at night. Additionally, many envision a variety of vehicles, and the current diversity of motor vehicles that populate our roadways would tend to bear out the desirability of this approach. Such ideas are not put forth by actual PRT vendors, however, since it currently would be counterproductive for them to develop and offer a confusing array of alternatives. Then there is the matter of routing. Long commuter type routes would call for faster vehicles or possibly vehicles for groups, if there were sufficient numbers of people with common origins/destinations to support this scenario. Therefore there would seem to be a very strong case for creating a track that is compatible for all or any of these vehicle types or speed ranges. This logic leads to a track profile where smaller vehicles are compatible with the track of the largest anticipated vehicle type. I submit that this “largest vehicle” would be GRT. (Group Rapid Transit) In the U.S. many cities have “Park and Ride” systems already in place, which aggregate passengers in outlying suburban locations. I would not want to rule out shuttling in these passengers in groups of, say, 10 or 12. Although GRT is a contentious issue in PRT circles, it should be noted that from a track design point of view, there is very little difference dimensionally between the two. Heavier gauge steel, more frequent supports, or simply spacing heavier vehicles more sparsely are ways to accommodate such a possibility. Whereas PRT track could be made smaller (in profile) than track for GRT, the size of the truss that is required to span reasonable distances can fit either. I would suggest that any such heavier track be used for arterial routes only, because cost is everything when it comes to expanding limited routes into meaningful networks, which, after all, is what PRT is all about. This modular approach recognizes that no one set of engineers is liable have all of the answers as to the very best vehicle design.(s) any time soon. Therefore I suggest removable running surfaces. This has the added advantage of reduced noise, a smoother ride, and essentially no chance of condensation. It also looks to a future with possibilities like Maglev. In order to accommodate the largest number of vehicle types I suggest something like this general shape.
The design allows for several centering means through the use of either convex or concave surfaces. My only concern is the fabrication of this profile, especially radiused sections for turns, because the required roll-forming equipment for that is not universally available on a local basis. Although I have tried to keep my designs easy to produce anywhere, this may be something that would need to be shipped in. I suspect, however, that many local job shops would be happy to fabricate the shape by splitting stock pipe and making curves by a tack-and-bend-as-you-go method.
Although I promised some further thoughts on use of tension cables to extend the span of track sections, I would like to put that off for the moment and return to standard truss design. As far as box trusses go, the round tubular type seems to be acknowledged as the strongest, and was the choice for the Skyweb Express demo track. I believe 90 ft. was the maximum span.
It has the disadvantage, however, of being relatively complicated to fabricate, because the pipe ends must be cut to match a curved surface. (I have seen the tube ends are flattened in some applications) I have wondered about this for some time, as it seems that square tubing would be so much easier, since it just needs to be mitered. Nonetheless, in most demanding situations, such as cranes, these complex joints are almost universally used. Here is a picture showing he complex cut. These have just been tack-welded.
In the illustration below I have taken a similar box truss but substituted those plates for the verticals. Because the plates are only rigid in one direction, I have stiffened them by joining the upper and lower gussets into a single piece. I have also used simple miter cuts on the diagonal pipes, again, using gussets to reinforce the joint. This is not exactly to scale and the diagonal and horizontal pipes don’t even touch in this picture, (much weaker) but this is just a conceptual drawing. The general design should, however, be very strong, cheap and easy to build anywhere where CNC flame-cut plate and pipe is available.
In the next picture the red surfaces indicate the running surfaces for a PRT bogey, taken from Post 83.
If it looks like I haven’t decided exactly what becomes part of the truss and what becomes removable, you’re right! I have not. There’s also a radius missing. It’s a work in progress. Also not shown is the skin, and provisions for electrification, communications, carrying utility wires. There’s plenty of work to do, and now you all know were I stand at the moment. For shorter spans or hanging sections, BTW, the design would be completely different, but, as they say, “One step at a time.”