Tuesday, May 24, 2011

124> While We're Still on the Subject...

Since we were discussing switching in the last post, and I have had a few ideas percolating about the subject for quite some time, I thought I might flesh them out in a drawing. The bogie shown above is nowhere near complete, but shows, at least conceptually, the hardware for aligning with the track and switching. The motors are direct drive, hub type. (in the wheels)

What I have attempted to do here is to illustrate a couple of possible solutions to what might be called the “too many guide wheels” problem. (Note: As I am getting this ready for posting, a review of past posts reveals that I have written MUCH more about this subject than I had remembered, with a great many similar designs as well. So to see some variations from the past, I would refer there reader to posts 67, 69, 79, 83, and 90.) OK: First let me remark about the problem itself. 

In the illustration from the last post, I show how a single pulley shaped wheel can be replaced by 3 wheels, a trade that seems of dubious value on its face. (Last frame, click to enlarge) Given the durability of some of the new plastics, in many cases it might not be worth it. I have been, however, primarily designing systems to more fully explore the requirements of the track. There is desperate need for standardization in PRT, and all designs have limitations. If a certain aspect of a track/bogie design inherently creates a speed or weight limit, this should be defined, quantified, and, if possible, overcome. I have therefore endeavored to design for very fast and heavy loads, with the thought that the track can always have lighter iterations if it is known that this will forever be sufficient. Designing for propelling such loads fast, silently, smoothly and safely no matter what (epic weather comes to mind) is a whole different sport than for more stripped-down systems, but it seems foolish to build the latter if the former can be built for nearly the same cost. Unlikely as that may be, only an exploration of the issues can reveal the truth. A good design must assume that the highly stacked luggage will fall, just as the passenger lunges to stop it, just as an extreme gust of wind happens, while the vehicle is just curving into an intersection. My efforts are not unlike the logic that brings car makers to the race track, where lessons are learned from pushing designs far beyond what will ever be expected in the field.

One point about flanged wheels; any material hard enough to roll on reduced points of contact will tend to transmit vibration, if not simply generate noise. Rubber wheels, on the other hand, wear faster but absorb vibration and noise. One possible compromise is to mount harder plastic running or guide surfaces on a intermediary rubber part, such as a large diameter ring or bushing.
While the pulley shape holds the wheel securely on the track, it does so by either allowing the friction of angular contact or by concentrating those points of contact onto a minuscule footprint. This is why roller coasters don’t use them. Yet the flanged wheel concept, or some version of it, is in wide and successful use in lots of applications and is often the best choice. Ordinary railroad track, for instance, is a variation that recognizes that the double flanges of the pulley design are redundant and that opposed singly flanged (steel) wheels will do. So what is really the minimum of flanges or wheels that is necessary for PRT? Surely the 24 opposing wheels suggested by my last post are not all needed. For one thing, it seems unlikely that 4 wheels should be needed to hold PRT down; Gravity should do that perfectly well. And we have seen with the railroad example that the wheel flanges themselves may not all be needed. A few down. What else can be done?

Note that track has spread for switching, although it is mostly cut away.
Also note that the widened "ceiling" is missing the guide. 

These illustrations explore a couple of options worth considering. First of all, the disadvantages of flanged wheels come from the effects of continuous hard use. In PRT, many of the flanges (or,alternatively, the corresponding opposing wheels) are only used in switching. Why then, would they wear excessively? The fact is they wouldn’t. Plastic flanges or wheels should work just fine for switching, even in high speed systems.    

Another situation is that guide wheels for switching must either turn continuously or engage and disengage. If they are to remain engaged, good practice would have them be large enough to not rotate at hyper speeds. At 100 mph, for example, a four inch wheel must turn over 5000 rpm, yet four inches is still way too big to start instantly rotating upon engagement. I have posted about this problem previously. By the way, one idea is not to motorize them but rather to use wind forces to keep them turning. Such wheels could be configured with turbine-like blades and since there is substantial captive air in the track that must be channeled around the bogie, keeping them turning should be easy. In this example, however, the main strategy is to minimize use of the steering guide wheels and to keep the centering guide wheels (the purple ones) spinning continuously.

Here I have brought back a very old idea… magnetic switching. In these illustrations, the eight steering guide wheels are small and intended as backup only. The main steering is from the electromagnets (red) attracted to the steel switching strip. (blue) In these illustrations it can be seen that when there is no switching, neither the drive wheel flanges nor the steering guide wheels need get any wear since lateral control is maintained by guide wheels (purple) which are always engaged, save for the moment where one side or the other ceases contact for a few seconds due to the track widening as it branches into two directions. I have included a fifth “hold-down” drive wheel, which provides the geometry to inhibit forces that would otherwise twist the bogie inside the track. (extreme sideways wind gusts for example) It has a plastic groove down its center to receive a rounded, bogie-centering guide, which, like the drive wheel flanges, will get minimal use.  Because I contemplate the potential for very steep or vertical  (elevator-like) travel capabilities, this “hold-down” wheel could help facilitate that purpose as well, were such a scheme ever considered worthwhile. (There would be an addition traction means for this beyond those five wheels, however, such as ordinary "cog railroad" methods) Note that that upper rounded track guide (that fits into the hold-down wheel recess) is missing from the last picture, where the track has been widened as it would for an "off-ramp". That piece would resume further down the track for the each divergent branch. 

I am becoming more and more inclined to give up the idea of pneumatic tires in favor of semi-solid rubber. I really don’t think the bumps created by well-engineered expansion joints warrant that kind of cushioning, and I don't believe custom rubber castings are all that expensive. The flanges would be of a long wearing plastic, such as is used for casters and roller coasters. They can be replaced separately from the rubber. One of the keys to this system is the recognition that the flanges and the rest of the wheel need not be a unified piece or of the same material.
Finally, this track gives a nod to the typical roller coaster track architecture in that the design, as shown, involves bending round pipe only, so there is no compound bending, as would be the case with angle steel or square tubing that must curve sideways and up or down at once while keeping its profile plumb and level. The various steel profiles are shown as unwelded, separate pieces. This arrangement makes banking the track so easy that it begs the question whether it is worthwhile having the self-banking characteristics exhibited by my (and some other) suspended vehicle designs. That, to me, is more about budget,business plan, timing and politics etc. A general purpose bank would do no harm unless it was at the wrong angle for what ended up being the running speed at a future date, if the vehicles could not self-bank. But that is a debate for some other time. Also there is no trussing or triangulation shown as would generally be the case for any larger spans. Nor is there covering over the track in these examples. I kept it minimal for clarity.

In summary, this general design provides full, secure containment of the bogie on either side of a track that is widening to form a "Y" even without bottom support from both sides. (It is fully “half-track” capable) It has no more than seven wheels in active contact at any one time, and those seven are optimized for constant duty. All flanges and steering guide wheels are for (more or less) extraordinary events. Switching guidance, centering within the track, and securing the bogie during extreme events are different issues and the wheel style, profile and materials can reflect this. My general recommendation is use detachable flanges to ensure safety from extraordinary twisting or inertial forces, but to minimize their use by making them redundant in general use. This is done with the centering guide wheels shown in purple in the illustrations. In switching, temporarily engageable, self-turning wheels are used but may only be a secondary safety system if magnetic means are used as the primary way to get the bogie to hug one side-wall or the other. Discontinuous top guides can also be employed.

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