Thursday, July 26, 2012

144> Compromise

This is a post I have been struggling over for several weeks.  It touches on so many subjects that it is difficult to compact it into a single post.  It is basically a review of the wheel geometry issues that have taken up so much of my time and an overview of the design philosophy and limitations we face.

There are three “must have” criteria that are central to the design - inexpensive, unobtrusive and easy-to-produce track, extreme 3D maneuverability, and speed.  There are also two additional criteria that overlay the others.  These are mechanical simplicity and forward compatibility.

Each of the three initial priorities is easily attainable, but at the expense of the other two.  This leads to tough choices.  Maneuverability in 3D lends itself to a short, wheel-driven bogie.  Speed indicates large wheels, or maglev.  Minimal track cost means no maglev or LSM, and minimal track size means smaller guide wheels.  All in all, the best choice seems like some kind of wheel driven system, but there is no easy answer as to what the best guide wheel placement would be.  Making the track a bit bigger or smaller, or making the speed bit faster or slower completely changes the optimal geometry.  What is sad is that sometimes these kinds of things are never resolved.  Consider rear wheel vs. front wheel drive for automobiles. Apparently now rear wheel drive is making a comeback.  Clearly this kind of ambiguity does not engender a standards-based approach, something that was central to this whole project.  That does not mean that there isn’t a best way to design for a specific speed or track size.  It does mean, though, that a relatively exact track size, speed, and maneuverability must be decided upon before the best geometry can be determined. 

And for the second two design priorities … Well simplicity seems like it would go without saying, but actually it jumps right in with the initial three;  there are ways to partially remedy the conflicts between track size, speed and maneuverability that involve the addition of parts.  This is, of course, not unusual. No engineer wants to add parts unnecessarily, but functionality may demand it.  Trying to design an automatic transmission with five moving parts is obviously a fool’s errand.  Yet mechanical complexity is surely the enemy of any brand-new, yet-untested PRT system, so I have spent considerable time on such foolish pursuits!

Lastly, there is the matter of forward compatibility.  Since PRT track of this type represents new and presumably permanent infrastructure, there is a need to “get it right.”  The last time this happened, with the railroads, there was little incentive to get everything standardized around the very best engineering paradigm, but there was every incentive to get track put down ASAP so that money could be made transporting goods.  This led to many now-obsolete track specifications and, to this day, those early design choices still limit what rail can do. Too late now!

So where does all of this leave us?  There is a real maze of options to sort through.  All of the designs I have shown are basically workable, and in my mind there is a near tie between them, and all have drawbacks.  One thing that would be useful is to get a handle on exactly how small guide wheels can be and still perform well in continuous duty at specific, higher speeds. New urethane formulations, like Dupont’s Hylene PPDI have greatly extended the life expectancy of similar wheels used in rollercoasters, although I have heard of some costing well over a thousand dollars each.  Also such wheels almost universally run on round (pipe) track, and so quickly wear in the center before spreading out the load a bit.  How would they perform against a flat surface? How wide could the wheels be?  Is a very wide wheel of small diameter a decent substitute for a narrower, larger diameter one with an equal wear area?  How about asymmetrical guide wheel placement?  Could that allow for a reduced track size?

All fixed guide wheels must contact outwardly, from the center, and steering guide wheels must engage from the outside, towards the center of the track.  If the steering guide wheels double as running guide wheels, they can be large (allowing for very high speeds) the track can be small, and if both sides retract,  very tight maneuvers can be accomplished.  But this creates the problem that was pointed out regarding the “diamond track” design.  While large guide wheels outside of the track can essentially eliminate any speed limit restrictions, one side must disengage at every interchange. Is this a price worth paying for speed and smaller track? Probably, but that sure doesn’t mean I like it.

Perhaps it is possible to include designated zones on the bogey or track where future guide wheels could go.  This might enable simple, straight-forward designs for speeds that have traditionally been considered sufficient without foreclosing the option of higher speed routes in the future.  For the record, I see little problem with switching schemes such as Anderson’s nearly up to highway speeds, assuming a similar track size.   

I started this project with the belief that it was only the desire of PRT companies to quickly get to market that lowered the performance bar.  I was hoping that with persistence and an open mind, a truly simple design would emerge that could do it all.  I have learned that it will be a bit more complicated, and there will be performance limitations that will have a real impact on functionality.  These will be hard trade-offs, and the many judgment calls will mean that such a design will be ill-suited for any kind of standardization.  After all, any design that prioritizes the above criteria differently is apt to prompt significant changes throughout. Thus any standard will only emerge by the usual Darwinian means, with all of the uncertainty and waste that such experimentation entails.

For the moment I feel like starting from scratch yet again, despite the fact that it seems like I am spinning my wheels. (pardon the pun) Sometimes you get too close to a problem and can’t see the obvious. Also, it is time to “try on” at least a few, reasonable limitations to the design’s capabilities. Perhaps in the next post we can delve into the three criteria, and discuss what those limits should be and why.  ‘Till next time!