Monday, September 17, 2012

146> Big Wheels

Anyone who has followed this blog at all (especially recently) knows that I have been obsessing about speed and guide wheel size.  It all started when I realized, some time ago, that the bogie’s guide wheels, used for steering and just to keep the bogie centered on the track, were of a diameter that is inconsistent with the goals automotive speeds, and minimum maintenance, noise, and track profile size.
 Once you start thinking about commuting speeds and distances, you really need to be thinking in terms of parts that will last for tens of thousands of miles and while still being economical to replace. 

In this pursuit I have identified several key insights.  One is that the movable steering guide wheels must always engage inwardly, and so they can engage the track from its exterior.  This means they can be of diameters that would be difficult to fit otherwise.  This is, however, made less important by another key insight, namely that the duty cycle of steering guide wheels is extremely short-lived.  Switching tracks represents a very small percentage of the time that a vehicle is in operation.  In other words, steering guide wheels to not have a diameter problem, since they are infrequently used.  Another key is that the track surfaces that actually engage the various guide wheels need not be continuous.  Therefore guide wheels can be disengaged when not in use.  This leaves the constant-duty guide wheels which center the bogie on the track as the only ones with the size problem.

The centering guide wheels can face either inwardly or outwardly, but if engaging inwardly they must disengage for every fork – this is a compromise that is unnecessary if we put them on the inside of the track, engaging outwardly.  This, then, presents a problem with space because symmetrically opposed wheel sets each only get one half of the track’s internal width.  If that width is, say, 24 inches, the wheels must have a diameter not exceeding 12,(305mm) and that is pushing the limits a bit.  Sure, there are miracle plastics, but they are not exactly cheap.  Yes, it is close, and somewhat of a judgment call, but experience has taught me to design things better than they need to be.  A faster system, skinnier track, or a little less vibration/noise are all laudable goals.

Following this logic then, one way forward is to break from the symmetrical, mirror-image paradigm of ordinary vehicles so that these outwardly facing guide wheels can be nearly as big as the track is wide. I first proposed such non-symmetry way back in ’09, in post 56. The stacked guide wheels shown above are another, more maneuverable approach.

One footnote for those new to the site… if you have stuck around this far…  For simplicity’s sake there is a lot missing from the illustrations, so here’s a bit of explanation. This is the wheel geometry for a bogie from which a PRT passenger compartment may be suspended.  It is missing almost everything but the various wheels and surfaces under discussion.  The bogie is designed to switch tracks without requiring any movable track parts, as is typical in PRT designs.  The drive wheels are self-turning via hub motors.  

By stacking the guide wheels as shown above, they can be much larger than previous designs, and so allow those faster speeds, less noise/vibration, more infrequent and cheaper replacement, smaller track or any combination of these attributes.  I would go for a little of all of the above.  The results should be speeds up to 85 mph (187 km/h) on cheap semi-hard rubber or ordinary urethane (replaced every other time the drive wheel tires are replaced) with an interior track width of under 20 inches. (500 mm)

This design takes full advantage of the basic observations I have mentioned regarding these matters, namely, the fact that guide wheels used for steering are only used for a fraction of the time while centering guide wheels are in constant duty.  Therefore steering guide wheels can be a fraction of the size of centering guide wheels.  The track itself has completely different engaging surfaces for different purposes.  In the image above I have made the discontinuous elements of the track that are used in switching only dark blue. 

In this illustration it can be seen that no steering guide wheels need to be engaged away from switching points. The upper steering guide wheel’s duty is replaced by a larger fixed wheel held captive by a pair of diamond guides. Here we rely on the fact that the pressure on these guides is very low, or else the urethane sidewalls of this wheel would be the first to go… alternatively a pair of inwardly angled wheels could be used if the front and back wheels are separated enough to allow the space.  I want to study tight track curvature applications more before committing to this. 

This pic shows the track’s divergence at a “Y”, with the bogie being fully supported while traveling in a “half-track” mode. Unfortunately, the upper steering guide wheel is nearly hidden, but it is engaging the left pipe.

Finally, here is how we could further increase our speed for still longer range travel. For high-speed travel where there are no interchanges, ALL of the regular guide wheels could disengage by having their
 engagement surfaces end. Instead, all centering would come from two sets of exterior, retractable wheels, as shown the diamond track of post 141, In this scenario these wheels would retract far in advance of any interchanges so speed through interchanges would be limited to (roughly) the regular top speed. This is, of course, well down the road. An upgrade path!

One last observation: The sprockets on the main wheels that I show are for climbing, not for drive chains. This detail… where the bogie can be fully supported by “pinion” gears driving over a guiding “rack,” means that the floor (drive surface) of the track can be removed, as well as any other internal surface feature that would restrict maneuverability. Thus the bogie can turn extremely tightly, including up or down, without the guide wheels hitting anything, so long as it isn’t too long.