Sunday, March 28, 2010
I would like to offer an update on the effort toward standardizing a track design for hanging, (gondola style) PRT. I have toyed with a few variations of the Acronym “SMART” which could stand for “Small-scale Modular Rapid Transit”, or “Standards-based Modularized Automated Rail Transport”, or some variation like that. Anyway I like the idea of a calling it the “SMART” system, and it will eventually describe an architecture that can broken down into separately contractible (and maintainable) parts, such as track, vehicles, controls and stations. This is to reduce the total vendor dependency, which municipalities have so far rejected. (post 77) In the last year I have shown many iterations of track for such a system, exploring various possible uses, including indoor and outdoor freight, high speed, group transport, platoons etc. trying to ascertain the most versatile dimensions and layout.
One “fly in the ointment” that I have recently identified is the matter of slope vs. overall height, from the bottom of the cab to the top of the track. In post 16 I show a vehicle being lifted vertically by an external chain. While complete vertical lift is not absolutely necessary, and is obviously less energy efficient than simply lifting passengers instead, I can still see where this might be advantageous. In a system that depends on pervasive local access for its usefulness, the need for inexpensive, barebones stations is great. Vertical travel capability permits the combination of the minimal footprint of elevator-equipped stations with the absolute station minimalism of bus stops. I don’t want to design that out. On the other hand, this requires a swing-arm of a height that is at least half as high as the passenger compartment is long. For example, an 8’ cab hung from the middle (4’ back) needs a 4’ pivot arm. If the vehicle has an auto-style “sit-down” cab, that is, say, only four 4’ high, that is still eight feet to the bottom of the track. Although this is advantageous from the point of view of keeping curious hands out of harm’s way, the problem is that I have previously identified a general track height of about 36 inches as a good minimum size for high speed, high-performance bogies. Add this to the 8’ previously noted and now we have a minimum overall height of 11’. Can this still go into buildings? My guess is yes, much of the time, although it is close enough that it would probably often be necessary to rework some of the support structure for the floor above. It would obviously be better to be able to simply hang track between floors.
So what is a PRT designer to do? Well for one thing, perhaps recognize that 100 mph vehicles probably wouldn’t be docking in office buildings. That would be a lot of power (and motor weight) for getting around downtown. Can a slower system translate into something that can fit into buildings better? Perhaps.
In the pictures above, note that the wheels have the flange typical of ordinary railroad cars. Whereas this high-wear part is normally made of steel, which is too loud for PRT use, in low speed environments I believe many abrasion resistant plastics would work. (Nylon comes to mind.) The flange can be separately replaceable. Note that this allows the elimination of the upper guide wheels. I considered a tilted wheel design instead, but with this design the wheels can span the track’s slot so that the bogie cannot physically fall out of containment on a “Y” or merge, provided there is a cantilevered “frog”. This continuous support also lessens strain on the reduced guide-wheel arrangement. The wheels are smaller, in this case being about 15”, and are based on readily available 3 kW hub motors for scooters. The top speed would be under 40 mph, but with dual-bogie designs (8 wheels, like a railroad car) the speed could be greater. Also 3 kW is not a magic number. There are a few more powerful hub motors out there in the same size range and none employ water-cooling, which boosts performance greatly. I want to note that do not know the nature of the structural differences between these low voltage, high amperage motors and the high voltage low amperage motors that would be better for electrified rail (rather than battery) systems… Can anyone shed some light on this? Anyway, this bogie style could run in the track for the high-speed and GRT designs I have previously shown. This design allows the track height to be reduced down to about 24 inches, enabling greater ease of installation in buildings.
More research needs to be done, though, on the average clearance between floors. There are a number of competitive building methods, and I imagine there are strong regional differences as well. For example, steel girder buildings tend to leave long (and tall) floor support trusses in place, supporting poured concrete floors, whereas concrete based building methods usually have more columns and therefore shorter spans for supporting the floor, which may be tensioned with cables in a more unified approach. I have never really spent much time poking around above the ceiling tiles in strange buildings. Perhaps I’ll pose as an exterminator sometime… “Bugs, Mam,” I’d say looking down from the step-ladder with a grim expression, “Big Ones!…”
I guess, if push came to shove, I would favor limiting the slope angle in lieu of building accessibility. In the meantime this gives reason to question any aspect of a standardized track design that adds unnecessary height. I have begun a study to determine how much angle is possible with how much swing-arm reduction and how that would affect station design. I’ll be keepin’ busy.