Sunday, January 18, 2009

16> How to Lift a Podcar

I want to start by noting the obvious difference between the designs I have shown and those of the other two hanging pod designs I am aware of, Beamways and MISTER. Neither has the vertical connecting beam I have illustrated. Besides the obvious advantages of having the track being high and out of reach, the connecting beam has one other advantage.

I have long wondered if PRT vehicles, (OK, Podcars), would need a transmission of some sort. Clearly the advantages that have led to their universal adoption in automobiles would seem to apply here as well, although I am led to understand that electric motors are a bit more forgiving in this regard. Anyway, my concern was about the on-board power needed for steep ascent, and my thoughts went to roller coaster design, where the cars are pulled up the track by a means outside of the vehicle, so there is no need to carry around a heavy motor.

I have taken a design philosophy that is consistent with open-source by seeking to not foreclose options that someone else might find useful, and the roller coaster method seems to have enough merit to at least keep as a possibility, Then the question becomes, “how steep?” Obviously straight up is as steep as possible, so my attitude is “why not?”
One problem that our societies must someday face is our land usage footprint. The future is UP!

Since vertical ascension, however, is not part of any immediate plan, I will not explore the matter further, other to say it’s not challenging, engineering-wise. Now you know another reason for the “beam”. This design also has some comfort (g-force) advantages, and possible collision safety advantages. One challenge, however, is with how to deal with loads that are highly unbalanced, front to back. Inventors, engineers,.. we need you!

By the way, that brings up one obvious problem with this site and format. You can’t post pictures. For the time being, if you have a pic that you think would help out the cause, send it to me at and if it’s worthy, I’ll make a post out of it. Meanwhile, I would be interested in suggestions on how to get a past this problem for good. Wiki? Forum? Tell me what you think!


Bengt Gustafsson said...

While the connecting beam has some advantages that you mention it also has distinct disadvantages:

Higher spacing between layers at grade separated crossings.

Significanly higher torsional moments on the guideway.

The lifting function could be placed in the guideway as you mention, where it is a first moving part in the guideway, as switches don't move. This could pose problems with wear and maintenance and reliability in bad weather.

There is also another problem with vertical stations if that vertical pole extends all the way to the ground. Then each station berth needs a pole and an associated switch once you get up to horizontal and need to get on the main track. In this case the station also has very low capacity, with turn around times of about 2 minutes, maybe.

If the track is to turn horizontal to get a normal horizontal station track with multiple berths the radius of this vertical curve needs to be < 2.5 m to ever reach vertical before starting to turn horizontal again. To allow this type of radius in a vehicle is not easy, I have spent many hours trying to figure out how.

Finally, if the vehicle is to ascend vertically using its own motors the stall torque needs to be able to keep the vehicle at a stand still at any height, which means that the motor needs to be very large. Furthermore the coefficient of friction must be very large in all weathers to make sure that the vehicle does not slip on the vertical track!

DSmith said...

I'd wondered about why the longitudinal pivot is so far from the guideway in your design. In most hanging designs have both the longitudinal and transverse pivots nearer the guideway to reduce the moment-arm at the wheels when climbing a grade. I see now the flexibility of this.

However, the transverse pivot would normally rely on gravity to keep it aligned under the track. In this vertical orientation the transverse pivot would need to be locked somehow.

DSmith said...

I would also wonder about the additional cost of maintenance for these vertical lifts. A nice feature of these PRT systems is that the guideways are completely passive. All of the active components are in the car itself.

Dan said...

Dan The Blogger Responds...
Dave, I think this response to Bengt (that I had written earlier) will give you some additional insight,

Welcome back, Bengt-
You raise a lot of important issues: Your first point about crossings is one that had completely slipped by me lately. In earlier notes I had come up with a figure of 2.43m (8ft.) from the ground to the bottom of the track. I must say, however, I am greatly troubled by the prospect of having the track too close to the ground for a variety of reasons. How low do think is practical? I am worried about graffiti, climbers, hands inside the track, tall vehicle accidents, traffic clearance beneath track for stations between closely spaced driveways, and the visual impact. I am also worried about the practicality of raised and/or walled stations. (to limit access to pods or track) I do not mean to say they cannot exist, Indeed, raised or walled stations are probably the most practical design for northern climates, but I am trying to design a specification here, not a system, so I am hesitant to commit to any particular system architecture.

As for torsional stress on the guideway, that does not worry me. When drawing for this blog, I do not put in every piece, because it would give the reader too much to digest. One such piece would be a vertical roller, centered within the “double plate cab hanger” shown in the post about switching. The purpose of this roller would be to center the wheels, in the track. Regrettably, I also drew the guide wheel assembly a bit too small. The larger, more robust guide wheel would be fitted with opposed tapered roller bearings, such as those that absorb the steering forces on a car. Between the vertical centering roller and this guide wheel assembly, sideways forces greater than the entire vehicle weight could be absorbed without undue wear.

Finally, as I mentioned in my post, I have no immediate plans for vertical travel, I just did not want to preclude the possibility. I certainly do not mean to say that I generally advocate, at this point, vertical track for stations. Again, I am not trying to design a system here as much as identifying options that should not be ruled out. After all, some cities have very mountainous terrain. The worst thing that could happen, from my point of view, is to encourage a standard that is structurally incapable of meeting the needs of certain cities. This may be a troublesome option, but I suggest not designing it out of a track design just yet.

Bengt Gustafsson said...

I'm not planning to have unwalled stations in the Beamways system. But it would be possible to have a ditch under the rail when the track runs at ground level (i.e. when vehicle floor is level with the ground for entrance and exit). This would reduce the risk of being able to reach inside the track and also would deter from walking where vehicles may appear. And thirdly, if you do get hit by a vehicle you can't be squeezed between the ground and the vehicle floor as easily.

Dan said...

Bengt- I know this is not your problem as you don't anticipate the need for un-walled stations, but I have to make the point that the problem with "ditches" in the warmer climates where open-air stations have appeal is flooding/drainage.
I have been considering the reverse, a crushable or slideable elevated platform with a ramp that would tend to "give" rather than allowing crushing injuries. Luckly my designs are consistent with a "tilt" switch, which could shut down the system...