Tuesday, January 19, 2010
First a reiteration of the big picture. Designing a PRT system as logically as possible must start with a track that won’t put unnecessary limits on future usefulness of the system. Unfortunately, in an effort to get a product to market, most PRT venders have produced designs that fall short of having the potential to do anything but be small players in the transportation mix. Current systems, by and large, are not versatile enough to be used for anything other than supplemental vehicles in very densely populated city areas. That may be a perfectly fine business model, but the resultant designs fall far short of embodying PRT’s transformative potential. Where would the internet (for example) be today if it relied on proprietary networking cables rolled out according to the business plan of a single company for specific, cherry-picked markets? Standards based PRT track design is sorely needed.
I recently posted bogie design with tilted wheels. The purpose of design effort was not so much the bogie itself, but to see if such a design would require modifications to the track profile. Actually it has, but only slightly. The general design first described in post 39 has stood up to many challenges (outlined in subsequent posts) including multiple weight classes, speed ranges, station types, freight and industrial uses, propulsion types, turning radii and pitch angles, etc. What remains to be done is to explore braking and traction issues, to address the issue of forking (adding) track with minimal deconstruction, and general track construction methods.
All of that being said, here are some explanations regarding that tilted wheel design. First of all, it is a response to the challenges of high speed. The design makes no sense for anything less than, say, 30 mph. It gets much more attractive at speeds over 80. The rationale is this. At very high speeds the guide wheels really get a workout. In order to provide a smoother ride and make less noise, all wheels should be made of a material with some give. (rubber or plastic, not steel) Such materials wear out, especially on very small wheels. The bearings, too, take a beating. Centering the bogie without guide wheels is a challenge, however, because of issues that are particular to the remedies. Flanges on wheels can wear and heat up. Having a wheel running in a trough generally means that all wear occurs only on a very limited ring around the wheel. The same is true if the trough is in the wheel. (Pulley wheel profile) While this can be tolerated to a degree, the issue is most acute on the drive wheels, which support the full weight of the vehicle. V-shaped wheels in a V groove wear rapidly because the wheel has multiple diameters contacting at once. This creates wheel slippage.
In the illustration below it can be seen that the angled wheels, through gravity, work to minimize any contact with the flange, which has a radius to minimize friction wear, and could be made of a harder material. The inserted detail shows how the bogie becoming off-center causes the entire wheel (and whole vehicle) to be lifted against gravity.
These drive wheels are characterized by their large diameters, which have plenty tread surface to distribute the wear, and revolve at slower speeds than smaller wheels would. It is assumed that these wheels would use tapered roller bearings, like cars, and so cannot tolerate very high rotational speeds like ball bearings, but can better support the vehicle weight and the sideways “thrust” forces associated with fast tight turns.
In the illustration above note that the running surfaces have been replaced by half-round, or bull nosed rails. (shown in white) While it is obvious that such a design would greatly increase wear on both rails and wheels, such a profile also allows an extremely tight turning radius. My thinking is that at very slow speeds, such as for station maneuvers, the wear will be a minor factor. After all, by definition, the sharper the turn, the slower the speed and the less distance traveled. What about medium turns? One thing that occurred to me is that such inserts could provide a banking angle, which would treat the bogie like it was going straight. With wheel motors the wheels toward the outside of the curve can be made to rotate faster, facilitating (if not actually causing) the turn. This is a work in progress…I don’t have all of the answers at this time.
This all raises another very interesting point. In the last post, it was pointed out that the steering guide wheels would come into and out of contact with running surfaces as needed, so they would not spin and wear unnecessarily. Here we have described other running surfaces that are not of a continuous, unchanging profile. There are other situations as well, such as very steep slopes, which might call for special “sticky” rails or other inserts. Having various inserts has a lot of advantages. They may be replaced and upgraded. They can be precisely finger-jointed to allow for thermal expansion to eliminate the repeating noise and vibration associated of expansion joints in train tracks and some roads. They allow one basic structural track profile to perform many functions without modification. They can be made of materials (such as stainless steel) that are too expensive to be used structurally. They can be rubber mounted for noise and vibration control. It becomes easier to add a diverging or converging track to a previously completed one when the running surfaces are modularized into precisely sized components. One interesting application for inserts is the technique used by Disney to detect any breaks in the tracks of their rides. They fill the tracks with compressed air. If there is any break or crack the pressure drops and they know it immediately and can stop the ride.
Finally, a note about the fifth wheel shown below; (in the center of everything else) There are several possible functions for such a part, from centering and holding down the bogie to additional braking, power and traction. Such a part can eliminate the possibility of wear on the wheel flanges altogether on straightaways. The truth is, however, that to do the design I had to either add it or not. Because it is easier to take it out than add it later, I added it. It could, in theory, prevent extraordinary forces, such as extreme cross winds or earth tremors from lifting the bogie inside of the track. It has a smaller diameter than the main drive wheels but it supports no weight, so ball bearings would suffice and wheel-wear would also be minimal. It is largely redundant, I know, but this is all a work in progress, and, as I mentioned before, it is really all about the track anyway.
Posted by Dan at 9:25 PM