Sunday, December 14, 2008

11> Personal Rapid Transit Drive Unit

Here is a sample drive unit. I have left off the track-switching units for clarity. There would be one on each end. (I don’t say “front” or “back” because the unit can move forward or backward equally) I know that my earlier renderings show a “car” with a front and back, but I have evolved my thinking. Notice how the vehicle weight pinches the track between the wheels. Most of the complexity is due to need for sharply turning track, both vertically and horizontally. Note the dual motors. This is because with the unit hinging in the middle the power transmission would be extremely complicated. Also note the offset bearings, which enable dimensional changes. Furthermore, because I envision brushless motors, (RPM is externally controlled and very precise) they may work reliably in tandem. Brushless motors are commoditized products anyway, and this adds a level of redundancy. Furthermore, it occurs to me that half of this unit could be used for light parcel or mail delivery. If six-wheel drive seems a bit excessive, remember that the wheels, being long wearing solid hard rubber, do not have the traction of pneumatic tires. That is just as well, as this design has no differential gears. I envision a track insert designed for slight slippage on curves.


Mr_Grant said...

Why chains? What kind of chains? What scale is the diagram? You could be talking about quite a serpentine arrangement.

As a bicyclist who does all his own maintenance, I am highly acquainted with the need to regularly replace chains due to stretch, because stretch accelerates wear on cogs (also need regular replacement).

Trek has just come out with a new drive system that uses a carbon fiber belt instead of a chain. Perhaps that is something to consider if it lasts longer.

BTW, I've linked to you at the News page of


Dan said...

Dan The Blogger responds:

Thanks David, for your interest. And thanks for linking to me.

The short answer is ANSI 50 roller chain, possibly 60. An ANSI 50 chain is 7/8 inch wide, 4880lb test. ANSI 60, 1.106” and 7032lb. test. Note the idlers, to take up slack. Also note that I have little room for a wide belt, as shown in the illustration for the post on switching. I would like to keep the slot on the bottom of the track narrow to give as much wheel support during switching as possible, and to make entrée difficult for birds. As for cog wear, point well taken.

Actually the best solution would be to eliminate the chains altogether, with wheels that are also motors. (the shaft is stationary and the motor itself rotates, as is commonly seen in ceiling fans) I believe that the “Beamways” PRT concept system takes that approach. The only problem with that is the cost and (perceived) risk factor. When it comes to city councils and transit authorities, (in U.S. cities anyway), there is little tolerance for either.

I would be happy to get a design which most engineers would sign-off on as having a probable life expectancy of a hundred thousand miles between tear downs, made from off-the-shelf parts, available anywhere, so any decent mechanic could keep one running. The design, however, must not preclude future use of more exotic, efficient, expensive components. (like carbon-fiber belts) It is my task to design something to be redesigned by people smarter than me. That is why there are no dimensions yet.