Fitting Parts Together in the Real World, Introduction to the Hot Rotor

June 18, 2001

Last month we discussed building the shaft-mounted components. Photo A shows our finished parts mounted with 35x62x14 mm bearings in our snowmobile case. (Click to view full size).

Three parts not shown are the 60x4 mm bearing end-rings, end seals, and shaft-cooling fan. We plan to order in 2.5-inch pipe for the end rings, turning down the pipe to 60 mm, then using a parting tool bit to cut the finished pipe to 4 mm widths. Seals will be ordered from Zatkoff to match a hardened and polished spacer between the end bearings and flanges/pulleys. The 4-6 inch shaft cooling fan will be made from either 1/8 inch aluminum or 1/16 inch steel and filled to the shaft between the hot rotor flange and seal spacer.

For this month's work we'll concentrate on the hot rotor disk pack. The theory behind the Tesla turbine is simple. All objects are subject to "skin effect" anywhere in vacuumless space. Fluids such as air, water, oil, etc. tend to bond loosely to any surface. In aerodynamics studies we learn that this "boundary layer" extends several millimeters perpendicular to the surface, exerting less adhesive force as we increase distance from the surface.

The Tesla turbine uses this surface adhesion effect to absorb and transfer the energy of high velocity gases into mechanical shaft power which can then be used to generate electricity or move a vehicle. 

By stacking a number of (highly polished) disks with narrow spacing between them, a high velocity gas directed in a tangential stream between the disks will transfer most of its energy to the disk pack (and finally to the shaft). The only other factor to keep in mind is that there must be an entry point for the gas (nozzle) and an exit port at the center of the disk pack. (See Figure A)

There are several methods for securing the disk pack to to the shaft. Some experimenters simply fit the disks directly to the shaft using a compression nut on the shaft end and a square key fitted to a keyway milled into the disks and shaft.

Our disk pack uses a modification of Tesla's advanced turbine design which secures the disks and spacers to a shaft-mounted flange. This allows us to build, assemble and balance the disk pack as an assembly. Using this approach gives us more freedom to experiment with various disks and spacers.

Figure B shows a fully assembled disk pack on our flange. (Click on picture to view full size.)

Figure C shows the same assembly exploded for easier viewing of the washers and star spacers.

We're showing seven disks (two at 0.1875 inch and five at 0.0625 inch thick, 9.75 inch diameter) which will give us approximately 20 horsepower for running a 10 KW generator head. The disks must be highly polished stainless 316 or 4140 carbon steel, or any similar material able to handle at least 50,000 psi of tensile load. A high polish on the disk surfaces guarantees greater fluid adhesion, and results in higher efficiencies. 

Disks are generally 0.0625 inch thick, and the spacers are between 0.03125 inch and 0.0625 inch thick. The narrower the spacing between the disks, the greater the efficiency and torque.

Although it is possible to build these parts in your shop, it will be much quicker and easier to have them laser-cut from a shop so equipped. If you do the work yourself just make sure the disks fit your shaft or flange very closely. Also, when boring the (6) disk pack assembly bolt holes in the flange, disks and spacers, use an indexer or rotary table on your mill or drill press to ensure accurate placement of the holes every 30 degrees. The 0.1875 inch to 0.25 inch holes drilled around the periphery of the disk pack for securing the round washers also require accurate indexing.

Finally, when bolting the disk pack together use six high-strength 0.25 inch to 0.3125 inch bolts for the flange, and either bolts or threaded inserts/rivets for the spacers/washers.

Well that about wraps it up for this month; getting these somewhat complex spacers and disks milled yourself or made up by a fabricator should keep you busy for a while. Until next time, keep those metal chips flying!