Shaft Basics & Pulling it all Together

April 2, 2001

This month we are going to take a quick look at where the design is headed and then examine a couple of ways to turn a shaft and hang parts on it.

Figure 1 shows a CAD model of the Sachs case we are using for our build along with bearings, spacers, a hot rotor assembly, shaft cooler and output pulley. This gives us a pretty good idea of the case related working parts -- excluding blower, combustor, and hot rotor cover.

The first and most important step before turning the shaft is to plan out placement of the parts along the shaft length, then decide how the bearings, flanges, pulleys, etc. will be attached to the shaft.

The first example (Figure 2) shows a pressed-bearing assembly. 

Since we are designing a relatively low horsepower turbine, the outboard or over-hung weights and radial loads will now be excessive. A 1.125-inch (end shaft) diameter will be sufficient up to about 20,000 rpm. This will allow us to use low-cost 30mm and 35mm ball bearings in the assembly.

Figure 3 shows a stepped shaft using 35mm bearings on the inner races and 30mm bearings on the outers. (Click on image to view full size.) 

The dimensions shown are for the Sachs case, the shaft being symmetrical on both ends. If another case is used, the cuts in the shaft must be made to correspond with bearing placements in that case. Simply use the old crank assembly to figure out bearing placements, etc. Also when turning a shaft for pressed bearings the race or area of the shaft that the bearing contacts must have an interference fit of .0001 - .0005 inches. So a 30mm bearing must be turned to 30mm + (.0001 - .0005 inches). This requires a high-precision lathe to hold that kind of tolerance. If you don't have high-precision equipment, it may be better to have a shop do the work for you.

If you are going to mount the bearings yourself, it's much easier if you put the shaft in your freezer for a few hours and heat the bearing to about 300 degrees F just before assembly. Remember to use a spacer between the bearings to keep shaft axial movement in check.

Next, cut tapers on the shaft just ahead of the end threads. Tapers need only several degrees to be effective, and only about one inch in length.

Finally, cut threads on the two ends leaving about one inch to an inch and a half for the flange nuts.

Figure 4 shows an alternative shaft design using a tensioning system for mounting and centering shaft components. Even though this design uses more components, it allows the use of lower precision tools for turning the parts. There is also more flexibility in positioning components along the shaft, so more experimentation is possible. The shaft is simply a straight piece of rod that is turned down slightly along its length to ensure roundness. Finally, the ends are stepped and threaded for the last inch to inch and a half.

Although we will cover component fitting to this type of shaft in more detail later in the series, Figure 5 shows generally how flanges, bearings, etc. are centered and held in place with locking collars.

In a tensioned shaft assembly the end flange nuts compress spacers and locking collars along the entire length of the shaft, so it enhances both stiffness and overall strength. This arrangement also allows the use of 35mm bearings in all four positions so our shaft diameter is larger, able to handle larger radial loads.

Next month we'll look at how to build and assemble all of the remaining shaft components...