How to Build & Assemble Remaining Shaft Components

May 2, 2001

Last month we covered the basics of turning shafts. Keep in mind that within certain load constraints, there are several ways to fit shafts to bearings, flanges and pulleys. Even though interference fits are the simplest systems, tolerances are difficult to maintain on hobby-level equipment.

The design we are using employs a system of locking collars to secure the shaft to bearings and other components. Photo A shows the shaft we turned in our shop -- complete with threaded ends and retaining pin holes. (Note: Click on the photo to view full size)

In order to use low-cost ($12.50 each) high-speed (14,000 rpm) ball bearings, we turned this shaft down to 28.5 mm, allowing a 6 mm gap between the shaft and NTN 6007 (35 x 62 x 44 mm) bearing.

Depending on the locking collar design, you could make the fit much tighter, but this gives us a good place to start. Bear in mind the shaft diameter and metallurgy have a lot to do with how much radial load you can place on the shaft end, so keep the shaft diameter around 1.125 inch or greater for the targeted 10-30 horsepower. If you already made your shaft to 1.25 inch (about 32 mm) you can still use the NTN 6007 or go up to a 40 mm bearing.

This month we'll take a look at all of the shaft mounted components and the basics of designing and cutting them for any fit. Remember -- we are showing a design for our shaft size and bearings. Simply modify the parts for your shaft and case.

Figure A shows our shaft with all of the axial mounted components. Bearings have been left out to show the locking collar sets more clearly. On the far end of the shaft we see a hot rotor flange, on the other end a drive pulley, with bearings and spacers between. The large 7/8 - 14 nuts on the two ends compress all of the parts between them. Axial loading of the bearing locking collars and flange/pulley collars centers and secures the components to the shaft. The spacers simply transfer the load and locate the parts along the length of the shaft.

Without going into a lot of word-filled detail, the following pictures of our 3D CAD models should be self-explanatory. Keep in mind, we are working with a Sachs 440 case; the bearing locations for your case may be different. Also, as you compress the locking collars, some movement of bearings will take place -- you may need to use shim washers between some components, so make up or order washers to fit your shaft in various thicknesses (0.5 mm - 5 mm).

The bearing locking collar consists of three pieces. The center ring is turned to slip inside the bearing inner race, a narrow slit cut on one side of the ring to facilitate expansion of the ring under pressure. If too much radial load distorts the bearing inner race, use a solid ring without the expansion slit. All three rings are cut with matching tapers (about 40 degrees) which fit together and convert a certain amount of axial load (from the end nuts) to radial load to secure the bearing to the shaft.

The spacer set (Figure C) is simply a set of bushings used to maintain proper distance between shaft components, and to transfer axial loads to the shaft. The bushing center bore should be just enough to easily slip them over the shaft. Outer bushing diameters should be about 6 mm greater than the I.D. Lengths will vary depending on final bearing locations for the particular case being used.

In the foreground of the shaft assembly (refer to Figure A) we see a pulley and locking collar (Figure D). Again we use a taper fit between the pulley and the collar. The collar has a slip fit over the shaft and is slit along one side to allow easier compression on the shaft. The mating taper is cut into the pulley (about 7 degrees taper). Just ahead of the pulley is a bushing spanning the distance between the pulley and the end nut & washer. Alternatively, several pulleys or one long pulley with several belt ways may be used for additional belts.

Going to the back end of the shaft, you'll see our shaft cooling disk fan (Figure E). This device prevents excess heat from reaching the synthetic nitrile seals on the hot end of the turbine. This fan is simply made using 1/16-inch nickel aluminum, stainless or 4140 carbon steel plate with 1/8-inch spacers between. A snug slip fit over the shaft will be sufficient for these light weight disks.

Just beyond the shaft cooling fan is the hot rotor flange (Figure F). The flange with a slip fit outer plate clamps the hot rotor disks using (6) 5/16 or 3/8 inch bolts. Tapered collars are used on both ends of the flange to center and clamp the flange to the shaft. The taper angel cut into both the flange and collar can vary between three (3) degrees and seven (7) degrees to to gain the highest possible clamping force on this part.

(Figure G) Last but not least, we cap the two ends with heavy duty washers and fine thread nuts. When the components are finally assembled, these end nuts will be cranked down to approximately 50-100 ft. lbs. to lock all of the shaft components to the shaft.

Next month we'll see how all these parts fit together in the real world, and we'll start the hot rotor section.