Prop Shaft Gears / Final Assembly


The prop shaft is completed in this section. I have chosen for my own odd purposes to cut a spur gear in the "fat" portion of the prop shaft. This is the major diameter situated between the two rear bearings and the big front bearing. The primary reason to do so is to some day possibly "tap" power from the prop shaft and into an accessory gearbox which can drive perhaps a generator or mechanical oil pump. Additional gears inside the gearbox would do the same thing, but then to get the power outside the gearbox would require a pretty elaborate seal to prevent oil leakage. Also, cutting gears is fun, and I took this as an opportunity to do so! Remember, I am a model engineer first, and live for this type of customization. Mike Murphy has done a tremendous job with the plans, so if you want to get your turboprop running, don't EVEN consider messing with stuff like drive collets, custom prop plates, and cut gears. FOLLOW HIS PLANS and you will be better off.

  Cutting a gear requires a dividing head, and can be done in a lathe but a mill is a better choice. A 0.5625" / 14.29mm OD was turned in the prop shaft major OD to begin. The dimension is mathematically derived thus:

Spur gear OD blank = (NumTeeth + 2) / Diametrical Pitch

Working backwards from 14mm, the basic planned OD, I found the closest whole number of teeth to be 16 which delivers the Gear blank OD of 0.5625". Thus, the shaft was turned to this OD and a 16 tooth, 32 DP gear was planned. I happened to have a 32 DP cutter for 16 to 18 teeth, a happy coincidence. Otherwise a gear cutter will run $25 to $40 U.S. The setup here is typical - a dividing head set up for 16 divisions, shaft mounted w/tailstock support, and everything carefully centered. Total depth of cut is 0.067"

The cuts MUST be made short of the whole depth. Initial cuts can be made of .03", then .02", etc, as being a form cutter, the deeper you go the more material is removed and the cutting pressure increases. Make the last cut 0.003" or so for finishing.

Shown here is a cut of perhaps 0.015" to test the setup and be sure my hands and brain were talking this day. The blue helps visualize the cutting action.

After the final cut, it is a joy to remove and clean up the gear section. Note that the cut does not extend into the shoulder which abuts the large forward bearing inner race... this would have prevented full contact of the shaft with said race. The smaller rear bearings, which will be inserted in the top of this photo, do not have this limitation.

A fine wire brush in a dremel makes short work of any burrs.

For final assembly, have on hand the two rear prop shaft bearings and the loctite, along with the driven bevel gear. Assemble as per Mike's plans.
I like his securing method and have used it before. Here the two holes are drilled for the set screws. They are tapped and the set screws loctited in place. Consider this whole assembly permanent.
The final rear portion of the prop shaft! I have a useable 32 DP 16 tooth gear with a working face of perhaps 0.25"

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