This one is for the engine building gurus...

Ok, I've been trying to find a way to make a simple prop driver without turning a taper in the crank shaft and the prop driver. I found this old plan for the "Little Dragon" that seems to solve the problem. What's the disadvantage of making a prop washer as described in these plans?

Thanks

can't really see what's been written there but ASSume it's tapped to match the crank thread?? if so that would probably be just fine and not so different than what's done with the prop hubs on a lot of big gas engines.

disadvantage is that the shaft is threaded all the way back to the shoulder and would probably bend/break right there pretty easily. might be better to drill/tap the crank and use a replaceable threaded stud for this part.


dave

It wasn't easy for me to read the drawing, but if the drive washer is threaded to the shaft, that is where it will break every time you hit the dirt. For a low speed, low power application it is probably OK. I don't like tapered designs either, they can be troublesome to reassemble. The FOX style is my favorite. The crank has 4 equally spaced lobes up front, and the drive washer is stamped to fit snuggly around those lobes. To top it all off, the drive washer is actually a front dust cover that wraps around the front of the case. Some of the best work FOX ever did.

SERCEFLYER,
The enclosed is from The Little Dragon redrawn in Cad and then converted to a PDF file. The file calls for a washer to be tapped #8-32. My personal opinion is that this prop drive washer would not last very long at 1/16" thick -- easy to strip threads. Also agree that by threading the drive shaft all the way acts as a stress riser.
Gene

Not only is it a breaking point, but it also cocks the rest of the threaded part of the shaft minutely when you tighten the prop washer. I still think this arrangement will work for low powered 1/2A and below.

It's been so long since it was published that I'm not sure, but wasn't the "Little Dragon" created to be a high school shop project to teach youngsters how to machine a working engine. Perhaps ease of construction was preferenced over practical use. Still...it's a nice looking little engine.

George

Uh-huh, now the secret's out... ya'll see the roots of my engine design!

Seriously though... SERCEFLYER I'm curious. Why are you hesitant to machine a taper for the crank and prop driver? In my opinion, for the average home shop, there isn't a better way - unless you have a way to broach shaped holes like the square (Fox) or "D" shaped (OS FPs). I know tapers on prop drivers have been discussed and frowned upon here, but the fact is, they're really little fuss and they work.

Although I have yet to try it, another method is to use the tapered & split collet between the crank and prop driver - like on the majority of the engines nowadays. At least your crank could remain parallel and you'd only have to experiment with making and fitting (or risking) the small parts.

-Joe

PBJ,

Yeah, maybe I should stop grumbling and get on with finishing the Midge! I've become a bit of an arm-chair machinist: I just really like to figure the advantages and disadvantages of various designs. I'm sure you made a lot of improvements on the Little Dragon/EZE design-- I take it you used a tapered prop driver? (I'll be ordering your plans at some point....)

SF

was one of the lessons how to thread the shaft all the way to the shoulder?? don't look too easy to me..

another good method is like the drawing below. crank ends flush with the driver unless you make an extra thick driver just for the drilling operation. easiest when both the crank and driver are of the same material. if not then a dummy driver can be made of steel and a dummy crank of alum so the drill bit doesn't wander over into the softer of the two metals.

i have used the same thing but with three pins on a 54 cc gas engine


dave

Would it be possible to turn up an alum. driver and put a small keyway in the crank and driver?
Maybe a shoulder on the crank to stop the driver touching the crankcase, or just a driver and set screw like this. Maybe 2 set screws to balance it all up. Might need to "dimple" the crank to stop the driver sliding back on the crank when the prop nut is tightened or just machine a shallow groove around the crank for the grub screws to seat into. Maybe run the screw right through the crank? A small split pin even. I seem to recall owning an engine that had a split pin through the driver, might have been an early FROG 80 diesel. Don't know if Zagnut meant this type of thing?

yeah, i suck at explaining things and even more at drawing what i tried to show above is basically the same as making a keyway in the crank and driver but instead of a square key i use a round pin. much easier as you don't need to broach the driver. and yes, the driver seats against a shoulder on the crank.

set screws also work fine but can cock the driver sideways if it is a hair loose on the crank. on one engine i have a long 2" hub that slides over the protruding crank and because the hole came out a bit too large i used three set screws at the rear spaced 120 deg. apart to adjust runout at the front...


dave