Friction clutch for small engines

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Brian Rupnow

Design Engineer
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While I was working on the dog clutch for my recently built drag saw, I put out a request asking for a good design for a friction clutch that could be "miniaturized". Some kind hearted soul from another forum sent me a link to a one page document, which shows setting up instructions for exactly what I was looking for, which surprisingly enough was a friction clutch used on a drag saw. It must be about 100 years old, and will totally make your head explode trying to figure out how it works. It is a great design, because it not only engages the clutch, but a secondary and very beneficial effect is that it "traps" the clutch handle and won't let the clutch accidently disengage until the operator wants it to. This is my interpretation of the mechanicals of this clutch, and an attempt to build one.
http://www.herculesengines.com/hercules/NewManual/page_58.html
 
These two models show the clutch engaged and disengaged. the green expander cone is slid along the shaft by an outside handle (not shown) and when it engages them it spreads them open, thus making them rotate and put pressure on the red friction disc. the yellow flywheel is keyed to the shaft. The blue hub with sprocket mounted on it floats on the shaft.
qrGpQr.jpg

ymIrt1.jpg
 
In about two minutes I am going to jump up and grab a piece of mild steel and make this expander cone. It's cold outside and good wife and visiting daughter are off to the mall. I don't have any 01 stock this large in diameter, so I'm not going to harden it.
YPFBEt.jpg
 
That's Great
I'm looking to combine my engine with a car :D
 
I was originally going to make this clutch with three comma shaped arms. However, looking at the slitting saws I have, I decided to make it with four comma shape arms. This way I can cut the 1/8" wide slots in the blue hub all the way across the face.
jSKx0D.jpg
 
Now you have a design that is very interesting to me. I wuz gonna take yer two arm and make it more powerful but you have done it here!

Excellent work! The magic of CAD....

Thanks,
Pete
 
I don't post very many set up shots anymore, but this is a worthy one. No, it's not a Maltese cross, it's the hub that will hold the four comma shaped arms. There is more work yet to be done on it.
za2OwH.jpg
 
Now, is that a beautiful thing or is that a beautiful thing? I chickened out at the very last moment and put the slots in with a 1/8" carbide endmill instead of using my slitting saw. This part is steel and I didn't want to take a years life off my slitting saw. I took 0.010" deep cuts with the mill running at 1300 rpm and hand cranked very cautiously so as not to snap the endmill off.
DyHfxS.jpg
 
Now it's my turn to ask a question. What can I buy for a friction disc. The disc stands alone and is not attached to anything, simply centered on the shaft. In a perfect world, I would like something about 1/16" to 1/8" thick. Whatever it is, it has to not only have very good friction surfaces on it, but must be durable and capable of holding it's shape. It needs to be 2 1/4" diameter. If I don't get a good answer to this question, I may default to a piece of 1/4" plywood for a friction disc.
 
Have you looked at clutch pack material for limited slip or perhaps motorcycle clutches? You many have some luck finding some in that diameter.
 
Funny- this ad popped up in response to your question about what to make clutch disc out of
Screen Shot 2018-09-24 at 11.10.43 AM.png
 
I went down street on a shopping trip today, and bought a 3 foot length of 1" x 1/8" precision ground 01 steel to make the comma shaped pieces from. Also bought a couple of very healthy small springs, one of them will work on the end of the clutch shaft, not sure which yet. Also picked up a sprocket which can be bored out to fit the appropriate clutch hub. I sawed off four 1 1/2" long pieces of the 1 x 1/8" 01 material, lined up the sides and one end, and clamped them in a set of welding vice grips. I will put a dab of mig weld on the ends which will be trimmed away after the fact, so I can machine all four pieces at the same time to ensure that they are all the same.
b2OeCe.jpg
 
I'm curious--those of you who have suggested leather, how would you use it. Would you glue it to the face of one of the discs, or just let it free float between the two metal discs?
 
Brian, you'd probably have to epoxy the sucker onto a sacrificial piece( make sure you have afew of this as spares) just like a car dry clutch plate.
Hope this helps.
 

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