Cylindrical interference fit?

[great white]

Hello.

I'm not sure if this is a "beginners" question, but here goes:

I'm building a chain drive for what was a CVT (IE: snowmobile clutch) driven transmission.

I've turned the center section out of an aluminum wheel (CBR600RR) to get the sprocket carrier and retain the "cush drive" (functions as a damper for vibes and shock loading):





I cleaned up the center bore to be dimensional true and it works out to 2.136 - 2.137".

The transmission input shaft is 1" OD:



So I'm machining an "adapter" to go from the hub bore to the transmission input shaft. The adapter is mild steel (1018 IIRC):



The hub goes on the middle "step" of the adapter. The smaller diameter is for the sprocket carrier and is a slip fit as there will be another piece that will secure it into the cush drive hub. It's got a few thou clearance as the sprocket carrier needs to "float" a bit to "self align" and absorb drive line loads.

The transmission input shaft is keyed, so retention and handling the power there is a fairly simple matter.

The hub to the adapter is a different story though. The Aluminum hub does not have enough "meat" in order to incorporate a keyway and there is also no suitable place where I can add a pin or bolt to mechanically lock the hub and adapter together.

So, from what I can deduce, I'm left with doing an interference fit.

What's a bit above my pay grade is calculating how much of a friction fit I need.

I plan to use heat to get my fit. Right now, the adapter shoulder is 0.091" larger than the bore in the cush drive hub. I stopped there in order to work out what I need for a final dimension as an interference fit.

The hub is (essentially) a cast aluminum piece, the adapter is mild steel. The final dimension is right around 2.137".

I took the hub and heated it to 450C and the clearance opened up to 2.141-2.142", so it gains about 0.004-0.005 when heated.

The joint needs to hold around 40-50 ft/lbs of torque at most and there's just a bit over 3" of shaft and hub length in contact once assembled.

Once assembled, it will never have to come apart again.

It seems that an interference fit on a drive component is "usually" around 0.004-0.005" in this type of assembly, although the actual calculation of the needed dimension is just outside of my knowledge level.

Any ideas on where I shoudl be shooting for on dimensions?

If it was all steel, it would be an easy fix; a bead of weld from the mig and done.....

 

[4ssss]

When I was shrinking carbide into dies, it was never more than .001-.0015 difference. There was a formula, but it was determined by the diameter, and I've forgotten over the years. I think you'll be safe with .001-.002

 

[JimDawson]

I think a 0.004 to 0.005 interference fit would hold it. For extra security, 3 grub screws, drilled and tapped after assembly of the parts would add a lot. Tapped right into both parts.

 

[great white]

I think a 0.004 to 0.005 interference fit would hold it. For extra security, 3 grub screws, drilled and tapped after assembly of the parts would add a lot. Tapped right into both parts.

Grub screws are a decent idea, except I've only got about 0.162 aluminum thickness on the shaft and about 0.250 at the back where it rests on the next steel step. I'm not sure if it would add strength or take it away to drill those three holes....

 

[great white]

I've also thought of maybe using an anaerobic adhesive (IE: locktight red, blue, etc) to increase the holding power, but I'm not certain if i should or not considering the aluminum part is going to be very hot when assembled....

 

[JimDawson]

I don't think locktight would like the heat.

 

[benmychree]

Press or shrink fits are both allowed .001" per inch of diameter, as a rule; shrink fits have considerably more holding power than press fits due to the lack of smearing of texture of the finish such as happens with press fits.

 

[great white]

So, I’m going to try a 0.004-0.005 shrink fit.

Admittedly, my measuring may be slightly suspect, so it may be a little closer than that.

With that being said, I’m measuring about 0.006 right now, but a trial fit has the appearance of it being closer than that...

 

[Cadillac]

When suggested to use set screws I thought of this way as I see on drive Assy. pretty frequent. After you put your hub on the shaft with whatever interference fit you decide. Drill your setscrew parallel with the shaft at the seam of the OD of shaft and ID of hub. It’s like putting a threaded keyway in place if that makes sense. You can use a 1/4-20 screw and still have clearance. You can also do them 180* for two which would definitely lock that hub. And it’s not permanent for future Maint. Which you wouldn’t need a press fit either just a slip fit. Looks like a interesting project your working on?

 

[great white]

When suggested to use set screws I thought of this way as I see on drive Assy. pretty frequent. After you put your hub on the shaft with whatever interference fit you decide. Drill your setscrew parallel with the shaft at the seam of the OD of shaft and ID of hub. It’s like putting a threaded keyway in place if that makes sense. You can use a 1/4-20 screw and still have clearance. You can also do them 180* for two which would definitely lock that hub. And it’s not permanent for future Maint. Which you wouldn’t need a press fit either just a slip fit. Looks like a interesting project your working on?

I see what you are saying, but I don't think I have enough "meat" in the aluminum component for it. I only measured .162" thickness where I would have to put the screws. Even if I and be precise enough to only use half that thickness, that means it's going to only be .081 after I'm done. I think that's just a bit too thin....