Joining back iron with Aluminium case

hias9

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I plan to have a rotor (similar to common 205mm ID outrunner DD motor rotors) machined out of aluminium but as back iron for the magnets I need a 3-4mm thick iron ring.
When shrinking the case onto the steel ring, there would not be enough friction at high temperatures because the aluminium will expand more than the iron.
When glueing it in, heat conductivity from the inside to the outside would get worse. Not sure if thermally conductive epoxy would be suitable for this.
All components of the motor will be able to take at least 185 degrees Celsius continously, so in operation the iron ring could see temperatures of 120 degrees celsius or higher.
Statorade and cooling fins will be used.

Which method to join the iron ring with the aluminium case would you choose?
 
Did you do the math on how much thermal expansion difference there will be at different temperatures yet? A heated slip fit of a few hundred degrees for assembly could be fine at that size / temp range. This would be my first choice to calculate / rule out.
Other options I see are built in flex sections or segmented back iron if going mechanical fastening. There are aluminum / steel hybrid disc brakes (search "Brembo dual-cast") and segmented iron aluminum flywheels you could look at for design references. I'm not sure how adaptable this tech would be to what you're trying to do though. The aluminum sections could be the segmented /relieved sections too (or the iron if you were thinking opposite.)
Any adhesive would have to be flexible and stay strong over the temperature swing and thermal expansion difference. That probably means a fairly specific adhesive gap that could be tough to achieve. Some holes in the iron would allow adhesive through the iron to the aluminum to help glue the 3 different components together.
A mechanically fastened setup that has a gap filled with a thermally conductive (at least slightly flexible) filler could work too.
 
Yes, I expect that the aluminium would expand up to 0.25mm more than the iron will expand.
I don't think only shrinking would be a good idea in this case.
I also definitely don't want to drill holes through the back iron. Segmented back iron would probably cause gaps where there won't be any back iron when it expands.
Are you aware of a suitable glue with high thermal conductivity?
 
.25 mm seems doable for that size. It's worth doing the thermal expansion calculations for it to check.

There are lots of options for adhesives but I don't have any I can recommend. Look at 3m and Loctite. There are some recommendations in this forum you can search for (not mine.)
 
Yeah you can interference fit it. Cylinder bores are made this way often. Block (alu) is heated in oven, new cylinder liners ( steel, iron) installed, cool down block, and it is locked in place. Forever.

I could give you exact dimensions you would want both of them to be. Depends on the diameter of the insert ( give me the outer diameter of the iron ring you want, and I will tell you the diameter, to bore the aluminum out to, for a long lasting, well engineered interference fit,). You will have to heat and press the liner in. Do it right, and it will NOT move. Not until the thing melts, (thousands of deg. ). It is doable. Tell me the outer diameter of the steel ring ( to the 0.00x inch.. to the thousandth of an inch) and I will tell you the internal diameter you want to cut the aluminum to.

Or sleeve retainer, if you are cheap. I would try that. They make some VERY tenacious anaerobic loctite sleeve retainer.... and I think there is certainly published specs for the clearances they can cover ( dimension) to make your rings to bond together with.

So. Sleeve retainer or press fit.

Expansion ratio depends on diameter and material.
 
Thanks a lot!
Are you sure it's that simple to calculate and are the tensions inside the aluminium a problem?
The spoke flanges will also be a part of the aluminium part, so spokes will pull it to the outside.

The measurements after joining the parts should be:
Iron ring ID: 205mm
Iron Ring OD = Aluminium case ID (where the ring sits) : 213mm
Aluminium (6061 or 6082) case OD: 219mm (could change that if required)

Cooling fins will be bolted around the outside of the aluminium case.
 
It's that simple if the calculations are done properly, with the correct materials.
It's a great feeling to do the calcs, get the pieces made, heat the aluminum, then drop the 2 together. Once cool it's science but feels like magic.

They make brake rotors that are similar to the suggestion. Those will have repeated temperature swings 5-10x what you're expecting.
Also, aluminum engine blocks typically have iron sleeves. Lots of decent stressed examples in similar or larger sizes than you're taking.
 
Will the inner part (iron ring) be slightly compressed or will the diameter stay exactly the same and only the diameter of the aluminium part increase after joining the parts?
 
Yes, the outer housing will be compressing the inner ring. You wouldn't be able to measure the difference in size (distortion) of the 2 assembled without very pricey tools.

The difference in size cold between the parts will be around a few pieces of paper thick.
The only downside to this is typically some increased cost machining the pieces to a high tolerance
 
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