Termal impact on rotational parts 1

[Azzazil]

Hello, I have issue with part that is in picture below,

I have fixed POM-C shaft and rotational ring from aluminum. As you can see gap between shaft and ring is Φ39-Φ38.8 = 0.2 mm where is contact between them but they ensure rolling of ring on shaft under room temperature without any issues. But when I have temperature around 90°C due to thermal expansion of POM-C and Aluminum critical gap of 0.2 mm is at 90°C 0.115 mm and at this value I got ring stuck and unable to rotate ring unless I cool it below 90°C. Is it root cause in this case that gap between part at value of 0.115 mm is too small and I need to make it bigger or ring should rotate even with gap of 0.115 mm and issue for stuck has different root cause?

 

[3DDave]

POM-C thermal expansion coefficient is roughly 14e10-5*1/K which is 140 PPM/K
Aluminum thermal expansion coefficient is roughly 23 PPM/K

so over 90°C - 20°C I would expect 70K*((140-20)ppm/K)*39mm/1e6 mm = .328mm of expansion. This gap appears to be only .2 mm.

That remaining amount is resisted by (.128mm/39mm)*1e6 = 3282 PPM compression.

POM-C appears to have a modulus of elasticity roughly 2000MPa, which is 2000*1e6N/M^2. Divide by 1e^6 and it is 2000N/m^2/PPM of deformation -> 3282 PPM *2000N/m^2/PPM = 6.6MPa pressure. So, yeah, that will provide some friction.

(edit: giving Pascal his capital letter.)

 

[dvd]

Also have undefined fillet radius / chamfer.

 

[Tmoose]

What kinds of radial and axial/thrust loads are involved?

Rpm, and lubrication ?

Do things warm up due to the process, of from friction generated by by the shaft and ring. Either thrust load or radial load ?
If the design is mostly finalized I'd be looking at carving out the POM to provide some radial flexibility or adding a steel ring or struts to resist the POM thermal expansion. the POM.

 

[Azzazil]

@3DDave thanks a lot it looks that I had wrong values for thermal expansion coefficient :/
Anyway I want also to understand your calculation since I am not familiar with unit with abbreviation of PPM/K I suppose this is pressure you are mentioning?

That remaining amount is resisted by (.128mm/39mm)*1e6 = 3282 PPM compression

What is formula for this? I understand that you divide amount that was in clash 0.328mm-0.2mm with diameter of ring and probably since you have units in PPM you multiply it with 1e6.

OK I get it that I have clash now with temperature on 90°C, what should be minimum gap to prevent stuck or hard rotation of ring on shaft?

Also have undefined fillet radius / chamfer.

I have chamfers on edges and fillets on corners if you think on that? Didn't want to add that in sketch because I didn't have time to make it more detailed.

What kinds of radial and axial/thrust loads are involved?

Rpm, and lubrication ?

Do things warm up due to the process, of from friction generated by by the shaft and ring. Either thrust load or radial load ?
If the design is mostly finalized I'd be looking at carving out the POM to provide some radial flexibility or adding a steel ring or struts to resist the POM thermal expansion. the POM.

For load and RPM I am not sure don't have correct number on my mind but load is radial. It is lubricated with grease which is not aggressive to plastics.
Heat comes manly from environment where is shaft and bearing positioned, and there is no possibility to move them in some colder environment.

Yes design is mostly finalized I am considering to increase nominal gap which will probably be bigger on room temperature but during the heat impact it won't stuck. Or change material with one that doesn't have big coefficient of thermal expansion but I am not sure what is good choice.
Thanks also for your advice to carve out POM.

 

[3DDave]

ppm = parts per million. That is, if a thing is marked off in 1 million parts then a 1K change results in a change of overall size by an amount equal to one of those parts, whether it's inches or meters.

Gap > original diameter + expansion. You have the formula for amount of expansion.

 

[Azzazil]

@3DDave thanks for clarification of that part . Also one more question since I have geometrical design as it is, beside gap redefinition is it good idea to change material, to be same material as ring so they both have same coefficient of thermal expansion and minimal possibility to get stuck?

 

[3DDave]

That would equalize expansion, but there's not enough information to say why the part was POM-C to begin with. I have no idea what any of this is to do.

 

[Azzazil]

Since I have low rotational speed, I went for cheap thermoplastic material after checking some usual materials used for plastic bushing and bearings I decided to go for POM-C. I can go with bronze or steel but I am not sure what are their tribological properties when they are in contact with Aluminum.