Heavy Shrink Fit 4

[nanobot29]

Are there any risks in using a heavy shrink fit on a 4340 steel disk? i heard of rules of thumb that suggest 2 to 3 thou per inch but my application requires a shrink fit of 0.035 inch interference on a 7.000 inch diameter rotor. The reason i need such a large shrink fit is because the rotor is spinning at 18000 rpm which is causin the disk to expand by over 0.030 inches.

i calculated all the stresses and fatigue life. streses are ok as long as i heat treat to a min yield of 200 ksi.

Thes disk is 1 inch thick and has a 15 in OD.

is there anything else i need to consider? is it possible to achieve a 0.035 inch diametral interference in a 7 inch rotor? thanks

 

[MikeHalloran]

How thick is the scattershield?



Mike Halloran
Pembroke Pines, FL, USA

 

[nanobot29]

oh great, i need a scattershield? even though i wont be spinning the parts?

 

[MikeHalloran]

If there's a shrink fit between them, it will be difficult to spin the rotor without also spinning the disk. Could you reveal a bit more about what the assembly is intended to do, without giving away the store?



Mike Halloran
Pembroke Pines, FL, USA

 

[nanobot29]

This first prototype wont be spun... neither disk or shaft. I simply want to confirm the manufacturability of he heavy shrink fit. I want to make sure that the disk wont warp or crack during installation. The actaul assembly is part of a 5 megawatt permanent magnet motor. The disk is part of a thrust bearing assembly. The disk needs to have enough interference fit on the shaft to prevent lift off at full speed. The production "scattershield" will be a 4 inch steel wall housing

 

[KENAT]

How are you planning to assemble it? Are you going to cool the shaft and heat the disk? Have you calculated what temp difference you'd need to get the force of fit you want? Do these temperatures affect the properties of your parts in a way they may fail during assy?

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[nanobot29]

Kenat,
yes, i calculated the temperatures i need. I'm planning to heat the disk to 700 degF and cool the shaft down to -20 degF. The disk is tempered at 800 degF so i figured i would need to stay below that. i'm not 100% sure what happens to the shaft at -20 degF.

 

[mechanic6]

wow!thats quite the project.

if you assemble with a .035" interference fit and the disk expands by .030" then your effective interference will only be .005".
it would not be safe even at 1800 rpm.

i have done lots of 10 to 20 ton press fits cold on 5" and 6" dia shafts,no problem.

you may want to use a much higher interference if you are going to heat one part and shrink the other, .035" would be a light press fit on a 7"dia shaft with both parts at the same temp.

no matter how you do assemble this,it must be checked for perfect
squareness ie:zero runout.

5MW electric motor,just curious what is this connected to?

M6

 

[kingnero]

Mechanic6:
"you may want to use a much higher interference if you are going to heat one part and shrink the other, .035" would be a light press fit on a 7"dia shaft with both parts at the same temp"


A quick check through a spreadsheat gives in interface pressure of 410 Mpa. Which I wouldn't classify as "light"...

Due to temperature issues it will be hard to increase the interference... But using large enough force (large shop press) you'll be able to press fit both parts (in combination with both heat effects) with an even larger interference - if possible and if the shaft may be scored/marked during the assembly.

Are there no alternatives?
Does it need to be removed/replaced?
Change of design maybe, so the shaft has a "rim" or a step up so the disk can be bolted (or welded) to the edge?

 

[racookpe1978]

But both parts are going to be heating up at the same rate (or very nearly the same rate) and reach the same final temperature.

If you start with a 0.010 heavy shrink fit (overlap) you will end up with a 0.010 (+/- .002 or so) overlap at final temperature. That 0.030 overlap (a compression force on the shaft) adds a tremendous pre-loaded tension stress on the outer part even before centrifugal forces and dynamic loads are begun.

 

[Wrenchbender]

Not meaning to complicate the issue, but at some point you may want to consider some analysis in fracture mechanics - things failing at much less that yield from the presence and growth of a flaw(s) to a critical size. This sounds ripe: high hoop stresses, start & stop cycles, high strength steel - just saying...

"Very interesting indeed, but tell me, Mr. Faraday, of what use is this electricity?" "Sir, you will soon be able to tax it."

 

[nanobot29]

Thanks for the feedback guys,

i did indeed perform the necessary calculation. I'll definitely look deeper though, i maye be missing something. I calculated the radial and hoop stresses due to the shrink fit and centrifugal forces and took into account the stress concentrations. My safety factor is around 1.3 assuming a yield strength of 198 ksi. The fatigue life was calculated to be 450,000 cycles (we need 50,000).

im not 100% sure what this motor will be driving... my guess is that it will be powering a compressor or turbine.

i did look into other alternatives such as machining the disk and shaft from one piece or bolting a disk onto a flange. The stresses at the bolt holes turn out to be very high. machining from one piece is still an option but it looks like it becomes a very expensive shaft, plus we would have to heat treat the entire piece which im not sure we can do since there is post machining involved.

 

[HDS]

Cast it as one piece then machine the critical areas. Talk to the caster about the need for very uniform density.

 

[nanobot29]

can i get 200 ksi yield strength on casted parts? The stresses will reach 150 ksi just from spinning.

 

[rb1957]

cast would be Really expensive, unless

 

[Cockroach]

This has been a wild read.

0.035 inch interference fit, cast iron with 200 ksi yields, pre-heats to 700F, is it just me or are the alarm bells getting louder?

I think there are several design issues in need of serious addressing.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[rb1957]

i think casting is a bit of a red herring, as impactical as machining. 200ksi steel is notorious for being brittle (ie little tolerance for notches).

wasn't this why key-ways were invented ?

what about cross bolts, taking the torque in shear ?

 

[MikeHalloran]

You don't want cast parts anywhere in the rotating assembly.

Have you got a mentor there? Cockroach is right; there are already too many ways for this to end badly, it's getting uglier, and we can't help you properly from way out here.





Mike Halloran
Pembroke Pines, FL, USA

 

[nanobot29]

yes Kenneth i Agree... thats the purpose oh this thread. i was hoping that other engineers can point out issues that i havent considered.

 

[nanobot29]

RB1957
thanks for the tip regarding 200 ksi steel. any suggestions as to how hight i can heat treat while maintaining some ductility?
I looked at key ways but the hoop stress is way too high on the inner diameter. Adding a key way is introducing high stress concentrations that puts the disk past its yield.

MikeHalloran
My mentor is scratching his head too... I'm thinking i may have to get a consultant in here. i'll stay away from cast parts in rotation, thanks