Estimating stresses in a aluminium housing for an electric motor with cooling fins
Estimating stresses in a aluminium housing for an electric motor with cooling fins
(OP)
Hi
I am trying to estimate the stresses developed in an aluminium cylinder exposed to an internal pressure from shrink fitting it on to a steel cylinder. For reference think of a housing for an electric motor.
The outer tube was heated to 120degC under assembly and the steel core was lowered in without any issues. The yield strength in the housing is stated to be 200Mpa but we experienced a rupture when the parts cooled.
I have done a hoop stress calulation and checked it with using stress-strain relation (Sigma = E*epsilon) and both give approx. 120Mpa stress. Here i have assumed that the outer circumerence of the tube is smooth and 640mm.
Inner diameter = 580mm
Outer diamter (circular part) = 640mm
Outer diameter fins = 780mm
the thickness of the fins is equal to the section without fins
The size of the interference fit is known. Approx 0.7mm over the diameter
¨
http://files.engineering.com/getfile.aspx?folder=e...'.JPG
The question is: Does the fins affect the stresses developed in the "non-fin" part ? In other words: would the stress on the inner surface be smaller if the tube was constructed with a uniform cross section of ((640-580)/2 = 30mm) rather than having a variable cross section of 30mm and 100mm ?
Cheers.
I am trying to estimate the stresses developed in an aluminium cylinder exposed to an internal pressure from shrink fitting it on to a steel cylinder. For reference think of a housing for an electric motor.
The outer tube was heated to 120degC under assembly and the steel core was lowered in without any issues. The yield strength in the housing is stated to be 200Mpa but we experienced a rupture when the parts cooled.
I have done a hoop stress calulation and checked it with using stress-strain relation (Sigma = E*epsilon) and both give approx. 120Mpa stress. Here i have assumed that the outer circumerence of the tube is smooth and 640mm.
Inner diameter = 580mm
Outer diamter (circular part) = 640mm
Outer diameter fins = 780mm
the thickness of the fins is equal to the section without fins
The size of the interference fit is known. Approx 0.7mm over the diameter
¨
http://files.engineering.com/getfile.aspx?folder=e...'.JPG
The question is: Does the fins affect the stresses developed in the "non-fin" part ? In other words: would the stress on the inner surface be smaller if the tube was constructed with a uniform cross section of ((640-580)/2 = 30mm) rather than having a variable cross section of 30mm and 100mm ?
Cheers.





RE: Estimating stresses in a aluminium housing for an electric motor with cooling fins
Specifically good clear close up pictures of the fracture and the fracture surface would move the discussion along nicely.
.7 mm / .028" diametral interference. Do you need that much for your function?
Did the fracture occurred in a jagged line between 2 fins ?
Is the finned housing a sand casting?
The thick/thin geometry I wistfully presume your part has could cause geometrical stress concentrations.
That same wistfully presumed geometry could cause immense casting quality issues.
3 down, 17 to go.
http://info.cpm-industries.com/blog/bid/178197/5-Q...
RE: Estimating stresses in a aluminium housing for an electric motor with cooling fins
That 0.028 is interference fit is likely the probable.
The motor housing primary stresses - WITHOUT the shrink fit! - are to be taken by the steel motor housing right? The startup torque, running torques, vibration, weights, bearing loads from the load and shaft - everything, right? I'd expect that 0.028 shrink to have fouled up the bearing alignment or bearing cups as well.
RE: Estimating stresses in a aluminium housing for an electric motor with cooling fins
My first post so I will take that into concideration the next time.
Regarding the lack of pictures/information about the fracture:
I didnt want to move the discussion into finding out what caused the fracture. My main objective was to find out if my approach to the strength calculation of the housing was correct or not. We are hopefully going to do this again some time and it would be nice to have confidence in the calculations that has been done.
This is the formula that was used to estimate the surface pressure from the shrink fit. But I am, as initialy stated, a but uncertain to what effect the fins have in this case.
http://files.engineering.com/getfile.aspx?folder=1...
We Choose an interference of 0.7mm to make sure that we obtain a shrink fit when the temperature of motor rose to operating condition of around 70 degrees. Since the aluminium expands 3 times that of the steel core (assuming uniform temperature)
Regarding the fracture:
I have attached a photo of the fraacture and SEM taken of the fracture face. The conclusion is that there is a casting defect. My knowledge of fracture mechanics is limited, but that SEM photo does not look like anything i have seen before....
Fracture:
http://files.engineering.com/getfile.aspx?folder=0...
SEM:
http://files.engineering.com/getfile.aspx?folder=f...
RE: Estimating stresses in a aluminium housing for an electric motor with cooling fins
RE: Estimating stresses in a aluminium housing for an electric motor with cooling fins
When you shrink fit you have to yield the Al, it will then expand from its new size.
After enough cycles the Al will be loose on the steel.
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P.E. Metallurgy, Plymouth Tube
RE: Estimating stresses in a aluminium housing for an electric motor with cooling fins
RE: Estimating stresses in a aluminium housing for an electric motor with cooling fins
RE: Estimating stresses in a aluminium housing for an electric motor with cooling fins
I don't think that it is a powder metal. The Company we hired in for the manufacturing uses pure alumunium where they add the alloying elements themself to obtain the correct material composition. We checked the composition and found it to be similar to AL-6063. They did a stress test on a sample from the same cast and it had a yield strenght of 245MPa.
We normally buy machined components and they don't have cooling fins and this approach with shrink fit has worked just fine for us. When we get the equuipment in for refurbishment after 10 years of operation we still have to use heat/force to extract the core from the housings. Although, i agree that it is too large in this case.
I dont see why the Aluminium has to yield, as long as we are in the elastic range of the material? To some extent there will be some yielding occuring, but we wont loose the whole shrink fit.
Does anyone have of you know of some sourch of knowledge on calculating stress in a variable cross section pipe under stress?
RE: Estimating stresses in a aluminium housing for an electric motor with cooling fins
Did they also do a stress-strain curve on the material? Without knowing the modulus you can't make any useful calculation.
RE: Estimating stresses in a aluminium housing for an electric motor with cooling fins
They did a sample parallel to the casting in which they have stated the strength of the material.
RE: Estimating stresses in a aluminium housing for an electric motor with cooling fins
A stress-strain curve is still needed to calculate the expected stress. Tensile yield strength by itself is useless.
Is there an alloy it was supposed to be made of?
3 down, 17 to go.
RE: Estimating stresses in a aluminium housing for an electric motor with cooling fins
Run the numbers and see. One of the problems with trying to hold this fine balance is that it forces you to work to tighter tolerances which drive up costs. If you want the parts still tight at 70C then you need to figure out the sizes at that temp and see if you can get an elastic fit that will work.
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P.E. Metallurgy, Plymouth Tube