Heat Expand/Shrink Fit

[BEMPE16524]

Hi All,

I'm going to use this formula for my problem.

δ = α.L.(Δt)

δ = Total deformation desired (in or mm)
α = Coefficient of thermal expansion (in/in ^OF or mm/mm ^OC)
L = Nominal length of the part being heated (the diameter for a cylinder) (in or mm)
Δt = Temperature different (^OF or ^OC)

Now let say i have a 250mm shaft, by looking at the shrink fit table, i got following value:
Shaft: 250mm (+0.126/+0.094)r6
Hole: 250mm (+0.052/0)H7

My question is, what should be my desired total deformation for the hole in order for me to make the shrink fit?
Is it 250.126-250.052? or 250.094-250.052? plus more tolerance?

My logic sense says it doesn't matter as long as i can expand the hole to required temperature.

My second question is, what about the shell thickness? how does it affect the expansion process?

R.Efendy

 

[btrueblood]

My question is, what should be my desired total deformation for the hole in order for me to make the shrink fit?
Is it 250.126-250.052? or 250.094-250.052? plus more tolerance? My logic sense says it doesn't matter as long as i can expand the hole to required temperature.

You need at least the first amount, or risk having the assembly bind before you can get the parts fully assembled (as soon as you take the hole out of the oven/flame, it starts shrinking, and it will shrink even faster once it makes contact with the shaft and the temperatures start to equilibrate). I prefer having double the tolerance, if I can, to give more time to get the parts lined up and make the assembly. On a 10" part I'd want as much wiggle room and time as possible, but I've never pressed something that big before.

My second question is, what about the shell thickness? how does it affect the expansion process?

Your equation assumes that the entire part containing the hole has some uniform high temperature. If all you do is heat the i.d. of the hole to a depth of say 1", in a 24" x 24" structure, then the hole won't expand much since the colder material surrounding it prevents that.

 

[racookpe1978]

If your male part goes in the female part (the hole), then why are are assuming the rod is heating at all?

Cool the 10 dia rod/cylinder/pipe/axle (?) as much as possible (liquid N2, CO2, dry ice, salt and water bath), and heat ALL of the metal around the female opening so the hole can expand. Dry run your assembly several times because, as noted above, you have only one chance to get the two together and in the right spot on the shaft.

 

[rb1957]

i think you need to check both ends of the tolerance ...

maximum interference = 0.126-0 = 0.126mm ... checks that the highest possible stresses are acceptable and is this interference physically possible?
minimum interference = 0.094-0.052 = 0.042mm ... is this minimum acceptable to design/stress ?

if you freeze the biggest shaft will it fit into the smallest hole ?



another day in paradise, or is paradise one day closer ?

 

[BEMPE16524]

thanks for your replies.i forgot to tell you that i cant cool down the shaft because the shaft is a stator of a motor which is imported from other country.

also the housing will be a 6061 T6 Alum.