Need Help!- Will its size reduce after cooling?

[Ashish B]

Hello,

I am using a shaft of EN47 material. I selected it because the thermal coefficient of expansion is 14.4

So, if I cool the shaft, then will it reduce its size in the same proportion (14.4)

Thanks

[HuFlungDung]

Think, man, think, this is not the first time you've asked a similar question. What does the coefficient 14.4 mean? Do you understand the units that describe what the coefficient is?

[Ashish B]

Hi,

I am very much confused about what to do? Please help me with my dumb questions...I need some proper guidance.

Also, yesterday I keep Shafts of SS410, HSS & MS in refrigerator for 24 hours. I was assuming that it would shrink by at least 10 micron, but they did not.

Is the refrigerator temperature insufficient for cooling the shaft?

Need help
Ash

[Ashish B]

Finally got a LIQUID NITROGEN vendor in my local area.

I am going to take shaft (of various materials) along with micrometer. I will check prior size & than put NITROGEN on to it & check the deviation (with micrometer).

Will let you know the results asap.
Thanks
Ash

[pirko]

Dear,
we are aerospace industry. For aluminum alloys we use automatic cooling (heating) attachment for cooling water. This held temperature by 1 degree C. We need this stabile temperature for accuracy.

[HuFlungDung]

In the inch system of units, the coefficient of expansion is described as so and so many inches of expansion per inch of length per degree F.

In the metric system the units are so and so many meters of expansion per meter of length per degree C.

In either system, don't overlook the fact that the chart values that you find have an inferred multiplier:
10^-6 = .000001

So a metric chart value of 14.4 written out in full is .0000144 meters displacement/meter of material length/°C. This is equivalent to .0000144mm/mm of material length.

Now if you think back to your high school physics, you'll understand that units of measurement can be manipulated just like fractions. If you have inches (or meters) in the numerator and denominator, they both cancel each other because they equal 1.

(interesting sidelight: so the only difference between the coefficients in metric versus inch, is the ratio between 1°F and 1°C which is 180:100 = 9/5)

So you need to know what diameter you are intending to measure across, and the net temperature difference from room ambient temperature before you can figure out how much expansion you will get when measuring with a micrometer at ambient temperature.

So once you have the understanding of what the coefficient represents, you only need to scale it up to find the measurable difference. If you think of a 100°C temperature rise, and a diameter of 10mm, then you have a scale up factor of 100 * 10 = 1000.

So look at the coefficient: .0000144 and move the decimal three places right, resulting in 0.0144 mm measurable change.

[Delw]

Contact China, or mexico I am sure they have similair problems maybe your 3 countries can network and produces Quality parts for once.
but then again it might hurt us here in the US too, as if you correct it , there wont be any need for us to reworked your scrap/junk that you all send over to the cheap american companies who send you the work in the first place.

Delw

[Ashish B]

Hi HuFlungDung,

Thanks for such wonderful essay on co-efficient of thermal expansion/contraction. Really hats off for such easy, fruitful expression of THERMAL EXPANSION.


I will try it with a small example -
Shaft Dia = 2 mm
Co-efficient = 0.0000144
I am using liquid Nitrogen. Its temperature is -200 degree celsius.
So, Scale Up factor = 2 * 200 = 400

Hence, Contraction will be 400 * 0.0000144 = 0.005 mm
I hope that I was not wrong.

Thanks
Ashish

[HuFlungDung]

Yes, you've got the calculation correct.

Now in terms of what a typical human is capable of assembling by hand, you'd need a minimum of .025mm clearance to actually do a very quick assembly. With some sort of guide mechanism to align the parts for perfect axial alignment, you might do it with .01mm clearance. Keep in mind that the bearing wants to shrink very quickly when it touches the shaft as they exchange heat in the blink of an eye.

[Ashish B]

Hi,

Once again, thanks for your comments. I am feeling much more confident about SHRINK FITTING Now.

Thanks for your sharing your wonderful knowledge.