Engineer Info Wanted...Temperature effects on Aluminum, Stainless & Titanium metals?

[Dueydog]

I own all three and the titanium seems to be the least effected by hot or cold temperatures.

For a control purposes I used a .689 for all three barrels and 10 balls from two different types of paint. I practiced Sunday in 100 degree weather and the aluminum barrel seemed to constrict to a smaller bore or the balls were bigger, but when I checked the titanium barrel the balls fit perfectly.

I didn't check the stainless until today in my house at 78 degrees, used 10 balls from each bag of paint, and all three barrels where identical at .689 bore.

There is a discrepency somewhere between 100 and 78 degree temps for the aluminum barrel.

I have read Tom Kaye's info and about everything else out there.

I tried two searches without results so please...NO FLAMES!!!

And please...NO OPINIONS! Only FACTS!

Thanks,
Matt

[lonelymag]

for one, the barrels diameter will expand, not contract. it may seem like they contract because the balls will expand as well, and to a greater degree, and cause a tighter fit. the expansion coefficients of all three materials are much much smaller than that of the plastics of the ball and the fills.

also, just because the air temperature is 100 F, does not mean that the metal of the barrel will be at that temperature. most likely the average temperature of the barrel will not increase more than a few degrees above room temperature.

[SeeK]

for one, the barrels diameter will expand, not contract. it may seem like they contract because the balls will expand as well, and to a greater degree, and cause a tighter fit. the expansion coefficients of all three materials are much much smaller than that of the plastics of the ball and the fills.

also, just because the air temperature is 100 F, does not mean that the metal of the barrel will be at that temperature. most likely the average temperature of the barrel will not increase more than a few degrees above room temperature.

If the air temp was 100 F then room temp would be 100 F. It would normally feel colder to the touch but since the room temp is higher than body temp it would be warm.

[The Spanish Inquisition]

good idea, but you need to measure the inside diameter with an "inside micrometer" if you want to be exact / have accurate data. Also like was said the differences you noticed were positivley due to the temperature's effect on the paint, not the barrel. What would be nice is if you measured the diameter of some different paintballs "with a mircrometer" and then record each ones temperature.

[Cristobal]

You are right when you suggest that aluminum has the highest temperature dependence, followed by steel, then aluminum. However, you should not see a noticeable difference between barrels made from the various materials. The amount that any of them will deform because of temperature changes is negligible for the range of temperatures they will see when playing paintball. The more important factor will be the tendency of the paint to swell or shrink based on temperature and humidity.


But to answer your question more scientifically:

Material behavior under a change in temperature is classified by a property known as the coefficient of thermal expansion, typically listed as "alpha". To find how much a material with initial length L (at temperature T1) will deform when the temperature changes to T2, you multiply as follows:

L*(T2-T1)*alpha = change in length

approximate values for the coefficient of thermal expansion per degree of temperature change are as follows (for Fahrenheit and Celsius scales):

Aluminum: 13 x 10^-6 / F or 24 x 10^-6 / C

Steel: 9.6 x 10^-6 / F or 17 x 10^-6 / C

Titanium: 5.3 x 10^-6 / F or 9.5 x 10^-6 / C
*values come courtesy of Mechanics of Materials, Second Edition by Roy Craig, published by John Wiley and Sons, 2000.

These numbers are so small that you would have to have a significant change in temperature for the change in length to be appreciable.

For example: the inner circumference of the barrel of .689 bore is 2.165 inches. For an aluminum barrel, with a 25-degree F change in temp, the change in circumference would be: 2.165 x 12 x 10^-6 x 25 = 6.49 x 10^-4 inches = 0.000649 inches. That is simply not noticeable.

(Keep in mind that your average sheet of printer paper is about 0.004 inches thick and that it is extremely unlikely that all your barrels measured exactly 0.689 to begin with either -- how much will depend on their quality, but I guarantee you it will be more than the difference due to temperature effects)

[Dueydog]

Redkey
Registered User Join Date: Jan 2002
Location: North of Seattle
Posts: 165

coefficient of thermal expansion

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Materials will shrink or expand as heated or cooled. The amount of expansion or contraction is stated as the change in length over the orginal length per degrees of temperature.

Steels have a Coefficient of Linear Thermal Expansion of between 0.000009 and 0.000012 in/in per degree C

Stainless Steel has a CLTE of about 0.000017 in/in deg C

Aluminum Alloys have a CLTE of 0.000022 in/in per degree C

Ti has a CLTE of 0.0000085 in/in per degree C.

in your example of 37.7 deg C to 25.5 deg C the change in temperature is -12.2 deg C.

to help with the visualization of this...
12 inches of Steel will shrink by 0.0015 inches
12 inches of SS will shrink by 0.0024 inches
12 inches of Aluminum will shrink by 0.0032 inches
12 inches of Ti will shrink by 0.0012 inches

since the barrel cross section is a hoop we'll need to unwind it to find the effective length of it. The inner barrel diameter is 0.689 inches which means the circumference is about 2.164 inches. (length = 2*pi*r)

so the change in the length of the unwound barrel will be....
Steel = 2.16373
SS = 2.16355
Alum = 2.16342
Ti = 2.16378

reversing the calculations gives bore diameters of the following
Steel = 0.68874
SS = 0.68868
Alum = 0.68864
Ti = 0.68875

so, as you can see the Ti barrel shrinks the least. However, the difference in diameter is about 0.0001 inches.

I suspect that any differences you noticed were due to the paintballs changing size as they are much more affected by changes in temperature and humidity. The gelatin shell has a CLTE of about 0.00003 in/in per degree C. I'm not sure on paint chemisty but I'll bet that it shrinks and expands with temperature changes.

hope this answers your question


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There is a descrepency between the temps for the aluminum barrel.

I understand that paint size changes, but why would the same paint still fit the titanium barrel on the 100 day and not the aluminum one?

And then fit the aluminum in the cooler 78 degree temperature.

The titanium one fit the same on all paint on all days.

What else could cause this?

Thanks again for all the great info,
Matt

[lonelymag]

what else? the barrel might not be exactly the size specified. and how are you determining fit? placing a ball in and pushing on it, one moves more easily than the other? as tom has suggested in a few posts: balls seem to orient themselves so as only the equator touches. this being the mode of least resistance, possibly balls fit ever so differently because of the way youve placed them in the barrel.

cristobal - i looked in my basic thermo book and didnt find a chart of material properties with the thermal expansion coefficient. and i cant find my materials book, do you know of a website with that info? thanks.

[Dueydog]

what else? the barrel might not be exactly the size specified. and how are you determining fit? placing a ball in and pushing on it, one moves more easily than the other? as tom has suggested in a few posts: balls seem to orient themselves so as only the equator touches. this being the mode of least resistance, possibly balls fit ever so differently because of the way youve placed them in the barrel.

cristobal - i looked in my basic thermo book and didnt find a chart of material properties with the thermal expansion coefficient. and i cant find my materials book, do you know of a website with that info? thanks.



Some of the paint would barely push through the aluminum with a swab!! Then went right through the titanium one on the hot day.

Then all the balls went easily through all barrels in the cooler degrees, which is why I wanted to know how much the metal varies.

Is the finish that much better on the inside of a titanium barrel VS Aluminum VS Stainless?

Thanks,
Matt

[vf-xx]

www.matweb.com <- good source of Materials info.


Another point of interest is how thick the walls of the barrel are.

The metal should be expanding in all directions, but the barrel geometry 'MAY' cause an uneven expansion. I'm too tired to look up calculations right now, but the best way to test that would be to have multiple barrels with the exact same geometry, just different materials.

[Dueydog]

6 hours in 100 degree day:
Ten paintballs ALL FIT the .688 titanium boomstick nicely.
Same ten paintballs would lodge in the .689 aluminum CP barrel.
Had to use a battle swab to get them out they were so stuck.

After sitting in the house for about 24 hours @ 78 degrees:
Same ten paintballs AGAIN fit the .688 titanium boomstick nicely.
Same ten paintballs also fit the .689 stainless Freak insert nicely.
Same ten paintballs NOW fit the .689 aluminum CP barrel nicely.

I'm not concerned about exact bore and I understand paintballs fluctuate with temp and humidity.

My question is what is going on with the aluminum barrel?

As stated titanium fluctuates less, but is too small to matter.

So, what makes the titanium barrel fit paint more often than the aluminum one?

What makes the aluminum barrel change more than the titanium?

Thanks,
Matt

[Cristobal]

I'm not concerned about exact bore and I understand paintballs fluctuate with temp and humidity.

My question is what is going on with the aluminum barrel?


My guess is that the bore of the Al barrel is actually smaller than the Ti barrel. When the paint is at room temp it fits both fine -- but the "margin" of fit is bigger for the Ti barrel. When the paint expands in the heat the first barrel to see this will be the smaller bore Al barrel. I highly doubt that either barrel is expanding enough from room temp to 100F to cause the phenomena you are seeing.