machine shop theory.

[colbystephens]

so i've been thinking about expansion of metal under heat, and i came up with a question. here's the scenario: you're creating a force fit with a shaft and a clearance hole, and you heat up the object, and freeze the shaft to do this. pretty basic. so clearly when you heat up the object with the clearance hole to make it bigger, why does the metal expand unilaterally, and not bilaterally causing the hole to shrink as the entire shape of the object expands?

 

[Leper]

I have wondered that myself.

 

[spearchucker]

Take a metal ring and cut it so that you have coincentric circles that are one molecule thick. Take the outermost ring and cut it and role it out flat so it looks like a bar. When you heat it up the molecules want to move away from each other and the only way for that to happen is if the bar gets longer. Just imagine standing in a line shoulder to shoulder and then everyone stretches their arms out so they're now finger tip to finger tip. Now if you take that metal bar and put it back into the shape of a circle it will have a larger diameter than what you started with. Do the same for the next coincentric ring until and you can see what will eventually will happen. Hope that makes sense.

 

[theperfectgarage]

I spend enough time wondering when things DONT work, When things do work, I stop thinking.You should prob. do some upgrade to your truck or take your lovely avatar mate to bachlor for the weekend. This will help stop your thinking. But seriously, it seems to me that if heat a chunk of steel with a hole in it, the steel will expand ( leaving the hole now larger ). But I am a simple man.

 

[southernspeed]

Take a metal ring and cut it so that you have coincentric circles that are one molecule thick. Take the outermost ring and cut it and role it out flat so it looks like a bar. When you heat it up the molecules want to move away from each other and the only way for that to happen is if the bar gets longer. Just imagine standing in a line shoulder to shoulder and then everyone stretches their arms out so they're now finger tip to finger tip. Now if you take that metal bar and put it back into the shape of a circle it will have a larger diameter than what you started with. Do the same for the next coincentric ring until and you can see what will eventually will happen. Hope that makes sense.

I concur....good analogy!

 

[colbystephens]

yeah, that makes sense. so essentially, the ring is not expanding radially out from the center, but rather it is expanding in circumference. thus, the added "length" of the circle forces the circumference to increase, thereby increasing the dia. of the hole. good info.

 

[colbystephens]

I spend enough time wondering when things DONT work, When things do work, I stop thinking.You should prob. do some upgrade to your truck or take your lovely avatar mate to bachlor for the weekend. This will help stop your thinking. But seriously, it seems to me that if heat a chunk of steel with a hole in it, the steel will expand ( leaving the hole now larger ). But I am a simple man.

she is pretty cute, huh? she happens to like me thinking tho. by the way, have you ever done ostrich racing at the ostrich farms between tucson and phoenix? when i lived in phoenix, i always wanted to do that. now i'd like to take my avatar mate to do that. very original.

 

[tiny2085]

If you have a solid bar and you heat it, it will expand from a center point outward, take that center point out and it will still expand from the center point.

Does that make sense to anyone but me ?

 

[theperfectgarage]

If you have a solid bar and you heat it, it will expand from a center point outward, take that center point out and it will still expand from the center point.

Does that make sense to anyone but me ?

That works for me.

 

[colbystephens]

i understand now the answer to my original question- so i see what you're getting at, tiny2085. however, using this theory, I posed the argument to your statement and say that the reason a rod would expand outward is because all of it's atoms are pushing away from eachother uniformly in a spherical manner, then take the center out of the rod and it would seem it could allow a new area into which it could expand, thereby shrinking the hole. thus, i came up with my original question. i think the explanation spearchucker used is probably more accurate and complete - leaving little room for argument.