We must have had the same professor. Mine tried to boil alcohol out of a plastic beaker on a bunsen burner. What beautiful flames! Lawrence Lile cc: Sent by: pic Subject: Re: +AFs-OT+AF0-: Coefficient of Thermal Expansion microcontrolle r discussion list 09/27/01 12:54 PM Please respond to Lawrence Lile Thanks, Michael! Actually, I did pay quite a lot of attention in Physics, and never for a minute thought I would never use this stuff. I note the volume coeff. of expansion is three times the linear coeff., but the area C of E is not something most people mention. Thanks for clarifying this. Now, If I could just find a reference that actually gives the C of E for these materials I am using ... Nobody ever threw an eraser at me, although I did have a Chem professor who musta slept through Chem I, because he put a non-pyrex gallon beaker of water on the bunsen burner at the beginning of a class, saying he'd show us an experiment once it reached a boil. When it burst, it soaked all his papers and most of the front row! --Lawrence Lile ----- Original Message ----- From: "Michael Vinson" To: Sent: Thursday, September 27, 2001 10:35 AM Subject: Re: [OT]: Coefficient of Thermal Expansion > Lawrence Lile wrote, in part: > >I've got an interesting problem in thermal expansion. We have a 304 > >stainless steel shaft running through a High Density Polyethylene bearing > >block, with about 0.020" clearance. [...] > >Once found, I'm trying to figure out what to do with it. Would the inside > >of the plastic bearing contract at the: > > > >1. Coefficient of expansion rate or > > > >2. Coefficient of expansion times Pi or something like that, since it is a > >circle? > > > >Hmmm. Shouldn't have slept through Physics. > > As a former physics professor, I've seen more sleeping engineering > students than I can throw a stick (or a chalkboard eraser) at. But > now you see why your physics professor begged you to pay attention > when you were a student (and you, like all engineering students the > world over, scoffed, "I'll never need to know this stuff."). > > At any rate. To determine if a disk will fit in a circular hole > (approximate both as 2-dimensional), you need the coefficient of > area expansion. You may not find this listed for your materials, > but fortunately you don't need to, because, as a simple argument > shows (I'll spare you the physics details), the coefficient of > area expansion is 2 times the coefficient of linear expansion, which > you *will* find listed (this only applies to isotropic materials, > of course). So, you measure the shaft cross-sectional area Ao at > a reference temperature, compute the area A at your working > temperature via A = Ao(1 g Delta-T), where g is the coefficient > of area expansion and delta-T is the temperature difference. Do > the same thing for the hole, and the difference gives you the > clearance (or overlap) in area units (sq. cm, for example). Special > note: When you cool an object so that the material contracts, if > it has a hole in it, does the hole get bigger (as the material > recedes away from it) or smaller (since everything is shrinking)? > The answer is: it gets smaller. So when you cool down your assembly, > *both* the shaft and the hole are shrinking, but, evidently, the > hole is shrinking faster. Do the calculation. > > Wake up! Class is over! > > Michael Vinson > > Thank you for reading my little posting. > > > _________________________________________________________________ > Get your FREE download of MSN Explorer at http://explorer.msn.com/intl.asp > > -- > http://www.piclist.com#nomail Going offline? Don't AutoReply us! > email listserv@mitvma.mit.edu with SET PICList DIGEST in the body > > -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listserv@mitvma.mit.edu with SET PICList DIGEST in the body -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listserv@mitvma.mit.edu with SET PICList DIGEST in the body