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From: Jerry Avins on 29 Dec 2009 16:23 Rune Allnor wrote: > On 29 Des, 21:39, Jerry Avins <j...(a)ieee.org> wrote: >> Muzaffer Kal wrote: >>> On Tue, 29 Dec 2009 14:37:29 -0500, Jerry Avins <j...(a)ieee.org> wrote: >>>> (I repeat: why are manhole covers round?) >>> Doesn't that question assume all manhole covers are round? There are >>> plenty of mhc which are square or rectangle (and some are hexagonal, >>> star shaped etc.) >> Yes. What disadvantage do those have? Are they often found in street >> accesses? Why [not]? > > Around here manholes down to water drains are round, while > manholes to underground electrical cables are rectangular. The rectangular ones are needed for access to and by certain equipment. Round ones never need to be retrieved after having been dropped into the hole. Elliptical manholes are used in pressure vessels because, while they have no stress-raising angles, they can be inserted so that pressure holds them more firmly in place. Terra-cotta qanat liner sections are elliptical for the same reason. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
From: Muzaffer Kal on 29 Dec 2009 18:25 On Tue, 29 Dec 2009 15:39:33 -0500, Jerry Avins <jya(a)ieee.org> wrote: >Muzaffer Kal wrote: >> On Tue, 29 Dec 2009 14:37:29 -0500, Jerry Avins <jya(a)ieee.org> wrote: >> >>> (I repeat: why are manhole covers round?) >> >> Doesn't that question assume all manhole covers are round? There are >> plenty of mhc which are square or rectangle (and some are hexagonal, >> star shaped etc.) > >Yes. What disadvantage do those have? Are they often found in street >accesses? Why [not]? I think your main reason would be that they can't fall in their own hole no matter how hard one tries. Another reason is that they're much easier to roll than the pointy ones which usually need to be lifted to be carried. -- Muzaffer Kal DSPIA INC. ASIC/FPGA Design Services http://www.dspia.com
From: glen herrmannsfeldt on 29 Dec 2009 18:34 Tim Wescott <tim(a)seemywebsite.com> wrote: (snip) > Because they're easier to turn out on a lathe, of course! That was the reason we were told we needed to learn about Bessel functions. Objects made on a lathe have cylindrical symmetry. -- glen
From: John Monro on 29 Dec 2009 19:12 Muzaffer Kal wrote: > On Tue, 29 Dec 2009 15:39:33 -0500, Jerry Avins <jya(a)ieee.org> wrote: > >> Muzaffer Kal wrote: >>> On Tue, 29 Dec 2009 14:37:29 -0500, Jerry Avins <jya(a)ieee.org> wrote: >>> >>>> (I repeat: why are manhole covers round?) >>> Doesn't that question assume all manhole covers are round? There are >>> plenty of mhc which are square or rectangle (and some are hexagonal, >>> star shaped etc.) >> Yes. What disadvantage do those have? Are they often found in street >> accesses? Why [not]? > > I think your main reason would be that they can't fall in their own > hole no matter how hard one tries. Another reason is that they're much > easier to roll than the pointy ones which usually need to be lifted to > be carried. It is a popular misconception that a round manhole cover is the only shape that will not fall in. In fact you can design a regular polygon cover with any number of sides that will not fall in. For example, a square cover can be used and it will not fall in if you make its width more than the diagonal of the hole. The problem is that the cover will then have an area that is more than twice that of the hole opening and, more importantly, will weigh twice as much. An obvious first step would be to trim off the corners to make an octogon, and if we repeat this process a sufficient number of times we end up with a circle. The advantage of the circular cover is that it only has to be slightly bigger than the hole opening. The circular shape minimises the area and weight of the cover. Regards, John
From: Randy Yates on 29 Dec 2009 19:37
Jerry Avins <jya(a)ieee.org> writes: > Tim Wescott wrote: >> On Tue, 29 Dec 2009 14:37:29 -0500, Jerry Avins wrote: >> >>> Randy Yates wrote: >>>> Jerry Avins <jya(a)ieee.org> writes: >>>> >>>>> Randy Yates wrote: >>>>>> Rune Allnor <allnor(a)tele.ntnu.no> writes: >>>>>>> [...] "He who thinks his education has finished is not >>>>>>> educated. He is finished." >>>>>> I have found that education exposes one's own ignorance. >>>>> Isn't that its most important purpose? >>>> Good question. I think most people hope it prepares them for a career. >>>> What I was trying to say is that you don't know how ignorant you are >>>> until you get illuminated. >>> Exactly. What we wrongly believe we know hurts a lot. When we perceive >>> our ignorance, we can be careful or use the library. >>> >>>> I know I still want to continue to study (e.g., some more math) but it >>>> comes down to time and money. If I won the lottery I'd probably become >>>> a permanent student! >>> If you don't study your surroundings as you walk, you are in danger of >>> stepping into an open manhole. (I repeat: why are manhole covers round?) >> >> Because they're easier to turn out on a lathe, of course! > > An important secondary reason. To save material? A round cover is the least amount of material for a given minimum radius. -- Randy Yates % "Rollin' and riding and slippin' and Digital Signal Labs % sliding, it's magic." mailto://yates(a)ieee.org % http://www.digitalsignallabs.com % 'Living' Thing', *A New World Record*, ELO |