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From: Randy Yates on 29 Dec 2009 23:25 John Monro <johnmonro(a)optusnet.com.au> writes: > Randy Yates wrote: >> "steveu" <steveu(a)coppice.org> writes: >>> [...] >>> Any sufficiently large object is not going to fall down a small hole. I >>> think reasonableness of size is implicit in the argument. >> >> A two-foot diameter would require a lip size of about 5 inches. Is that >> unreasonably large? It doesn't seem all that unreasonable to me. > > It makes the cover twice as heavy. Well, not TWICE as heavy, is it? First, there must be some lip, so we should be comparing the difference between minimal lip and big lip. Second, it wouldn't be the same thickness as the center portion. Assuming the center is 1 foot radius, the little lip is 0.1 feet and the big lip is 0.5 feet, and the lip thickness is 1/2 the center, we'd come up with a ratio of 1.625/1.105, or 47 percent heavier. If I did my math right... Yeah, that's significantly heavier, but it's not the slam dunk Steve was making it out to be, in my estimation. Anyway, this seems ever more so to support my point that it is weight (and material and cost), not geometry, that decides this. -- Randy Yates % "How's life on earth? Digital Signal Labs % ... What is it worth?" mailto://yates(a)ieee.org % 'Mission (A World Record)', http://www.digitalsignallabs.com % *A New World Record*, ELO
From: Randy Yates on 29 Dec 2009 23:31 Randy Yates <yates(a)ieee.org> writes: > John Monro <johnmonro(a)optusnet.com.au> writes: > >> Randy Yates wrote: >>> "steveu" <steveu(a)coppice.org> writes: >>>> [...] >>>> Any sufficiently large object is not going to fall down a small hole. I >>>> think reasonableness of size is implicit in the argument. >>> >>> A two-foot diameter would require a lip size of about 5 inches. Is that >>> unreasonably large? It doesn't seem all that unreasonable to me. >> >> It makes the cover twice as heavy. > > Well, not TWICE as heavy, is it? First, there must be some lip, so we > should be comparing the difference between minimal lip and big lip. > Second, it wouldn't be the same thickness as the center portion. > > Assuming the center is 1 foot radius, the little lip is 0.1 feet and the > big lip is 0.5 feet, and the lip thickness is 1/2 the center, we'd come > up with a ratio of 1.625/1.105, or 47 percent heavier. If I did my math > right... > > Yeah, that's significantly heavier, but it's not the slam dunk Steve > was making it out to be, in my estimation. > > Anyway, this seems ever more so to support my point that it is weight > (and material and cost), not geometry, that decides this. I should say, "..., not hole-fall-through-ability"... -- Randy Yates % "Midnight, on the water... Digital Signal Labs % I saw... the ocean's daughter." mailto://yates(a)ieee.org % 'Can't Get It Out Of My Head' http://www.digitalsignallabs.com % *El Dorado*, Electric Light Orchestra
From: Jerry Avins on 29 Dec 2009 23:33 John Monro wrote: > Jerry Avins wrote: >> John Monro wrote: >>> 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. >> >> Only with an impractically large flange. Cast iron, the usual material >> of choice. is sufficiently brittle to make large flanges unwise. >> >>> 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. >> >> Square (or even rectangular) with width greater than diagonal? How? >> >>> 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. >> >> Oh. I see. That explains round. >> >>> 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. >> >> And makes it easier to move by rolling. But when the opening must >> accommodate a rectangle of a given size, a rectangular cover will >> weigh less than a round one. I believe that that matches Rune's >> electrical case. >> >> Jerry > > And of course in the electrical case you then need to forget the whole > idea of designing the cover so that it can't be dropped down the hole. I > wonder how important this consideration has been in fact? Maybe the > deciding issue for street manholes was that the parts can be finished on > a lathe, as I think you mentioned way back in the thread. A cover's not fitting through the street opening is important, but not important enough to override all other considerations. Old Franklin stoves make it clear that machine finishing is optional. > An (almost) related issue comes up concernng mine shafts. In Australia > we had gold rushes in the ninteenth century, with people streaming in > from all over the world. Apparently, while the Europeans dug rectangular > shafts the Chinese dug round ones. Local opinion is that the reason for > the round shafts was that the Chinese miners believed that rectangular > shafts allowed evil spirits to lurk in the corners. Another possible > reason for not wanting four-sided shafts is that the number four is > condidered by many Chinese to be unlucky. > > On the other hand, it has been observed that you need to remove less > dirt and rock when digging round shafts, so there may have been this > more practical reason for the Chinese to prefer them. We dig round tunnels through hard rock, but rectangular passages through material that requires pit props. Mining is hard enough work so that aesthetics probably has little influence on how it's practiced. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
From: Jerry Avins on 29 Dec 2009 23:37 Randy Yates wrote: > Jerry Avins <jya(a)ieee.org> writes: > >> Randy Yates wrote: >>> Jerry Avins <jya(a)ieee.org> writes: >>> >>>> 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. >>>> Right on both counts. There is a minor manufacturability advantage, >>>> especially for the receiving surface. >>> Doesn't a manhole cover have to have a lip? You can make a square cover >>> that can't fall into its own hole: set >>> >>> L > ((sqrt(2) - 1) / 2) * d >>> >>> where d is the inside distance and L is the lip size. >> Cast iron is too brittle to allow large lips with reasonable >> thickness. > > Well what about unreasonable thicknesses? The point is, it's not really > the "can't fall through" argument, is it? The constraint is something > else, like "too heavy" or "too expensive." OK. We could also build bridges by laying pipes along the river bottom and covering them with an earthen dam. I tried to stay with "practical". Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
From: Jerry Avins on 29 Dec 2009 23:42
Randy Yates wrote: > John Monro <johnmonro(a)optusnet.com.au> writes: > >> Randy Yates wrote: >>> "steveu" <steveu(a)coppice.org> writes: >>>> [...] >>>> Any sufficiently large object is not going to fall down a small hole. I >>>> think reasonableness of size is implicit in the argument. >>> A two-foot diameter would require a lip size of about 5 inches. Is that >>> unreasonably large? It doesn't seem all that unreasonable to me. >> It makes the cover twice as heavy. > > Well, not TWICE as heavy, is it? First, there must be some lip, so we > should be comparing the difference between minimal lip and big lip. > Second, it wouldn't be the same thickness as the center portion. > > Assuming the center is 1 foot radius, the little lip is 0.1 feet and the > big lip is 0.5 feet, and the lip thickness is 1/2 the center, we'd come > up with a ratio of 1.625/1.105, or 47 percent heavier. If I did my math > right... > > Yeah, that's significantly heavier, but it's not the slam dunk Steve > was making it out to be, in my estimation. > > Anyway, this seems ever more so to support my point that it is weight > (and material and cost), not geometry, that decides this. Assuming that radius implies roundness, any size lip is adequate. Jerry -- Engineering is the art of making what you want from things you can get. ����������������������������������������������������������������������� |