I spent one summer working on cable TV plant up on poles and also down in manholes.

Workers don't roll manhole covers. Workers don't lift 300 lbs. We used a manhole cover tool that DRAGS the cover off the hole and drags it back.

And the cover is round because the hole is round!

Bob

 

Well, as Roff noted, if you are allowed free reign with the width of the lip that the cover sits on when in place, then you can do it with many shapes. If you want to maximise clearance through the hole and use the least material to make the cover, then I don't think any shape but a circle or a regular polygon which approximates a circle fairly closely can be guaranteed to never fall through its own hole.

This is my line of reasoning--and please shoot any stupid points down. I'm just thinking about this because it's fun (I'm a sad, sad man):

It seems to me that a circle is the only shape which has equal maximum and minimum lines of bisection. So for any non-circle, there is a way that you can rotate it so that the minimum width of the cover passes through the maximum width of the hole.

This is using ideal shapes of two-dimensional planes. The thickness of the cover and the width of the lip will both matter. If you are allowed to make the lip width greater than the difference between the shape's maximum and minimum widths, you can make other shapes work. But then you increase the amount of material you need and reduce the room for access through the hole.

You could also increase the thickness of the cover to a greater fraction of the shape's size, but then you're also wasting material.

I think that a circle also uses the least material overall (as Robert-Jan noted) and as a couple people have also said, the holes are round.

I'd like to see that picture too.


Torben

 

I did a little research. It turns out that a Rouleaux triangle will also work. I had never heard the term.

__________________
Ron (aka Rube)

 

I suppose you're right! It kind of hurt my brain trying to think of a non-circular shape that wouldn't, then hurt it again sorting out why the non-circular would still fall through!

I better brush-up on my spatial reasoning skills. I'm hoping to join the army here soon, and part of the entry test is a portion I guess designed to gauge some aspect of recruits' aptitudes. It's multiple choice, but it involves looking at pictures of shapes and determining what goes where, e.g. choosing the right two dimensional fold-out that will fold up into a given three dimensional figure. Not the sort of thing you'd find on your average job application, I don't think!

 

Well now I don't know what to think! Round triangles?!

 

OK, that's pretty neat. After work today I'm going to mess around with that idea.

Thanks for the link!


Torben

[Edit: Got a break and did a little more reading and doodling--turns out that many regular Rouleaux "polygon" (puffy polygons) with odd numbers of faces have this property. Live and learn. I think this is pretty cool. Thanks again for the link!]

 

That is how the Wankel rotary engines are designed. But they are not very fuel efficient in the RX-7's.

Sounds like you guys are coming up with the other 9 answers I gave as well lol. The two last I gave were : preference and "The first guy made them round, so the second and so on made them round". I guess you could say heritage.

 

Wikipedia has a nice discussion of manhole covers, including some reasons they are round.

__________________
Ron (aka Rube)

 

Perhaps a circle is just a Rouleaux polygon with (2*infinity-1) sides.

__________________
Ron (aka Rube)

 

Thats not necessarily true. One of my bosses enjoys a six figure salary, has about .5 million dollars in company stock at reduced price, travels to europe and japan at least a dozen times out of the year and has a nice pension/retirement bonus. He spends half the time in meetings, enjoys 5 weeks plus of paid vacation and enjoys calling the plays. yes, he's been there for 25 years or so at Lexmark, but he seems happy and content. I couldn't see what more a man could want.

slow and steady wins the race. cheney is a great example. the guy will retire filthy rich. he also has deep political connections within the GOP party, so he gets to make the big decisions and noone messes with him. same with mccain, he started off as a foot soldier for reangan look at him now, one step away from being president of the US. of course, these guys PUT IN THEIR TIME! we're talking a minimum of 20 years service.

 

Well, your boss sounds like he's in a nice position, but I wouldn't say that spending half my time in meetings is a good thing, and 6 figures is anything over $100K, not really a big deal.

__________________
Ron (aka Rube)

 

Why doesn't it surprise me that you would admire Cheney?
Join the Navy. You'll live longer.

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--- The days of the digital watch are numbered. ---

 

I was going to say the airforce. but i'd avoid serving altogether. i'd refer to design military hardware instead.

 

Airforce would be OK as a COM/RADAR tech, though you tend to lose your hearing. Hank just doesn't strike me as someone who would follow an order to kill without question. But maybe I'm wrong.

__________________
--- The days of the digital watch are numbered. ---

 

That'd be my preference, and the Navy tends to have great bands, but the local Artillery needs a trombone player!