Manhole Covers: Newsletter Challenge (08/09/05)

That if you have round manholes covers then there is no "right way round" to refit them...and more importantly, since the inside of the manhole is lipped to support the cover and thus is smaller than the cover, there is no way you can drop a circluar cover down a manhole shaft.

Square and oblong ones can be turned so that they fit through the hole and could be dropped on workmen below, quite apart from the inconvenience of trying to retrieve it and having to hold up traffic because there's a hole in the road.

I think this holds true for all straight sided polygons.

Incidently, the design also saves money as the area of a circle diameter D is less than a that of a square of sides D, where D is the nominal clearance size needed for a workman to pass through the hole, bearing in mind that the ladder is best mounted against one of the flat walls of the shaft not across the corner. Then again, most shafts are of circular cross section, I believe!

Manhole covers are round so they can't fall in. You can also spec manhole covers at - http://process-equipment.globalspec.com/SpecSearch /Suppliers/Manufacturing_Process_Equipment/Tanks_V essels/Manholes_Manhole_Covers?SrchItem=1

Most of the points in favour of a circular manhole seem to have been covered. But if manhole covers require machining (at least, I believe, the lips do), then a circular job is easier machined than other shapes.

As written in the 8/16 issue of Specs & Techs from GlobalSpec:

First, you praise her for being so observant. Then you tell her that while not all manhole covers are round, quite often they are for practical purposes: a round cover can't fall back into the hole if removed, but a square one could accidentally be turned and dropped in (could be an unpleasant task to have to retrieve it). Also, as manhole covers can be heavy, round ones are easier to move — they can just be rolled. And when putting back into place, round requires no alignment.