Why are manhole covers round?”

A number of possible explanations for why manhole covers are round include:

• A round manhole cover cannot fall through its circular opening, whereas a square manhole cover may fall in if it were inserted diagonally in the hole.
• Round tubes are the strongest and most material-efficient shape against the compression of the earth around them, and so it is natural that the cover of a round tube assume a circular shape.
• The bearing surfaces of manhole frames and covers are machined to assure flatness and prevent them from becoming dislodged by traffic. Round castings are much easier to machine using a lathe.
• Circular covers do not need to be rotated to align them when covering a circular manhole.
• Human beings have a roughly circular cross-section.
• A round manhole cover can be more easily moved by being rolled.
• Tradition.
• Supply. Most manhole covers are made by a few large companies. A different shape would have to be custom made.

source: wikipedia

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