Wednesday, November 24, 2010

Why are manhole covers round you ask? Here's why!

Originally meant as a psychological assessment of how one approaches a question with more than one correct answer, the problem has produced a number of alternate explanations, from the pragmatic ("Manhole covers are round because manholes are round.")[6] to the philosophical.
Reasons for the shape 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. (A Reuleaux triangle or other curve of constant width would also serve this purpose, but round covers are much easier to manufacture. The existence of a "lip" holding up the lid means that the underlying hole is smaller than the cover, so that other shapes might suffice.)
  • 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.
  • Similarly, it is easier to dig a circular hole and thus the cover is also circular.
  • 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.
  • If a cover had corners and were bent that would create a protruding point that could puncture tires.
  • Most manhole covers are made by a few large companies. A different shape would have to be custom made.
SHOOT: I would have answered that the circular shape is safest in terms of tyre traffic and also best in terms of wear and tear [on tyres and on the cover].  The cirular shape is also stronger in the same sense that engineers build curved dam walls.   It's also a great platform to stand on when Scotty decides to beam you up [or down], but only before or after rush hour.

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