❓ why
Why are manhole covers round?
engineeringdesign
Quick Answer
A circle cannot fall through a slightly smaller circular hole, and round covers are easy to roll and need no alignment.
The Full Story
A circular cover has the same width in every direction, so it cannot drop through its own opening however it is turned. A square cover, by contrast, can slip through diagonally. Round covers can also be rolled rather than lifted, which matters because cast-iron covers commonly weigh 45–135 kg. They can be dropped back in any orientation, and round shafts are easy to dig and strong against the pressure of surrounding earth.
Key Facts
1.Cast-iron manhole covers often weigh more than 45 kg (100 lb).
2.Reuleaux triangles also have constant width and could work as covers too.
3.Some cities, including parts of San Francisco, have used triangular or square covers with hinges.
YouTube Angle
Suggested video title for this topic:
"Why Every Manhole Cover Is Round"