Why are manhole covers round?
Manhole cover - Wikipedia, the free encyclopedia
** Manhole cover **
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KrakÃ³w manhole cover (note integral hinge).
Ancient Roman sewer grate made out of lime sandstone, 1st century AD,
excavated at Vindobona.
Modern manhole cover in Rome with SPQR inscription.
Painted manhole cover in Matsumoto, Japan.
Pick holes in manhole cover, Palo Alto, California, USA.
Triangular manhole cover from Nashua, New Hampshire atop a circular manhole
that is replacing it.
A *manhole cover* is a removable plate forming the lid over the opening of
a manhole, to prevent anyone or anything from falling in, and to keep out
unauthorized persons and material.
· 1 History
· 2 Description
· 3 Circular shape
· 4 Security and safety
· 5 Manhole covers and race cars
· 6 Propelled into space
· 7 Use for navigation
· 8 Dover Engineering Works
· 8.1 Shell Commission and the design of GATIC Cover
· 8.2 Jack Bourne â the Jack Screw Lifting Key
· 9 Gallery
· 10 See also
· 11 References
· 12 Further reading
· 13 External links
[icon] This section requires expansion. /(June 2013)/
Manhole covers date back at least to the era of ancient Rome, which had
sewer grates made from stone.
Manhole covers usually weigh more than 50 kilograms (110 lb), partly
because the weight keeps them in place when traffic passes over them, and
partly because they are often made out of cast iron, sometimes with infills
of concrete. This makes them inexpensive, strong, and heavy. A manhole
cover sits on metal base, with a smaller inset rim which fits the cover.
The base and cover are sometimes called "castings," because they are
usually made by a casting process, typically sand casting
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