Manhole cover

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Painted manhole cover in Matsumoto, Japan.
Manhole cover in Rome with SPQR inscription.

A manhole cover is a removable plate forming the lid over the opening of a manhole, to prevent anyone from falling in and to keep unauthorized persons out.

Contents

[edit] Description

Ancient Roman sewer grate made out of lime sandstone, 1st century CE, excavated at Vindobona.

Since the era of ancient Rome, sewer grates made from stone have been used to keep people from falling into the sewage and to catch anything large that might otherwise fall in.

Pick holes in manhole cover, Palo Alto, California, USA.
Kraków manhole cover (note integral hinge).
Manhole cover, USA 1912

Manhole covers usually weigh more than 100 pounds (roughly 50 kg), 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 and strong but 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 made by a casting process.[1]

They usually feature "pick holes", in which a hook handle is inserted to lift them. Pick holes can be concealed for a more watertight lid, or can allow light to shine through. A manhole pick or hook is typically used to lift them, though other tools can be used as well.

Manhole covers are generally made using sand casting techniques.[1] The United States is widely recognized as the world leader in the manufacture of manhole covers and construction castings. In recent years, India has become a major player in the industry because of cheaper, lower-grade iron and lax labor and safety laws.[2] These practices have driven many manufacturers in other countries out of business.[3] Due to extremely low wages and relative lack of regulations, Indian castings have overcome even the high cost of shipping such heavy objects.

Although the covers are too large to be collectible, their ubiquity and the many patterns and descriptions printed on them has led some people to collect pictures of covers from around the world.

Despite their weight and cumbersome nature, manhole covers are sometimes stolen, usually for resale as scrap, particularly when metal prices rise.[4][5]

[edit] Circular shape

The question of why manhole covers are typically round, at least in the U.S., was made famous by Microsoft when they began asking it as a job-interview question.[6] Originally meant as a psychological assessment of how one approaches a question with more than one "right" 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, where as a square manhole cover may fall in if it were inserted diagonally in the aperture (A Reuleaux triangle or other curve of constant width would also serve this purpose, but round covers are much easier to manufacture.)
  • 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 horizontal boring mills.
  • 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.
  • It's easier to dig a circular hole.
  • Tradition
  • Supply. Most manhole covers are made by a few large companies. A different shape would have to be custom made.

Other manhole shapes can be found, usually squares or rectangles. Nashua, New Hampshire may be unique in the U.S. for having triangular manhole covers that point in the direction of the underlying flow. The city is phasing out the triangles, which were made by a local foundry, because they are not large enough to meet modern safety standards, and larger triangles cannot be found.[7]

[edit] Manhole covers and race cars

Modern racing cars create so much vacuum due to their aerodynamics that they can lift a manhole cover off the ground. During races on city streets, the manhole covers must therefore be welded down to prevent injury. In 1990, during the Group C World Sportscar Championship race in Montreal, racer Jésus Pareja's car struck a manhole cover that was lifted by the ground effect of the car he was following, causing his car to catch fire.[8]

[edit] Propelled into space

According to urban legend, a manhole cover was accidentally launched from its shaft during an underground nuclear test in the 1950s, at great enough speed to achieve escape velocity. The myth is based on a real incident during the Operation Plumbbob nuclear tests, where a heavy (900 kg) steel plate cap was blasted off the test shaft at an unknown velocity, and appears as a blur on a single frame of film of the test; it was never recovered. A calculation before the event gave a predicted speed of six times Earth escape velocity, but the calculation is unlikely to have been accurate and they did not believe that it would leave the Earth in reality. After the event, Dr. Robert R. Brownlee described the best estimate of the cover's speed from the photographic evidence as "going like a bat!!"[9] [10]

This incident was used as part of the technical justification for the Orion project.

[edit] Gallery

Trondheim manhole cover, Trondheim, Norway

Prague

German standard manhole cover with infills of concrete in Berlin.

Lithuanian SSR standard manhole cover of the city water main in Vilnius.

[edit] See also

[edit] Notes

  1. ^ a b Manhole Cover: How Products are Made, enotes.com.
  2. ^ Timmons, Heather, and J. Adam Huggins. "New York Manhole Covers, Forged Barefoot in India". The New York Times, November 26, 2007. Accessed December 19, 2008.
  3. ^ "Made in India", Little India. Accessed December 19, 2008.
  4. ^ Liu Shinan. "Severely punish theft of manhole covers". China Daily, December 21, 2005. Accessed December 19, 2008.
  5. ^ Mickle, Bryn. "Hundreds of manhole covers stolen around Genesee County". Flint Journal, July 14, 2008. Accessed December 19, 2008.
  6. ^ a b Poundstone, William (2003). How Would You Move Mount Fuji? Microsoft's Cult of the Puzzle – How the World's Smartest Company Selects the Most Creative Thinkers. Little, Brown. ISBN 0-316-91916-0. 
  7. ^ Shalhoup, Dean."Nashua’s triangular past up for auction". Nashua Telegraph, May 5, 2007.Accessed December 19, 2008.
  8. ^ Classic Cars Race Results Database
  9. ^ Learning to Contain Underground Nuclear Explosions By Dr. Robert R. Brownlee - June 2002
  10. ^ Operation Plumbob at the Nuclear Weapon Archive

[edit] References

[edit] External links

[edit] Collections of photos

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