Manhole cover

Kraków manhole cover (note integral hinge).
Ancient Roman sewer grate made out of lime sandstone, 1st century AD, excavated at Vindobona (present-day Vienna)
Modern manhole cover in Rome with SPQR inscription.
Painted manhole cover in Kumamoto, 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.

Manhole covers date back at least to the era of ancient Rome, which had sewer grates made from stone.

Description

Manhole covers are often made out of cast iron, concrete or a combination of the two. This makes them inexpensive, strong, and heavy, usually weighing more than 50 kilograms (110 lb). The weight helps to keep them in place when traffic passes over them, and makes it difficult for unauthorised people not having suitable tools to remove them.

Manhole covers may also be constructed from glass-reinforced plastic or other composite material (especially in Europe, or where cover theft is of concern). Because of legislation restricting acceptable manual handling weights, Europe has seen a move towards lighter weight composite manhole cover materials, which also have the benefits of greater slip resistance and electrical insulating properties.

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 techniques.[1]

The covers usually feature "pick holes", into which a hook handle tool 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, including electromagnets.

Although the covers are too large to be easily 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. According to Remo Camerota, the author of a book on the subject titled Drainspotting, 95% of Japanese municipalities have their own cover design, often with colorful inlaid paint.[2]

Despite their weight and cumbersome nature, manhole covers are sometimes stolen, usually for resale as scrap, particularly when metal prices rise.[3][4][5][6]

Shape

Circular

The question of why manhole covers are typically round (in some countries) was made famous by Microsoft when they began asking it as a job-interview question.[7][8] 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 alternative explanations, from the tautological ("Manhole covers are round because manholes are round.")[7] to the philosophical.

Reasons for the shape include:

Other

Other manhole shapes can be found, usually squares or rectangles. Nashua, New Hampshire, may be unique in the United States 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 a manufacturer for larger triangles cannot be found.[9] Some manhole covers in Hamilton, Bermuda, are triangular, and hinged. Some triangular water-main covers also exist in San Francisco.[10]

Security and safety

Because of concerns about unauthorized access to underground spaces, manhole covers may be locked down, or even temporarily spot-welded in place. This practice has become routine in some locales, as advance preparation for official parades and similar events attracting large crowds or important people.

In urban areas, stray voltage issues have become a significant concern for utilities. In 2004, Jodie S. Lane was electrocuted after stepping on a metal manhole cover, while walking her dog in New York City.[11] As result of this and other incidents, increased attention has been focused on these hazards, including technical conferences on stray voltage detection and prevention.[12]

Interaction with race cars

Because of their aerodynamic design, some modern racing cars create enough vacuum to lift a manhole cover off its recess. During races on city streets, manhole covers must therefore be welded or locked down to prevent injury. In 1990, during the Group C World Sportscar Championship race at Circuit Gilles Villeneuve (located in a public park in Montréal, Quebec), a Brun Motorsport Porsche 962 struck a manhole cover that was lifted by the ground effect of the car he was following, a Courage C24 Porsche. This caused the trailing Porsche to catch fire, and safety issues ended the race shortly afterwards.[13]

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 900-kilogram (1,984 lb) 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 not likely to have been accurate. 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 out of hell!"[14][15]

Use for navigation

A robotics research paper in 2011 suggested that robots could examine the shapes of specific manhole covers and use them to calculate their geographic position, as a double-check on GPS data.[16]

Manufacturing process

The manufacturing process of metal manhole covers consists of six steps:

  1. Design and simulation
  2. Patterns
  3. Moulding
  4. Melting
  5. Finishing
  6. Quality control.[17]

Gallery

See also

References

  1. ↑ Manhole Cover: How Products are Made, enotes.com.
  2. ↑ "Japanese manhole covers" (website), Governing Magazine, September 29, 2011, retrieved 2016-01-09
  3. ↑ Liu Shinan. "Severely punish theft of manhole covers". China Daily, December 21, 2005. Accessed December 19, 2008.
  4. ↑ Mickle, Bryn. "Hundreds of manhole covers stolen around Genesee County". Flint Journal, July 14, 2008. Accessed December 19, 2008.
  5. ↑ http://twcnews.com/content/news/722111/city-officials-investigate-rising-theft-of-manhole-covers--storm-grates/?ap=1&MP4
  6. ↑ http://washington.cbslocal.com/2014/03/10/15000-in-manhole-covers-stolen-in-va/
  7. 1 2 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.
  8. ↑ Davis, Jim (2009-02-10). "Why Are Manhole Covers Round?". Joblossguide.com. Retrieved 2011-04-08.
  9. ↑ Brooks,David. "Nashua’s triangular covers historic and unique". Nashua Telegraph, Aug. 21, 2011. Accessed Aug. 21, 2011.
  10. ↑ "picture of triangular water-main in San Francisco". Maa.org. Retrieved 2013-12-05.
  11. ↑ Ramirez, Anthony (January 19, 2004). "East Village Woman Was Electrocuted on Street With Metal Plate, Medical Examiner Says". New York Times. Retrieved 2009-12-03.
  12. ↑ "(Homepage)". Jodie S. Lane Public Safety Foundation. Jodie S. Lane Public Safety Foundation. Retrieved 2011-06-02.
  13. ↑ "Classic Cars Race Results Database". Wspr-racing.com. Retrieved 2011-04-08.
  14. ↑ "Learning to Contain Underground Nuclear Explosions By Dr. Robert R. Brownlee - June 2002". Radiochemistry.org. Retrieved 2011-04-08.
  15. ↑ "Operation Plumbob at the Nuclear Weapon Archive". Nuclearweaponarchive.org. Retrieved 2011-04-08.
  16. ↑ Marks, Paul (2011-02-08). "I, for one, welcome our manhole-seeking overlords". New Scientist. Retrieved 2011-04-08.
  17. ↑ "6 Steps of Manhole Cover Manufacturing Process". Maria Infiniferro. September 25, 2015. Retrieved September 28, 2015.

Further reading

External links

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