|
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 . 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.〔(Manhole Cover: How Products are Made ), enotes.com.〕 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. Despite their weight and cumbersome nature, manhole covers are sometimes stolen, usually for resale as scrap, particularly when metal prices rise.〔Liu Shinan. ("Severely punish theft of manhole covers" ). ''China Daily'', December 21, 2005. Accessed December 19, 2008.〕〔Mickle, Bryn. ("Hundreds of manhole covers stolen around Genesee County" ). ''Flint Journal'', July 14, 2008. Accessed December 19, 2008.〕〔http://twcnews.com/content/news/722111/city-officials-investigate-rising-theft-of-manhole-covers--storm-grates/?ap=1&MP4〕〔http://washington.cbslocal.com/2014/03/10/15000-in-manhole-covers-stolen-in-va/〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Manhole cover」の詳細全文を読む スポンサード リンク
|