A manhole covers 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. 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 unauthorized people without suitable tools to remove them. Manhole covers may also be made from glass-reinforced plastic or other composite material.
A manhole cover sits on a 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. 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.
Reasons for the shape might include:
- A Round manhole cover cannot fall through its circular opening, whereas a Square manhole cover might fall in if it were inserted diagonally in the hole.
- Round tubes are the strongest and most material-efficient shape against the compression of the earth around them.
- A Circular manhole covers a given diameter has a smaller surface area than a square cover of the same width, thus less material is needed to cast the manhole cover, meaning lower cost.
- 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 the machine using a lathe.
- Circular covers do not need to be rotated to align with the manhole.
- A round manhole cover can be more easily moved by being rolled.
- A round manhole cover can be easily locked in place with a quarter turn which makes them hard to open without a special tool. Lockable covers do not have to be made as heavy, because traffic passing over them cannot lift them up by suction.
In reality, there is little danger of lids falling through the hole with any shape of manhole because small shelves, or lips, are built into the hole to help hold up the cover, effectively reducing the hole’s diameter. This wasn’t quite right as it stood. There’s a ledge because the cover has to sit on something, and one has to be pretty clumsy in the first place to let it slide diagonally. But when it happens there’s a difference between round and square and the ledge plays no big role in it. A round manhole cover cannot fall down the hole because the lip is smaller than the cover thus the cover cannot ever fall into the hole. A square or rectangular cover has the same lip that the cover sits on but it is fairly easy for a cover in the shape other than a round cover to slip sidewise and fall down the hole when installing the cover.
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Other manhole shapes can be found, usually squares or rectangles. Because of concerns about unauthorized access to underground spaces, manhole covers may be locked down, or even temporarily spot-welded in place. 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. Manhole closings are protected by a manhole cover, a flat plug designed to prevent accidental or unauthorized access to the manhole. The access openings are usually circular in shape to prevent accidental fall of the cover into the hole. Manholes are generally found in urban areas, in streets and occasionally under sidewalks