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why must the sewer manhole cover be round?

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1. From the point of view of mechanics, the forces on the round manhole cover and the support point are uniform, while the force points on the square or rectangular sewer manhole cover are close to the point of force application.
2. Because the mouth of the sewer is round.
3. The round manhole cover can be moved by one person because it can roll on the ground.
4. Because the round well cover is not easy to tilt, it can better protect the safety of pedestrians and vehicles.
5. Because each diameter of the circle is equal, if the well cover is made circular, the cover will not fall into the well regardless of the angle. In other shapes, there will always be one with the longest inner diameter, such as a rectangle, whose corner line is longer than the side length. If the manhole cover is made rectangular, it is very likely to fall from the angle of the corner line.
Manhole cover performance characteristics
1. High strength: Magnesite inspection has high compressive, bending and impact strength, and toughness. After long-term use, the product will not be crushed and damaged, which can completely prevent the "urban black hole" accident.
2. Beautiful appearance: The surface pattern of the magnesite inspection manhole cover is exquisitely designed, the color is adjustable, and it can beautify the urban environment.
3. Easy to use and light weight: The product weighs only about one-third of cast iron, which is easy to transport, install and repair, which greatly reduces labor intensity.
4. Anti-theft: Magnesite inspection manhole cover has no recovery value and is natural anti-theft; according to customer needs, a lock structure is provided to prevent theft of property in the well.
5. Strong weather resistance: Magnesite inspection manhole cover can be used normally in the environment of -50 ℃ ~ + 300 ℃ through scientific formula, advanced technology and perfect technical equipment.
6, acid and alkali resistance, corrosion resistance, wear resistance, rolling resistance of vehicles, long service life.
Reference source: Baidu Encyclopedia-Manhole cover
The hole under the lid is round because the cylindrical shape can withstand the pressure of the surrounding land most. Moreover, the sewer manholes meant that there was enough room for one person to pass, and the cross section of a person climbing down the ladder was basically round. So the circle naturally becomes the shape of the manhole. Square lids are also seen on some square openings. The lid is larger than the entrance and has bars around it. Usually the cover is metallic and very heavy. We can imagine a two-foot-wide square opening with a rung from 1 to 1.5 inches wide. In order for the manhole cover to fall in, it is necessary to lift one end and then rotate it by 30 degrees so that it is not hindered by the rungs. Then the manhole cover is at an angle of 45 degrees to the horizon. At this time, the shifted center of gravity is enough to let the manhole cover fall. The round ones have no such concerns.
From an engineering perspective, the shape of the manhole cover depends entirely on the shape of the opening it covers.
First, the manhole is located on the road, and people who walk every day come and go. When designing, we must pay attention to the safety of pedestrians, and the child cannot fall into the manhole. If it is designed as a triangle or a square, although Gaier is a little bigger than Sakai Iguchi, it may still fall. And if it is circular, because the diameter of the circle is equal, as long as the lid is a little larger, it will not fall.
Secondly, since manholes sometimes need to be manually combed or wired, etc., the area of ??manholes is required to be as large as possible. In these figures, the circle has the largest area when their perimeters are equal. At the same time, the round shape conforms to our body shape, which is convenient for staff to enter and exit.
Third, the sides of triangles or squares are made up of straight line segments, and the corners formed are sharp, which can easily cause injury if touched during repairs. The circular edge is an arc, so this problem does not occur.
Fourth, the reason why the cover of the sewer is round is the structural force requirement:
To withstand loads from uncertain directions. Imagine that if the cover of the sewer is square or other shapes with edges and corners, when the impact load of the wheel acts on it, the edges and corners are easily damaged. The reason why the sewer inspection well is made circular is also because of the consideration of force-the arched structure can effectively resist earth pressure
  However, since the area of ??a circle is the largest in a figure with the same perimeter, the most material is used. Therefore, from the economic point of view, some places have been designed to use manhole covers of other shapes.
 This question is interesting. I think the well cover is designed to be round for convenience. You think, if the manhole cover is square, when you cover the wellhead, you must align the four corners of the well cover with the four corners of the wellhead. The manhole cover is so heavy. Isn't it troublesome to cover the manhole cover? It can be covered in any shape. It doesn't need to be aligned, which is much more convenient.
From a geometric point of view, the round manhole cover is to prevent the manhole cover from falling into the well, because the circle has the largest chord, that is, the diameter; and other manhole covers may be destroyed into the well, so except for special cases, general manhole covers They are all round!
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