Because in the case of equal perimeters, only the circle has the largest area! And if it is square, although it is usually larger than the wellhead and will not fall, it will still be deformed if the load is too large, that is, it may fall! But the round one won't, because its diameter is equal and it will still get stuck at the wellhead no matter how it changes! Because no matter how it turns, it is round and evenly stressed, and it will not fall down because of different forces on both sides; Convenient transportation, you can roll; The circle has no direction difference and is convenient to install; Save materials, the same large area, use less materials; Circular sewer openings can reduce the probability of irregular objects (bicycles, stones, etc.). ) The stolen cover falls after being stolen; Roundness can reduce the possibility of pedestrians falling down because they look at the sewer cover after being stolen. The eyes are round and the mouth of the sewer is round, which can reduce the eye fatigue and physical fatigue caused by uneven stress on the eyes due to looking at objects of other shapes, thus reducing the possibility of falling into the sewer. In short, there are many benefits.

Save materials. For the same material, the cross-sectional area of a circle is larger than that of a square. In this way, when it rains, the flow is large. The fastest flow rate. The circular hole conforms to fluid mechanics when it passes through rainwater. Can form a vortex, the fastest flow rate. Convenient round manhole cover will never fall into the well. Easy to move and roll. Easy to process. When making a mold, you can use a lathe to do rotary processing, which is fast and low cost. The intensity increases. Round things are stronger than square things. For example, chickens are round when they lay eggs. This is why eggs don't like to break. Will not fall into the well. The diagonal of a square is 1.4 1 times the side length, and the diagonal of a rectangle is also greater than the side length of either side. Only circles have the same diameter. ?