Why luke lids

Why are the hatches of the hatches round?
This question, it would seem simple, is actually fraught with an interesting engineering history. We are so used to round hatches on the streets that we rarely think about their shape. But it is not accidental, and the choice of a circle is far from a whim of designers.
1. Prevention of the fall:
The main cause of the round shape is security. Imagine that you are trying to drop a square or rectangular hatch into the mine. There is a high probability that he will get stuck, turning diagonally. The round hatch, due to its symmetrical form, will always fall strictly vertically, without stuck and not creating a danger to passers -by. This is especially important in emergency situations when time plays a decisive role.
2. Strength and ease of use:
The round shape provides the highest strength with the minimum use of the material. It evenly distributes the load, making the lid more resistant to deformation under the influence of external factors, such as passage of transport. In addition, the round cover is easier to carry and install, since there is no need to control its orientation. Just roll and get it!
3. Saving materials and simplicity of production:
The manufacture of round lids is often cheaper and simpler than square or rectangular. This is due to the absence of angles that require additional treatment and increase the risk of damage. It is easier to stamp the sheet metal in round shapes, which accelerates and reduces the reducing process of production. As a result, round hatches are an optimal solution that effectively combines safety, strength and economy. And all this - for the benefit of you and me, passers -by, who daily pass by these inconspicuous, but important elements of urban infrastructure.