The extended nut of the cover is made into a circular shape for ease of use. In machines, the area where the extended nut of the cover is installed is sometimes not spacious enough, and the space for turning the nut with a wrench is also quite narrow. The circular extended nut of the cover only requires turning the wrench 60 degrees at a time to gradually tighten it, whereas the corner extended nut requires 90 degrees each time. This means that the circular shape requires less space for tightening, but the octagonal extended nut, due to its smaller contact area with the wrench, tends to slip easily and is rarely used. Therefore, the circular extended nut is more convenient to use. As for the wrench, the handle and the jaw of a regular wrench can form a 30-degree angle. When the space for installing the extended nut is extremely narrow and the wrench cannot move freely, the six-sided extended nut only needs to be turned once to tighten it, then flipped and turned again. By repeating this process a few times, the extended nut can be securely tightened.