Why are manhole covers round?

The subject took on a life of its own when Microsoft began asking this question in job interviews. The inquiry was meant to test the prospective employee’s psychological make-up, poise, and ability to think on one’s feet, rather than elicit a correct answer.

A round manhole cover can’t fall through its circular opening. On round manholes, there is a “lip” holding up the cover, which means that the underlying hole is smaller than the cover. A square, rectangular, or oval manhole cover could fall in if it is inserted diagonally in the hole.

There are other reasons. Manhole covers are round because manholes are round. Manhole tubes are round because that is the shape that is the strongest against the compression of earth around it, plus round is the shape that is the most efficient in materials used.

Even more reasons. Round castings are easy to machine compared to square or rectangular castings. Manhole covers are heavy, weighing in at over 50 pounds, and round manhole covers are easy to roll. Rolling is easier on the back than carrying.

Also, round manhole covers don’t have to be aligned in any special way when replaced by workman or if accidently dislodged by traffic.

Manhole covers around the Midwest, including here in Tomah, are made by the Neenah Foundry Company in Neenah, Wisconsin, over in the Fox River Valley. Neenah Foundry has been making manhole covers since 1872.

Manhole covers go back to the time of Christ. The Romans used square sewer grates made of lime sandstone. Triangular manhole covers are found in Bermuda and a few in New Hampshire and San Francisco.

Manhole covers are heavy enough so that cars can pass over them without any problem. But very low modern racing cars create enough vacuum to lift a manhole cover off the ground.

It is the classical Bernoulli Effect of a fast air flow creating a low pressure. The higher pressure beneath the cover is enough to lift it straight up. That scenario came true in Montreal in 1990 during the World Sportscar Championship Race. A Porsche lifted a manhole cover, which struck another Porsche behind it. The trailing Porsche caught on fire and the race had to stop.

Manhole cover theft is a serious problem in India and China. When night descends, scavengers will scour the streets for manhole covers, take them up, and sell them to scrap iron dealers.  Manhole cover theft in Tomah has not been a problem. It does require a special tool, a pick, to get them out.

Concerning questions asked by Microsoft management in job interviews. Here are a few more. How do you measure 4 gallons of water using only 3 and 5 gallon jugs?  How many gas stations are there in the United States? How many golf balls can fit in a school bus?




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