How precision could a steel ball be?

Steel ball, also be known as bearing ball, steel ball bearing, bearing steel ball and various of names. Steel ball is considered as the roundest product in the world. Perfect round also comes high precison, but how precison could a steel ball be?

How precision a steel ball could be?

A friend named Brad G on Quora told me that he once read a Japanese magazine says that “if a steel ball was the size of the earth, an imperfection would be no greater than the height of the Eiffel Tower”. This is indeed interesting and I do want to verify this.

There are three important specifications to define the roundness and precision of steel balls: a. variation of ball diameter; b. deviation from spherical form; c. surface roughness. Just like the name, variation of ball diameter is to check the changes of diameter from different position; deviation from spherical form is to check the deviation between the steel ball and perfect spherical shape; surface roughness is to check if the surface is smooth enough.

Steel ball for bearing

However, what should the height of imperfection be? Since the diameter of earth with Eiffel Tower would be bigger than other area, the right parameter should be variation of ball diameter. So let us start calculation.

Take our products for example, the best precision of our 2″ steel ball is G20. Which means the variation of ball diameter is no more than 0.5 μm. We can calculate the proportion is approx. 1:101600. And we know the diameter of earth is 12742.02 km, thus, the height of imperfection should be no higher than 125.4 m. And the height of Eiffel Tower is 276.1 m (without aerial). Well, the height of Eiffel tower is more than twice than the value. Let us change to G40 2″ steel balls! The variation of ball diameter of G40 steel balls is no more than 1 μm. Thus, the proportion is approx. 1:50800. Therefore, the height of this imperfection should be no higher than 250.8 m. Only 25.3 m tolerance to the height of Eiffel Tower.

Thus, if a G40 2″ steel ball was the size of the earth, any imperfection would be no greater than the height of Eiffel Tower!