# If a steel ball (ball bearing) was the size of the earth, would any imperfection be no greater than the height of the Eiffel Tower?

If a steel ball (ball bearing) was the size of the earth, would any imperfection be no greater than the height of the Eiffel Tower? That is a very good question. We believe that steel ball is the roundest item in the world, but how round is it? Let us verify this:

Since it says the height of imperfection, I think it should be surface roughness in the technical specification of bearing balls. Take our product for example: The best precision grade of our 2" steel ball bearing is G20. Which means surface roughness is no more than 0.032 μm. We can calculate the proportion is approx. 1:1587500. And we know the diameter of earth is 12742.02 km, so our calculation shows the height of imperfection should be no higher than 8.03 m. And the height of Eiffel Tower is 276.1 m (without aerial). Well, it seems something is wrong.☺

If the imperfection means the tolerance of roundness (Variation of ball diameter and Deviation from spherical form). The requirement of G20 is 0.5 μm, the proportion is approx. 1:101600. Then the height of imperfection should be no higher than 125.4 m. We can see it is closer, but not enough.

Change to G40 2" steel ball bearing, the tolerance of roundness will be 1 μm and the proportion is approx. 1:50800. Then we know the height is 250.8 m, only 25.3 m tolerance to the height of Eiffel Tower.

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