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WHY THE STEEL
BEARINGS WEAR AND THE PLASTIC BALL DOES NOT WEAR?
The surface area of a sphere
is equal to four times pi times the square of the radius.
The surface area of a
circle is equal to pi times the square of the radius.
The ball in the Explorer
is about 1.5 inches in diameter, so the radius is about .75 inches.
The surface area is about
4 x .56 x 3.14 or 7 square inches.
The bearings in the
Explorer are molded in place and do not turn.
So, the ball always rubs
on the same place on each bearing – in other words, the area of wear on each
bearing is the one place where the ball touches it.
The surface area of each
bearing where the ball touches when the bearing is new is a 'pinpoint' - let's
say .001 inches in diameter.
The surface area of a
circle .001 inch diameter is equal to pi times radius squared or 3.14 x .0005 x
.0005 or .0000008 square inches per bearing.
At this time the wear
surface of the ball compared to that of three bearings is 7 divided by
(.0000008 x 3) or about 3 million to 1
Yes, the ball also wears
but the wear is spread out over a very much larger area and is not easily seen.
(The spots on the
bearings are not actually flat, but concave - the same curvature as that of the
surface of the ball)
When you begin to feel
the effect of flat spots on the bearings, the area on each bearing where the
ball touches is about 1/32 inch in diameter (.03 inch). The total wear area of
the three bearings is about .002 square inches – not very big, but the ratio of
the ball to the bearings has dropped from 3 million to 1 down to about 3
thousand to 1. Now the ball drags and the surface of the ball has lost some of
the shine. Yes, the ball does wear.
In extreme cases, the
worn area on each bearing is 1/16 inch (.06). The ball to bearings ratio drops
to about 800 to 1. Now, the ball drags a lot and may be so badly worn that even
new bearings are not enough. The ball may now look dull and have many fine
scratches. The ball needs to be replaced or to have its surface restored.
How to prevent this,
or at least slow down the wear.
First, keep the ball and
bearings (and your hands) clean. Look at my listing for the trackball
maintenance kit.
Second, install bearings
that are very smooth and very durable.
On the listing page,
click on the link to my detail page about Bearing Materials
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