Our experimental activity will be to attempt to produce a perfectly
spherical ball using Alloy 158, a low melting point alloy used in the
manufacture of eyeglasses. Due to its lower melting point and its lower
percentage of lead, this alloy is much safer than the standard solder that
we had originally planned to use.
One method of producing lead shot is by dropping molten lead from a tower
and allowing it to freeze before it hits the ground (Runnels). The molten
lead's surface tension pulls it into a sphere, but air drag during its fall
deforms it into a slightly-teardrop shape. The ball bearing industry
generally produces their balls by grinding and polishing a lump of metal
until its sphericity falls within required specifications (Peters). In a
free-fall environment such as in NASA's KC-135A training aircraft, a ball
produced by the drop method should freeze perfectly spherical assuming it
does not touch anything before it hardens.
To dispense alloy 158 we will use a metal syringe wrapped in nichrome wire
with sufficient voltage applied through a Variac to keep the alloy's
temperature a few degrees above its melting point of 158 F. This will
ensure that the alloy cools and freezes quickly enough to prevent it from
touching the sides of the container while it is still melted. Additionally,
keeping the temperature as low as possible, 159-165 F will lessen the
hazards involved with molten metal.
Ball size will be from 1/8" to 3/8" in diameter. We will narrow this down
to 2-3 specific sizes that will be selected during the testing phase since
we will be able to experimentally determine the freeze times of various
sized balls in the lab once our equipment is built and on-site. Preliminary
calculations place the freeze time for a 3/8" ball at about 7 seconds if
dispensed at 165°F.
We will produce a set of control balls in the lab using the tower method and
a set of reduced gravity balls during the flights. We will then use two
methods to compare their sphericity: by measuring both sets under a
microscope having a calibrated scale in the eyepiece, and by measuring them
in three dimensions using micro-calipers.
Our team's objectives in performing this experiment are to characterize the
improvements in sphericity of metallic balls created by cooling in zero
gravity over those produced the same way on earth. Another is to explore a
practical way to manufacture highly spherical metallic balls. In the
future, this may prove the best way to produce precision ball bearings that
could last indefinitely.
After our equipment is built and on-site, we will manufacture a set of
control balls in the lab using the tower method described above. From this
experiment, we expect to produce a set of slightly teardrop shaped balls due
to the air resistance the ball will experience during its drop.
Our expectation is that in the Microgravity environment of the KC-135, we
will produce more spherical balls than were possible using the drop method
on earth. This will be confirmed in the lab using a microscope with a
calibrated scale in the eyepiece and measuring with micro calipers in three
We hypothesize that the largest cause of imperfections in spherical balls
formed using the drop method are due to air drag from movement through the
air. In zero gravity, we can eliminate this effect to test this, so that
balls will be formed with surface tension as the only significant force on
the ball surface.