open the valve over a 90 degree arc. It only took a few
tests to determine that this would not be an easy solution.
Ensuring that the shock cord stretched evenly throughout
the arc so that it would pull evenly throughout
the opening was very challenging, and we were
unable to achieve reliable valve opening even at
zero pressure. Once again, we needed to amend
Bruce Springsteel and the E
Perhaps it was the provenance of our giant
springs that had us thinking trucks, but one day
we had a flash of inspiration when thinking about
a truck trundling down the highway. We thought
about how leaf springs deal with large forces, and
can provide a lot of force without a ton of deflection. Trusty
McMaster-Carr offered a decent selection of strips of spring
steel, and we thought of creating our own torsion spring
by layering it. Calculating the force from deflecting
a thin plate of spring steel is actually quite tricky, so
to refine our spring steel design we engaged in our
preferred design process of trial and error.
We tried out several thicknesses of spring steel
and found that the 0.050” thickness strips were
in the goldilocks zone of being stiff enough to
promise a lot of force upon deflection, and flexible
enough to allow us to create the arc we needed.
Our custom torsion spring — which we referred to
as Bruce Springsteel — was fixed at the back end
to the center back cannon mount and on the front
end to the tip of the valve lever arm. We slotted the
spring steel with a pneumatic disc cutter at the end
that connected to the lever arm because it was the
easiest way to ensure that each layer of the spring
could line up properly (since the arc of the inner layer
actually ended up being significantly shorter than the
arc of the outer layer).
After cutting countless layers of spring steel for
the various experimental incarnations of the valve
actuation spring, we can tell you with confidence
that spring steel is a real pain to work with. We killed
many drill bits making the mounting holes. Carbide
tipped masonry bits actually proved very effective in
drilling the holes in the spring steel, but just a few
holes would completely destroy the bit.
Each time we refined the geometry or wanted
to try a different number of layers, we had to test
the cannon to ensure that we were, in fact, making
an improvement. To test the cannon, we needed
a working trigger that would allow us to lock the
valve closed when we wanted and pop it open on
command. That was our second big design challenge.
With our torsion spring under torsion with the
LATE NIGHT TESTING.
ELEVENTH HOUR REPAIR TO A LEAKY TANK.
72 SERVO 09/10.2018