Video: Tom’s friend Robert’s Kinetic Sculpture

This is a kinetic sculpture build by Tom’s very talented friend Robert Swarner. Robert came into Tom’s shop in Santa Cruz in the early 90’s wanting to learn about sewing so he could build his own climbing equipment. They’ve been friends ever since. For a brief period of time Robert sewed for Tom, but they found drinking margaritas together was more fun. For fun, Robert has been an artist, a machinist, a sky diver and a big-wall climber in Yosemite, and for a living flies helicopters.

If you can prove to Robert you understand what this thing does, we’ll send you a Friend of Tom pouch. Submit your summary in 45 words or less to: darcy@tombihn.com

Click Read More to read Robert’s explanation of the sculpture.


“This is a mobile. The rotating part is on a nut that spins down that screw
under gravity. The computer watches its position on the screw and when it
gets to the bottom, it turns on the motor, spinning the screw to return the
nut back to the top, ad infinitum.

I wanted to know if I could propel the nut that way using only its momentum
to push against. Usually if you’re moving a nut by turning a screw, you
restrain the nut against rotation. Here I’m just turning against momentum.
I also wanted to see the sinusoid motion of a rotor reacting to wind, and
the reaction of a Delta 3 hinge in a system that is slow enough to watch.

The system is unstable without the rotor bit, and the aerodynamic drag that
goes with it. When I ran it without the rotor attached, the nut accelerated
down the screw, the motor came on, and further accelerated the rotation of
the nut, and something bad would have happened if I hadn’t turned it off.
Because of aerodynamic drag, the nut decelerates to its terminal speed while
it’s descending. Even though it accelerates while the motor is turning, it
reaches a stability in its RPM during the descent. This made me really
happy we don’t live in a vacuum. Aside from being short of breath, it would
be terrifying to move things around without drag.

The hinge that I mentioned is common in tail rotor designs. The problem
with rotor systems moving on edge like a Frisbee is that the airfoil sees a
headwind half of the time, and a tailwind the other half. This is why a
Frisbee always rolls to the retreating side.

The Delta 3 hinge allows the blade to flap as this is happening, only in
this case the flapping axis is 45 degrees from the axis of the rotor. If
the airfoil flaps away from the centered position, the blade is forced to
pitch back toward that centerline, and it is self-stabilizing. You can see
the rotor reacting to gusts of wind, and sailing up and down. The hinge
helps to stabilize it back into a flat orbit.

The airfoil is a cut section of an Agusta helicopter main rotor blade.
Everything but the lead screw, the nut, and the blade section was
manufactured by me.”
— Robert

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