Trolley Modeling in N Scale

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Giant Metronome ~ Sunday, July 12, 2015

This was the first year I was actually responsible for designing and building (as opposed to just re-assembly) of our team's props. It probably would have made sense to stick with basic backdrop-type shapes for my first attempt at this, but that would have been dull. To go with our musical-themed program, we decided to built kettle-drums, which you have already seen (and which were challenging, but basically an exercise in 3D geometry), and a giant working metronome. This involved some mechanical engineering, which being employed in the software field, I have no business messing around in, but the idea was too cool not to try...

Actually, just making the large frame to support the metronome, which we decided to make a kind of bay-window shape to give the skaters an enclosed place to do a quick costume change, was a bit of a challenge. PVC pipe is ideal for ice skating props in a lot of ways, particularly in that it's light and cheap; but it turned out to be a bit too flexible (maybe I should have used 1" instead of 3/4") and the first version of these bay-window frameworks flopped all over the ice in the first practice where we tried to use them. A lifetime interest in truss bridges paid off here, though, and with some additional interior members and diagonal reinforcements, the frames became much more stable.

I had several ideas about how to make the giant working metronome swing. My first idea was to just put a weight on the bottom of the swinging beam and let it swing naturally as a pendulum, but even with a fairly good bearing (recycled from an old bike wheel) it slowed down too rapidly from friction to make it through a whole show. I thought about giving the beam an extra push on each cycle with a solenoid (maybe one on each side), but my Dad suggested a design that looked more promising, which is easier to show than describe:

One complexity is that, since I didn't want the mechanism to show, I put it on the inside of the prop and used the pivot to drive a larger, visible metronome on the outside.

In the end, it all came together and worked flawlessly for the duration of the six-minute show.

Here's a few more technical details in case anyone wants to try something like this:

  • I used a motor with built-in reduction gearing. They are easy to find on Amazon.
  • For the pivot I used a very long bolt, and secured the pendulums on both sides with lock nuts.
  • For the pivot bearing I used scooter/skateboard bearings. (I chose a diameter of bolt to fit).
  • The visible pendulum beam is made out of spray-painted yardsticks, which you can get pretty cheap at any hardware store, because they will probably have the name of the store on them. This is convenience source of lightweight wood that I will keep in mind the next time I'm working on benchword for a layout.

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