Dump Truck
First Generation
This went from an idea to a project as I started developing the Men at Work diorama. The goal was to pair it with the excavator so that there's always material to remove from the work site: after it fills up the dump truck, the dump truck dumps it all back into the hole from whence it came. Now, I realize that moving aggregate materials around breaks one of my tenets on animation, but I have a one-word defense: fun.
The first thing that had to be addressed with the GHQ truck kit was the fact that it was not designed to be animated—there weren't any protruding parts that could be drilled to make pivots. I had to make pivots using bits of Gold Medal Models industrial ladders, which have holes to mount the safety cages. I cut the ladders at the points marked above left to produce pivots that I then glued into #80 holes drilled in the truck frame and dump bed, above right. A piece of brass wire completed the pivot. The gate was articulated the same way.
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The mechanism is a linear drive powered by a rack and pinion gear set. The rack turns a bell crank, which lifts the hydraulic piston under the dump bed; limit switches prevent overtravel.
First Gen Revision
The more I watched the dump truck operate, the less satisfied I became. The problem was the gate: as the body tilted, the gate would abruptly pop open and then jiggle rapidly, which ruined the realism of the action. So I devised a fix.
First, I added a tiny eyelet (1) to the bottom corner of the gate; then I fashioned fine wire (2) that engages the eyelet and is soldered to a lever (3). The lever pivots at the base (4) and is linked via a connecting rod (5) to a pin (6) in the bell crank (7).
Here it is in action:
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Second Generation
I was not happy with the revision, particularly how it looked; worse, nothing was performing satisfactorily. Finally I had to accept the fact that I needed to start over, and take a whole different approach to the animation. I chose a different model truck—Showcase Miniatures, this time—and I immediately realized this was exactly the right move. The design of this kit allowed me to make the articulations much more robust (the first gen model was always a hairsbreadth away from failing). The truck chassis has nice big pins at the dumper body pivot point, and it was a simple matter to add brass rods to create functional pivots (below, left). And I took an entirely different approach with the gate: rather than four tiny eyelets and fine wire, the gate now has a length of brass tubing bonded to it, which rotates around a brass rod that's bonded into holes drilled in the sides of the dumper body (below, right), none of which is visible from normal viewing angles.
Another problem with the first design is that the dumper body was lowed by gravity, which meant it was free to swing upward too far and thus had the potential to be damaged. This time, the piston rod is pinned to the body (below, left and right).
Starting over also meant I could plan a better means of animating the gate. Instead of an awkward kluge of visible wires and levers, the gate is now moved via a virtually invisible 0.005" stainless music wire threaded through fine tubing mounted to the side of the truck frame (below, left). The wire then passes down to the mechanism behind the front wheel (below, right).
Driven by a low-RPM motor (1, below), a large gear (2) creates all of the truck movements: a pin mounted on the gear (3) lifts the dumper body via a lever (4), linkage (5) and rod (6). The linkage resolves a major problem with the first gen mechanism: this time the rod is pulled up, instead of pushed up—enormous difference! Meanwhile, the gate's music wire runs along the truck frame (7), drops down through the base plate (8), passes through another guide tube (9) and is moved by a lever (10) with a cam follower (11); the cam (12) was cut from an identical large gear.
Building something twice always provides opportunities for improvement, and now the dumper works like a Bic pen: first time, every time.
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Cosmetically, I fixed a mistake in the Showcase Miniatures kit: I could find no reference image that showed dual tires on the lift axle; they are usually smaller single tires. Fortunately I never throw out spare parts left over from other kits, so it was simple fix. I was a bit disappointed that the kit didn't include mud flaps, so I ordered their mud flap kit. Also disappointing was the half-round relief horn on the cab, so I shaved it off and installed a spare Atlas Trainmaster diesel horn. And finally, I added working brake lights (red 0402 SMD LEDs).
Alternate Animation
After I dismantled the first gen assembly, I came up with an idea how to re-use the model truck for a different animated effect, inspired by a message board comment. This would be a truck simply rocking back and forth, causing the gate to slam repeatedly so as to knock any remaining material loose. Quite unexpectedly, it actually became one of my favorite animations. Why? Because it perfectly met two of my animation rules: it's subtle—no big splashy movement—and it's unusual, not something one would expect to see animated. (As an aside, this is also an example of an animation that would benefit from a sound effect.)
To move the truck back and forth, I made a slider (1, above) from square telescoping styrene tubing mounted to the underside of a base plate (2); the slider is linked (3) to a crank (4); the truck is then bolted to the slider with 00-90 screws that pass through slots in the base. The gate was a little trickier: its motion is irregular. To create its syncopated movements, the gate is moved by a fine wire linked to a lever (8, below) under the base plate; the other end of the lever (5, above) has a follower (6) that rides a cam (7) attached to the crank on the motor. Just getting the shape of the cam right took more time than the rest of the build combined; I must have made about five or six of them. As a finishing touch, I added a microswitch triggered by the slider to turn the brake lights on briefly as the truck approaches its direction change.
Out of necessity, I took a few shortcuts. For instance, the wheels do not turn—piles of material will block them from view. The truck has no left front wheel: its space is occupied by part of the gate linkage. And since it will be something of a background model, many details were omitted.
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Alternate Alternate Animation
As I created the scenery around the above dump truck, it occurred to me that the truck might appear to be stuck in the mud. So, bear that in mind as a possibility when developing a scene.