# Drive design

Here is a drawing I put together in Google Sketchup showing my idea for the module drive:

Each rod has a spring which tries to pull it in. If the brass bar shown on the right side is slid to the left, then the rod will pull in if it can. Pushing the bar back to the left would reset all the rods on that layer. The slots cut into the bar have bushings in them, which are on shafts that go vertically. Various other bits prevent the bar from moving vertically.

The bar on the left works the same way, except it is slid towards the front to activate its rods.

The funny-looking box in the lower left moves up and down, and serves to activate each bar, one after the other. If the box moves up, the bars will activate by means of springs. If the box moves down, it pushes all the bars back to their unactivated state. I think I'm going to put bearings on the bars to reduce the friction against the box. This way, the force required will be closer to the theoretical force required.

Each bar is 1/4" thick, and they are 1/2" apart. Each bar needs to slide 5/8" in order to activate or deactivate, and so each bar must do that within 1/2" of the box moving up or down, otherwise you'd get the next bar moving before the previous bar is finished moving. So unfortunately this leads to a situation where the box requires a force of 8/5 times the force required to move a bar: this is a wedge, and the equation of a wedge has mechanical advantage proportional to the length of the slope divided by the length of the short side. The slope is 0.8", making the advantage 1.6... advantage to the bar, that is. For pushing the box, this is an inverse advantage of 5/8.

The advantage to the bar over each rod is 0.7 due to the 45-degree sawtooth. Thus, the advantage of the box to each rod is 0.44, meaning that the box requires about 2.3x the force to move a rod.

Today at NextFab I used the laser cutter to cut the casing out of acrylic with wings as shown above. I also cut a piece of brass down to size and cut the slots in it. I didn't have time to use the CNC mill to cut the sawtooth in yet. That will be for next week.

Hopefully I will get some torsion springs this week to attach to the rods.