I made this to enjoy higher rpm, louder noise, and higher pitch when operating my air raid siren. :D
After I printed myself MlePh's air raid siren (THANKS!) I gave the crank a good spin and ran in to hard friction before even reaching what I'd call 'mid rpm'. Turns out the slop between the crank handle and bearing allowed 'gear1' to move too much, to where it would hit gear3, and gear3 would rub the inside of the gearbox. So I made this shim to eliminate that slop. With the shim installed, my crank handle is very snug against the bearing. Now my siren operates smoothly all the way to max rpm (the hardest this healthy middle aged man can crank it anyways... which is imho scary fast when you consider the materials and momentum in play).
Since printer tolerances vary... before printing this yourself, first measure the actual amount of slop that needs to be taken up on your assembled siren. Then add ~0.3mmm to get your desired shim thickness (I used 0.3mm to ensure I could get a tight fit against the bearing when I tighten the bolt down). If your optimal thickness is different than 2mm (thickness of the attached stl file), do some simple math and scale the z axis up or down in your slicer (desiredThickness / 0.02 = z scaling percentage).