I've been experimenting with 3D printed mechanical clocks, and I really want to print one that is fully assembled. Just take it out of the machine, dust it off, hang some weights, and have it work. But to make an efficient clock, you need to minimize friction - how hard is this? To find out, I've measured the friction of a plain bearing made in three of the materials that Shapeways offers.
Here is my experimental setup:
The horizontal beam is attached to a 5 mm shaft, and it can pivot by sliding in the holes on the upper and lower arms. By hanging a large load weight from the lower arm, measuring how much weight at what distance will make the beam move, I can estimate the friction in the bearing.
The results: Alumide was best with 0.14, followed by Strong and Flexible at 0.17, and Frosted Detail at 0.30. With a drop of oil, the Alumide got 0.07, the Strong and Flexible got 0.11, and the Frosted Detail got 0.20. I tried loads as high as 1kg and as low as 100g, and in all cases it was linear.
The alumide showed an interesting effect - it started out at a considerably higher friction, but after spinning it for a minute or so under a 1kg load, the friction was much reduced. I suspect that's because it starts with a rough surface, but after a short time, it polishes itself, and you have a somewhat smooth aluminum bearing surface.
Both the Alumide and the Strong and Flexible had a "textured" feel to their motion. Especially after the oil, the alumide was a little inconsistent in its motion - a weight would make it go a short distance and stop, but if you pushed it past that it would move further. The Frosted Detail, especially under higher loads, had a grabby feeling to it - similar to when a sliding a wet finger makes a squeaking noise, but lower frequency. The oil didn't help this much.
The oil I used was "Liquid Bearings" synthetic clock oil. I applied one drop to each of the 4 bearing surfaces, and waited an hour or so. The Strong and Flexible and Alumide plastics are both very porous, though, so I'm curious how long the oil will last. I'd also like to try other lubricants, such as silicone grease or graphite powder.
Version 1 of my clock used printed bearings, but it wouldn't run due to gear clearance issues. Version 2 and 3 both used ball bearings. The ball bearings work well, and probably have a much lower coefficient of friction. But they're expensive, annoying to assemble, and don't tolerate any misalignment. For heavily loaded joints, the ball bearings probably win, but in lightly loaded joints, the constant drag of the grease may be more than the linear drag of a plain bearing. Anyway, my calculations (and some experiments) show it should be feasible to make a clock with only plain 3d-printed bearings, so I'm gong to try that next.