Reduce the extrusion multiplier / flow rate and print at 100% size (no down scaling).
While printing the printer presses the filament of one layer onto the material of the layer below, if the flowrate is too high (100% might also be too high) the filament gets sqashed to the sides, meaning that the printed lines get wider. At one point the lines are too wide and will touch, even though there was a clearance of 0,2mm or alike set in the 3d file.
If you scale down the model you reduce the clearance aswell making the above described issue more likely.
Reducing the temperature might help a bit, but I’d go for more cooling, instead of less temperature. You still need the layers to stick to one another so you shouldn’t go too crazy with cooling and temperature dropping. Some cooling may help to get cleaner edges and it prevents “stringy” prints, in which the nozzle oozes material out during travel moves, those strings can cross the outlines of two seperate parts, causing them to fuse together => nomore movable!
Lastly I would go for another material.
PLA has a very rough surface and wears down quickly (ie in the bearings or in a gearbox). Nylon is by far the best -yet affordable- material out there to get moving parts printed, however it’s not that easy to print. ABS is fine too, but also there’s the problem of ease of printing. ABS tends to warp quite a lot and you might tinker quite a lot with it.
This is, why I would go for Polyester based materials (PET, XT, T-Glase, …). My personal preference is XT by Colorfabb, not only because of the price (3dhubs discount!!!), but also because it’s fairly easy to print, available in many colors and Colorfabb’s headquater is in the Netherlands. To my knowledge this is where 3dHubs (and you, Arnoldas) are setteled so there aren’t any trans-continental shipment costs added to the order.
As the last possibility you should look for models with larger clearance. In many cases the designers of those parts upload their models multiple times with clearances from 0,1-0,5mm or so.
Use a hammer (preferably with a rubber head, or covered with something, that’s not rock solid metal) and gently hit the 3d models, the vibrations and soft shocks can cause minor connections between moving parts to loosen, whereas those would stick together if you’d apply just your manpower 
If you print stuff like planetary gearboxes the chances are, that the middle part has a hexagonal cutout (or something similar), this would allow you to take a large allen key and then you could try to crank up the gearbox.
Anyways good luck with the projects and let us know, if you experience some new issues or if you manage to get those parts printed.
Cheers,
Marius Breuer
