20pcs V623ZZ Miniature Steel V- Pulley Rail Ball Bearing Miniature Pulley Bearing Pulley 3x12x4mm for Rail Track Liner Motion System

I am building a crane for an RC project. These make perfect little pulleys .

Good quality item

So these are tiny pulleys' and they are nicely made with a bearing inside of each and roll very smoothly. My use for them, is to guide 3d printing filament and they are perfect for this. I use 1 on each side, with one pulley being adjustable for easier filament changes. Very nice, quality pulleys'.

Not 3mm bearings, holes closer to .5mm.0/10 don't buy currently having to run to lowes to get new hardware for the project because of this.

I bought these for a linear guide along some din rail. I plan to use four of them in total to make up for their small size. Each bearing should be supporting about 12.5 lbs. I will update my review after I have used them for a bit, if they hold up for awhile I'll bump this up to 5 stars. So far they seem to be pretty solid and smooth. They're very quiet at the low speed I'm using them for. The actual measured outside diameter of them is at most 50um off for what I received.I deducted one star (really should be half a star) because the bag they came in was all greased up. Not the inside of the bag... the outside. Never seen this before.

The lube that was in them is really thick, so I needed to flush it out with a thin oil. They then spun freely.