It's the cheap Linear bearing that they make. The more money you pay, the better bearing you get. I deal with these type bearings all the time and these are pretty much what came with my Geeetech Prusa but they are ok for the time being. Look on ebay for some rexroth or other name brand. The other trouble is, too, that these carriers are so small, the bearings are also very small which might make the difference too.