Typical inexpensive Chinese products. Good in some respects, a little poor in others. When it comes to machinery often the component needs to be disassembled and adjusted.
The FK15 bearings I purchased for my RF45 Mill CNC are no different.
When I assembled the mill, I had too much backlash on the Y axis. It took a while to find it, but I chased it down to the fixed end bearings. After tearing down the assembly, I noticed a few things:
(1) The angular contact bearings were mounted in tandem (both facing the same way). This is no good for a lead screw because the lead screws on a CNC typically move two ways on a CNC.
(2) Lack of grease. The bearings are oiled, but no grease. This is *not* suitable for a CNC unless you have an oil line to these bearings (which I highly doubt)
(3) Unable to apply preload. The AC bearings are stacked together, but unable to apply any preload because the inner and outer races are in contact with each other. A shim is required on the outer races in order to apply preload. I made a 0.025″ shims on the outer race and placed the bearing back to back.
Once I did the above, the fixed bearings worked fine.
Don's Technology Blog 