The screws come already screwed in, was very hard to unscrew them (probably used some pneumatic screwdriver).
Expanded at maximum and the tap completely tight, they are unstable, they are rated to bear 15Kg, but my speakers have less 2Kg each, yet the adjustment fell a few centimetres.
So, don't worth the money, even cheap, I've thought they could bear the kilos, so could not be that bad! But they are.
Thank you, Thomann, for your warning attached to the product and for the care you've shown to warn about the rubber bands that might chemically react with the floor in time!