CUPS don't detect USB printer

[Artiom]

I have 2 usb printers. An old hp deskjet and a new samsung ML-1640.The first works fine with hplip dirver. For the second I have installed the official Samsund driver via AUR.  But CUPS don't see it. It is recognized by the kernel. I see /dev/usb/lp0 device created (root.lp rw-rw---) but still nothing in CUPS .

After searching similar problems on the net I have found a hint that changing permissions on the lp0 device can solve the problem. So chmod +rw lp0 and it WORKS.

But evidently after turn off, turn on of the printer lp0 will have its default permissions again. I tried to find the cause of the problem by comparing the system behavour with my printers. I found that it's identical. The devices are created with the same permissions.
I found also that changing the group of the device to nobody or owner to daemon have the same effect as chmod +rw (it works).

I see by ps -ef command a print task owned by daemon (and I suppose nobody group). I think that this print process try to access /dev/usb/lpx device. The same thing when I print to HP printer...

Could you explain me what happens and how to fix it?