Acrylic Prusa I3 pro B - assembly issues - Z axis

[darmar]

Hello,
this is my first 3D printer, and my first 3D printer assembly.

I finished with the building following the instructions and I'm having a problem with the Z axis.
When in the low position (first 5cm from the bottom) is gets stuck and I have to manually (by hand) spin the threaded rod (one or sometimes both) to make it move, and I have to spin it constantly until it passes those 5cm (or so). After that, is flows to the upper part of the rod without any problem.
The same, of course, when I try to move it again to the bottom. When within those 5cm from bottom motors begin to groan.

I checked both rods (threaded and plain) and as far I can see they are NOT bended. I even tried rotating them, etc.
Also, what I can see, that the couplings are bending just a little bit while rotating.
I checked that small screw on couplings are tight on the flat side of the motor shaft.

Is there is anything else I could check to fix this?

Thank you,
regards

[William]

You can check if there are some bad thread on the rods.
Then change another driver for Z axis to have a look.
And refer to this post to see if you've missed something:
http://www.geeetech.com/forum/viewtopic ... 22&t=16917