Recently one of the Z-axis threaded rods on my Prusa i3 started making noises. I added a small amount of silicon grease (the type used for garage door screw mechanisms) to the part of the rod which comes in contact with the nut and that seemed to help.
The rod may have come with a very light coating of oil but it could have been worn off with all the finger contact as I've calibrated several times.
I was wondering what others do, specifically what type of lubrication do you use on vertical threaded rods?
