I recall from an old machine design class that 90% of the torque applied to a fastener is used to overcome underhead and thread friction. Having said that, you have to consider what the TRUE design/operation intent of threaded rod is - and it is not necessarily power transmission.
Given that your biggest problem is friction (galling is from friction), you first would want to look into the lubricants used. Silicon grease is slipper BUT it is not a grease suitable for extreme pressure (EP) applications like that which you are putting it. Motor oil is designed for force feeding into a clearance space - it lubricates via hydrodynamic lube film theory, you are asking for BOUNDARY LAYER lube film creation and maintenance.
IT would seem that the type and quantity of lube that you are applying is not helping. Moreover, although they might work as a power transmission device, the threaded rod you're using may simply be too soft or posess the wrong surface finish/characterisitcs/coating/platings to frictionally slide without galling.
Even though the nut threads "look good", that does not mean that they will survive relative high speed relative motion between the nut and screw without galling. It may take more than what you're doing to generate a nut thread successfully - at least to serve as a ball nut.
You may have some cheap grade five or less metal instead of some decent stuff. Cut threads are typically not as well finished/smooth as rolled thread. If the budget allows, even inexpensive rolled thread rod is better than hardware store cut thread rod. A rolled thread ball screw is far superior to any screw thread.
Finally, steel nut against steel screw might not be the best for wear/scuff resistance. There is good reason why acme ball nuts are often bearing bronze of some type whereas the screws are steel - think about it for a while and you should be able to understand why.
With the above info, you can either redesign or shop until you can find a screw thread that will survive. If it were me, I'd look into moving up to a ball screw.  |