Lubricating smooth rods?

Hello.

I'm building my first reprap (Prusa i3 rework), but noticed that the LM8UU bearings don't ride smoothly on the 8mm smooth rod (make grating sound and have fair bit of fritction). This is not an assembly issue: I tested a lone bearing on a free rod, and it has the same effect. I'm not sure it's safe to run the printer like that.

Should I try lubricating the smooth rods (machine oil for example)? Or does this mean I simply have bad rods and need them replaced? These are some tool steel smooth rods I got from McMaster Carr. They look ok, though some visible rusting has developed along some areas, which makes me skeptical about how well these will work.

I had the same problem. I got 8mm smooth rod that was tight against the lm8uu bearing that I purchased. I decided to purchase 5/16" smooth rod and they worked just fine. It is hard to know if the problem is really the rods or the quality of the bearings. So, I did what worked for me. The 5/16" rods are slightly smaller than 8mm.

Milt

I would take the rods off and cuck them into a drill then attack them with at least a oil soaked rag. Depending on how bad the rust is I would sand them lightly then give them a polish. You want the rods as smooth as possible.

Thanks Mike

I can try replacing with smaller rods, but that's a pain in many ways (wait for delivery, pricey, etc.). Also, doesn't it seem like correctly sized rods SHOULD work with correctly sized bearings?

You mean actually place it into a drill or drill press and then use an oiled rag or sandpaper to polish it? Will any oil work? Also, is there a way to protect it from rusting afterwards (e.g. lubricate)?

Quote
anvoice
I can try replacing with smaller rods, but that's a pain in many ways (wait for delivery, pricey, etc.). Also, doesn't it seem like correctly sized rods SHOULD work with correctly sized bearings?

You mean actually place it into a drill or drill press and then use an oiled rag or sandpaper to polish it? Will any oil work? Also, is there a way to protect it from rusting afterwards (e.g. lubricate)?

Yes just slip then end into a drill and tighten it down, easiest way to clean and polish smooth rods. I tape the end in the chuck to help prevent damageing them and keep the speed low to start. I happen to use tri-flow bike oil on mine because it's what I have on hand, but really any fairly light oil should work. If you polish them with any waxed based polish you shouldn't have any rusting issues..or just lube them once and awhile.

Thanks Mike

Thanks, I'll give that a try today or tomorrow and report how that turns out.

Edit: Seems to have worked pretty well, the bearings run smoother after polishing on a drill with some fine sand paper. Can't say it's perfect, but I'm wondering if that could be due to the rods themselves not being perfectly straight, and the rust just wasn't helping. Thanks for the help!

Edited 1 time(s). Last edit at 09/03/2014 04:18AM by anvoice.

1200 to 2000 grit sandpaper and polishing paste afterwards will do the job. Use CRC or WD40 or some minor number car oil and apply a decent surface. Just about any oil will go as long as it's not grease.

Tool steel rods are quite often ground centerless, and while the accuracy may be high, surface finish may not. All my HSS toolbits have very rough grinded surface and I would not use them as guides. An ideal rod for printers is cold drawn, since this deforms the metal, and depending on the production level or purpose, leaves next to mirror finish. Hardware sold H9 surface cold drawn rods only need a final polishing with 2000grit or directly to polishing paste to bring out mirror.

push the rods together side by side, if there is a bend you will see light shine through. if it's the bearings it is likely that you have dropped a ball or two.
If you take the bearing off the rod and give it a shake by your ear and it sounds like a baby's rattle then it's time for a new bearing, you should only hear a very slight movement as there should not be much space between the balls inside the bearing.

if your smooth rod is rough cut on the ends and has not been smoothed down in anyway then the edges readily dig into and pop out a ball if you don't hold the bearing straight and jus shove the rod through, when installing bearings, it's a good idea to give the smooth rod a gentle turn as you "Slide" the bearing on, too much force and your back to where you started.