Saturday, 7 January 2012

Bearings, Bushings and Bars

My last post started a discussion about why I got only a few hundred hours of use from PLA bushings and in particular commercial IGUS bushings. I think I mounted the IGUS bushings well enough. I printed PLA holders and reamed them to a 10mm bore, which gave a nice press fit.

I had intended to use a small self tapping screw to retain the flange but found I didn't need them. That is what the two holes are for. They are triangular because they are polyholes.

The holders have slotted screw holes and were screwed to the underside of my Prusa's Dibond Y carriage. I started with them loose and then tightened the screws as I ran the axis up and down to ensure they were aligned well. I then applied lithium grease.

When first fitted they had no slop and very low friction. After a few days of continuous use the holes in the bushings had elongated and there was noticeable slop. At that point I replaced them with LM8UU bearings in prototype bearing holders I designed for the Mendel90.

These have run for thousands of hours with no noticeable wear. They do have more friction than bushings though. It seems higher to start with but they seem to "wear in" quite quickly and it drops.

My suspicion was that the surface quality of the stainless steel rods that I used was to blame, so I have just had a look with a microscope.  I used a cheap USB "Traveller" microscope from Aldi and a times 4 objective lens. The magnification is much greater than that though when photographed and blown up to screen size. 

Here are a couple of pictures of an off-cut from the stainless steel rods I used on my Mendel: -

Obviously you can only have a small strip in focus due to the curvature of the rod but you can see it looks far from smooth. The difference between the pictures is mainly the lighting angle.

Here is a mild steel rod bought on eBay, sold for Reprap use, so probably typical of what most people use: -

Quite a lot smoother, so hopefully most people get better life from PLA bushings than I did.

Here is a bright steel rod from a 2D printer, or maybe a flat bed scanner, I can't remember which, but it will have used bushings: -

It seems to have a finer grain structure but doesn't look particularly smooth.

And here is a "precision round rail (Induction Hardened)" sold for use with linear bearings that I got from Zapp Automation.

It looks the best out the four, so I guess you get what you pay for.

I think for soft bushings to last you need high quality rods. LMUU bearings seem to be more tolerant.


  1. It would be interesting to see what a rail or rod looks like under the microscope after many months of running on flat bearings or Linear's?

    Have you tried felt bushings at all?

    On one machine I have Ground rails on the Z, Stainless on the Y and Mild steel on the X, only because I had these to hand, and the ground rails were too short to use them anywhere else. the mild steel makes a real mess, I'm always cleaning the rods, one day I'll change them out.

  2. One has to spend time reading through Igus' documentation. In particular, some of their bushings do require hard & smooth ground surfaces, while other materials perform well on softer rails such as aluminium. And then there's the operating temperature that is really important. Once you think you have a winner, you make sure you are within the correct loading and operating speed ranges. And then there are the finer, oft overlooked points: are the bearings used regularly, or will they be sitting idle for extended periods? And so on...

    My bottom line: I wouldn't waste time trying to use polymer bearings in a one-off application, nor even a small series production. It takes serious commitment to ensure that you have the right bushing for the job, especially if the case at hand as any "exotic" characteristic that sets it apart from a textbook case.

    Also, in my experience, hardened and ground shafts are always a good idea for guide rails. It's one of those cases where you either pay a little more upfront or a lot more in time...

    Please keep up the amazing work! Was so glad to see your posts over the holidays :-)


  3. I'll be building another Printer for my university next month, and having built both a Sells and Prusa, i'd love to build a 90 next, it looks like the best options out there right now. Are you going to release the files for the sturdy model anytime soon?


    1. Hopefully this coming week.

      All the models are built from the same files, just a configuration option.

    2. Nice! I'm looking forward to it!

  4. Any updates on the release of the files and build instructions?

    1. I hope to release the files this week. Build instructions will follow.

    2. Awesome! I can't wait to start printing these out. :) I personally believe this design has a LOT of potential.

  5. When I see the lm8uu holders I just must say that I love how design fuses thru the community in RepRap world :-) I will pull them back to Prusa Mendel. THX

    1. Yes it is just like evolution and OpenScad code is the DNA.

  6. Hi i have been working with the Polymorph modeling plastic recently i have noticed that when overheated it will stick to almost anything. Except a electrical blanking plate witch it sticks to very well whilst hot but then snaps off of easily when cooled leaving a glassy surface.

    The plate handles the heat of my gas soldering iron well so maybe of use a solid build base.

    Cheers Andrew

  7. I have been in contact with Igus. They have sent me several different bearings/bushings which I am going to test. During our talks I asked about what type of lubrication to use with their bearings. Specifically, the lubrication I had been using was a synthetic "white lithium grease" which is a plant based "high oleic soy oil". FWIW, she was unsure about this "white lithium grease" and said that lubes other than the recommended ones may damage the bearing. She suggested I stick with one of the lubes listed below.

    Waylube Oil
    Petroleum-Based Grease
    3-in-1 Oils

    Not recommended:
    PTFE Sprays
    Silicone Oils, Grease, Spray

  8. I'm really excited in your design. I can't wait to build it.

  9. News! Need some news! Checking your blog 5 time a day. How are things going? I want to give this design a shot? Is it living up to your expectations?

    1. Yes the prototype works flawlessly and doesn't visibly shake like my Mendel and Prusa.

      I have updated the design so it can have wood screws into wood, machine screw tapped into acrylic or with nuts.

      I have designed a bottom limit switch with fine and course adjustments. I have modelled all the twists and turns in the belts so the BOM length should be accurate.

      I redesigned the fixing blocks to have slots to allow some inaccuracy in the holes and also made them use the same screws as the other parts. I have also unified the thickness of all the screwed parts so they use the same length screws.

      I have modelled all the wiring holes, yet to do is the cable clip holes.

      I have modelled the plastic strips that guide the ribbon cables, again to get the BOM lengths correct and make the BOM and assembly diagram include every mechanical part, even the bulldog clips.

      I have redesigned the D-type connector bracket on the extruder to eliminate the over priced and odd sized hex pillars that are normally used to clamp them together. The bracket now clamps the connector and has captive M3 nuts for clamping the plug. I am now working on the other half as the prototype uses a D-shell hacked to take the ribbon cable.

      So I am close to finishing the design and then I have to set up the make files and scripts and learn GIT.

    2. And I also modelled the fan and fanguard and created the ability to show exploded views.

      I added nut traps in a few places they were missing, spring washers and tweaked a lot of hole sizes to account for that horizontal holes need to be a bit bigger than vertical holes due to the staircase effect.

      As they say, the last 10% of any design takes 90% of the time.

    3. I think this is a good design. Moving away from lower part count, towards being a better printer. Take your time, and get it right.
      All so, if you need help with anything, documenting or doing a pre-build. I am hear.
      thanks for all the work you have done in 3d printing

  10. I've been using acetal bushes to run on A2 stainless steel rods. I'm surprised that acetal (or the related plastic 'Delrin') hasn't been mentioned. Acetal is strong, dimensionally stable, has excellent sliding qualities and machines beautifully. It slides very smoothly on the rods which I polished with metal polish and lubricated with dry silicone lubricant. This was for an automated camera 'focus stacking' rail.

    The camera carriage is driven by a stepper motor through an A2 stainless steel threaded rod. The thread form is rolled to give a good finish, which I further improved by polishing. The nut for the carriage is made of acetal, which runs very smoothly on the rod with imperceptible backlash.

    I wonder if the above arrangements would be suitable for a 'RepRap'?


  11. BTW the bushings didn't last long because they need Hard chromed linear shafts, OR anodized aluminum shafts.

  12. Any news on how things are going? Anxiously awaiting files, bom and build instructions :)

    1. I have completed the design today, so very soon. I have to set up make files to make the STLs and DXFs and tidy up the directory structure, etc.

  13. You might want to check oilite too. At there is a thread by microcarve describing a MDF style CNC machine using oilite bearings on precision shafting. It's 111 pages long. Here is a pointer to one of the posts:

  14. Hi nophead !
    Near from finishing printing all the parts for a Mendel90 STURDY version, I would like to use Acrilyc, can you tell me what would be the recommended thickness ?
    Am I the first one making a M90 in Argentina ?

    1. The files in the mendel directory are for the acrylic version with 8mm rods. I used 6mm and 10mm for the base. The plastic parts have bigger screw holes because I tapped M4 holes in the acrylic but use wood screws into MDF for the sturdy.

      If you want to make 10mm rod acrylic version you should have changed the config and to a cross between the sturdy and the mendel configs and that would change all the plastic parts and sheet drawings. If you use the sturdy parts the screw holes might be a bit tight and the sheet drawings will have the gantry slightly too far back as it expects it to be 12mm thick, not 6mm.

  15. Thanks very much for your advice !!!!
    the firm that provides de acrylic, only makes laser cuted pieces in only one thickness, so I would use 10 mm for all. So the gantry would be only 2 mm far back.
    Best regards.
    Merry Christmas !!!!