Maintenance-free bushings?

Hello Everyone,

I know, I have been flooding the forum with all kinds of new topics lately. I am sorry about that, but I am in the process of collecting parts for my own printer (modified Prusa i3) and to be honest, I just lack the real-world experience about many mechanical parts.

So, in search of a lubrication-free linear guide system, I found a lot of products like plastic bushings, V guide rails and so on. One of them was this Misumi linear bushing which is supposed to be maintenance-free (in the German version of the site) or Oil-free (linked US version).
A little bit of research has shown that those black dots are most likely graphite inserts which act as a dry lubricant and other manufacturers claim that you can run these lubrication-free.
Now I am a little bit confused because Misumi recommends to lubricate these bushings (cannot find the exact page in their catalog again) before first use. Does that mean that I in fact should not run these lubrication free and the low maintenance refers to the period of re-lubricating?
Can anyone please explain/clarify this product to me as I would be interested in keeping my whole printer lubrication-free?
Also, will the linear rails stay as "clean" as e.g. when I use polymer bushings? After all, these things seem to constantly apply graphite or PTFE to the rail.

Edited 1 time(s). Last edit at 12/28/2012 06:31PM by uGen.

I would not worry to much over the bushings, I built my Prusa (1) using printed bushings, then later moved to using L8UU linear bearings on hand polished 8mm rod. These I lubricate by spraying PTFE lubricant on to a cloth and rubbing it along the rods at the start of a printing session.

My advice would be to use the simplest method for your first printer and once it is up and running and you have some experience then look at more costly options. Misumi parts are excellent quality, but they are not cheap.

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Yeah, I agree with you on using "standard" solutions on the first printer. However, my first printer is not the first one I have used. I have about half a year of experience with a Prusa Mendel and a Mendel90 respectively and have helped in the construction of 10 Mendel90s in an art school project which lead me to think about alternative solutions to linear guides because one of the main problems was heat-induced change of consistency of our lubricants (which were not exactly suited for this application to be honest). Also, oil-free would mean clean hassle-free transport. As the Prusa i3 design is quite portable, this is nearly a given for me.

I have some work related experience with plastic bearings. They can be a good choice but like everything else, represent tradeoffs. They won't, most likely, be as low friction as ball bearing based linear bearings. They will have some level of "stiction" i.e. the tendency to stick then slip when starting motion...more, in my experience than ball bearing based bearings.

They do have the ability to self-lubricate (which is important to you) and therefore are more maintenance free. For long life and low friction, they do require the correct metal rod to run on in terms of hardness, surface finish, material, coating, etc.. (so do ball bearing based bearings). If the surface finish is not correct (somewhat surprisingly, too smooth a finish can be a problem for some plastic bearings), they can present higher friction and/or wear out more quickly. They, like most plastics, can "creep" (deform and take a set) until load. They can load with abrasive dust (if there is some in the environment) and wear the rod.

My personal experience has led me to avoid the use of plastic bearings *unless* there is a consideration such as self lubrication or the need to work in harsh chemical environments, based mostly on the increased friciton and stiction that seem to come with them.

Of course, I have not tried them all and am not an expert...more my two cents based on some experience and your mileage may vary. I am curious if others are of the same opinion(s).

-mars bonfire: I also have looked into Igus plastic bearings quite extensively. Unfortunately, most of them do not easily hold up to 15000+ km of use on the X axis as calculated by the Igus service life calculator, which is why I am looking for alternative solutions. These 15000 km are representative for 1000x15km axis movement per print (a surprisingly high mileage!). However, they are the ideal solution for the Z axis where they are supposed to hold up to over a million full sized prints @ 300mm axis length.
The stiction issue sounds interesting, I will look into that.

My current plan by the way is to use V-Rails (Open Rail) on X and Y, as the wear should be extremely little as calculated by the datasheets of Bishop-Wisecarver and LinRol (a local brand (?)). Parts shouldn't be too expensive either, but the rails may still have to be lubricated for maximum performance.

Wow...15Km per print! Sounds like such an incredible number (high) but I have not tried to calculate it. I am not sure how much you need to worry about the stiction issue for this application. I think of it as the ratio of the static to dynamic coefficient of friction. If the value it 1, than roughly speaking, no stiction. Practically, if both coefficients are really low, it kind of doesn't matter...the friction force is so low, other forces may dominate. I never would have thought of the wear issue as being in play but your numbers are interesting. Off to clean and lubricate my ball bearings...

Yeah, it surprised me as well, but if you calculate it as follows:
- 200mm print bed, filled so that 150mm are filled (let's say you want to print multiple objects)
- 50,000 actuations (to and fro) per print (source: German RepRap forum, Traumflug's second post, adjusted it a little bit)
-> 50,000*300mm = 15,000,000mm
-> 15,000,000mm / 1,000,000 = 15km

After thinking about these values a bit again today, it is rather unlikely that one would print multiple parts every day. And not every print is that big. So let's say that one "standard" (as in 15km) print per day is the average (before, I based service life assumptions on much higher throughputs of 2 to 3 prints a day, 365 days a year). Under these modified circumstances, a plastic bushing could in theory last about 3 years - which honestly is quite amazing for something that is conceived as poor and cheap material.

Stiction seems to be ok for Igus bushings if you follow the implementation rules. Some people use them for their camera sliders and claim that they get smooth results (some say otherwise, but I have seen people applying forces way up on the camera when actually you should apply it as close to the bearings as possible).

Here is the link to the Igus calculator in case you are interested: Igus Drylin Expert 2.0

Another interesting point would be the extrudability of their plastic stock. I know that they also sell raw material and have asked them about this, but so far, I did not get any answer back. It would be awesome to print your own bushings with an optimized plastic instead of PLA

My machines run almost 24/7 but I can only remember needing to lubricate linear bearings once. They seem pretty low maintenance to me. I used car engine oil as that should handle the temperature.

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