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JohnFox

Advantages of different types of bearings

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Are there any engineers out there?

What are the advantages of using the phosphor bronze slide bearings on the X & Y axes and the roller bearings on the Z?

The Z axis roller bearings seem to be quite a source of trouble yet the X & Y slides are problem free, ( although I shouldn't tempt the fates ).

Just curious.

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Probably the biggest difference between linear bushings (the bronze slide bearings) and linear bearings (ball bearings) is the fact that bushings can rotate around the shaft, while bearings can not.

So, the Ultimaker XY gantry demands for bushings per definition because the 8mm shafts rotate. The printhead on it's 6mm shafts however can use linear bearings because the 6mm shafts don't rotate. The same applies to the 12mm Z shafts.

Now - AFAIK (this is where my expertise ends, I'm speaking from my own observations and experience. Not professional knowledge) - linear bearings are in general better than bushings when it comes to precision linear movement, which is why one generally uses linear bearings - unless the shafts rotate.

Now, why do many reprap printers have problems with their linear systems? Because they are made from cheap chinese parts that probably don't even match up with the shafts. And, some users probably never greased (not oiled, but greased!) the insides of their linear bearings before mounting them (myself included...).

I'm a huge fan of Misumi parts. They deliver awesome japanese quality far beyond what's called "industry standard". Let's just say they offer a great performance / price ratio at insanely high prices :D

Why am I telling you this? Misumi has very detailed technical specifications for their products. I basically learned everything I know about that stuff from them.

So, their linear bearings (and bushings) are specifically made for shafts that are produced with "g6" tolerance.

If you were to use a "h6" tolerance shaft instead, you would not get the same near perfect performance the product was intended for.

Now, if you buy shafts and bearings from reprap stores, you have no clue what kinds of bearings and shafts these are. You don't know if they match up... Other bearings might be made for h6 shafts, but you might be buying cheaper h9 (or not specified at all) stuff. If the quality and specifications don't match up - you will get poor performance.

When I bought my first  set of Misumi shafts (#PSFJ) and linear bearings (#LMUW), it was so crazy to feel the difference between that pair and the UMO standard parts. There was perfectly smooth movement without any notable play on one side, and bumpy, wiggly movement on the other.

Note that there is a price difference of probably a few hundred percent, so I'm not saying the UMO is a bad product because it isn't - for the price you're paying for it. It could be so much better - and it would be so much more expensive...

Now, I already mentioned greasing. Misumi's linear bearings come packaged in a sealed bag, all oiled up for protection against corrosion. Before mounting them, you're supposed to de-grease them (optionally) and then apply a specific type of litium-soap-grease to the roller balls. This is not the same thing as oiling a shaft, because that lithium grease is specifically useful for keeping the roller balls lubricated under high mechanical load. Standard "white machine oil" can't do that job. So, how many repraps are greased up properly? Mine isn't (and I intend to correct that for my upcoming printer builds)...

Personally, I don't like the bronze bushings. You have to keep them clean and well oiled or they will degrade quickly. I also think they would degrade faster when you use them for the Z axis because of the large cantilever force. Linear bearings also don't particularly like these forces, but good quality bearings can take them without problems. The larger diameter also makes them stronger - so even if 10mm shafts would be more than sufficient for supporting the platform, the 12mm bearings can take much more cantilever force and will probably never degrade significantly.

By the way, bushings are not cheaper than linear bearings if you're comparing the same quality levels.

I hope someone else can also comment, this is an interesting and not-as-simple-as-it-sounds topic...

Edited by Guest
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Some more details:

Misumi PSFJ8 (link don't work because it contains [brackets] ) is an induction hardened, hard chrome plated precision shaft with g6 tolerance. Unless most competitors, they specify EVERYTHING, including the maximum "straightness" deviation of their shafts. (These babies are perfectly straight...)

Technical information about Misumi shafts

Misumi LMUW8 is the matching (double length) linear bearing.

This document tells you how to select the correct shafts for your linear bearings, and also the lubrication thing I mentioned before.

As you can see in that document, the tolerances for both the shafts and bearings are in the single micrometer range - this is quite far from what I've had on my UMO.

Here's an example for a bronze bushing. As you can see, there are different tolerance options available, and each has to be matched with a suitable precision shaft. You shouldn't just go to ebay and buy "something" - you won't know if these parts match at all...

More technical info on these bushings

By the way, I just noticed that in english (I usually work with the german Misumi page) they call the linear ball bearings "linear bushings" -.-

This is kinda difficult to translate from german, because there we use two different words... "Buchse" for the bronze bushings, and "Lager" for the linear ball bearings / bushings...

/edit:

Found something interesting here... :)

Edited by Guest
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jonnyb

Thank you very much for the reply, interesting and explains a lot.

( I'm a biochemist by background not an engineer ).

 

Some more details:

Misumi PSFJ8 (link don't work because it contains [brackets] ) is an induction hardened, hard chrome plated precision shaft with g6 tolerance. Unless most competitors, they specify EVERYTHING, including the maximum "straightness" deviation of their shafts. (These babies are perfectly straight...)

Technical information about Misumi shafts

Misumi LMUW8 is the matching (double length) linear bearing.

This document tells you how to select the correct shafts for your linear bearings, and also the lubrication thing I mentioned before.

As you can see in that document, the tolerances for both the shafts and bearings are in the single micrometer range - this is quite far from what I've had on my UMO.

Here's an example for a bronze bushing. As you can see, there are different tolerance options available, and each has to be matched with a suitable precision shaft. You shouldn't just go to ebay and buy "something" - you won't know if these parts match at all...

More technical info on these bushings

By the way, I just noticed that in english (I usually work with the german Misumi page) they call the linear ball bearings "linear bushings" -.-

This is kinda difficult to translate from german, because there we use two different words... "Buchse" for the bronze bushings, and "Lager" for the linear ball bearings / bushings...

/edit:

Found something interesting here... :)

 

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