My experience with the IGUS bushings RJZM-01-08 on the y axis  

My experience with the IGUS bushings RJZM-01-08 on the y axis

Hello everyone,

I recently bought three IGUS RJZM-01 drylin bushings to replace the stock LM8UU bearings on my MINI+ and I wanted to share my experience with the community.

Reasons for buying the IGUS bushings

When I first assembled my Prusa Mini+ kit, I was really impressed how quite the printer was compared to my old one. But overtime, the printer became noisier, especially the y axis. It started making grinding and squeaking noises, especially when the bed was moving fast (e.g. while printing the infill). I tried different solutions to counteract the problem (yoga mat underneath the printer, special feet printed from PLA, moving the printer to a sturdier desk...), but these only had a small effect. The only thing that was working reliably for me was removing the y-bearings every couple of months and regreasing them. Then, the printer would run much quieter for some time, until I had to regrease the bearings again. I always took extra care to apply the grease exactly like it is shown in the assembly manual, but every time I removed the bearings there was literally no grease left in them. I also checked if the balls in the bearings are correctly oriented against the rod, which they were. At some point, I even ordered a new set of bearings from Prusa to make sure the bearings are not faulty, but this did not improve the situation. I should add that I am not a heavy user, sometimes I do not use the printer for a month or longer and I mostly print small structural parts, so I would not say that I stress the machine a lot.

Recently I did some research on the internet and quickly found out about the IGUS drylin bushings. There appear to be three options to replace the LM8UUs:

• RJZM-01-08: Bushing with aluminum shell (which to my research does not require a press fit). They are not a 1:1 replacement for the LM8UUs – they are 1 mm longer and 1 mm thicker than the standard LM8UU.
• RJZM-02-08: Bushing with aluminum shell (which to my research does not require a press fit). In contrast to the RJZM-01-08, these are a 1:1 replacement for the LM8UUs regarding the size.
• RJ4JP-01-08: Bushing without aluminum shell, entirely made from plastic. They are the cheapest option, but according to my research require a press fit and/or snap rings.

However, the information I found was inconsistent - some users say that their bushings work perfectly and are really quiet, others report that the print quality was reduced and they have a lot of play. For example, according to this this thread on the Prusa forum (according to @spark), the RJZM-01-08 seem to be the best option, since the RJZM-02-08 and RJ4JP have more play. On other websites the exact opposite is stated. Also, there is no specific opinion regarding the mounting – some users simply use the RJ4JP with cable ties and are happy, others say that the RJZM-01/02 require a press fit, for which I could not find any information on the IGUS website.

Overall, my research left me confused on which bushings to choose. In the end I decided to order myself three IGUS RJZM-01-08 and the fitting aluminium rods (2xAWMP-08 with 279 mm) for a total of 48 € (including shipping) so I could replace the bearings on the y-axis and perform some tests for myself. I choose those because the RJZM-02-08 are a bit uncommon and you can find much more information about the RJZM-01-08. Also, I could not believe that the RJ4JP do not require a press fit as some users claim. I only ordered the bushings for the y-axis, so the following paragraphs only apply to this axis!

Baseline measurements

Before disassembling the printer, I went ahead and created some G-Code which moves the x- and y axis at different speeds. I took videos with my smartphone so I could compare them to the IGUS bushings later. Sadly, I did not take any decibel measurements as I did with the lubricated LM8UU and the IGUS bushings later.

Rod measurements

After I ordered the IGUS bearings, I read some more online and some people suggested that the rods that Prusa ships with their printers are apparently not of the highest quality and the diameter of them is not consistent, which could cause the noise I was hearing. Even though I had already ordered the bushings, I wanted to check this as well.

So I took my digital caliper (which is of good quality, but has not been calibrated in years) and took three measurements in a row on each side and the middle of each of the y-axis rods. Here are the results:

Rod 1:

• Left: 8,00/8,02/8,01 mm -> mean: 8,01 mm
• Middle: 7,99/8,01/8,00 -> mean: 8,00 mm
• Right: 7,99/8,03/8,02 -> mean: 8,01 mm

Rod 2:

• Left: 8,05/8,05/8,02 mm -> mean: 8,04 mm
• Middle: 8,03/8,02/8,05 mm -> mean: 8,03 mm
• Right: 8,01/8,01/8,00 mm -> mean: 8,01 mm

I am not an expert regarding rods, but in my eyes this looks pretty good. The second rod is a bit on the thicker side, but I would say there is nothing to worry about.

So, I left the printer disassembled and waited for the IGUS bushings to arrive.

Installation

The IGUS bushings finally arrived and the quality seemed to be pretty decent. The aluminium rods were also precisely cut to length, so I started the assembly... until I ran into problems :D.

First up, since the bearings are 1 mm thicker than the LM8UU, I had trouble mounting them to the y carriage. I knew this when ordering, but I thought "well, this should still work, it is only a mm". Turns out, this was a bad assumption, because it did not work. I could not mount the bearing clips flush with the y carriage (surprise!), so I had to leave some space (see the attached pictures). Also, there was some pressure on the bearing clips since the bushings widened the clips.

But well, for a first test, I mounted them as good as I could with the stock bearing clips. The next issue is faced was with the rods. The original steel rods have a slight chamfer at each side which allows them to be inserted into the holes in the plastic moulds easily. The aluminium rods did not have them and it was therefore pretty hard to guide them inside the holes. So, I took a small file and added the chamfer on the aluminum rods myself. This way, the assembly worked without any other issues.

First impressions

After reassembling the rest of the printer and tightening all the screws, the first thing I noticed was that the bed was indeed wobblier than with the stock LM8UU. I would say there was around 1 mm of play in the x direction. I also noticed that the bed tilts a bit during homing (see slow motion clip), which was probably a result of the play in the axis.

So, I was off to a bad start. I still went ahead and ran the same G-Code as I did with the LM8UU. The result was that the noise was different – more of a slight rubbing noise than the crackling and squeaking I had before. But in terms of the amount of noise, I would say there was not a huge improvement. You can still hear the printer if it is running, but the noise might be less annoying.

To summarize, the noise was improved a bit from my perspective and since I would never need to relubricate the bushings, I was tempted to leave them in the machine. But I did not really like the play I had in the y axis, so I went ahead and printed two calibration cubes to see how they would turn out. We will come back to those later.

Back to the LM8UU

When I put in the IGUS bushings, the LM8UUs were already running for a couple of months since the last lubrication and the printer was starting to get loud again. I decided that it would be a good idea to relubricate the LM8UU and install them again with the original steel rods to get an apples-to-apples comparison to the IGUS bushings.

After reinstalling the LM8UU, the bed was again stiff with no noticeable play. Also, the bed did not tilt during homing, so, the precision of the LM8UU is by all means better than the IGUS bushings.

After running the same test G-Code, I noticed that the squeaking and crackling was once again gone. To obtain some numbers, it appeared to me that I could use a decibel app on my phone. I recorded the measurements for the IGUS bearings (put them back again just for the test) as well as for the regreased LM8UU. Sadly, I did not take the measurements for the un-greased LM8UU and I was not going to remove the grease out of the greased bearings just for this test.

+--------------+-----------------------+------------------------------+
| Speed (mm/s) | IGUS RJZM-01-08 (dBA) | Re-greased Prusa LM8UU (dBA) |
+--------------+-----------------------+------------------------------+
|           20 |                    26 |                           24 |
|           40 |                    26 |                           26 |
|           60 |                    31 |                         34,5 |
|           80 |                  27,5 |                         28,7 |
|          100 |                    27 |                           31 |
|          150 |                    26 |                           32 |
+--------------+-----------------------+------------------------------+

As can be seen, there is not much difference. The IGUS bushings are a bit quieter at medium and high speeds, whereas the LM8UU are better at low speeds. I mostly print with medium speeds (60-80 mm/s of bed movement), where the IGUS would be better, but not by much.

Calibration cube comparison

As mentioned above, I printed two calibration cubes using the IGUS bushings to check if the play in the axis would be a problem. I also went ahead and did the same with the re-greased LM8UU. I used Polymaker PolyTerra PLA for this test with a layer height of 0.15 mm.

The prints came out good, however, I need to get myself a filament dryer, since there are quite a few blobs on the surface of the prints (and no, these are sadly not the seam positions). Apart from that, there is no noticeable quality difference between the ones printed with the IGUS bushings and the ones which were printed with the LM8UU (see attached pictures). Regarding the measurements, here they are (they should have 20 mm in each direction):

• IGUS X/Y/Z (1): 20,01/19,97/19,95 mm
• IGUS X-Y-Z (2): 20,00/19,95/20,00 mm
• LM8UU X-Y-Z (1): 19,99/19,97/20,01 mm
• LM8UU X-Y-Z (2): 20,00/19,99/20,06 mm

I used the same caliper mentioned above and took three measurements in a row, calculating the mean afterwards. Overall, the measurements seem to be decent for all the cubes. The tolerance in the y-axis appears to be slightly reduced with the LM8UU, but this could also be just a coincidence.

Conclusion

So, what is the end result of all those tests? Honestly, I am not so sure. On the one hand, the IGUS bushings are a bit quieter than the LM8UU and not needing to regrease them regularly is a plus for me. On the other hand, there is still the play in the y-axis. Even though the calibration cubes turned out well, I am not 100 % convinced that I will not have any issues down the road with different models. Also, there is the mounting problem, for which I would need to find a solution in case I want to use them more permanently. I could go ahead and buy the RJZM-02-08, but in the thread linked above the tolerances were even worse than the RJZM-01-08. Or maybe I get lucky and they are better than the ones I currently have - I do not know.

Overall, there are just too many ways how this could turn out, so for the moment I decided to re-install the LM8UUs and continue using them. Even though they need more maintenance work, they seem to be more reliable due to the smaller tolerances. I think I will not order any other IGUS bearings for know and instead maybe look for a higher quality grease which will last longer. There is also the option of getting the original Misumi bearings, but I have not investigated those yet. If you have any advice for me regarding the IGUS bushings, a better and longer-lasting grease or the Misumi bearings, please let me know.

Thanks for reading through and happy printing.

Best Regards