Metal pulley and id...
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Metal pulley and idler anyone?  

Trusted Member

Pulleys and idlers in Robox are injection molded plastic (Guys, really?)
And idler is toothless.
Not that toothless idlers are inherently bad, but in that case they are.
Only 5~6 belt tooths touch idler in Robox simultaneously.
With 2.8mm wheel radius and 2mm belt pitch, we get loads of vibrations and noise.
I find it second most powerful noise generator after linear bearings.
I found 12-tooth GT2 aluminum drive pulley easily (see pictures in next post) but toothed idler of that size doesn't exist anywhere.
Literally, nobody manufacture it. To make things worse, it must be made of brass or bronze to work correctly on steel shaft.
I roughly modeled it (see pictures in next post) and asked few manufacturers if they are interested.
They are. ;)
If I get those idlers manufactured, would you be interested in a set?
I was thinking of a set consisting of:
- 2 aluminum drive pulleys
- 2 brass idlers
- 2 belt tensioning springs (if they will fit, to be checked)
The price on that will depend on manufacturing cost, it should be around 20USD/set + PP (rough estimate)
I don't want to make profit on it but I want decent pulleys and minimum manufacturing quantities are 50 or more.

Posted : 4th April 2019 6:58 pm
Trusted Member

Pictures follow.

Posted : 4th April 2019 6:59 pm
USA Robox Support Moderator

@palindrom The drive pulleys for the belts are not a wear item since the rubber on the belt is softer than the plastic. I recently took apart a printer that had almost 3 feet of filament strands wrapped around the drive pulley. The belt was so tight I could barely move the bed by hand and when I released the locking screws the belt popped out of the teeth like a broken guitar string. The bearings on the Y motor were destroyed from the additional forces. There was no damage to the teeth on the pulley. They are loaded in compression for the most part. There is little to no gain to change the part to aluminum from a strength viewpoint. If your pulleys have been damaged it is due to the belts being too tight for a long time. The Robox belts don't need to be very tight to maintain proper accuracy since the printer doesn't generate a large amount of inertia. 

The toothless pulleys get noisy from the pulley on the steel pin, but they don't really contribute to the noise appreciably and a little lubrication causes them to quiet right down. Smooth pulleys are also used on the Prusa MK3, in the same configuration, only aluminum, and that printer is nearly silent. In fact, comparing the MK2.5 Prusa to the MK3, the structure is nearly the same and the only real difference is the Triaminic stepper drivers. They use the same rods and bearings, same as the Robox, they use the same belt as the Robox, they use the same pulleys as the Robox except aluminum. There is a huge noise difference between the two printers. The only real difference between them is the stepper drivers and the drive voltage, which the Robox already uses 24 VDC. 

I am not trying to say your idea doesn't have merit. In my experience there are four major noise contributors to the Robox: 

1. Linear bearings.

2. Fans

3. Stepper drivers.

4. Z limit tab. 

The noise reductions from changing the pulleys is negligible in my experience. And yes, I have done it. The pulleys on my printers run on ball bearings, so they are silent. There was only a tiny noise reduction. I underwent about two years of experimentation to try to get the Robox as silent as possible, just to see what I could do, which is where my data comes from. I went to polymer bearings and that showed the largest noise reduction. Fans were next. With just those two changes my Robox units, I have 6, and they all have well over 2000 hours on each of them, dropped to nearly the same noise level as the Prusa MK2.5. Minor additional gains were made with the bearings on the belt pulleys and with a really expensive solution to the Z limit tab that didn't work very well. 

Also, note that you can't run aluminum on steel without some type of bearing surface. The aluminum will gall and weld the two parts together. This means you will need to have a bearing surface against the steel axle pin to avoid this. Just lubrication won't be enough. At a minimum you will need hard anodization and that won't last very long.

Posted : 5th April 2019 1:45 am
kev.gill liked
Trusted Member

@bhudson, I found mine pulleys eccentric. Movements are slightly wavy.
As a test for idlers, I twisted belt on gantry (just before idler) and moved head near its left limit. Noise difference was substantial. This is why I decided to search for parts.

The aluminum pulley on the motor will work just fine, as this will be static contact.
I'm aware of steel-aluminum interactions, that's why I'm getting brass (or bronze) idlers.

I already fitted Igus bearings everywhere and this worked perfectly.

Did you manage to quiet down fans without compromising their efficiency?
And did you replace head fan as well?
This sounds like a good step forward.

Posted : 5th April 2019 8:11 am
Prominent Member

@palindrom talking of head fans, I replaced some in a external hard drive enclosure with these, and they are the same size as the head fan, a little pricey, but very quiet. Not sure if you can use them though, but the tech looks interesting.

Posted : 5th April 2019 1:45 pm
Marcin liked
New Member Customer

Alternatively, you could use a larger fan and an adapter to reduce down to 40mm.

There's some adapter designs on thingiverse.


Regarding replacement linear bearings, has anyone tried the PTFE lined bushes?

I used these on another printer and the quality improved noticeably and the noise reduced massively.

Posted : 5th April 2019 2:36 pm
USA Robox Support Moderator

@palindrom Sorry, I meant idler. The aluminum drive pulley will be fine against the shaft, except that it shows a higher chance of coming loose. Make sure the grub screw gets tightened every 3-4 months as the aluminum will move over time. Brass or bronze idlers will be fine with lubrication. I didn't see the material called out in your post; I just saw the picture of the aluminum idler.

I would like to know how the slight eccentricity possible with the plastic idler causes vibration. This does not seem to fit my understanding of how a pulley works. Any eccentricity should be neutralized by the balanced forces on the idler, especially at print speeds.

I would either carefully deburr the edges of the pulleys and idlers or plan to replace your belts periodically. Sharp edges will dig into and wear on the belt teeth, leading to additional slop in the system. 

I was not able to quiet the fans; I had to replace them with a much more expensive version. The majority of the noise sources on the Robox are from the need to keep the price at a point that consumers would still be willing to buy it. The parts work fine, there is nothing operationally wrong with them, they are just optimized for price and performance and not noise. I replaced the electronics fan and ambient fan but the head fan doesn't provide enough noise to justify the expense and work to move the fan to a new head when the head wore out. 

I would also not use springs to tension. This will mean when your belt wears out you will not notice as the springs will take up the slack and you will just get wrong dimensions until the belt completely fails.

@kev-gill Yes, PTFE and polymer bushings have been used. They show lower noise but increased wear over time even if you get an aluminum or steel shelled bushing, which is about 6-10 times the cost of the normal linear ball bearing. There is also the need to look at higher-tolerance linear rods, which increase the price of the linear rods substantially. Linear ball bearings can adjust to tolerance ranges that lined bushings cannot, leading to increased noise and poor accuracy in extreme cases.

Posted : 5th April 2019 5:11 pm
Trusted Member

I had to put printer together but on first occasion, I'll take a look on the edges of that pulleys.
And I had to replace thermal fuse while doing this. I love when all of this happen when I need to do something. Lovely ;)

I haven't thought about belt stretching and consequent scaling issue - good point. I have steel reinforced belt, but it's a bit stiff so I'm thinking of fiberglass reinforced belt. Shouldn't stretch.
Hopefully ;)

Posted : 5th April 2019 9:02 pm
USA Robox Support Moderator

If you have to replace the thermal fuse, your thermistor is not seated properly. You need to check the actual bed temperature against the thermistor reading or you will continue to blow fuses. You will also need to replace the thermal paste every time you disassemble the bed or reseat the bed thermistor as it dries out over time and can lead to erroneous thermal readings.

You should not need a tension spring with a steel belt or a fiberglass belt. A steel belt will be even worse for your wear issues because of the stiffness of the steel wires inside. They will also fatigue faster than the fiberglass due to the small bend radius around the drive and idler pulleys. I have only had to replace three belts in 5 years on my 6 Robox printers, so the fiberglass should be fine. I also track calibration over time and do not notice significant scaling issues until a belt is close to failure.

Posted : 6th April 2019 6:42 pm

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