Why only 3 bearings for the Y bed?
So I tore my Y down to mod the belt adjustment and I figured while I was in there I would try some Drylin bushings (no grease = cleaner parts). I don't remember this with the bearings, but the bed torques quite a bit (enough that I can't fix Y ghosting) when it starts and stops (maybe a good .5 to 1 degree of rotation!). There is definitely a lot of slop in these Drylin bushings, but looking at it if there was a 4th bearing on the right hand side it would be a lot more stable anyway. I also don't think the single U bolt mounting is doing much to stabilize things. I'm playing with some different mounting options for the Drylins now, but its not looking promising.
Anyone know why Prusa went with the 3 bearing approach rather than sticking a 4th on there? Are there mods to sneak a 4th in there for more stability/torsion control?
RE: Why only 3 bearings for the Y bed?
A 4th bearing will be far more likely to cause binds if they are not aligned correctly. 3 offers some degree of self alignment.
If your drylin bushings have too much slop, you got the wrong ones. Some of them need to be press-fit into a housing to squeeze down to the proper size. You'll either need to find the part numbers for the correct ones or try the new alternative on the market, vesconite. Search here for my post on those.
RE: Why only 3 bearings for the Y bed?
My way to address the rotational play is to apply a dampened spring from the front left corner of the bed frame to right front corner of the base frame. I used a large rubber band, and it worked amazingly well. New bearings reduced the play, but wear (as play) is already starting to show again: less than 2000 hours this time, so I suspect the rods are shot. My next maintenance cycle will be new Misumi rods and bearings.
RE: Why only 3 bearings for the Y bed?
And as Vintage is saying, twist in the Y rods is likely because of the distance between and the mounts being what they are. The bearings are also held somewhat loosely by the u-bolt, and this allows the bearings to rotate and flex a bit to accommodate any misalignment. The same approach is used on the X-Axis - but the rods are better aligned and wear less.
There have been a few discussions of user mods to replace the rods with true linear bearings ... maybe some day we'll see something 😎
RE: Why only 3 bearings for the Y bed?
A 4th bearing will be far more likely to cause binds if they are not aligned correctly. 3 offers some degree of self alignment.
If your drylin bushings have too much slop, you got the wrong ones. Some of them need to be press-fit into a housing to squeeze down to the proper size. You'll either need to find the part numbers for the correct ones or try the new alternative on the market, vesconite. Search here for my post on those.
As long as you left them loose until the rods are secured in place and then tightened up the bearing holders I would think that should take care of any alignment issue no?
They are supposed to be the right ones, but I've seen complaints on a few of the Amazon options where people didn't get what was sold or what even the package said. I went with a set that didn't have such complaints. I did print some other options last night including a range of press fit holders. The "straps" do nothing for the play in the bushing, but I think it would definitely be more stable than the U bolt. One of the press fit sizes seems to take most of the slop out (maybe indicating that I didn't get what I was sold), but the torsion issue is still there (and maybe even worse?).
I also tried printing a PLA bushing option, but it's pretty clear that is not going to be smooth at all.
I'll glance at your thread. I've seen it, but as the name meant nothing to me I assumed it was a filament vendor and haven't opened it. If I can't get these bushings to play nice, however, I'm just going to go back to the factory bearings (they just need to be greased).
The Drylin do seem to be working well enough on the X and Z axis though. I think the X drive screws are helping stabilize and slop there and while I can clearly move the extruder more than I think it should allow, it doesn't seem to be effecting anything. Just my Y is showing issues (ghosting).
RE: Why only 3 bearings for the Y bed?
My way to address the rotational play is to apply a dampened spring from the front left corner of the bed frame to right front corner of the base frame. I used a large rubber band, and it worked amazingly well. New bearings reduced the play, but wear (as play) is already starting to show again: less than 2000 hours this time, so I suspect the rods are shot. My next maintenance cycle will be new Misumi rods and bearings.
I'm having trouble visualizing your solution. Have any pics showing it?
RE: Why only 3 bearings for the Y bed?
Think Rube Goldberg, and that should help.
Tie a long rubber band [1/8" x 6"] to the left front bed stand-off. Stretch it across the printer and tie it off at the right front corner of the frame.
Dress it to avoid rubbing on rods and belt.
It wasn't pounds of pull, a few ounces maybe. All I can say is it eliminated a bunch of bed rattle and improved print surface quality. New bearings got me back to normal and I removed the band... but now getting close to thinking I need to install the mod again. lol.
RE: Why only 3 bearings for the Y bed?
Very cool. I'll give it a try the next time I'm disassembling my printer.
RE: Why only 3 bearings for the Y bed?
Beauty of the rubber band mod was it requires no disassembly 🙂
RE: Why only 3 bearings for the Y bed?
Think Rube Goldberg, and that should help.
Tie a long rubber band [1/8" x 6"] to the left front bed stand-off. Stretch it across the printer and tie it off at the right front corner of the frame.
Dress it to avoid rubbing on rods and belt.
It wasn't pounds of pull, a few ounces maybe. All I can say is it eliminated a bunch of bed rattle and improved print surface quality. New bearings got me back to normal and I removed the band... but now getting close to thinking I need to install the mod again. lol.
Yeah still not getting it. At least not in a manner that makes sense. What I'm thinking would be stretched when the bed is all the way back and slack when it was all the way forward with a tension curve as it traverse the Y range. Doesn't seem like that would really help (or at worse might cause different torsion issues) which is why I think I'm picturing it wrong.
RE: Why only 3 bearings for the Y bed?
Very cool. I'll give it a try the next time I'm disassembling my printer.
That should have gone to a different thread. I wonder how it landed here. A subsequent edit didn't work.
Anyway.
I've recently spread the Y rods on my printer 0.1mm further apart to 170.10mm to pretension the bearings and reduce bed play *.
* That sounds weird. 🙂
RE: Why only 3 bearings for the Y bed?
Yes - the torque the band applies is not constant as the bed moved fore and aft, but a large rubber band is long enough to do the trick. The one I used was sized for holding small trash can liners in place. About 1/16"x 1/8" section, 4" diameter or 6" when held flat...
Here are pics - a print in process so not ideal time to attach things solidly, but should give you an idea. I printed like this for about a month while replacement parts were arriving. You could attach the band to the right rod, too, and press it back until the bearing or belt holder hits it - that would equalize the tension as best it can.
RE: Why only 3 bearings for the Y bed?
Novel idea!
Might give this a go Myself.
Joan
I try to make safe suggestions,You should understand the context and ensure you are happy that they are safe before attempting to apply my suggestions, what you do, is YOUR responsibility. Location Halifax UK
RE: Why only 3 bearings for the Y bed?
Don't use the igus bearings. People have tried in the past and it didn't work out well. Use Vesconite instead: https://www.vesconite.com/3d-printer-promo/?mode=web
RE: Why only 3 bearings for the Y bed?
Don't use the igus bearings. People have tried in the past and it didn't work out well. Use Vesconite instead: https://www.vesconite.com/3d-printer-promo/?mode=web
and https://shop.prusa3d.com/forum/general-discussion-announcements-and-releases-f69/anyone-tried-vesconite-yet--t29061.html for my thread on them. I'll add another post this weekend with my observations after a few weeks.
RE: Why only 3 bearings for the Y bed?
Well it looks like one of the press fit bushing holders might have worked after all. Seemed like the size that was tight enough to hold the bed still(ish) was too tight for them to slide easily. Today (frustrated I just left them on the rods overnight), however, they moved easier.
So I bolted them onto the chassis (which appears to have removed the rest of the torsion issue) and gave it a whirl through the settings menu. Without the bed on it seems to be able to move the Y axis just fine. So tonight I'll try to spend another few hours getting the bed back on and leveled and see how it does.
RE: Why only 3 bearings for the Y bed?
So I gave up and went back to the linear bearings after cleaning then and packing them with white lithium grease.
The bolts I used to attach the carriers were a couple mm too long. In my testing and XYZ calibration all was fine, but a couple passes into leveling the bed I started to have crashes. I finally tracked it down to the bolts for the carriers nicking the frame (the black plate for the uprights) just enough to trigger crash detection. Unfortunately poor light and less than stellar vision led me to believe I didn't have the nyloc nuts sunk completely. So I tightened things down more which drew the nuts up more and cracked the carriers.
After that not matter what I did I kept having crash detections even simply moving the Y axis back and forth. I suspect that I deformed the bushings enough to be a problem. So after a few iterations of tearing down and rebuilding the Y axis, I gave up and went back to the linear bearings. All has been happy and I'm back to printing again.
I've run about 30-40 hours since getting the Y sorted back out and the X and Z (still Drylin) seem fine, though I am hearing "clacking" from the X carriage from rough moves as the play lets it move a bit. No sign of it impact the prints at all though so I'll continue to live with those bushings until a problem arises or it's time to replace them.