Here is what I see that is "different". When the belt is on a down stroke (moving counter-clockwise), it pushes the pressure bearing out, when it's on the up stroke it relaxes more, and the bearing pushes the belt in more.

That tells me the belt has more authority over the bearing, and It should be the other way around.

That means the spring is not strong enough.

On my printer, the bearing seems to have the same pressure in both direction.

I'd second this was worth chasing up: if the tensioner is moving, it means the relative length of the belt on each side is changing, which means the relative angle of the two pulleys is changing with direction... Which comes out as backlash. Ideally the belt doesn't change shape with direction, the aim of this tensioner is to squish it in enough that it doesn't?

Here is what I see that is "different". When the belt is on a down stroke (moving counter-clockwise), it pushes the pressure bearing out, when it's on the up stroke it relaxes more, and the bearing pushes the belt in more.

That tells me the belt has more authority over the bearing, and It should be the other way around.

...

On my printer, the bearing seems to have the same pressure in both direction.

Thanks for that info.
I did have a problem of step skipping with the next stronger spring I had in hand.
Maybe I should try this mod again when I get bearings for the back rod like you have to decrease the pressure on the Y motor a little.

I think this mod is better than moving the Y motor down instead, since because it creates more surface for the belt on the pulley, you probably need less tension to achieve the same result. I think this is jagowilson's theory also if I understood him correctly.

That tells me the belt has more authority over the bearing, and It should be the other way around.

That means the spring is not strong enough.

On my printer, the bearing seems to have the same pressure in both direction.

This is a very promising mod, so I hope I'm doing something wrong as I don't get any improvement when  trying this myself. I printed the STLs, got the appropriate bearing, screw it in, screwed the printed parts to the motor and  tried few different springs.

Result without this mod: https://lh6.googleusercontent.com/-5X-w … 1600/2.jpg

I've also recorded a video of how it works.

One things I noticed was I had to flip the pulley on the y motor so the bearing would align perfectly with the belt.

one thing I see in your video, not seen in others machines is the huge adjusting knob for 'Z' stop. is it touching either shaft/rod for 'Z' or 'Y'? that may be a point to watch as any slight friction on them can cause deflection as they are easy to turn/move.

@ Claghorn: Your links are not working.

Thanks! I fixed 'em now. That's the trouble with having my local copy, everything works when I check it from here :-).

I did a bit of back tracing and found the actual web page.  The linksyou posted are of your internal LAN, not the external (WAN) address. 

Here is the link to the actual project page:

https://home.comcast.net/~tomhorsley/ha … nsion.html

You might also want to fix the link to the image.

More details here:

http://home.comcast.net/~tomhorsley/har … nsion.html

I gotta make a slightly longer spring now and see if it gets even better with more force, but it is definitely headed in the right direction (actually didn't seem to make much difference with a more powerful spring).

Now I'm wondering if a bearing on both sides would work even better? I'm imagining something like a scissors arrangement with a bearing on both sides of the belt (if there is room) and a tension spring or bungee at the bottom to try and close the scissors on the belt.

Thanks for sharing the STLs.

Though I'll be annoying and again ask for top-down photo of a circle print after you installed this mod. The last one you shoot was very close just a bit front view rather than top view.

Yes, this is getting annoying. 

I did the test, posted a photo, as close as I could get to it, but that's it.  If that's not good enough for you, sorry.  It's good enough for me.  I see no reason to do it again, and I'm certainly not taking the glass off and putting it on a scanner.

However, it could still be an issue with newer batches of Solidoodle 3 and SD4, like swtiching to different quality parts on the printer.
Otherwise wardjr either got lucky with his specific Solidoodle 3 for not suffering from this problem or his SD3 still has some things privan's was missing to print fine without any mod.

Anyway, I'm really excited someone was able to fix the problem finally.
Though I'll be annoying and again ask for top-down photo of a circle print after you installed this mod. The last one you shoot was very close just a bit front view rather than top view.

BTW, have you also repaced the side bushings with flanged bearing as well, or only that pillow in the middle of the rod (Thingiverse link?)

So pirvan, what awesome circular things are you going to do with your printer now?

I've been planning on printing a few models that go together in pieces via ball joints  Like this guys here:

before this I usually didn't attempt to print anything that had circles, meshing gears or ball joints.  Now I can.

Er... that's not gonna happen.  If you want to test the effect of the tensioning mod without a pillow block, you can try that on your own printer.

That's not what I want to test. I wanted to know if the bearings had any positive effect on the circles and how much they contributed to your solution vs this mod (because this mod by itself doesn't seem to help me at all) before I would buy and wait for my own flanged bearings to ship.
The stock SD4 pillow is easy to remove. I guess the custom one you are using isn't. Sorry then, I didn't know.

Well then, can you please post a straight top-down photo of a circle you have printed after this mod? The photos are at a bit of an angle and hard to tell for me.

I'm glad at least one of us Solidoodle 4 owners has reached a point where hes satisfied with the quality of the prints. Even if the rest of us don't know exactly what the solution is. Will just order stuff online and add one upgrade after the other I guess.

So pirvan, what awesome circular things are you going to do with your printer now?

Maybe it's the combination of the two bearings in the middle + this mod that made it work for you?

It could very well be.  My original circles were a lot worse than yours.  At small sizes, they weren't even circles.

Er... that's not gonna happen.  If you want to test the effect of the tensioning mod without a pillow block, you can try that on your own printer.