I was finding myself having to re-tighten the X belt quite frequently on my PrintrBot LC v2, I tried upgrading to a higher quality belt with less stretch, but that only helped a little,. After looking at the design of the two bridges, I realized that mechanically what you have here is a compression bow since the belt is off set from the carriage rods. In this you have two problems, one the bot is made of wood, wood warps even when not under tension, and I could see it with the naked eye, a definite bowing of the two bridges. The second problem this generates is that the movement of the X carriage itself is tugging on the "bow" ends whenever acceleration happens along the "string"(belt) of the bow. This tugs back and forth on the two "bow" ends during printing causing little variances in the warp of the two bridges towards the belt especially at higher movement speeds. (grab a bow and pull on the string along the strings length and watch one end of the bow bend toward the center of the string and the other end of the bow loosen a bit and "de-flex" this is happening a little every time the print head accelerates,. Causing what I can only think to describe as a warble in the print finish especially at corners where acceleration shifts.
I neglected to research otherwise I would have noted that others had made various widgets to tackle this, and a chat with a local printer expert let me know that the original PrinterBot had a wood plank to help fight the bow,. (No idea why they decided to remove it in the LC, it's quite needed). Regardless I had already started working on my solution to the problem and wanted to finish what I had started.
So while this is by no means an original fix it's my take on the fix,.
Four parts (two printed):
The shoulder: Mounts behind the right bridge, Requires a 1 inch 6-32 screw, and a 6-32 square nut, 4 1.25 inch 6-32 screws (you will need to be able to cut/grind one of them to be flush with the nut), and 4 6-32 bolts. For strength & rigidity I recommend 2-3 parameters with 2 top 2 bottom solid layers. PLA is strong enough, though ABS would likely be better.
The Claw: Snaps in around the X motor, can be left loose, but there are also holes in the design to tighten the ends of the claw together with a zip tie if you wish. Two walls in all directions is plenty strong on this part.
The Rod: this can be just about any 1/4 inch diameter rod material, aluminum, carbon fiber, fiberglass, etc,. Though I would caution against wood for hopefully obvious reasons, of course wood would still be better than nothing. I choose 1/4 carbon fiber rod as its strong, light, ridged, and at my hobby shop 1/4 is the cheapest size in the ball park of what I was looking for.
The Mini Nuts: because the X belt tension thumb screws take up some space behind the right bridge when the carriage is all the way to the right and the "Shoulder" takes up some of this space you may need to change the wooden thumb screws to smaller profile plastic ones (also provided),. Though this is a good thing for another reason at least on the LC, move your carriage all the way to the left,. Look at the bottom of the bottom most thumb screw,. It's likely rubbing on the belt,.. bad,. These new mini nuts solve both problems. They will work with the stock hardware, though I suggest adding rubber washers just under the new mini nuts to keep them from loosening on their own from back and forth vibration.
You will have to disassemble a portion of your right bridge to install the shoulder, though I found it fairly easy to do especially with a power screw driver. The screw sticking out of the back of the bridge that's towards the top and closest to the extruder will need to be made flush with its nut to not impact the movement of the carriage. Do cut this prior to installation as the dremmel will heat the bolt plenty hot enough to scotch/damage wood.
The claw just uses compression to be held in place,. Push it on from the side of the motor, it should flex just enough and then snap on. Be careful of the wires that may be just under the motor.
Push the 1/4 inch rod though the shoulder hole then into the claw hole.
Add the square nut and 6-32 bolt to the shoulder.
Now I am playing with different ways of installing this, but so far, I have found that if I loosen up the belt, loosen up the two bridge plates that hold the big X rods in place, and then try to balance everything out with a caliper I can then tighten down the X rods, tighten down the new shoulder bolt while gently pushing the carbon rod towards the motor just a hair. And then last, tighten the belt, things get really, really smooth all of a sudden.
At this time I have chosen not to epoxy the new support rod at the motor claw, and iv also not zip tied the motor claw, though the holes are there to do that if you wish. So far this loose but linearly firm system is working wonders on my print quality. Though I am considering putting some kind of foam or thin padding between the motor and claw as there is a new harmonic signature coming out of the hollow carbon fiber rod,. For now im finding it somewhat novel to have this new noise, down the line ill likely remove it by lessening the possible vibrations from the loose claw either by zip tie or padding.
WARNING: I have not tested the Shoulder with the old wooden thumb screws, I fixed the belt rubbing first then designed the shoulder, I tried to give as much room as possible for the original bulky thumb screws, but I have not yet confirmed that they would not hit something on the shoulder if the carriage was all the way to the right (please let me know the results if you test this).
If your dialed in right, both printable parts should print just fine without support, even with the shoulder having a fairly large bridge to do (see photos)..