Update 2/11/17 Added pictures
For most Prusa I3 machines, getting sufficient tension on the belts can be tough. The X Axis belt is the most difficult. This is especially true if you have to take the belt off and try to re-install it, in it's now trimmed form, where you can't seem to get enough purchase on the belt to pull hard enough to get tension anymore. The RepRapGuru I3 (original or V2 with the display) is no exception to this problem. Spring clips are not a good solution, as they add their own artifacts to the print by allowing the belt tension to be variable. What the machine does have is a nice open through-slot on the left side of the X Axis assembly into which we can add our own tension block.
On my prints, I started seeing X-axis shifting, indicating the belt was slipping during the print which ruined several long print jobs. This was my solution.
On the left side of the X axis is the belt idler pulley, secured by an M3 bolt and lock nut. We can replace that static bolt with our tension block, allowing us to adjust the tension easily using two M4 screws. You can print this in PLA or ABS, either works well.
Print the part supplied here. In addition you'll need:
(2) M4 bolt x 30MM or larger, then sized to allow for adj w/out too much sticking out.
(2) M4 nut.
(1) Strip stainless steel, recommend K&S #87163 available online or at Ace Hardware for less than $2. Cut two 60mm lengths from the strip.
(1)M3 bolt or 3mm steel rod to act at replacement axle for the idler pulley. Cut and trim this to be exactly the same width as the inside of the slot where the idler pulley screw used to pass through, about 11 mm. Be ready to file this down to size until it's exactly the right length.
On each of the steel strips, drill identical 3mm holes at each end, along the centerline of the strip. See the assembly diagram above. I used a 1/8 inch bit which is just slightly larger than 3mm, and I taped the two strips together to ensure alignment.
Dismount the X axis belt and remove the M3 bolt and lock nut that secure the idler pulley.
Insert the two steel strips into the block, secure them in place by reusing the M3 bolt and locknut from the idler pulley. Insert the two M4 nuts into the nut traps in the block. Insert the M4 screws from the outer face and thread them onto the M4 nuts, but just flush, not all the way through.
Mount the idler pulley onto the exposed ends of the steel strips, passing the M3/3mm replacement axle through the pulley and through the drilled holes in the metal strips.
Insert the now assembled tension block assembly from the outside (leftmost) edge and into the slot. Thread the belt back through the pulley and secure/zip tie to the anchor block on the print head carriage. (you may want to instead pull the belt through the slot in the block and thread it through the pulley, and then pass the belt and block into the slot. Up to you...) Reconnect the belt to the print carriage, taking out as much slack as you can, but no longer needing to really apply force in doing so. That's what the tensioner is for.
- Using a screwdriver (or EVEN just finger tight, I've found is sufficient) evenly tighten down on each of the two M4 bolts. This will push the block away from the center of the machine, and tighten the belt. Do so until you have applied enough pressure to lessen the amount of deflection you can get if you try to pinch the belt together. Be reasonable though, and be warned: too much tension can pull the Z Axis smooth rods in towards each other. You could bend them, overload the Z axis bearings and/or the print carriage bearings, or distort your prints in other ways due to this tension and strain. I find that it does not require that much force now that I have the block doing the work for me.
I've supplied the Fusion 360 source file so that you can make adjustments for your application. If you come up with an improvement to it, please share it back! :-)
I'm not sure that the stock plastic idler pulley will give you enough clearance on the sides to make this work. I had occasion to change out the stock idler pulley, so I replaced it with an aluminum 20T dual bearing pulley which is what I used when designing the part.
you could also go toothless, like stock::
If you have the stock idler pulley, you could try thinner steel... it should be plenty strong at half the thickness. IE: http://www.acehardware.com/product/index.jsp?productId=106613346
PLA seems to be just fine.