Saturday, November 26, 2011

Belt fender and other ideas...

The traditional Mendel design has large stationary metal washers (fender washers) on either side of the 608 bearings keeping the drive belts on the bearings. The problem many people have experienced with this arrangement is the belt can rub and grip on the washer, causing resistance and leading to motor step skipping, that can ruin a print job.

Despite spending a lot of time aligning my pulleys with the belts on both the X and Y axes, ensuring my stepper power is set to an optimum level, and checking the carriages are running smoothly on their rods, I've still occasionally had the motors skip.

It's as much an experiment in 'design and print' as anything but I devised a clip-on T-bar that keeps the belt aligned on the pulley with minimum resistance. It can also be retro-fitted without removing the bearing.

Sketchup is proving an invaluable tool for a quick 'idea-sketch-print-modify-reprint' way of working. I went on to evolve the 'retro-fit' T-bar concept into the following design (below)... but it's heading back to the washer type solution which I'm trying to avoid. Ultimately, I may just have to strip out the bearings and fit proper rimmed sleeves to the bearings. But for now I'm happy to play with different ideas and possible solutions.

You can see the above design in use on my Y-axis, at the motor end of the printer (see photo below... excuse the working chaos!). Not sure it's any better that the washers, but again, it can be retro-fitted without removing the bearing, and it has less contact surface with the belt, and is a slipper material.

By the way... My acrylic sheet had lost it's grip. I couldn't get the first layer to take to it at all. I tried the blue tape but couldn't get plastic to stick to that if I paid it! (no heated bed yet). So I flipped my acrylic sheet (12mm plate - salvage) over and continued on the under-side. I print the first layer really close to the ground to force it onto the surface and it takes nicely. It does leave a lip around the base of the printed objects but that's not a problem for me at the moment.

I discovered later that a quick wipe of white spirit cleans the acrylic surface nicely and renews it's willingness to hold down the first layer. I still have issues though with larger objects which lift at the edges as the object cools. I do need to get a heated bed. I'm looking into the options.

In the interim, there was an idea on Thingiverse that adding 'mouse ears' to your design could help keep it stuck down to your print bed, if you didn't have a heated print bed. I had to try it out, and it seems to work. See the 'mouse ears' I added to the object in the photo below. That illustrates what I'm talking about.
The object is a replacement 'quick release tripod plate'. (It attaches to the base of you camera and attaches to your tripod... unless you loose it, and have to design and print another one! )
Here's the finished item... I'm delighted with the print quality and turn-around time on these one-off items!

The above exercise was a modified derivation of a plate componend of this item: I imported the plate.stl into Sketchup, increased the height so my longer blot would fit, and made it wider so it sat better into the receiver.

(Technical note: Nozzle .35mm, Layer Height (mm) .25, Extrusion Width (mm) .45, speed 40mm/sec, 20mm/sec on perimeter, Pronterface/SFACT.)
As always... questions and comments welcome, and happy printing!


  1. I like the belt fender. I know I could probably whip it up in Tinkercad or Sketchup, but did you post it to Thingiverse by any chance?

  2. I bet the fender would work a little better if it were rounded. A rectangle is really asking for friction with all those little irregularities.

  3. Hi Gary,
    Haven't thought of putting them on Thingiverse, but I can in a short while.

    Rounding the T-bar would probably help lower resistance even more. I think a quick rub of a file will probably do me at this stage rather than tricking around with the design and reprinting. :) Tks for the input.

  4. There you go Gary, Sketchup files up on Thingiverse

  5. I hate to sound critical, but I think that you are barking up the wrong tree with this one. I had the same problem you are trying to solve on my prusa. In the end I made 608 Idler Pulleys ( With a little adjustment these eliminate friction on the belts entirely.

    There are several designs that are similar kicking about, I am sure that they are all effective.

  6. You know what Blair, I think you are probably right. I just needed to hear it from someone else that ran into this problem. There may be nothing for it other than to print some full idler pully rims, take the pulley's off the rods and fit the rims! A little more effort but probably worth doing, as you've found. :)
    Tks for your input.

  7. See my later post for latest on belt fender design variant.

  8. Nice post. layer thickness .45 <<< Extrusion width I believe...

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  10. Tks for pointing that out Ahmet. I corrected that now.