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Belt Pulley design

I'm a bit wary about the integrity of the Pulley Belt fixing. The threaded portion of the bolt is a good 3 mm short, resulting in only a fraction of the threads of the bolt held within the nut. With the correct tension on the belts, the pressure on the bolt/nut assy is considerable. As these bolts described as "M6x20 Shoulder" (should be M5 btw) I really would like to see these as M5x25. That extra 5mm will expose some threads of the bolt above the nut. That can be compensated for with a split washer and a plain washer, making for a more sound mechanical construction. I'd like that plain washer to be oversized too.
The last thing you want is for the frame top to crack !! :-)


  • Hi John,

    It is hard t see fro your pic but there should be no washer between the lock nut and the chassis top. These nuts have been specially designed in because they have a surface that locks onto the polycarbonate and do not undo with movement and vibration.

    The shoulder bolts are correctly designated at M6x20. The designation refers to the shaft diameter and not the threaded section. M5x25 shoulder bolds would not work because the thread would be M4 and the length of the shaft (25) would not be the correct geometry for belt system.

    As I mentioned above, you should not place a different washer into this section. Not even a split washer. We have tested this particular fitting exhaustively during design.
  • My observation is that there are too few threads in the nut. I would really like to see more threads to have more confidence that the nut is secure.

    I did not fit any additional washers, simply no room for them!

    I understand that the diameter of the shoulder part of the bolt has to match the ID of the bearings, and you have done a good job to have those machined to size. The plain washer is needed to provide clearance for the bearing to run free. However, I found that the washer wouldn't seat concentrically, you can just see that in the photo for the assy closest my finger. (I just wonder if someone overlooked the dimensions of the skirt of those nuts?)

    I'll just have to drip in some 'thread lock'. Nail varnish is a good substitute for that! Don't recommend Super Glue - too messy.
  • edited July 2015
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