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[Sticky] NEW FOREARM: for everyone having problems tightening the timing belt (STL files attached)  


Honorable Member Admin
Joined: 5 years ago
Posts: 687
17/11/2020 8:56 am  
We have reports from users having problems getting the forearm’s timing belt a bit loose. We have detected that the problem comes from a slightly longer timing belt.
Knowing that and after prototyping many models, we have a final version.
Basically, I have split the UPPER- forearm into two pieces. Now you can adjust its total length just extending it and fixing it tightening the bolt indicated below. If you are having problems with your forearm, just print the STL files and mount the forearm as below. All the other parts stay the same. I will update the assembly guide ASAP.
As the total length may vary, the Arduino CODE has to be modified.
For the Arduino CODE (file configuration.h ):
 You will need to measure the GAP and add this distance to the default value in this line of the Arduino CODE:
// Robot configuration
#define ROBOT_ARM1_LENGTH 91.61 //92.84  FIRST SEGMENT   <-IF THE DISTANCE MEASURED IS 1mm, change the value  from 91.61 to 92.61
#define ROBOT_ARM2_LENGTH 105.92 //106.79 SECOND SEGMENT

Some photos of the new forearm below as a reference





Active Member
Joined: 1 month ago
Posts: 5
17/11/2020 1:18 pm  

This is perfect timing for us!  We got to this point in assembly last evening.  I think we will go ahead and swap this out now while it can be done easily.

IMG 7189

Active Member
Joined: 4 years ago
Posts: 14
23/11/2020 10:38 pm  

I was a little skeptical about getting the bearing out of the old pieces and put into the new printed parts but it was not to bad.  I found that putting the bearing on the table and then pressing with your palm on the part you could easily get the bearing into place.

I will say that the belt is still loose and that I think instead of printing two pieces that it should be one piece just made longer for those people that have the longer belts.  The bearing that is on the bottom of the Elbow is not adjustable and needs to move out with the elbow.

So making a piece that is designed to be longer would fix that and make the belt a better fit all together.


mappler liked
Honorable Member Admin
Joined: 5 years ago
Posts: 687
26/11/2020 11:03 am  


 The problem with enlarging the forearm too much is the stability of this element when moving. We are too close to get into the “resonance” distance due to forearm mass and inertia. We have tested longer arms and when “drawing/ writing” it starts to lose its accuracy (shake while performing certain movements). The 280mm GT2 belt, should be 280-ish belt. Right now, even a 281.3 mm belts can be fitted with the new design but not more than that.

Regarding the tightness… The forearm belt, actually, should be a bit loose. There are no horizontal forces that could compromise the precision of the arm and a 280mm belt does not suffer from “elasticity enlargement”. Besides that, as the robotic arm has been 3D printed… well, an overtighten belt will, in the long run, worn out the plastic parts

Mike, thank you for your feedback. Really. The experience and feedback of other Makers like you encourage us to keep improving the robots!