X axis belt tensioning

The X axis as standard uses a fixed bearing mounted on the right-hand Z axis carriage and a 3D-printed tensioner which attaches (at one end only) to the rear of the X axis carriage. One of the drawbacks of this approach in the standard build is that tensioning the X axis belt will lead to deformation of the Z axis rods. Essentially, as the belt is tightened, it will pull the Z axis rods closer, which is highly undesirable. 

If you plan to retain the standard tensioning method on the carriage, then I strongly recommend attaching collars to the right-hand side of the X axis rods against the Z axis carriage. 
8mm collars on X axis








My solution introduces two parts. The first is a clamp which fits to the rear of the carriage using existing mounting holes.
X axis belt bracket


An essential consideration here is that the location of the existing holes predicates the need for the clamping to happen on the bottom of the belt whereas the original was on the top.

This means that the direction of the X axis travel must be reversed in order to use this solution

This is a simple thing to change. It can be done either by a simple firmware change in Marlin or by reversing the cables to the X stepper. I don't propose to go into detail here, though, as it is very easy to find instructions online. 

X axis carriage bracket in situ

The second part of the exercise is to fit a modified bracket to the end of the X axis, on the far side of the right-hand Z axis carriage. 

This bracket includes a fitting for a 5mm ID 20T idler which has performed well in my tests. 


The pictures here show the device before and after fitting. It includes two 6mm tensioning screws which connect to captive nuts held within the body of the bracket. 

Importantly, these screws push against the ends of the X axis rods. This means that the tension on the belt is translated into pressure along the length of the rod and not (now) into the Z axis carriage. This implementation therefore brings two advantages: it provides a much more robust method of tensioning and also removes the problem of the original design in pulling on the Z axis rods. 

You can see that despite the fact that the problem should be resolved, I chose to leave the collars in place. One thing which they did show was the amount of flex in the original design. After the new tensioner was installed, there was a visible gap between the end of the collar and the carriage. A gap which wasn't there before!

These devices are downloadable from Thingiverse and Cults3D

Carriage Mount - Thingiverse  - Cults3D
Tensioner/Idler - Thingiverse - Cults3D

Addendum: When selecting the hardware for this, be sure to choose nylon locking nuts and not plain ones. This stops the tensioning being loosened by vibration.

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