Y axis belt tensioning

The standard belt tensioning on the A8 Plus uses a pair of pegs attached to one side of the base of the Y carriage around which the belt is wound and fixed with cable ties. At the opposite side to the motor is the idler which is fixed to a sliding metal bracket on the end of an eyed bolt attached to a wing nut. This allows the idler to be moved away from the centre, thus tensioning the belt by tightening the wing nut.

While this worked, it was far from ideal and left room for improvement. 

There were various solutions already available for this and I tried a number. The best (by far, in my opinion) solution for replacing the tensioning mechanism is this design by Indychus on Thingiverse. It is very solid and provides very good control of the tension. 

However, I found that the design intent to accommodate the standard idler brought about the same compromise as I'd seen with the X-axis. Specifically, the 3mm smooth idlers are really not up to the job.

So, my solution here was to rework the slider portion of the design such that I could use the 5mm ID idlers which were working successfully on the X axis. 

These pictures show the modified slider design before and after installation. It fits into the original design of fixed mount and works in the same way but allows for a much more robust and reliable 5mm ID toothed idler. 

The next change was to replace the standard rather poor belt clamp on the carriage. 

As with the X axis, I chose to use the same method of a design incorporating a toothed channel to hold the belt securely and squarely. 

This does require a fairly high degree of accuracy in printing but as long as this can be achieved, the part works very well. The key here is to print the part such that the "teeth" are printed on the Z axis, to gain advantage of the extra resolution available there. Certainly, I found that this was quite easily done. 

In line with the method I employed for the X axis, the new part was again able to be fitted to existing mounting holes on the Y carriage. In this case, these are the two existing holes for the original fixing pegs and their adjacent partners. This provides for four sturdy M4 mounts. 

The bracket clamps to the base of the Y carriage and the two free ends of the belt can be simply slid into the grooves such that they each reach almost to the centre. Once in place, the belt can then be tensioned.

The pictures here show the mount before and after fitting. The four M4 mounting screws are recessed into the mount and the belt sits above (below in use, of course) them. 

The final pictures show the two Y axis components together,  with the belt connecting them. The views show the same from above and from the side.

For anyone proceeding further with these upgrades, it is worth noting here that the upgrade to the endstop switches uses a modified version of the Y axis carriage mount which incorporates an actuator arm for the switch. 

If you plan to replace your endstops too, then you should use the modified version of the mount, not the one shown here. Refer to the page on endstops for more details. 

The parts for this are available on Thingiverse and Cults3D

Enhanced Tensioner slider - Thingiverse  - Cults

Y axis belt clamp mount    - Thingiverse - Cults

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

Addendum 2: When setting the initial belt length, try to make the length of the slider insert as short as possible without compromising its function. This helps to reduce the bending moment on the assembly. 

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