[BKruger (Bert)]

I know that owners of Dobsonians that want to convert their mounts to a Goto mount often have a need for a slip clutch. As mentioned in my previous post I have designed and 3D printed my own version of a slip clutch with a very small footprint.

The following Sketchup drawing shows the different components of the slip clutch. The pulley sizes can be changed depending on the drive ratios that you want to achieve. Please note that these type of clutches are available if you search for it – I was just not prepared to pay a ridiculous price for it – hence I have made my own.

I designed the GT2 pulleys by using tools available on OPENSCAD. I normally copy the STL file to Sketchup (which I know well), modify it there and export an STL file again for 3D printing. The clutch pulley in the drawing below needs to be screwed onto the Outer Shell of the clutch. You can also strengthen the Outer Shell with 4 small screws or bolts (use the small holes for that purpose). That would prevent the outer shell from shearing when the forces are applied. At this stage I am not using it as such but have been able to drive the scope without any problem (so far). I must however still buy long enough bolts/screws and fit them because this is definitely a weak area.

If you use more Inner/Outer Disks more force will be transferred to the Outer Shell and ultimately the Clutch Pulley (Which means the footprint can be small). I have only used three of each and they are only 2mm thick. It is VERY important for a very tight fit of the Inner Disks over the Nuts and likewise for the Outer Disk’s octagon shape to fit very tightly into the Outer Shell of the clutch. This is to rule out any play or backlash. I had to print a couple of sizes to ensure a tight fit because the dimensions of the 3D printed version is almost never exactly the same as the drawing measurement. Also note that in my application the Clutch Pulley buts up against the inner bearing part of the KP08 pillow block.

The clutch that I have used for this scope is very small (if you compare it to the size of the Nema 17 motor). That is because I had limited space in the area where it had to fit in. You can increase the sizes of the Inner/Outer Disks and the Outer Shell (to increase the area onto which pressure will be exerted). It is also important to slightly sand the flat surfaces of Inner/Outer disks - this will prevent slippage.

The drawing below shows my current configuration. However it is possible to re-configure the layout of the different components - it all depends on what you require in your specific application. If have printed the components by just using ABS with 70% print density. In my case I have also just used an 8mm threaded rod and not a rod with a shaft as shown in the drawing. If you can get an 8mm shaft and tap the thread onto the one end it will probably be the best solution.

Hope the above and the drawing makes sense but let me know.