Shaft couper solutions/recommendations


It seems a great solution. I would test it if you decide to share it


My attempt to connect the shaft. Super rigid coupler with key way. Cut the key way into the shaft with a dremel and 3d printed dremel holder and slider tool with a 10mm linear bearing that holds the shaft. The reduction gear also has a key.


Great idea! How did you cut the internal keyway in the coupler?


Fortunately, the coupler came with it already made, it is stainless steel. I did some research on how you would cut a an internal key way in a coupler: either on a lathe (should work for aluminium):
Or with a “braoching” tool and a press:


On alluminum (Ergal 7075) you can broach the keyway with a late, turned off, fixed mandrel, using a custom made HSS cutter and going in&out less than a tenth or few thou at the time, better if releasing pressure while driving out, to not make the tool dull. On steel I tried…I barely squared the hole! Carbide cutter and an heavier lathe would be needed.
Broaching tool doesn’t apply in our case, it’s a blind hole!
Very important: drill a side hole at the end of keyway to let the chips to excape! Or cut an internal groove same depth of keyway, if you prefer. I just sold my mini-lathe, was ok but too limiting for this project…and future ones!


As I understand it, these bearing have a seal for the bearing cage but not for the shaft itself. So the lip seals would still be needed unless the fit on the shaft is perfect.


What do you guys think about this coupler?

Would it stand the torque


I ordered a couple similar but have not used them because they look a lot like some that have broken under the high torque pressures in other builds. See Flo’s post on April 25 above. But It might be that the one you are referring to is stronger.


Any idea where to buy high torque flex coupler in US?


Here are two U.S. sources I have bought couplings from:


@sunrise305 I am having hard time to locate one with high tourque. I couldnt find any in macmaster with more than 4 nm max tourque


I ordered THIS COUPLING from SDP/SI but have not used it yet so cannot confirm its suitability. But it is rated 8 nm.



@sunrise305 Thank you. I think 8nm should be OK. Please let us know about your test results


What do you guys think about this one?

it is up to 12.5 nm torque


I have a couple of similar multi-piece couplers. It’s not clear to me what will keep the parts together when there is nothing else holding the prop shaft in place.


I guess I found a giid coupler. Just confirmed the 12.5 continuous torque and 25 nm max with the US seller:

I ordered one to start testing it


here is one way to do it.

2x deep grove bearings.
been running a motor with this configuration for half a year. quite a lot of parts but it works :slight_smile:


Best I can tell the unmodified PacificMeister “SealMount” component will not properly hold a multi-part coupler in place since there is noting to keep the parts in place. Aft of the coupler there are only two seals and one bearing., all of which allow the shaft to slide freely. Under normal operation conditions the forward prop thrust should keep a multi-part coupler in place. But – unless I am missing something – touch reverse and you may find the shaft and prop pulling aft and opening the coupler. There’s nothing to hold the three pieces firmly together. Thus in this build the specified fixed (or at least single piece) coupler is required.



HI Hiorth! I will change my blue alu coupler from PM for a new one. You’ve used the “jaw spider clamp”, do you think it is a better solution than the “rigid clamp”? This supplier provide both in custom size (and the price is ok)



i think it definitively is better to change to a more solid type of coupler. Ideally made from steel with a tight tolerance relative to the shaft. I think ideal material would be steel.

we have been using couplers from ruland and they are very good quality and have not failed yet.

also I would recommend using an adhesive like Loctite 648 to mount it.

if you are changing to a "jaw spider clamp" you must have additional support of the prop shaft in axial and radial direction. i would not recommend this if you are using 3d printed parts.