If you’re transmitting mechanical power via a piece of PVC pipe, chances are that things aren’t aligned perfectly. There’s also a good chance that your pipe transmits a certain amount of shock load, as seen in my Mountainbeest. In my ‘Beest’s case, there is a certain point as the motor turns that the driving shaft turns violently due to gravity.
After ruining a fairly expensive motor* due to this violent motion, and nearly losing another, it was time for a change. Check out the video (and other build info) after the “read more” to see how this improvised PVC beam coupler (AKA helical coupler) turned out. I’m quite happy with the results:
After this happened, I considered a few solutions, including using a different motor, or weights to offset some of the torque. I even considered a proper shaft coupler like this one from Amazon. Fortunately, I didn’t rush to buy something, or even start cutting until I’d had time to let the idea stir around in my head. My mind went back to something I’d seen on Hackaday a while ago, about a technique called “kerf bending,” using careful cuts in wood to allow it to bend. It would seem that a similar technique could be used on PVC pipe.
How to Make the PVC Pipe Shaft Coupler
The cuts shown in the video are meant to sort-of imitate** the cuts from the coupler mentioned above with a chop saw. What I tried to do was cut roughly halfway through the 1 inch PVC pipe, move the pipe over about twice the width of the saw, then rotate the pipe a quarter turn (90 degrees). The cut is made again and the procedure repeated until I had 8 cuts in the pipe.
Why 8? I don’t have an extremely good reason for this number, but it did give it two cuts at each of the four rotational positions. This symmetry seems like a good idea, and should allow it to flex evenly in all directions.
If you follow me on Twitter (if not, you should!), you may have noticed that I changed my profile to say: “Inventor of the PVC pipe misalignment coupling.” Maybe that’s true, I’m not sure. If you’ve done something similar, be sure to let me know!
Although what I have now with two motors is certainly better, I wonder if this shock absorbing PVC misalignment coupler would have helped with the original motor design. I still would have needed a better alignment system at the windshield wiper motors, but maybe it would have helped reduce the shock starting up.
*The motors ruined were ServoCity gearmotors as seen here in the 3 and 2 RPM varieties. Noted there is that if one is to stall these motors, damage can occur, so certainly I did this, and quite a bit worse.
Another motor was nearly ruined when I turned my back, and may have actually got caught on something. I took a gear out of another already ruined motor (melted from my soldering iron) and repaired it (good to keep standard parts).
Given what the motors are going through in this case, I can’t really complain. I’m still quite impressed the little motors can put out as much torque as they can.
**It would be interesting to see how more of a spiral cut would work on a properly set up lathe.