A Safe Drivetrain

Many of my projects are electromechanical, at least to some degree. I often find myself wanting to turn something with a stepper, a servo or a gear motor. Until recently I’ve generally done that by press-fitting a shaft of some sort (usually a dowel, or a Delrin rod) into whatever GF-cut thing I’m trying to turn. Often I’ll glue the joint if I’m worried more about it failing than I am about needing to disassemble it. As I was working on one such project the other day, it suddenly dawned on me that there’s a much better way. And since I haven’t seen it here, I thought I’d write it down.

I already use the GF to cut my dowel and Delrin shafts to precise lengths using a jig. What I realized is that I can do the same to make precise-length shafts with double-flats on the end(s). Double-flat shafts are great for transmitting rotation because a piece with a matching hole locks firmly onto the shaft.

But once I do that, if something jams, the torque transmission will be so good it’s likely to break the mechanism. Then it occurred to me I could easily add a shear pin coupling to prevent that. Here’s a typical example of the parts involved:

As you can see, ends of the round stock have been cut away to give them double-flat ends. Assembled it looks like this.

From the rear

From the front.

The flower-shaped thing is a sprocket for a wooden roller chain. It’s the part I want to drive by turning the shaft. The holes in it are for the shear pin(s). They line up with the holes on the drive disk. The driver is the bigger of the disks. The other is the retaining cap.

The drive disk is what gets directly driven by the shaft. It just slides over the end of the shaft. Its hole fits exactly over the double flat, so it can’t rotate independently of the shaft. Then the sprocket is slid on. It has a round hole in it, so it’s free to rotate relative to the shaft. Finally the retaining cap is slid on keeping everything from falling off. It’s press-fitted.

Once everything is assembled, the sprocket is rotated until the holes in it line up with the holes in the drive plate. At that point the shear pin(s) are inserted to lock the two together. In this example, there’s only one shear pin, a sliver of wood.

The result is that the sprocket is locked tightly to the shaft, but if something jams up, preventing the sprocket from turning, the shear pin will break, saving the mechanism.

All this is, of course, completely standard well-duh-level mechanical engineering, just adapted for use with the GF. The pattern is easy, adaptable and reliable. It’s become a regular part of my designs.


Never underestimate duh-level engineering.
Great idea


Thank you for presenting this. I love learning from fellow makers.


Very nice. I use pins all the time to line-up/attach layers together, but I never thought about using them as “shear pins”. I really like what you have done here.


The shear pin is a splendid idea. :sunglasses:


Added to the mental list. Oh who am I fooling… Bookmarked!


I feel like I just read through a Master Class introduction and I’m already feeling smarter! Thanks for the read!


Love it. There is so much more we can do with the :glowforge: than most of us do. Thanks for sharing and opening up some head space!


Excellent!!! What a great idea!

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