What's a Schmidt Coupling?

A project log for Paste Extruder using Moineau Pump

doctekdoctek 11/26/2014 at 05:080 Comments

"So what's hard about a Moineau pump?" you ask. "OK, you need some math, but big deal. That should do it." Seems like all that's left is to create the impeller and chamber using the profiles, then connect a motor and away you go.

There's just one small problem. If you look at the animations on the Gravesen site, you'll notice that the impeller follows an eccentric path as it rotates: there is no fixed center of rotation. So you can't fix a gear or shaft to the impeller and simply rotate it. Ugly! Any center you define and try to align with a motor shaft (for example) will move out of alignment before one pumping cycle is complete. I spent some time trying to come up with a way to use elliptical or eccentric gearing. While I was able to create a way to follow the correct path, the impeller needed to turn more than one revolution while the gearing would only turn one revolution. A frustrating failure!

What is needed is a mechanism that can accommodate shafts that are not aligned. Fortunately, such mechanisms exist. The one I liked best is called a Schmidt Coupling. I found a nice design for one on Thingiverse. Printing one of these helped me (a lot!) to understand the operation. The design was modified slightly to fit my Moineau pump. You can see it in action in the video.

Now how do we drive the Schmidt coupling ?