In my previous post, I discussed my latest project – automating reading and calibration of a pH and EC probe.  I completed my first go around of building the pinch valve.  The pinch valve will be used to start/stop the flow of solution into the container.

Thanks To Those That Went Before

Nick Johnson – THANK YOU.  Nick posted a YouTube video showing a pinch valve he made that used the HTX900 servo motor.  Nick also posted the CAD files he used on Thingiverse.  I used Nick’s work as the basis for this design and build.  The files from Thingiverse gave me many of the measurements I needed to build the enclosure.  THANK YOU.

James Youmatz – Fusion 360 support for bring up the spur gear addition (see below).  THANK YOU!

Adafruit – THANK YOU YET AGAIN.  For your detailed learning guides on servo motor stuff.

A Confession

Us Gals Learning About Servo Motors

While I am old enough to be a grandmother, sadly – this is the first time I have explored servo motors.  For you lucky young uns – oh the fun you must be having! :-).

Since I was a noob to servo motors, Adafruit’s explanation of servo motors was helpful, especially after I tried their simple sweep test.

Pinch Valve Design

Here’s the design I ended up with:


Here are the parts I 3D printed next to the HXT900:


Spur Gear

3D printing a spur gear to attach at the end of the rotor was a Fusion 360 challenge I had to overcome.  To make one, I started with a roller I quickly designed:


An image of the roller and spur gear:


  • The roller’s hole has a diameter of 3.2mm.
  • The diameter of the roller is 10.6mm.
  • The height is 8mm.
Now…how to put spurs on the roller?…In my Googling I stumbled across this post.  

James.Youmatz details the Spur Gear Generator…how cool is this?…:

“…there is a Spur Gear Generator that you can utilize in the Sample folder within Scripts and Add-Ins.”

These are the settings I used:


Arduino Code

I posted the test code I used at this GitHub location.  


Fail – Not Enough Torque 

Given the hose I am using – a piece of hose I snipped off from another project – the HTX900 did not supply enough torque to pinch the hose.  Perhaps I could find squishy enough hose.  However, the chances of leakage would be high given I am using a gravity set up for the solution flow.  Besides, now I know to look closer at torque requirements.

(side note: I had a disturbing flashback to being in school and having to “compete” on math problems that had a similar thought process to this explanation…i.e.: given this hose with these features and this motor with this torque – will the pinch work – umm…I dunno…can I try it? :-)…so here I try it.  The answer is .. no…).

The failure here is using a servo motor that does not have enough torque to pinch the hose (for now I will stay with the hose I am using).

The HTX900 servo motor has a torque of 22.20 oz-in (1.60 kg-cm) at 4.8V.  So I’m looking for something with “significantly” more oomph.  I just ordered this servo motor from Amazon.com.  I figured I’d think of it like going to a store today and buying one.  Amazon.com listed this servo as same day delivery and the specs are noted in the comments section as : This item will be worked angel in 90°-360° depend on your setting, not a circle. Speed: 4.8V@0.19sec/60°; 6.0V@0.18sec/60°. Torsion:4.8V@9kg-cm; 6.0V@11kg-cm. Working V:4.8V-7.2V .  Ah – the wonders of metal gears…

What’s Next

Once I get the servo motor, I’ll fit the enclosure and other parts and try pinching off the hose once again…hmm…I am sure I could just back of the envelope this but…well…watching stuff spin is more fun.