Tuesday, February 3, 2015

Ratchet and Pawl Mechanism

servo-driven ratchet and pawl

I've been working on a project that requires some precision motion control, rotating an object 360 degrees in 12 steps. At first, I tried using a stepper motor but it drew too much power, ran hot, and was too heavy.  Worst of all, it wasn't strong enough to turn my load. A servo would be a better solution but most servos are only capable of 180 degrees of motion. How do you get 360 degrees of rotation from the limited throw of a servo? A ratchet and pawl looked like it might work.

drafting the ratchet
I looked online for an existing design but I couldn't find anything that would work for me. So I drew one up in 123D Design. It was pretty easy; simply draw two circles to define the top and bottom of your teeth. Sketch one line from the center point to the outer edge of the circle. Then use the Circular Pattern tool to replicate that line around the circle as many times as you need (once for each tooth). Then use the 3 Point Arc tool to draw the back edge of the tooth and replicate that arc just as you did the lines. Lastly, just extrude and remove any unwanted parts. The arm and catch aren't nearly as critical, just sketch them to fit the profile of the teeth.

the finished design

With the design finished, I printed the parts in ABS at 230°C with a brim. Since the appearance isn't critical, I printed with a 0.2 mm layer height at 80 mm/s. I mounted the pieces to a scrap piece of lexan and used some small springs to tension the pawl and arm against the ratchet. Using an online calculator I determined that for a 30° arc, the servo arm needed to be about 19 mm long to give me the necessary linear motion from the ratchet arm.

servo connected to arduino

I used an arduino to test the mechanism. I simply modified the Sweep code from the Servo library and shortened the sweep from 180° to 30°. To my surprise, it worked on the first try. I modified the code a few degrees to dial the servo motion in perfectly. This is a nifty little design that should come in handy in the future. Next I'll print a hub to mount this to the part I need to rotate.

Files are available on Thingiverse and YouMagine.

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