Ratchet Knob (Unidirectional Mechanism)

A ratchet is a mechanical device that allows motion in one direction but not the other.

An example of this is the ratchet on a ski boot - a latch holds the strap in place, only allowing the strap to be tightened. Then, a latch release button is pressed to undo the strap.

For an upcoming project, I needed a mechanism that allowed a knob to rotate in one direction but not the other.

I also didn't want any other external part besides the knob itself. That means that all the mechanism had to be hidden internally, and an external button to release the latch was out of the question.

Well, what if we simply pull the knob to disengage the latch, and push the knob to reengage the latch?

So using two household items - clothespins and a broomstick dowel, we're going to make the ratcheting knob!

This is one of those things that is best to see in video to understand!

Check out this quick clip for a demonstration of the ratchet mechanism and run-through of all the steps below from start to finish.

Click here to watch on YouTube

We'll start with cutting half an inch off one end of our dowel.

This small piece has to fit snug inside an inch deep pot hole which we made using a spade drill bit.

Going back to the long piece of dowel, we're going to have to taper one of its ends into a windmill like shape.

So draw in a pattern just like as shown. Also draw in two lines (at half inch and 1 inch from the end) to tell ourselves where to stop tapering.

Symmetry is key here! It will make you a sturdier and smoother mechanism.

Now we take a Dremel (rotary tool) with a sanding drum attachment at an angle and shave off the bits we don't want.

The result should look as shown. You can now see how the dowel can turn one way but a latch with prevent it from going the other way.

Glue the little end back on with wood glue, clamp it in place and let it set for an hour.

To make our two latches, we're going to modify two clothespins.

Again, using the Dremel, shave off a corner to each of the two clothespins as shown.

Take a stack of pennies, half an inch thick.

Put it in the pothole to prop up the dowel so that the little end is completely inside of the hole.

Then we take one of our clothespins and latch the tapered side to one of the grooves in the dowel.

Using a nail, mark a hole into the block through the spring.

Then make a symmetric hole on the other side, and drill them both out like so.

Hammer both clothespins into place with a nail through the hole.

Keep the spring loaded by wedging something into the clothespin, like a pencil sharpener, and hammer another nail to keep the tension in the spring after the sharpener's taken out.

Do that to both latches.

Note: placing the nail closer to the dowel will put more tension into the springs, making the knob tighter to turn.

At this point, test the mechanism to see if it feels right.

Stick turns one way, locks the other way.

Push the stick into the pothole to disengage the latch, pull to engage.

Loosen/tighten the springs (by adjusting the placement of the secondary nails) as you see fit.

(And, don't forget to take out the pennies!)

Now we take our knob, and a long screw.
Cut off the head of the screw, and glue it into the knob with super glue.

Then glue the other end of the screw into the dowel and sink it down until the knob touches the surface of the block.

Give the glue some time to set before turning the knob.

That's it!

A mechanism like this has a lot of uses - wind up toys, Gachapon machines, etc.

Next week, I'm going to show you what I'll be using this for!

If you liked this Instructable, then perhaps you'll like some of my other projects!

You can check them out over at my YouTube channel.

New projects every Thursday! I'll see you next week!