Ratchet mechanisms are useful for ensuring movement occurs in one direction. Examples include the hand brake lever, socket wrench, and tie wraps. A nice way to explore how ratchets work is to create one out of cardboard. The following steps will show how to make a ratchet gear and two pawls that are easy to assemble into a neat handheld device that demonstrates how it works. It is fairly easy to operate with one hand a bit like a fidget (non-)spinner.

Supplies

corrugated cardboard - commonly used in packaging boxes for stuff ordered over the Internet
card stock, preferably in 2 different colours
A drawing pin or thumb tack
PVA glue
Epoxy resin, or super glue
Pair of scissors or exact-o-knife
Pair of compasses or a circle cutter
Pencil
Ruler
Protractor

Step 1: Cut Out Circles

Cut a circle 6 to 7 cm in diameter out of card stock
Mark out a number of equally sized wedges. 12 is a good number to work with.
Cut two circles of the same size out of corrugated cardboard

Step 2: Glue the Circles Together

Use PVA glue to stick the corrugated cardboard circles together. Pay attention to the orientation of the corrugations. Gluing the corrugations at right angles to each other would strengthen the final result
Glue the card stock circle on top of the other two
This completes the main disc that will become the gear wheel of the ratchet mechanism

Step 3: Cut the Gear Wheel Teeth

Mark a straight line at an angle across each wedge as shown - starting at right angles to each of the radial lines and ending at the perimeter. The aim is to make a saw-tooth pattern that wraps around the outside rim of the circular disc
Cut along the lines you just marked. You can use scissors for this step instead of an exacto-knife, however scissors tends to squeeze the gear teeth flat. This shouldn't pose a problem to the working of the final mechanism
Cut along the radial lines to remove a piece from each wedge on the disc. What remains will form the gear wheel of the ratchet

Step 4: Cut Spacers

You will also need at least one spacer of the same thickness as the gear wheel you just created. Make this by cutting two smaller circles out of corrugated cardboard. These can be optionally stuck together using PVA glue.

Step 5: Making the Pawl

An important piece of any ratcheting mechanism is the pawl which engages with the teeth of the gear wheel to stop it from turning in one direction

Start by cutting two long rectangular strips from card stock. Use a different colour from that of the gear wheel. The width of the rectangles should be roughly the same as three teeth on the gear wheel.
Punch a hole in one of the strips using a drawing pin. The drawing pin will be used as the shaft of the gear wheel. The hole should be near one end of the strip but not so close that the gear wheel would extend beyond the edge.
With the gear wheel in position on the drawing pin, mark a pencil line parallel to one of the teeth. Don't trace along the tooth. The line should be offset away from it but not too far away. Expect some trial and error with getting the positioning just right.
Draw an inverted L-shape above this pencil line. This will become the pawl.

Step 6: Cut Out and Test the Pawl

Cut along the inverted L-shape but do not cut along the initial pencil line. This line will become a creased edge.
Fold back the L-shape along the initial pencil line. Make another crease parallel to this fold line so that the inverted L-shape is standing upright
Bend the inverted L inwards towards the pin hole
Test the pawl design
- Insert the drawing pin
- Place the spacer discs on the pin
- Place the gear wheel on the discs
- Try turning the gear wheel forwards and backward to check that the pawl engages with each tooth.
The gear wheel should freely turn in one direction but be stopped by the pawl from turning in the opposite direction
If it doesn't work try adjusting the pawl - making the creases sharper or straightening them out. Failing that, get a new strip of cardboard and try positioning the pawl either closer or further away from the drawing pin.

Step 7: Ratcheting With Two Pawls

Once you've found a pawl design that works, use the other rectangular strip of card stock to create a 2nd pawl using the same dimensions as the first one.
Mount both strips on the same drawing pin.
Place the spacer discs on the drawing pin, followed by the gear wheel
Test that both pawls engage the teeth of the gear wheel. Move the rectangular strips by flexing and contracting them. Observe that the gear wheel turns in one direction when doing this.

Step 8: Assemble the Final Mechanism

Cut one of the rectangular strips into two pieces. One piece will have the pawl and pin hole. The other piece will be used to form a bracket around the gear wheel
To form the bracket, make a few creases in the strips of card stock. These should be spaced such that the strips can be folded around the gear wheel and spacer discs.
Glue the strips together. If you plan for sufficient overlap, the doubling of the layers of cardboard will add strength to the final device
Push the drawing pin through the pin hole. The previous gluing step might have covered the pin hole necessitating that you punch a new hole in the freshly glued layers.
Position the card with the 2nd pawl as before, and mount the space disc and gear wheel.
Fold over the rectangular strip on top of the gear wheel. Push the tip of the drawing pin through the top layers of cardboard
Glue the overlapping top layers of card. You might find that PVA glue is not strong enough to prevent the drawing pin from falling out, so use a drop of super glue or epoxy resin to secure the pin.

Step 9: Display Window Enhancement

Your handheld ratchet mechanism is now complete!

One useful modification you might like to make is to cut a small window in the bracket to display information that changes as the gear wheel ratchets forward one step at a time. You might find it easier to cut this window before the final step of gluing everything together.

The ratchet gear shown here displays the twelve animals of the Chinese Zodiac. You can make your ratchet to display other information, such as birth stones, star signs, emoji, select different colours, or to keep score in a game.