ALTERNATIVE GEARS
The ratchet is really another form of gearing. Unlike gears which can be used to speed up or slow down movement, the ratchet can only be used to slow things down and it happens in a very jerky manner. Below and opposite is an explanation of how they work.

 

For every 8 turns of the crank the ratchet wheel turns one complete revolution. This gives a ratio of 8:1.

CRANK
PAWL
RATCHET
PAWL
Below is a 3D drawing of a simple ratchet. There are not many moving parts to make but as with all mechanical things you do have to work out fairly accurately the size and distance of all the various components.
The pawl is rotated on a crank and pushes the ratchet wheel one notch for every revolution. It will take 8 turns of the crank to produce one complete turn of the ratchet wheel.
The crank turns clockwise which helps to push the pawl or catch down onto the ratchet wheel
This 2nd pawl is lightly sprung and locks into the ratchet wheel stopping it moving backwards.
If you add more teeth to the ratchet wheel it will take longer to rotate. If you have less teeth it will take less time.
TAKING ADVANTAGE
The ratchets disadvantage of a jerky movement can be taken advantage off. The automata below uses this movement to build up suspense before finalising in a big finish
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