A ratchet mechanism is a toothed wheel and a pawl. As the ratchet wheel turns, the pawl falls into the 'dip' between the teeth. The ratchet wheel can only turn in one direction - in this case anti-clockwise.

The pawl can be regarded as a small lever / latch that engages with the teeth of a gear. When the ratchet is turned in an anti-clockwise direction, the pawl slides over the teeth (with the aid of the spring),allowing the ratchet to rotate.
When the pawl is positioned between the teeth, it locks the ratchet preventing a reverse rotation .This is why the water bucket (see below) does not drop back into the well.
This unique mechanism allows rotation in one direction and prevents it in the other. This useful feature is applied to tools such as socket wrenches.

The water well seen below has a ratchet mechanism that allows the person to rotate the handle in an anticlockwise direction. The bucket of water is heavy and so the person can rest by taking his/her hands away from the handle. This is because the pawl has fallen into the 'dip' between the teeth and so the bucket cannot fall back into the well.

Ratchet mechanisms are very useful devices for example, they are used in mechanical clocks. They are also very useful when using a system, such as the one seen opposite, to lift heavy weights.

You are to design a ratchet mechanism to fit in the back of a delivery van. This system will be used to lift heavy packages from the van to ground level. Use drawings and notes to explain your design.

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