Linkages are an essential part of many mechanisms. They can be used to change direction, alter speed and change the timing of moving parts.
In the hammering man example two linked linkages are used to convert the small linear movement of the drive shaft (bottom left) into first a rotational body movement and secondly a fast hammer movement. Compare the speed of the hammer with the speed of the drive shaft!

The basic 4 bar linkage. All four bars make up a parallelogram. Two, equal length orange shafts and the distance between the joints on the red moving bar and yellow fixed bar being equal. The movement of the top arrange shaft exactly shadows the movement of the lower orange bar.
By changing these lengths and the lengths of the other bars different movements can be achieved.

Different Lengths. This time, two different lengths of bar, the two long bars, yellow and red are the same length as before.
Look at the tip of the red shaft, notice how it moves smoothly until the last second then flips to the right. The same effect is used in the Motley Man in my book Paper Automata to make him look up at the last moment of his bow.

Quite an extreme arrangement this! With the two long bars crossing over each other. A more extreme 'kick' in the orange bar this time at the end of the green bar's travel. Looks like a likely mechanism for a model!

### Bell Crank

The bell crank is used to convert the direction of reciprocating movement. By varying the angle of the crank piece it can be used to change the angle of movement from 1 degree to 180 degrees. The bell crank was originally used in large house to operate the servant’s bell, hence the name.

“Jeeves, where’s my tea?!”