Lengths of the crank and the connection rod are equal.

The crank, the connection rod and two short links creates a rhombus.

Equation of traced ellipse: (x.x)(a.a) + (y.y)((a – 2b). (a – 2b)) = 1 the coordinate origin O is the fixed revolution joint center of the bue crank.

Axis Oy is vertical. Axis Ox is horizontal.

a is length of the crank.

b is length of the short link.

The blue bar can not turn full revolution because of interference between the orange long pin (connecting the pink and green bars) and the violet and blue bars. So the mechanism can draw only less than one-half of an ellipse (left or right).