Watt’s curve

Two cranks of the same length turn about fixed pivots, a rod joins their ends, and the midpoint of the rod traces Watt’s curve. James Watt, who studied it in 1784, wanted a motion as nearly straight as possible for his steam engines, and near its crossing point the curve is almost a straight line. Jointed bars give the straight motion without a sliding guide, whose friction wears the parts.

r² = b² - [a sin θ ± √(c² - a² cos² θ)]²

On mathcurve.com as Courbe de Watt