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Straight Line Generators
Watt's Linkage
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When the radius rods (AB and CD)are of equal length, then the midpoint (P) of the connecting link (BC) traces a figure eight pattern over the full mechanism travel. This animation shows only the portion of the travel that approximates a straight line. The error (the divergence of the path of point P from the desired straight line) can be seen to the left of the line at the top of the stroke and to the right at the bottom. For an example of the way that Watt used this linkage in a steam-driven beam pump click here (under construction).
James Watt, a mathematical instrument maker at the University of Glasgow, devised this linkage to support the piston rod of his beam steam engine (about 1769). Straight line mechanisms were a key technology to the development of steam power before the advent of methods and materials for production of crossheads and their slides. The Watt linkage was popular in the 1950s to locate rear axles in racing cars.
Raphael paints wisdom, Handel sings it, Phidias carves it, Shakespeare writes it, Wren builds it, Columbus sails it, Luther preaches it, Washington arms it, Watt mechanizes it. - Emerson, Society and Solitude. Art.
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