11.6  Straight-line mechanisms

Guiding a point along a straight line with pins only, the problem that occupied engineers from Watt to Peaucellier. Approximate straight-line linkages (Watt 1784, Roberts, Chebyshev, Hoeken), the coupler curve and the size of the deviation over the stroke, computed and measured on a printed model. The exact solutions: the Peaucellier-Lipkin inversor in the plane and the Sarrus linkage in space. Why the problem mattered when flat surfaces and slideways were hard to make, and where these linkages survive: suspension (the Watt linkage on a rear axle), walking robots and the printed flexure guides of Compliant mechanisms.