Watt

GENERAL | ARTICLE

James Watt and his Linkages
Sangamesh R Deepak and G K Ananthasuresh James Watt’s name is synonymous with steam engines but there is another equally impressive facet to his technical accomplishments in the areas of design and manufacture of machines. In this article, we highlight his contributions to the design of linkages, which are assemblies of rigid bodies that performintricate mechanical motions. The linkages described here range from the ones that can draw a straight line to those that can replicate three-dimensional objects through purely mechanical movements. We use a compass to draw a circle and a scale (a ruler as it is sometimes called) for a straight line. We generate a circle with the help of a compass whereas we copy a straight line from a scale. Ifthe scale is not straight, so will be the line we copy. It is difficult and expensive to make a perfectly straight scale. It was more so before the industrial revolution when manufacturing techniques were not as good as they are today. Hence, craftsmen of that time explored alternate means for generating a straight line. A particularly vexing problem encountered by craftsmen of the mid 18th centurywas guiding a piston inside a cylinder of a steam engine. The first person to come up with a solution, although approximate but good enough for practical purposes, was James Watt. The approximate straight-line linkage that James Watt developed is known today as the Watt’s linkage. This and other inventions of Watt defined a field called linkage synthesis and occupied the minds of many engineers,scientists, and mathematicians since then. Thus, James Watt was not just an inventor who played a key role in ushering in the industrial revolution but was also the one who played a key role in transforming the image of able craftsmen into engineers and scientists. By discussing his contributions to linkages, we present a profile of Watt that is much more than the person who improved Newcomen’ssteam engine and made steam power generation portable.
RESONANCE ⎜ June 2009

(left) Sangamesh R Deepak is a PhD student in the Department of Mechanical Engineering, IISc, Bangalore. His specific interest is the design of innovative mechanisms using deformable bodies that can be held in static equilibrium in multiple configurations. (right) G K Ananthasuresh teaches in the department of MechanicalEngineering, IISc and is engaged in research on optimal design and mechanics concerning machines and structures, microsystems, biological cells, and proteins.

Acknowledgements: Professors Ashitava Ghosal (IISc), Amitaba Ghosh (IIT-Kanpur), Gregory Pennock (Purdue), and Anupam Saxena (IIT-Kanpur) read this manuscript and suggested improvements. The authors are grateful to all of them.Keywords James Watt, straight-line linkage, centrifugal governor, sculpting machine.

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GENERAL ⎜ ARTICLE

Watt’s Straight-line Linkage Let us turn to the question of why Watt required a straight-line linkage or other linkages that he invented. Figures 1a,b show a linkage (see Box 1 to know what linkages are) in the steam engine that was invented by Thomas Newcomen in 1712 – almost 24 yearsbefore James Watt was born. This linkage, which is simply a lever driven by steam power, was used to pump water out of mines. It had a wooden beam pinned to a base with sectors attached at its either end. Iron chains tangentially hung from the sectors and held a pump-rod and a piston-rod. Since a chain hanging from one point remains vertical due to gravity, the rods could be guided vertically. But achain will be straight only when it is pulled. We cannot push through with a chain. In a double-acting steam engine that Watt invented, there was a need to push and pull on the piston-rod. Manufacturing a snugly fitting and yet freely movable piston-cylinder assembly is one option. But it was not practicable at that time. Watt did have access to an able ironmaster, John Wilkinson, who could...