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In kinematics, the parallel motion linkage is a six-bar mechanical linkage invented by the Scottish engineer James Watt in 1784 for the double-acting Watt steam engine. It allows a rod moving practically straight up and down to transmit motion to a beam moving in an arc, without putting significant sideways strain on the rod.
The linkage actually used by Watt (also invented by him) in his later rotary beam engines was called the parallel motion linkage, a development of "Watt's linkage", but using the same principle. The piston of the engine is attached to the central point of the linkage, allowing it to act on the two outer beams of the linkage both by pushing and ...
Linkage mobility Locking pliers exemplify a four-bar, one degree of freedom mechanical linkage. The adjustable base pivot makes this a two degree-of-freedom five-bar linkage. It is common practice to design the linkage system so that the movement of all of the bodies are constrained to lie on parallel planes, to form what is known as a planar ...
The exception in this list is Watt's parallel motion, which combines Watt's linkage with another four-bar linkage – the pantograph – to amplify the existing approximate straight line movement. It is not possible to create perfectly straight line motion using a four-bar linkage, without using a prismatic joint. Watt's linkage (1784)
Watt's parallel motion generator consists of the four-bar linkage that has a coupler curve that traces an approximately straight line trajectory, combined with a parallelogram linkage that copies this straight line movement to a desired location. This configuration of six bars and seven joints has two four-bar loops.
To bridge the conflicting actions of the beam and the piston, Watt developed his parallel motion. This device used a four bar linkage coupled with a pantograph to produce the required straight line motion much more cheaply than if he had used a slider type of linkage. He was very proud of his solution.