[[Image:Watts Parallel Motion Linkage.gif|thumb|Animation of the parallel motion linkage.<br>
Dimensions (unit lengths ):
Vertical distance between ground joints <br>
Horizontal distance between ground joints <br>
Thus, link 1 (total distance between ground joints) <math>\approx \sqrt{4a^2 + 4b^2}</math>
]]
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.
Description
right|thumb|[[James Watt|Watt's parallel motion on a pumping engine]]
In previous engines built by Newcomen and Watt, the piston pulled one end of the walking beam downwards during the power stroke using a chain, and the weight of the pump pulled the other end of the beam downwards during the recovery stroke using a second chain, the alternating forces producing the rocking motion of the beam. In Watt's new double-acting engine, the piston produced power on both the upward and downward strokes, so a chain could not be used to transmit the force to the beam. Watt designed the parallel motion to transmit force in both directions whilst keeping the piston rod very close to vertical. He called it "parallel motion" because both the piston and the pump rod were required to move vertically, parallel to one another.
thumb|left|Hand-drawn diagram in [[James Watt's letter to his son. The sketch he included actually shows what is now known as Watt's linkage which was a linkage described in Watt's 1784 patent but it was immediately superseded by the parallel motion.
The parallel motion differed from Watt's linkage by having an additional pantograph linkage incorporated in the design. This did not affect the fundamental principle but it allowed the engine room to be smaller because the linkage was more compact.
It would have been possible to connect directly to the piston rod (the "Watt's linkage" design), but this would have made the machine an awkward shape, with a long way from the end of the walking beam. To avoid this, Watt added the parallelogram linkage to form a pantograph. This guarantees that always lies on a straight line between and , and therefore that the motion of is a magnified version of the motion of . is therefore the point to which the piston rod is attached. The addition of the pantograph made the mechanism shorter and so the building containing the engine could be smaller.
As already noted, the path of is not a perfect straight line, but merely an approximation. Watt's design produced a deviation of about one part in 4000 from a straight line. Later, in the 19th century, perfect straight-line linkages were invented, beginning with the Peaucellier–Lipkin linkage of 1864.
See also
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- Pantograph, part of what the Parallel motion linkage uses.
- Straight line mechanism
- Watt's linkage, the core of how the Parallel motion linkage works.
References
;General
- Linkages article in Encyclopædia Britannica, 1958.
- Parallel Motion article in Encyclopædia Britannica, 1911.
- Robert Stuart, A Descriptive History of the Steam Engine, London, J. Knight and H. Lacey, 1824.
Further reading
- How Round Is Your Circle? (Bryant and Sangwin, 2008) contains a chapter about James Watt's parallel motion mechanism