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 The Watt steam engine design became synonymous with steam engines, and it was many years before significantly new designs began to replace the basic Watt design. The first steam engines, introduced by Thomas Newcomen in 1712, were of the "a ...

. 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

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. In a letter to his son in 1808 describing how he arrived at the design, James Watt wrote "I am more proud of the parallel motion than of any other invention I have ever made."Franz Reuleaux, ''The Kinematics of Machinery'' (1876), page 4. The sketch he included actually shows what is now known as

Watt's linkage In kinematics, Watt's linkage (also known as the parallel linkage) is a type of mechanical linkage invented by James Watt in which the central moving point of the linkage is constrained to travel on a nearly straight line. It was described i ...

which was a linkage described in Watt's 1784 patent but it was immediately superseded by the parallel motion. Also available at https://www.gutenberg.org/files/27106/27106-h/27106-h.htm 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. The Newcomen engine's piston was propelled downward by the atmospheric pressure, and raised by live steam. Watt's device allowed live steam to be used for direct work on both sides of the piston, thus almost doubling the power, and also delivering the power more evenly through the cycle, an advantage when converting the reciprocating motion to rotary motion (whether through a crank or through a

Sun and planet gear The sun and planet gear is a method of converting reciprocating motion to rotary motion and was used in the first rotative beam engines. It was invented by the Scottish engineer William Murdoch, an employee of Boulton and Watt, but was patent ...

system).

Principle of operation

See the diagram on the right. is the journal (bearing) of the walking beam , which rocks up and down about . is the piston, which is required to move vertically but not horizontally. The heart of the design is the four-bar linkage consisting of , and and the base link is , both joints on the framework of the engine. As the beam rocks, point (which is drawn to aid this explanation, but is not a marked point on the machine itself) describes an elongated figure-eight (more precisely, a

lemniscate of Bernoulli In geometry, the lemniscate of Bernoulli is a plane curve defined from two given points and , known as foci, at distance from each other as the locus of points so that . The curve has a shape similar to the numeral 8 and to the ∞ symbol. ...

) in mid-air. Since the motion of the walking beam is constrained to a small angle, describes only a short section of the figure-eight, which is quite close to a vertical straight line. The figure-eight is symmetrical as long as arms and are equal in length, and straightest when the ratio of to matches that of to . If the stroke length (that is, the maximum travel of ) is , then the straight section is longest when is around and is .Neil Sclater and Nicholas P. Chironis, ''Mechanisms and Mechanical Devices Sourcebook Third Edition'' (2001), page 136. 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 A pantograph (, from their original use for copying writing) is a mechanical linkage connected in a manner based on parallelograms so that the movement of one pen, in tracing an image, produces identical movements in a second pen. If a line dr ...

. 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 The Peaucellier–Lipkin linkage (or Peaucellier–Lipkin cell, or Peaucellier–Lipkin inversor), invented in 1864, was the first true planar straight line mechanism – the first planar linkage capable of transforming rotary motion ...

of 1864.

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.

Description

Principle of operation

See also

References

Further reading