Parallel motion linkage

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.

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.

Since the motion of the walking beam is constrained to a small angle, F describes only a short section of the figure-eight, which is quite close to a vertical straight line.

If the stroke length (that is, the maximum travel of F) is S, then the straight section is longest when BE is around ⅔ S and AB is 1.5 S.[3] It would have been possible to connect F directly to the piston rod (the "Watt's linkage" design), but this would have made the machine an awkward shape, with G a long way from the end of the walking beam.

Animation of the parallel motion linkage.
Dimensions (unit lengths a, b ):
Link 2: a + a
Links 3 & 5: b + b , 2 b
Links 4 & 6: a
Vertical distance between ground joints ≈ 2 b
Horizontal distance between ground joints ≈ 2 a
Thus, link 1 (total distance between ground joints)
Watt 's parallel motion on a pumping engine
Hand-drawn diagram in James Watt 's letter to his son. [ 1 ]
Schematic of Watt 's parallel motion: A and G are fixed hinge joints while F is not a joint but merely signifies the point on the linkage which follows a lemniscate . Its motion is magnified in D by the parallelogram ▱BCDE .