Watt's linkage

[4] This linkage does not generate a true straight line motion, and indeed Watt did not claim it did so.

However, for rotary motion a linkage that works both in compression and tension provides a better design and allows a double-acting cylinder to be used.

Such an engine incorporates a piston acted upon by steam alternately on the two sides, hence doubling its power.

[4][6] An example of Watt's linkage can be found on the high and intermediate pressure piston rod of the 1865 Crossness engines.

Watt's linkage is used in the rear axle of some car suspensions as an improvement over the Panhard rod, which was designed in the early twentieth century.

Both methods are intended to prevent relative sideways motion between the axle and body of the car.

Watt's linkage approximates a vertical straight-line motion much more closely, and it does so while consistently locating the centre of the axle at the vehicle's longitudinal centreline, rather than toward one side of the vehicle as would be the case if a simple Panhard rod were used.

In Watt's arrangement, however, the opposing curved movements introduced by the pivoting Panhard rods largely balance each other in the short vertical rotating bar.