Watts Linkage

A Watt's linkage (also known as the parallel linkage) is a type of mechanical linkage invented by James Watt (19 January 1736 – 25 August 1819) to constrain the movement of a steam engine piston in a straight line.

The idea of its genesis using links is contained in a letter he wrote to Matthew Boulton in June 1784.

I have got a glimpse of a method of causing a piston rod to move up and down perpendicularly by only fixing it to a piece of iron upon the beam, without chains or perpendicular guides […] and one of the most ingenious simple pieces of mechanics I have invented.

James Watt
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Animated depiction of Watt’s linkage.

This linkage does not generate a true straight line motion, and indeed Watt did not claim it did so. In a letter to Boulton on 11 September 1784 he describes the linkage as follows.

The convexities of the arches, lying in contrary directions, there is a certain point in the connecting-lever, which has very little sensible variation from a straight line.

James Watt

Car suspension

Watt's linkage used in a 1998 Ford Ranger EV

The 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 intend to prevent relative sideway motion between the axle and body of the car. The Watt’s linkage however approximates a vertical straight line motion more closely.

It consists of two horizontal rods of equal length mounted at each side of the chassis. In between these two rods, a short vertical bar is connected. The center of this short vertical rod – the point which is constrained in a straight line motion - is mounted to the center of the axle. All pivoting points are free to rotate in a vertical plane.

In a way, the Watt’s linkage can be seen as two Panhard rods mounted opposite of each other. In Watt’s arrangement however, the opposing curved movements introduced by the pivoting Panhard rods are compensated by the short vertical rotating bar.

The Watt's linkage can also be used to prevent axle movement in the longitudinal direction of the car, this is however more common in racing suspension systems. This application usually involves two Watt's linkages on each side of the axle, mounted parallel to the driving direction.