From Wikipedia, the free encyclopedia
Animated depiction of Watt’s linkage
Hand-drawn diagram by James Watt (1808) in a letter to his son, describing how he arrived at the design.
[1]
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.
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.
Car suspension
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 sideways motion between the axle and body of the car. Watt’s linkage however approximates a vertical straight line motion more closely, and does so while locating the centre of the axle rather than toward one side of the vehicle, as more commonly used when fitting a long Panhard rod.
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, Watt’s linkage can be seen as two Panhard rods mounted opposite 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 linkage can be inverted, in which case the centre P is attached to the body, and L1 and L3 mount to the axle. This reduces the unsprung mass and changes the kinematics slightly. This is used on Australian V8 Supercars.
Watt's linkage can also be used to prevent axle movement in the longitudinal direction of the car; however, this is 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.
References
- ^ Franz Reuleaux, The Kinematics of Machinery (1876), page 4.
See also
External links
Wikis
Quiz
Search
