Watt mechanism

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The Watt mechanism (named after its inventor James Watt ) is a coupling gear . It is composed of the watt rod and the watt parallelogram :

The implementation of the movement through the Watt linkage and the enlargement of the stroke through a parallelogram are two independent mechanisms that can also be used separately. This functional separation was blurred for a long time, so that in numerous older publications the Watt mechanism was referred to as a Watt parallelogram.

Other mechanical constructions that, like the Watt linkage, convert a rotary swivel movement into a straight movement, are the Peaucellier Inversor and the Chebyshev parallelogram .

Mode of action

Watt linkage

Watt linkage (animation)

The Watt linkage is the actual mechanism that converts the movement of a point on a circular path into a movement on a lemniscate . In some areas of application, the Watt linkage is therefore also referred to as a lemniscate link (see applications below). For small deflections , the lemniscate's path is approximately straight.

The upper diagram shows the sequence of movements of a watt rod without a parallelogram. Here the end points of L1 and L3 move on circular paths, point P on a lemniscate.

In the diagram below, the Watt linkage consists only of the two fixed points A and G, the two levers  AB and EG, which can rotate in A and G, and the lever BE, which can be rotated at its end points (!) With the two levers mentioned connected is. When lever AB is turned back and forth, point F moves up and down almost in a straight line. And vice versa: if point F moves up and down in an almost straight line, the lever AB is turned back and forth.

Watt parallelogram

Watt's linkage and parallelogram (scheme)

The parallelogram is formed by the points BCDE in the diagram, with all four corner points being rotatable joints . It has the function of a pantograph - since points A, F and D lie on a straight line with a suitable length of CD and the distance from A to D is always proportional to the distance from A to F, so D performs the same movement as F, only "enlarged". If F moves in an (almost) straight line, so does D - the point connected to piston  H.

Strictly speaking, the parallelogram would not be necessary to operate a machine - the rod that starts at piston H could also be connected directly to F. However, this would mean that the piston stroke would be much smaller or, in order to achieve the same stroke, points B, E and above all G would have to move much further to the right, which would significantly increase the space requirement of the machine.

Applications

The invention had an effect very quickly beyond the steam engine, because the conversion of types of movement is generally of central importance in technology .

The Watt linkage (without the parallelogram) is used variously on cars with rigid axles or with twist beam axles for wheel guidance in order to prevent longitudinal or sideways movement of the axle body. The vertical movement of the axis thus corresponds much better to a vertical straight line than when using a Panhard rod, for example . Installed lengthways, it also acts as a brake buckle compensation .

In the same way, the Watt linkage under the name Lemniskatenlenker is used in rail vehicles to prevent the wheel axles from moving in the longitudinal direction of the train.

Historical derivation

Original hand drawing by James Watt in a letter to his son dated November 10, 1808 about the development process for the Watt linkage.

In order to increase the performance of the steam engines he designed , James Watt looked for ways to successively flow steam into the cylinder of his engine from both sides in order to be able to generate an active upward movement of the cylinder in addition to the active downward movement of the cylinder that was customary at the time. In this case one speaks of a double-acting cylinder . For this, Watt needed a rigid coupling between the cylinder and the balancer ; the previously common coupling by means of a chain was only able to transmit downward pulling forces from the cylinder.

The Watt mechanism, invented by Watt in 1784, contains the Watt linkage, which converts the rotational movement of the balancer into the linear movement of the cylinder. This usually requires a size adjustment: this is done by the Watt parallelogram, which scales the movement of Watt's linkage in such a way that the piston rod of the machine can be connected to the balancer swinging back and forth around a fixed pivot point .

James Watt wrote of his invention: "Although I do not care about fame, I am more proud of the parallel movement than of any invention I have made."

Individual evidence

  1. ^ A b Franz Reuleaux: Textbook of Kinematics . The practical relationships of kinematics to geometry and mechanics. tape 2 . Friedrich Vieweg and son, Braunschweig 1900, p. 302 ff . ( (here online) [accessed on August 7, 2011]).
  2. ^ Franz Reuleaux: Theoretical Kinematics . Fundamentals of a theory of mechanical engineering. tape 1 . Friedrich Vieweg and son, Braunschweig 1875, p. 6 ( (here online) [accessed on April 26, 2010]).
  3. ^ A b James Patrick Muirhead: The Life of James Watt . with selections from his correspondence. John Murray, Albemarle Street, London 1858, XIX, p. 294 ( limited preview in Google Book search).

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