Straight line mechanism

In the late seventeenth century, before the development of the planer and the milling machine, it was extremely difficult to machine straight, flat surfaces. For this reason, good prismatic pairs without backlash were not easy to make. During that era, much thought was given to the problem of attaining a straight-line motion as a part of the coupler curve of a linkage having only revolute connection. Probably the best-known result of this search is the straight line mechanism development by Watt for guiding the piston of early steam engines. Although it does not generate an exact straight line, a good approximation is achieved over a considerable distance of travel. Exact linkages were later discovered, but not in time to be put to as much use.

A Roberts linkage
A Sarrus linkage
Peaucellier–Lipkin linkage:
bars of identical colour are of equal length
Cardan straight line mechanism

List of linkages

Nearly straight line linkages

Perfect straight line linkages

Eventually, several linkages were discovered that produced perfect linear output:

The earlier Scott Russell linkage (1803) translates linear motion through a right angle, but is not a straight line mechanism in itself.

Rolling mechanisms

These mechanisms produce a straight line as a rolling curve, not as a coupler curve – i.e. they use gears or belts, instead of just bars connected by rotary joints.

See also

Sources
  1. https://web.archive.org/web/20180418124753/http://kmoddl.library.cornell.edu/model.php?m=139 Reuleaux Collection, Cornell university
  • Theory of Machines and Mechanisms, Joseph Edward Shigley
  • Cornell university (archived) - Straight-line mechanism models
  • Alfred Kempe (1877). How to Draw a Straight Line (PDF). Macmillan – via University of California at Irvine.
  • Daina Taimina. "How to Draw a Straight Line - a tutorial". Cornell University.
  • Simulations using the Molecular Workbench software
  • bham.ac.uk - Hart's A-frame (draggable animation) 6-bar linkage


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