2.12 THEGRASHOF CONDITION
The fourbar linkage has been shown above to be the simplest possible pin-jointed mechanism
for single degree of freedom controlled motion. It also appears in various disguises
such as the slider-crank and the earn-follower. It is in fact the most common and
ubiquitous device used in machinery. It is also extremely versatile in terms of the types
of motion which it can generate.
Simplicity is one mark of good design. The fewest parts that can do the job will usually
give the least expensive and most reliable solution. Thus the fourbar linkage should
be among the first solutions to motion control problems to be investigated. The Grashof
condition [8] is a very simple relationship which predicts the rotation behavior or rotatability
of a fourbar linkage's inversions based only on the link lengths.