Overconstrained mechanism

The mobility formula evaluates the degree of freedom of a system of rigid bodies that results when constraints are imposed in the form of joints between the links.

The calculated mobility for the lid relative to the car body is zero, yet it moves as its hinges (which are pin joints) have colinear axes.

A well-known example of an overconstrained mechanism is the parallel linkage with multiple cranks, as seen in the running gear of steam locomotives.

The Sarrus mechanism has one degree of freedom whereas the mobility formula yields M = 0, which means it has a particular set of dimensions that allow movement.

[2] A general spatial linkage formed from four links and four hinged joints has mobility which is a highly constrained system.

James Watt employed an approximate straight line four-bar linkage to maintain a near rectilinear motion of the piston rod, thus eliminating the need of using a crosshead.

Trammel of Archimedes with three sliders.
A multi-hinged door is an over-constrained mechanism.
A Sarrus linkage.
A Bennett's linkage
Watt steam engine .