# Overconstrained mechanism

Watt II overconstrained mechanism - ver.1
Watt II overconstrained mechanism - ver.2

An overconstrained mechanism is a linkage that has more degrees of freedom than is predicted by the mobility formula. 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 connecting the links.

If the links of the system move in three-dimensional space, then the mobility formula is

$M=6(N-1-j)+\sum_{i=1}^j f_i,$

where N is the number of links in the system, j is the number of joints, and fi is the degree of freedom of the ith joint.

If the links in the system move planes parallel to a fixed plane, or in concentric spheres about a fixed point, then the mobility formula is

$M=3(N-1-j)+\sum_{i=1}^j f_i.$

If a system of links and joints has mobility M=0 or less, yet still moves, then it is called an overconstrained mechanism.

## Contents

A well-known example of an overconstrained mechanism is the Sarrus mechanism, which consists of six bars connected by six hinged joints.

A general spatial linkage formed from six links and six hinged joints has mobility

$M = 6(N - 1 - j) + \sum_{i=1}^j f_i = 6(6-1-6) + 6 = 0,$

and is therefore a structure.

The Sarrus mechanism has mobility M=1, rather than M=0, which means it has a particular set of dimensions that allow movement.[1]

Another example of an overconstrained mechanism is Bennett's linkage, which consists of four links connected by four revolute joints.

A general spatial linkage formed from four links and four hinged joints has mobility

$M = 6(N - 1 - j) + \sum_{i=1}^j f_i = 6(4-1-4) + 4 = -2,$

which is a highly constrained system.

As in the case of the Sarrus linkage, it is a particular set of dimensions that makes the Bennett linkage movable.[2] [3] Below is an external link to an animation of Bennett's linkage.