2.2 Constrained – Unconstrained Mechanism

“Constrained mechanism” may mean two different things in mechanisms literature.

1. It may refer to mechanisms in which F = 1.
2. It may refer to mechanisms whose degree-of-freedom may be greater or equal to one, but the number of inputs (the number of independent parameters) defined is equal to the degree-of freedom.

With Unconstrained mechanisms we mean those mechanisms with more than one degree-of-freedom and the number of inputs defined is less than the degree-of-freedom of the mechanism, but the motion is constrained by the forces and dynamic characteristics of the system. A good example is the differential of a car where the rotation of the wheels is governed by the moment acting on them. Due to these characteristics, when taking a turn, the inner wheel rotates less than the outer wheel. This unconstrained motion will create problems in icy conditions.

Above, two other examples for unconstrained mechanisms are given. One of the freedoms of the mechanism is usually controlled by a continuous motion. The other freedom is usually controlled by a spring and a key (not shown in the above examples). The motion is governed by both the continuous input plus the force or moment acting on the links under the spring force.