A rigid body free in space has six degrees of freedom: three translational and three rotational. It can move along its x, y, and z axes and rotate about its x, y, and z axes. When you add a constraint, such as a revolute joint, between two rigid bodies, you remove degrees of freedom between the bodies, causing them to remain positioned with respect to one another regardless of any motion or force in the mechanism. The constraints in COSMOSMotion remove various numbers and combinations of degrees of freedom.
For example, a revolute joint removes all three translational degrees of freedom and two of the rotational degrees of freedom between two rigid bodies. If each rigid body had a point on the joint that was on the center line of the revolute pin, then the two points would always remain coincident. They would only rotate with respect to one another about one axis: the center line of the revolute. The revolute joint is a single-degree-of-freedom joint because it allows a single rotation between the rigid bodies.
A cylindrical joint is a two-degree-of-freedom joint. You can create a cylindrical joint from the revolute joint by adding a translational degree of freedom along the axis of rotation of the revolute joint. There are also three-degree-of-freedom joints, such as spherical joints, which constrain all translations but allow rotations about all three axes.
When you submit your mechanism for simulation, ADAMS/Solver calculates the number of degrees of freedom in your mechanism as it determines the algebraic equations of motion to be solved in your mechanism.
When a mechanism has a closed loop, such as in a four-bar linkage, there can be redundant constraints. There are three redundant constraints in a four-bar linkage when all of the joints are defined as revolute joints. This is because each side of the loop (starting from ground) constrains the connecting rod to stay in the plane of the mechanism.
ADAMS/Solver attempts to resolve the redundant constraints automatically, and can do so easily for a four-bar linkage. For more complex closed loop linkages, we recommend that the connecting part that closes the loop be attached with a spherical joint on one end and a universal joint on the other end.