A joint is something like a hinge, which is used to connect two solids. It is a relationship that is enforced between two bodies so that they can only have certain positions and orientations relative to each other. To create a joint between two solids, choose the joint type from the Links Menu, the joint will be added to the Links Tree, and its properties will appear in the Properties Panel on the left.
Fixed: maintains a fixed relative position and orientation between two solids, in practice, using this joint is rare. If two bodies are needed to be glued together, it is better to represent them as a single solid when deciding that both of the solids hold the same properties during the simulation session.
Revolute: also called pin joint, is a one-degree-of-freedom kinematics pair used in mechanisms. Revolute joints provide single-axis rotation function used in many places such as door hinges, folding mechanisms, and other uni-axial rotation devices.
Prismatic: also called slider joint, provides a linear sliding movement between two bodies. The two joined solids have their rotation held fixed relative to each other, and they can only move along a specified axis, chosen by the user while specifying the joint properties.
Spherical: also called a ball and socket joint, is a joint in which the ball-shaped surface of a solid fits into the cup-like indentation of another solid. This type of joint allows the solid to move in a 360 degree angle—with more freedom than other joints.
Cylindrical: a two-degrees-of-freedom kinematics pair used in mechanisms. Cylindrical joints provide single-axis sliding function as well as a single axis rotation, providing a way for two rigid bodies to translate and rotate freely.
A selected Joint requires its properties to be set in order to work appropriately.
