fixedaxis

Fixedaxis (also written fixed axis) is a concept in mechanics and kinematics describing rotation about an axis that remains stationary relative to the surrounding frame. In this motion, the axis is fixed in space or in a rigid support, and every point of the body describes a circular path around that axis. The angular velocity vector is aligned with the fixed axis, and there is no translational motion of the axis itself.

This contrasts with moving-axis motion, where the axis can translate or rotate with the body. Fixed-axis joints are common in simple mechanisms such as hinges, wheels on a shaft, and doors. In a typical fixed-axis mechanism, one link rotates about a constant axis that is constrained by bearings or supports, enabling clean rotational motion with minimal axis drift.

In engineering design, fixed-axis elements simplify analysis because the motion can be described using a single degree of freedom (rotation about the fixed axis). The concept is central to revolute joints used in robotics, machinery, and civil engineering structures, where a shaft acts as the fixed axis for a wheel, pulley, or gear.

Related terms include hinge, revolute joint, and fixed-axis crank mechanisms. See also axis of rotation, moment of inertia, and plane motion.