What is axis of pure rotation?

   The concept of axis of of pure rotation  ( or instantatious center of rotation)  is a bit confusing. The text books in engineering mechanics( Irvin shames , Beer & Johnston etc) tells that the locus of points in a body, which is (are) having zero veloity.   This fits  well in case of a 2D motion. As a limiting case of this we are assuming the center of rotation is at inifinity for a rigid body moving in a straight line

    But in case of a 3D motion, there are every chance is there that such a locus may not exist. For example,take a case of a nut which  is going in a helical thread. In that we know that the traslationary motion and the rotation are in the same direction, and there will not be any point where velocity is zero. 

   In such cases  how we can define axis of pure roration. I feel that the locus of points in which the linear velocity in the plane perpendicular to the axis of rotation is zero can be considered as the axis ofpure rotation. 

Sreenath A M

Asst Professor

NIT Calicut