The instantaneous screw axis, or'instantaneous helical axis'( IHA ), is the axis of the helicoidal field generated by the velocities of every point in a moving body.
12.
The six parameters that define a screw displacement are the four independent components of the Pl�cker vector that defines the screw axis, together with the rotation angle about and linear slide along this line, and form a pair of vectors called a "'screw " '.
13.
When a spatial displacement specializes to a planar displacement, the screw axis becomes the " displacement pole, " and the instantaneous screw axis becomes the " velocity pole, " or instantaneous center of rotation, also called an " instant center ."
14.
When a spatial displacement specializes to a planar displacement, the screw axis becomes the " displacement pole, " and the instantaneous screw axis becomes the " velocity pole, " or instantaneous center of rotation, also called an " instant center ."
15.
A non-discrete screw axis isometry group contains all combinations of a rotation about some axis and a proportional translation along the axis ( in rifling, the constant of proportionality is called the twist rate ); in general this is combined with " k "-fold rotational isometries about the same axis ( " k " e " 1 ); the set of images of a point under the isometries is a " k "-fold helix; in addition there may be a 2-fold rotation about a perpendicularly intersecting axis, and hence a " k "-fold helix of such axes.
