Every action is opposed by an equal and opposite reaction. If the screw rotates, then the nut must translate for power transmission to occur. To react the force, one component must be fixed with respect to linear motion.

Because most screws are electric-motor- driven, the screw is usually constrained with respect to linear motion. Typically, it is installed on bearings that are mounted on the same base as the motor. The forces of load and acceleration created on the ball nut are thereby reacted by an axial thrust at the bearing end (or ends) of the screw. This is why the method used to mount the screw has a major impact on the system’s load capacity, stiffness, responsiveness, and speed.

Mounting of ball-screw ends is described as either supported or fixed. A supported end offers one focal point of the ball screw and does not react bending moments; instead, the angle between the axis of the screw and the rotary bearing increases. A fixed end can react moment loads because it is based on two rotary bearings sufficiently spaced apart so the ball screw remains perpendicular to the planes of the rotary bearings. Effective centers 1.5 times nominal screw diameter are usually enough.

Two advantages of a screw with a fixed end are greater column strength and higher critical speed. The advantages of a supported end are compactness and lower cost. A fixed end is generally harder to align and install than a supported end, so installation cost is a factor.

EK	EF
BK	BF
FK	FF
WBK