Abstract: A jack assembly of the ball screw or machine screw type includes a screw, a nut in driving engagement with the screw whereby rotation of the nut causes the nut or screw to move axially. A nut drive housing in which the nut rotates includes a worm gear transmission which may be driven directly or indirectly by an electric motor. The screw projecting through the nut is enclosed by a screw stem cover mounted on the housing which environmentally protects the screw. The cover and screw move axially relative to each other. In a preferred embodiment, the screw stem cover is made of plastic and is provided with slots extending axially in which are adjustably positioned magnetic reed switches. The switches may be clamped in place by a sleeve type clamp or the exterior of the cover may be flattened in the area of the slot for direct mounting of the switch. The stem cover may also have a flattened area for mounting of the required terminal.