Override control for axle locking apparatus of mobile crane

Abstract

A pair of hydraulic locking cylinders for a vertically oscillatable wheel axle on the carrier frame of a mobile crane are interconnected and controlled by a hydraulic double-locking valve which is responsive to the centered or off-centered rotary position of the boom-carrying rotatable upper section of the crane. The double-locking valve comprises a pair of normally closed ball-type check valves which are operated by a valve stem comprising independently movable upper and lower sections. The upper valve stem section is positioned by a cam on the crane upper section and the lower valve stem section is positioned by a pneumatic actuator which is controlled by a solenoid valve selectively actuatable by an electric override switch in the crane operator's cab. The upper valve stem is moved to down (valve open-axle unlocked) position by the cam when the crane upper is centered on the carrier frame. The upper valve stem moves to up (valve closed-axle locked) position when the crane upper is rotated off-center. The lower valve stem is selectively movable to up (valve open-axle unlocked) position from its lower (valve closed-axle locked) position by the pneumatic actuator to unlock the axle and allow the crane to be repositioned by the crane operator even while the crane upper section and its boom are off-center.