Series self-leveling valve with single spool for unloading and relief

Claim 1

A self-leveling hydraulic system controlling two or more functions including a boom and bucket; double-acting cylinders controlled by separate boom and bucket directional control valves and an open-center series circuit with the boom control valve located upstream of the bucket control valve with a pressure source supplying the valves in a series path across open-center positions of each valve to reservoir; a self-leveling valve including a flow-dividing valve means connected to the rod end of the boom cylinder which splits the flow from the boom cylinder into two flow paths; a first passage means connecting the first of said two flow paths with an end of the bucket cylinder supplying a sufficient flow to maintain the bucket at a level position as the boom raises; a second passage means connecting the second of said two flow paths with an open-center path downstream of the boom valve and upstream of the bucket valve whereby the bucket valve can be separately actuated concurrent with the boom valve and supplied with oil so as to override the boom cylinder if desired; a third passage means connecting the opposite end of the bucket cylinder with the second passage means whereby the bucket cylinder discharge is returned to the circuit upstream of the bucket control valve, and the improvement comprising: a servo-operated spool type valve means with dual functions as an unloading valve and a relief valve positioned in said third passage means normally spring-biased closed in a first position blocking flow in the third passage means and a servo means acting on the valve against said biasing means sensing pressure in the first passage means which in a second spool position opens flow in the third passage means when a minimum pressure level is reached in the first passage means and a third spool position when maximum pressure is reached in the first passage means which opens the first passage means to relief into the second passage means.