Hydraulic systems are known for converting fluid energy, for example, fluid pressure, into mechanical energy. Fluid power may be transferred from a hydraulic pump through fluid conduits to one or more hydraulic actuators. Hydraulic actuators may include hydraulic motors that convert fluid power into shaft rotational power, hydraulic cylinders that convert fluid power into translational motion, or the like.
Some hydraulic systems include a plurality of hydraulic actuators. For example, lift trucks for transporting materials may include a hydraulic motor to propel one or more drive wheels of the lift truck, in addition to an array of hydraulic cylinders that perform different functions, such as, for example, lifting a load against gravity, adjusting an angular orientation of the load with respect to the lift truck, and adjusting a distance between lift forks. Similarly, off-highway trucks for mining and excavation operations may include, for example, hydraulic motors to propel drive wheels and steer the truck, in addition to hydraulic cylinders to raise and lower a bucket.
In such systems, separate hydraulic actuators may have significantly different power requirements; and therefore, different corresponding pressure requirements to optimize system operability and efficiency. For example, the fork distance adjustment cylinder in a lift truck may have significantly lower pressure requirements than the lift cylinder in the same truck because, unlike the lift cylinder, the fork adjustment cylinder only has to move the weight of the forks and not the weight of a load bearing on the forks.
U.S. Pat. No. 7,222,484 (hereinafter “the '484 patent”), entitled “Hydraulic System with Multiple Pressure Relief Valves,” purports to address the problem of various hydraulic pressure requirements in a hydraulic system. The '484 patent describes hydraulic systems in which various hydraulic actuators have different operating pressure limits as determined by separate pressure relief valves. However, incorporating multiple pressure relief valves into a hydraulic system can be disadvantageously expensive, bulky, and unreliable. Accordingly, there is a need for improved hydraulic system pressure controls.