BACKGROUND OF THE INVENTION

This disclosure relates generally to hydraulic valve circuits for use with material handling equipment and, more particularly, to hydraulic valve circuits adapted for weight-responsive control of clamping members associated with material handling equipment having free lift masts.

Standard forklifts and other types of material handling equipment typically have mast assemblies for hoisting or lifting a load from one height to another, and such mast assemblies are typically configured to receive a variety of attachments that may be designed for handling particular types of loads. For example, load-clamping attachments such as carton clamps or paper roll clamps may be used, each having hydraulically controllable load-clamping members for imparting sufficient gripping forces on the sides of a load to allow lifting and carrying the load from one place to another.

Mast assemblies are typically one of two general types—“free lift” or “non-free lift.” Free lift masts permit lifting a load from one height to another throughout a “free lift” range of motion without a corresponding change in the overall height of the mast assembly. Lifting the load beyond the free lift range of motion requires the mast to telescope so as to extend the range of lifting. The mast may have several stages which telescope in succession, one after the other. Each stage will generally have one or more extensible hydraulic cylinders which, when activated, extend fully before activation of the one or more extensible hydraulic cylinders associated with the next stage. The hydraulic cylinders in each successive stage usually require higher hoist pressures for activation than cylinders of the preceding stage. Consequently, in a free lift mast having, for example, a free lift range of lifting motion and a main lift range of lifting motion, the main lift cylinder or cylinders will not begin to extend until the free lift cylinder or cylinders have reached their fully extended position.

By contrast, non-free lift masts begin to telescope immediately as the load is lifted. Such telescoping of the mast is undesirable in overhead constrained environments. For example, the interior of enclosed tractor trailers may be limited to, for example, an inside height of 104 inches. If the particular lift truck has a collapsed mast height of between 79 to 84 inches, as is common for counterbalanced sit-down lift trucks, there may be only 20 to 25 inches of vertical space available for the mast to telescope before further telescoping of the mast interferes with the ceiling of the trailer.

In part because free lift masts typically require stepped or progressively higher hoist line pressures for extending the mast beyond the free lift range of motion, principally hydraulic control systems adapted to utilize hoist line pressures for sensing load weight and correspondingly regulating gripping forces automatically in response to such pressures have not been achieved with such masts. Alternative designs using electronic controllers for such gripping force regulation have disadvantages such as higher unit costs and added system complexity, as well as the requirement for electrical conductors which must be movable in response to mast extension. Therefore, different hydraulic valve circuits are needed for automatic weight-responsive force control of load-clamping members associated with material handling systems having free lift masts.

