Some power machines including skid steer loaders, tracked loaders, steerable axle loaders, excavators, telehandlers, walk behind loaders, trenchers, and the like, employ engine powered hydraulic power conversion systems. In some power machines, the hydraulic power conversion systems utilize an open center series control valve assembly that receives pressurized fluid from a pump. This control valve assembly typically has multiple valve elements to port hydraulic fluid to different work functions on the power machine. For example, on a work machine with a lift cylinder that raises and lowers a lift arm, a tilt cylinder that controls a tilt position of an implement carrier and thus an attached implement with respect to the lift arm, and one or more implement work actuators, the control valve assembly may have three (although any number can be used) valve elements, often in the form of linear spools, to port hydraulic fluid to the different actuators on the power machine and/or implement. The term open center refers to a feature in a valve assembly such that when a valve element is in an unactuated position (such as the center position on a typical spool valve) or a partially actuated position (such as in a proportional spool valve), at least some hydraulic fluid is allowed to flow through the unactuated position to a downstream valve element.
The valve elements in an open center control valve assembly are arranged such that the first valve element that receives hydraulic fluid from a pump has priority over subsequent downstream valve elements. A traditional priority in a power machine such as a skid steer loader is that the hydraulic fluid is provided first to a lift valve element, which is used to selectively control the lift cylinder to raise and lower the lift arm. Subsequently hydraulic fluid is provided to the tilt valve element, which is used to control the tilt cylinder and then to the auxiliary or implement valve element and then out of the valve.
It is known that in certain open center hydraulic control valve assemblies, when downstream valve elements are actuated to provide fluid to a downstream actuator, back pressures can be raised to a point where functionality of upstream elements can be limited or compromised.
The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter.
USPTO Backgrounds
USPTO Backgrounds