1. Field of the Invention
The present invention relates generally to hydraulic pilot systems, and more particularly to a pilot hydraulic system in which pilot fluid circulates from a source of pressurized fluid, to the actuator ports of a fluid actuator, and on to a sump.
2. Description of the Prior Art
Prior art hydraulic pilot systems have conventionally supplied pressurized fluid to one port of a fluid actuator through a conduit and have then received fluid back from that one port through the same conduit. This type of pilot system functions satisfactorily in environments where the ambient temperatures are moderate; but, when subjected to very low temperatures, the fluid that is in the conduit becomes and remains quite viscous, resulting in sluggish operation, because the fluid in the conduit is not recirculated with the source fluid, and so the fluid in the conduit never reaches a normal operating temperature.
In particular, a conventional hydraulic pilot system is quite inadequate for use with tree harvesters that operate in the northern part of the United States and in Canada. Tree harvesters, such as are disclosed in U.S. Pat. No. 3,905,407, use hydraulic cylinders to hold or grapple, to fell or shear, to lift and position, to delimb, to top, to bunk, and to drop processed trees in bundles. To speed processing, it is highly advantageous to automate the processing cycle. Also, it is highly advantageous to provide logic which will top the tree in accordance with the tree size. A hydraulic pilot system is well suited to these automation and logic functions, providing that it can be made to operate satisfactorily in cold climates.