Abstract: A hydraulic yarding system in which the haulback and main drum are coupled to a primary drive motor and provided with a hydraulic interlock motor coupled to a planetary set in the haulback drive in an open loop hydraulic circuit. The hydraulic fluid from the interlock motor can be diverted to the hydraulic circuit for the primary drive motor through a pressure sequence valve and a directional valve. In this manner, regenerative driving of the interlock motor by the load pumps fluid from the interlock circuit selectively in the desired direction to the primary drive motor and supplements the power available to the primary drive motor. A preferred embodiment utilizes two pressure sequence valves in circuits to either the in-haul or out-haul fluid flow directions for the primary drive motor so that a diversion of fluid from the interlock circuit can occur at a lower pressure valve during out-haul.

Abstract: A small, variable-speed yarder is provided with main, haulback and slack-pulling drums, each containing a planetary drive transmission with provisions for dual power inputs. A variable interlock mechanism controls the relative speed of the haulback drum to the two inhaul drums (main and slack-pulling) and a differential drive mechanism controls the relative speed and/or direction of the inhaul drums.