This is a model of an excavator which was created using a combination of MapleSim's Multibody library and Modelon's Hydraulic library. The movement and functions of the hydraulic excavator are accomplished through the use of differential cylinders which are used to convert hydraulic flow into translational motion. Input signals are given to the directional valve of each hydraulic circuit to signal an extension or retraction motion. In the case of an excavator, a runaway load situation can occur when the directional valve shifts to retract or extend a hydraulic cylinder and the weight of the load forces the actuator to move faster than the pump flow. A counterbalance valve was added to the hydraulic circuits to control this runaway load.