ELECTRONICALLY CONTROLLED HYDRAULIC SYSTEM FOR LOWERING A BOOM IN AN EMERGENCY

摘要

An industrial lift truck has a boom that is raised and lowered by a first hydraulic actuator and a load carrier that is pivoted with respect to the boom by a second hydraulic actuator. In the event that the supply of hydraulic fluid for powering the actuators fails, the boom may be lowered by gravity by draining fluid from the first hydrauli c actuator. To prevent a load from sliding off the load carrier as the boom descends, th e load carrier is pivoted to maintain a substantially constant angular relationship to the ground. This is accomplished by selectively conveying fluid drained under pressure from t he first hydraulic actuator into the second hydraulic actuator. Changes in the positi on of the boom are sensed and, in response, the flow of fluid into the second hydrauli c actuator is controlled to produce corresponding changes in the load carrier position.