Automatic overload control for a counterbalanced lift truck

Abstract

When the load moment on a counterbalanced lift truck exceeds a predetermined magnitude, a blocking valve automatically prevents forward tilting of the mast and elevation of the load. The load moment is sensed by the pressure differential across the tilt jack and the blocking valve is actuated by the pressure differential. In order to prevent the blocking valve from becoming locked up, when the tilt jack is contracted to the end of its stroke, a spring-biased unloading valve is incorporated in the tilt jack piston. Abuse of the truck is reduced and operator safety is improved further by the provision of a second blocking valve which prevents lifting loads when the tilt jack is at the end of its extension stroke, that is when the mast is tilted forwardly as far as permitted by the tilt jack. In this condition the second blocking valve will close preventing further raising of the load. However, retraction of the tilt and lift jacks is permitted even though the blocking valves are in their closed positions. The second blocking valve is biased toward its open position by the pressure in the rod end of the tilt jack and toward its closed position by the pressure in the lift jack. The blocking valves are biased toward their open positions by adjustable spring means which permit the control system to be adjusted to meet the protection requirements of lift trucks with different load carrying capacities. Pressure modulating restrictors are provided in pilot lines to the blocking valves to prevent false actuation when pressure fluctuations occur.