Hydraulic circuit for forklift application

HYDRAULIC CIRCUIT FOR FORKLIFT APPLICATION 

(COUNTERBALANCE VALVE CIRCUIT)

1. Functional and Operational Requirements

• Forklifts are extensively used in industries for lifting and transporting materials and goods. They usually have a hydraulically operated forked platform at the front that can be raised and lowered so that to lift or move the heavy loads.

• This forklift application can be achieved by using a counterbalance valve circuit. 

• A counterbalance valve, also known as a back-pressure valve, is applied to create a back pressure or cushioning pressure on the underside of a vertically moving piston to prevent the suspended load from free falling because of gravity while it is still being lowered.

2. Circuit

Fig.16.7 depicts a typical hydraulic circuit for a forklift application using a counterbalance valve. This circuit employs solenoid-operated 4/3 DC valve and counterbalance valve.

3. Operation

Extension (Lowering): The pressure setting on the counter balance is set slightly higher than the pressure required to prevent the load from free falling. Due to this back pressure, the fluid available on the rod side of the piston opens the counter balance valve; and varies the spring-opposed spool, thus the extension stroke starts. The fluid coming out of the cylinder flows directly to the tank through the DC valve.

Retraction (Lifting): As the valve is normally closed, flow in the reverse direction cannot occur without a reverse free-flow check valve. When the load is raised again, the internal check valve opens to permit flow for the retraction of the cylinder.

Cylinder locking: At the centre position of the DC valve, pump is unloaded to the tank while the piston is held locked by the counterbalance valve preventing the flow to the tank. If the load is required to be held for a prolonged time, then a pilot-operated check valve has to be used in addition to the counterbalance valve.