A load can be held by a closed centre directional valve although it will gradually lower due to leakage across the spool.
A directional valve does not completely seal the flow between the different line connections. The valve's control spool has a very small radial clearance which allows a small amount of leakage. Over time this leakage will be sufficient for the cylinder to move its complete stroke.
A single directional valve must NEVER be used to control a vertical load. The speed of lowering is not regulated by the directional valve and the load will fall out of control.
Apart from the danger to human life this can also generate negative pressures which will encourage the ingress of dirt and may severely damage the hydraulic components.
Adding a meter out flow control valve will regulate the flow leaving the cylinder and therefore control its descent. The raise and lower speeds can be controlled by two different adjustable orifices.
Please note that adding a meter in flow control valve would only exaggerate the dangerous negative pressures.
Adding a pilot operated check valve to the cylinder will seal the flow when the directional valve is in its centre position. This will stop the cylinder dropping over a period of time.
It is normal that the PO check valve is located as close to the actuator as possible for safety reasons in the event of a hose or pipe failure. This normally means that the meter out flow control will be after the load holding valve in the circuit.
A simple cylinder circuit uses a flow control for speed control (meter out) and pilot operated check for load holding.
Experiment by operating the directional valve and change the flow control settings yourself.
Using a flow control and pilot operated check valve can work successfully on simple circuits where the load is relatively constant, where there is little or no inertia in the load and the structure is reasonably rigid.
Problems can also develop however if the flow control is mounted some distance from the actuator and therefore the pilot operated check valve, this means that the pipework between the two valves is not pressurised when the load is being held. When the pilot operated check valve is piloted open the column of oil between the load holding valve and the flow control has to be pressurised before the flow control valve starts to restrict and this could cause the load to initially drop.
A flow control valve will generate a backpressure onto the outlet of the pilot operated check valve when operated. When the PO check valve is piloted open from the other side of the cylinder during lowering the oil from behind the pilot piston is displaced into the outlet port, which then connects to the inlet of the flow control. The backpressure generated by the flow control valve will oppose the pilot pressure opening the PO check valve and if it is higher than the pilot pressure the PO check will close. As soon as the PO check valve closes, the backpressure will decay and the PO check will open again. This will cause an unstable lowering condition.
The problem of back pressure in pilot lines can be overcome by using an externally vented PO check valve so that the oil from behind the pilot piston is now piped separately back to tank. This solution often creates problems because it is often not easy or practical to run a drain line back for the PO check valve to tank when it is mounted by the actuator. Sun does offer a version that is vent to atmosphere that fits into the three port cavity but over a long period of time there is always the possibility of a small leakage of oil, which may not be acceptable.
An alternative solution to overcome the pilot pressure problems is to use a counterbalance valve. This also offers better load control.
When the load control valve replaces the PO check valve and speed control is still required the flow control is now transferred to meter the flow into the actuator. This is because the performance of the counterbalance valve, like the PO check valve, can be affected by backpressure.
This solution also eliminates the problem discussed earlier of the load initially dropping because the counterbalance is providing both load holding and load control. The only additional procedure required with the counterbalance valve is that the setting of the valve is high enough to support the maximum load.
It is important as with the PO check valve that the counterbalance valve is located as close to the actuator as possible for safety reasons in the event of a hose or pipe failure.
A proportional directional valve can be used to control the cylinder speed instead of the flow control valve. However, most proportional valves provide both meter in and meter out control and the meter out can generate back pressure onto the counterbalance valve and cause unstable operation.
Sun Hydraulics produce a range of externally vented counterbalance valves that are suitable for use with proportional valves.