Our latest blog - containing useful technical information from https://www.hydraulicspneumatics.com/ - looks at hydraulic valves and the common differences between the various types of load control valves available as well as how these valves operate to control loads.
There are two main types of load control valve, the counterbalance and pilot-operated check valve. The counterbalance valve’s main duty is to hold suspended loads and also sort out the over-running of loads. It is also referred to as a brake valve, when it is used in conjunction with a hydraulic motor.
A big advantage counterbalance valves have over their Pilot-operated check valve rivals, is that they can control an over-running load whereas as the pilot-operated valve does not have this functionality.
The counterbalance valve deals with overrunning by preventing a load from dropping and this process is carried out when there is no pressure in the line that goes to the cap-end port of the cylinder. So, in simple terms, if you have issues with an over-running system, it is best to opt for a counterbalance valve.
The process explained…
With no pressure applied to the cylinder end containing the cap end, fluid pressure is maintained by the counterbalance valve. The pilot lines in the counterbalance valve act on the various surface areas within the valve. The common ratios of this surface area are around 3:1 to 4:1.
Assuming that the ratio used in this instance is 3:1; the counter balance and cylinder rod-ends connection line acts on a piston area located inside the valve. The pressure in the valve would have to total 1,800psi to successfully counter a spring force of the same amount - 1,800Ibs.
As only 1,500psi is produced in our example, the force is substantially lower and therefore, the valve will remain closed. In order to lower the load amount, the volume at the cap end of the cylinder must be pressurised in order for the counterbalance valve to open. This is achieved as the surface area is around 3 times more than the internal pressure acts on.
The external pilot pressure also has less work to do as it only has to exert 300Ibs of extra force added to the 1500Ibs to get the valve to open with pressure rising to 100psi.
The weight and pressure combined will allow the valve to open, thus allowing the load to lower. If the load drops too fast a pressure drop would occur in the external pilot line. An uncontrolled drop of the load would be prevented as the spool of the counterbalance valve would be partially closed.
So, to summarise our blog; if you are looking for a solution for basic load holding applications, pilot-check valves often suffice and would be a cost-effective solution. However, if you add motion control to the equation, a counterbalance valve is essential.
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