Josh Cosford, CFPHS, The Fluid Power House (Cambridge) Inc.
Last month, we finished off with the start (is it possible to finish with a start?) of square polygon symbols, such as the relief valve. I’d like to continue where I left off with pressure valves. There are at least a half a dozen different type of valves—essentially just relief valves—with modifications for use in separate applications. See the diagram for an illustration.
Since most pressure valves are based on this valve, we’ll discuss the relief valve first. In example a) I’ve broken down the schematic symbol of a basic relief valve into colors. The red line(s) represent the pressure side of the valve (port 1), and the blue side (port 2) is for the low pressure (normally direct to tank). The spring is green, and is rated by how much pressure it takes to compress it, opening the valve.
The diagonal black arrow is the standard symbol for anything that is variable. In this case, the spring tension is adjustable by a screw, increasing or decreasing the pre-load on the valve. The purple arrow represents the desired flow path (from port 1 to port 2) should the valve open.
If you’ll remember last month’s column, we discussed pilot pressure and the relationship between springs and pilot pressure (Push O’ War). In this case, the spring is holding the valve closed with a thousand pounds per square inch worth of pressure. Pressure at port 1 can travel to the side and around the corner to act on the purple arrow.
When pressure reaches 1001 psi, the red line will push the purple line into the middle, opening up a flow path from ports 1 to 2. As long as pressure at port 1 is at or above the 1001 psi range (actually, it’ll stay open below 1001 psi, in some cases), the valve will keep bleeding fluid to tank.
Example b) shows a counterbalance valve. These are called “motion control” valves because they prevent the cylinder and load from retracting or dropping faster than intended. If the counterbalance valve was not installed, the pressure induced from the load could force fluid out of the bottom port (red side), dropping the load faster than you can pump fluid in the top port (green side).
The CB valve blocks flow out the bottom port until it sees a pressure signal from the pilot line attached to the top port. Because the valve is blocking flow in the bottom port (red side), flow into the top port (green side) is restricted. This restriction causes pressure to rise on the green side, which in turn pressurizes the pilot line going to the top of the counterbalance valve. The valve then opens, allowing the cylinder to retract smoothly.
Pressure at the pilot port doesn’t have to equal the spring setting of the valve, as the counterbalance valve opens with less pressure when acted upon the pilot port than the cap end port. The ratio can be anywhere from 3:1 to 10:1. So if the load induced pressure on the red side is 1000 psi, pressure will only have to rise to 250 psi up top for the valve to open, retracting the cylinder if we assume the CB has a 4:1 ratio.
The final valve I’m going to discuss is the pressure reducing valve. Of the various square pressure valves, it is the only one considered normally open. If you’re an electrical guy, normally open in hydraulics means “normally flowing,” and not an open contact as with electrics.
The reducing valve is intended to limit or reduce pressure at an actuator to limit force/torque, or to protect the actuator or sub circuit from over-pressurization damage. It works by blocking flow into and out of the valve. Diagram c) shows a simple circuit with a pressure compensated pump and two motors with their own directional valve.
Between each valve and motor is the pressure reducing valve. It will flow in either direction as long as pressure at port 1 remains below the spring setting. If the load at the first motor reaches 1000 psi, you can see the pilot line will push the arrow off to the right side, blocking flow from reaching the motor. Because the motor is still spinning, pressure at port 1 decays extremely quickly and pressure drops again.
The reducing valve opens to allow more flow in, and the pattern repeats. The action happens quickly and unnoticeably; the valve essentially holds pressure at the inlet of the motor at 1000 psi.
There are other pressure valves, such as the sequence valve, brake valve and other versions of the already described valves. The three I have covered represent 95% of the pressure valves you will come across.
Sequence valves are used to delay, or time, actuators or sub-circuits. However, due to the proliferation of inexpensive electronics, you can purchase a smart relay or PLC for not much more than a sequence valve. For that reason, I don’t recommend them.
Brake valves are the same as counterbalance valves, only used with hydraulic motors.
Next month, I’ll cover flow control valves and their related symbology.