Fluid Power Tips: Issue 12: Can you still handle the pressure?

Josh Cosford, CFPHS, The Fluid Power House (Cambridge) Inc.

Last month, I gave you the drill down on basic hydraulic symbology. It was literally as basic as I could get, so I’ll kick it up a notch this month. I not only covered lines, but also what the three basic polygons are (square, circle and diamond, of course). This issue will focus on single squares, most of which in the hydraulic library are pressure valves. I’m going to discuss pilot pressure and pressure differential, as well. Understanding the dynamics of pressure is key to understanding any hydraulic circuit.

Fluid can and will only flow from an area of higher pressure to lower pressure—“fluid takes the path of least resistance.” Without a path for hydraulic oil to flow, there is no flow, such as through a blocked valve. If you look at example in the picture, the pressure that may exist at 1 will not be able to create any flow through this pressure valve.

That particular graphic shows a basic pressure valve; a plain square, a spring on the right, a pilot line on the left and a directional arrow that will represent flow path from port 1 to port 2. All schematics are drawn in their natural or neutral state, as if the machine was off. In the case of this valve, the force of the spring keeps the arrow pushed off to one side, preventing flow from port 1 to port 2.

The pressure required to move the arrow to the right and start compression of the spring is 1000 psi. At 1001 psi, the arrow will have shifted and opened up a path for fluid to flow, as shown in example b. Fluid will flow from 1 to 2 so long as pressure is maintained in the pilot line at or above the 1000 psi range. The spring and pilot fluid are at a “push o’ war.” The spring is trying to force the valve closed, and pilot pressure is trying to force it open. Whichever force is higher (spring pressure or pilot pressure) will dictate the state of the valve.

To further demonstrate my “push o’ war” example, see example c. Some relief valves may have a spring drain chamber that could be hooked up to a line going straight back to tank. This allows any bypass oil that makes it way past the poppet or spool (more likely a spool) to drain without affecting the performance of the valve. If oil were to build up inside the spring chamber, the pressure created would be additive to the spring force, causing the valve to open at higher pressure or not at all.

Let’s say in this case, the drain line has a restriction in it, which is causing 100 psi of backpressure in the spring chamber. Now, the relief valve won’t open until pilot pressure reaches 1100 psi plus. This can be a bad thing, especially because you can be fooled into thinking someone changed the setting on the relief valve, or if the drain pressure reaches a thousand psi or more. This can cause damage to components from over-pressurization because the relief valve is locked shut.

It’s a concept you must be aware of when looking at schematics to understand what’s going on with a machine, but the concept isn’t always bad. By manipulating pilot and drain lines in hydraulic systems, you have freedom to build creative circuits. Looking at example d, we can see I’ve added a solenoid valve to the drain line of this relief valve. In its current state, drain flow is blocked, essentially locking up the spring chamber with fluid. Even 3000 psi at port 1 and the internal pilot line of the relief valve is still not enough pressure to overcome incompressible fluid trapped in the spring chamber.

As soon as we shift the solenoid valve to allow the spring chamber to vent to tank, the relief valve will shift and start to control pressure in the system, as shown in example e. The control of pilot and drain energy is quite common, especially in larger hydraulic systems. Because of the forces acting upon valve spools with high flows, electric coils are not strong enough to shift hydraulic valves. This is why pilot operated valves are used for anything above 40 gpm at best.

In example f, I’ve shown a standard pilot operated logic element setup. If we assume the area ratio of the logic element is 1:1, then any pressure at ports 1 and 2 higher than 15 bar will cause the valve to open. Pilot pressure essentially kicks the door open because it wasn’t locked very tightly. However, we can see at port one, we have a pilot line going to the solenoid valve and then back into the logic element. Any pressure at port 1 will be additive to spring pressure, resulting in the valve being locked closed regardless of how high pressure gets at port 1.

If we shift the solenoid valve (called a pilot valve, in this case), the pilot side of the main valve will be vented back to tank. Pilot pressure is no longer added to spring pressure and any pressure higher than 15 bar will open up the logic element. That logic element could flow 1000 gpm, and could be controlled by a tiny D03 valve that flows 15 gpm. 
 

Next month, I’ll do a session on the various types of pressure valves. Try to contain yourself until then!

Josh Cosford is a certified fluid power hydraulic specialist with The Fluid Power House (Cambridge) Inc. Contact him at joshc@fluidpowerhouse.com or call (519)-624-7109.