In this section, we look at a range of different hydraulic circuits and analyse how they work. We discuss the benefits and drawbacks of each approach including typical applications where they may be used.
Examine a range of load control circuits to understand some of the problems and pitfalls of moving loads with hydraulics. We consider each circuit and discuss the pressure balances that make them work. We show you how to experiment with our valve simulations to explore the circuits and better understand how they work.
Please note that the fluid stiffness values have been reduced in our simulation to better demonstrate the potential problems when using different load control circuits. This can be seen in delays when lifting the cylinder and oscillations when it's stopped suddenly. The issues do appear in actual systems but are only a problem apart for highly dynamic or high accuracy control systems. Examples include:
Hydraulic spool valves maintain clearances between the spool and bore to ensure there is no metal to metal contact and therefore the components do not wear out. Unfortunately, this benefit for the reliability does mean there will always be a small leak past the spool and all cylinders that are not supported will slowly fall the ground. Clearly, this is potentially dangerous and it is not permitted to lift by LOLER (The Lifting Operations Lifting Equipment Regulations) regulations to support loads with just a spool valve.
If a load is supported by a cylinder then there will be pressure against one port of the directional valve. When the valve opens to lower the valve this pressure will act to create a high flow and the cylinder will fall very quickly. This is completely unacceptable as the flow on the annulus side will try to create the same pressure drop except if will be negative. It should be expected that this negative pressure, generated in the cylinder bore, can be greater than the supply pressure available after the directional valve and therefore create aeration or cavitation, inside the cylinder. This is highly likely to create damage to the rods or bore and has the potential to damage the seals by trying to suck them out of their grooves, as seals are not designed to work with negative pressures.
Never allow a load to fall without some control on the loaded side. See meter-out flow controls below.
It is recommended, and normal, to drive a load down against a flow control orifice so that the system always retains pressure on both sides of the cylinder piston. Hydraulics is a braking technology so we drive the cylinder movement against a brake to control it.
Meter-out flow control valves control the flow as it comes out of the cylinder to create the control. They are set to provide the maximum cylinder speed which must be lower than the available pump flow.
Meter in flow controllers are appropriate in some situation but don't provide any load holding capabilities. The phrase to remember is 'If in doubt, meter out'.
The main issue with meter-out flow control valves is that when applied to the cylinder annulus side, the full-bore pressure is amplified through the cylinder area ratio, to give a higher pressure on the annulus area than is seen at the bore side from the pump. Cylinders using meter out flow controllers must be rated as a minimum, at the pump supply pressure times the cylinder area ratio.
Check valves can be used to support the load and stop the cylinder creeping downwards. When they need to be driven down a pilot supply from the driving side of the cylinder is used to open the valve and let fluid past.
Ideally, PO check valves should be located at the cylinder. This is to ensure that there is a minimum of fittings or hoses that could fail and cause the cylinder to fall.
Hydraulic fluid is generally considered to be incompressible i.e. when pressure is applied at one point in a pipe, the same pressure is seen at every other point in the pipe. In reality, hydraulic fluid is compressible and acts like a massive spring. If you hit the end of a cylinder it will bounce like a spring but just very quickly. No hydraulic systems should oscillate at less than 3-4 times per second, but generally, you may expect the cylinder to oscillate at around 10Hz. If you attach a good digital pressure recorder and stop the cylinder quickly enough you should be able to see the pressure ripple in the trace.
This circuit natural frequency is linked to the volume of fluid in the supply lines. Therefore if you increase the length of the supply lines you will lower the circuit natural frequency and reduce the ability of the system to respond dynamically.
When using PO check valves they require a clean signal on their pilot lines to say when they should be open or closed. However, when running proportional valves or meter out flow control valves, a standard PO Check valve may susceptible to pressure getting onto the rear of the poppets and causing the valves to oscillate as the cylinder lowers.
To remove the risk of unwanted back pressures acting on the poppets we can externally vent the PO check valves. This should provide clean switching but does require an extra drain line or 'vent to atmosphere' port which brings an additional weak spot where dirt or water may get into the system.
Counterbalance valves provide both load holding and load control. These can be used in place of the PO check valves, inconjunction with a flow control valve to control the speed. A meter-in flow control valve is used to avoid any risk of back pressure problems.
We can replace our directional valve and the flow control valves with a single proportional control valve, which will regulate the direction and speed of flow. We can now utilise the proportional valve's variable speed control via hand levers rather than spanner adjustment of the flow control valves. However, we will again have the issue of the line pressures potentially upsetting the control valves in the counterbalance valve because the proportional valve restricts the flow in the same way that the meter-out flow control valve does.
When using proportional valves it is recommended to use a vented counterbalance valve to remove the potential for back pressure problems. This will require an extra drain line or 'vent to atmosphere' port which brings an additional weak spot where dirt or water may get into the system.