I think everyone is familiar with the term “you can’t get blood from a stone.” This directly relates to the lack of system performance when under-sized sensing valves are used in a load sensing circuit. Several times a year we receive calls from people stating that they can’t seem to get the required or anticipated flow out of a valve or a particular leg of their circuit. |
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More times than not, the actual problem is that the full L.S. signal isn’t making it back to the L.S. pump or by-pass compensator, depending upon which type of system you have (open center or closed center). Several things can cause a weak signal including the use of too small of sensing valves or too large of bleed-down orifice. Fig.1 below shows a true static circuit where the use of smaller 04-size Load Sense valves would be acceptable. In this circuit the sensing portion is considered more or less static, because there is not a continuous flow of oil going through the sensing portion of the circuit. Also, the smaller LS04-B30 is adequate since there will be almost no pressure drop across the Shuttle Valve, thus the full signal will make it to the EP (by-pass compensator) valve ensuring proper differential is maintained across each leg of the circuit, which will assure proper anticipated flow.
Note that with a circuit like this, the EP spring should be slightly higher than the EC springs to assure proper functionality. Also, the damping orifices shown should be small enough to ensure stability, yet larger enough not to hinder response times.
Fig.1 above: Static Application of the LS04-B30
Fig.2 below shows a circuit using a L.S. pump with a built in bleed down orifice. Due to the fact that bleed down orifices in pumps typically range from .018”-.031” in diameter, you can expect a bleed down flow rate at 3000 PSI anywhere from .3 to 1 gpm. Having multiple sensing valves in series poses a bigger problem since the pressure drop is additive. This excessive p-drop across the sense valves reduces the margin or differential pressure set by the low standby setting on the pump. Reducing the differential pressure will reduce the flow out of each leg of the circuit. An easy fix for this is to make sure you use larger sense valves like the LS08-30 and CV08-20.
Fig.2 above: Dynamic Application of an LS08-30 and CV08-20
Fig.3 below shows another option for limiting the sense line pressure losses. Using a separate pressure compensated flow regulator instead of a fixed orifice will limit the loss of signal. If the circuit is utilizing a by-pass compensator (EPxx-S35), you can upgrade to an EPFRxx-S35 which has a pressure compensated bleed down orifice built into the spool. Either method will allow the use of the smaller, less expensive 04-size valves.
Fig.3 above: Dynamic Application for CV08-20 and CV04-B20
In summary, to avoid having to turn up the pump compensator or changing out to a heavier spring in your by-pass compensator, please consider proper sizing of the load sensing components.
Scalable, print-friendly PDF of the above circuit drawings: LS.pdf
CAD DXF of the above circuit drawings: LS.dxf
About the Author:
Scott Parker is a Senior Application Engineer at HydraForce.
He’s been developing Hydraulic Systems for over 20 years. Contact Scott
