Insane Hydraulics

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Hydraulic Myth NÂș2 - LS stand by pressure is the delta P

It is not uncommon for mechanics to consider the stand-by pressure of a closed center load sensing pump to equal the delta P. (the stand-by pressure being the pressure that you read at the pump outlet when the directional control valve is in neutral and the LS line is vented to tank).

It's not. There's a difference. Sometimes it' negligible, sometimes it's quite substantial. It all depends on the pump's control and swash-plate mechanism, plus the hydraulic circuit itself can have peculiarities that influence the stand-by.

You can have systems that require, say, 15 bar of delta P for correct operation, but will present stand-by of 30 bar. In such a case trying to "squeeze out" the 15 bar stand-by out of the poor controller will result in a badly adjusted system.

The best way to measure and, consecutively, adjust the delta P would be in the mid displacement range, when a pump is pumping, and you're taking the direct delta P reading with a reliable instrument.

Of course, for a known system with a known pump - you (or the OEM) can establish the correct stand-by value and use it to recommend the correct pump adjustment.