One of the advantages of variable-displacement pumps with hydraulic or electro-hydraulic displacement control, or load-sensing control, is their low standby pressure. Meaning, when pump output isn’t required, the pump ‘stands by’ at a relatively low pressure, and usually at zero flow (other than that necessary for the make-up of internal leakage).
From an efficiency (heat-load) perspective, the lower this standby pressure is, the better. Because leakage within the pump is a pressure drop and a pressure drop without useful work generates heat. This links back to the reason for having a low standby pressure in the first place.
But how low can you go? And is zero standby pressure best? In the case of a load-sensing pump, standby pressure below 15 bar (220 PSI) can result in control instability. So standby pressures lower than this are generally not recommended for closed center load-sensing pumps.
Variable pumps with hydraulic or electro-hydraulic displacement control have the capability to standby at zero flow and near zero pressure without control issues. But this has an inherent danger: without some internal leakage, (which requires a certain amount of standby pressure) friction within the rotating group can overheat the oil in the pump’s case, resulting in scuffing of lubricated contacts and possible seizure!
In other words, it is NOT a good idea to allow the pump to rattle around completely unloaded for extended periods. If this is likely to occur in a particular application then the housing of the pump should be flushed to keep it cool.
For example, Rexroth recommends that if their A11VO series pumps are on standby for greater than 10 minutes with zero flow, or at a pressure of less than 15 bar (220 PSI), then the pump’s housing must be flushed at the rate of 2 to 6 L/min (0.5 to 1.6 gal/min) depending on pump frame size. This advice means if standby pressure of greater than 15 bar is maintained, flushing is not required. In other words, 15 bar generates enough internal leakage to prevent the oil temperature in the housing from rising to a dangerous level.
So as you now know, allowing a variable-displacement pump to operate below its minimum required standby pressure can be a costly mistake. And to discover six other costly mistakes you want to be sure to avoid with your hydraulic equipment, get “Six Costly Mistakes Most Hydraulics Users Make… And How You Can Avoid Them!” available for FREE download here.